However, iron limitation sets an upper limit to primary productivity. Here we report on a considerably dense late summer phytoplankton bloom spanning 9000 km2 in the open ocean of the eastern Weddell Gyre. Over its 2.5 months duration, the bloom accumulated up to 20 g C m−2 of organic matter, which is unusually high for Southern Ocean open waters. We show that, over 1997–2019, this open ocean bloom was likely driven by anomalies in easterly winds that push sea ice southwards and favor the upwelling of Warm Deep Water enriched in hydrothermal iron and, possibly, other iron sources. This recurring open ocean bloom likely facilitates enhanced carbon export and sustains high standing stocks of Antarctic krill, supporting feeding hot spots for marine birds and baleen whales.The Southern Ocean is a major sink of anthropogenic CO This study reports a dense, late summer phytoplankton bloom in the Southern Ocean that accumulated unusually high levels of organic matter and supported feeding hot spots for birds and whales. The authors show that this recurring open ocean bloom is driven by anomalies in easterly winds that push sea ice southwards and favour the upwelling of deep waters enriched in hydrothermal iron. This originates from the unique connection between the Southern Ocean and the lower cell of the global overturning circulation3. The Weddell Gyre is a key region of the Southern Ocean where Circumpolar Deep Water (CDW) travels through the Antarctic Circumpolar Current (ACC) at mid depth4, is upwelled to the surface through wind-driven divergence5 bringing high concentration of nutrients and carbon to the surface, and modified to Warm Deep Water (WDW) on its path through the Weddell Gyre6. Later, it is entrained into the deep ocean as Antarctic Bottom Water (AABW) forming at the southern and southwestern borders of the Weddell Gyre7. A recent carbon budget across the Weddell Gyre8, based on observations of inorganic carbon, suggests that intense primary productivity in the region of the Weddell Gyre, away from continental shelves, can explain the carbon drawdown estimated from the carbon deficit directed out of the Gyre below the ACC 10. The strength of the resulting Weddell Gyre carbon dioxide (CO2) sink to depth, at an estimated ~50 Tg C yr−1, is significant when compared to the global ocean biological carbon pump, estimated at 430 Tg C yr−1 across the 2000 m horizon by Honjo et al.11. Satellite-derived ocean color and oceanographic campaigns have however thus far not been able to verify the level of primary productivity required to explain the proposed strength of the Weddell Gyre CO2 sink8.The biological carbon pump in the Southern Ocean, driven by phytoplankton primary production, plays a disproportionately important role in global climate at recent and millennial time-scalesEuphausia superba), marine birds, seals and both baleen and toothed whales. With over 500 million tons of E. superba thriving across its open waters12, the Southern Ocean ecosystem sustains a rich marine wildlife whose life cycles are linked to the short intensive primary production that takes place in specific areas of the Southern Ocean polar waters.Seasonal phytoplankton primary production sustains the Southern Ocean’s rich food-web, dominated by the Antarctic krill , in the Kong Håkon VII Hav, eastern Weddell Gyre ) and Particulate Organic Carbon (POC), as retrieved from a Chl a fluorescence and a backscattering sensor mounted on the Seaglider and calibrated with in situ Chl a and POC measurements (see Methods), were distributed from the surface to 50 m depth (to approximately the base of the surface Mixed Layer (ML)). A subsurface Chl a and POC maximum was however observed at the base of the ML southwards of 68.32° S  and the POC:Chl a ratio (g:g) in the ML was elevated14 .It is believed that we encountered the bloom as it was terminating given that the phytoplankton cells looked chlorotic under light microscopy, the maximum quantum yield of photosystem II, −3 at the base of the ML at the northern edge of the bloom and a maximum estimated POC concentration of 1080 mg m−3 at the base of the ML at the core of the bloom 18, naturally iron-fertilized open waters such as in the lee of the Kerguelen Plateau (maximum POC concentration of 384 mg m−3 and up to 22.5 g C m−2)19 or artificially fertilized open waters (up to 14.4 g C m−2 at the end of the large diatom bloom that was induced by the European Iron Fertilization Experiment (EIFEX) at the Antarctic Polar Front in the Atlantic sector of the Southern Ocean20).This open ocean phytoplankton bloom accumulated striking levels of organic matter over its ~2.5 months duration, with a maximum measured POC concentration of 327 mg moom Fig. . Given toom Fig. . Such hi21 profiles that have been collected in the open Southern Ocean from September 2014 to December 2020, i.e., more than 9500 profiles. We identified only 34 profiles  where the integrated POC over the upper 100 m amounted to more than 15 g C m−2 and up to 19.7 g C m−2 over the whole open Southern Ocean  are typically high and non-limiting. During our campaign, light and macronutrients were not found to be limiting throughout the open ocean bloom . Therefore, the production of such striking levels of organic matter in the open waters of the Southern Ocean must have involved an important iron source and a relatively low grazing pressure at the onset of the bloom as grazing can dominate phytoplankton loss in the Southern Ocean23. During our campaign, dissolved iron (dFe) was relatively elevated24, and probably not limiting, at the sea surface throughout the larger study area, ranging from 0.65 ± 0.2 nM at Astrid Ridge, 0.4 ± 0.1 nM at Maud Rise and 0.6 ± 0.04 nM at 2 stations studied south of the bloom we report here −2 to be between 3.8 and 33.3 μmol Fe m−2 by using lower and upper bound Fe:C ratios from Fe-replete and Fe-deplete Southern Ocean waters (2.3 to 20 mol:mol 10−6)25. Uncertainties on the lower and upper limit of this range are large as they do not account for the role of grazing or vertical export on POC in the ML23, and as POC is both constituted of autotrophic and heterotrophic carbon. A lower estimate could be obtained by considering that phytoplankton only represents 20 to 30% of the total POC26. With this consideration, the necessary iron to accumulate 5 g C m−2  was calculated to be between 1 and 8.3 μmol Fe m−2. An estimation of satellite-derived net primary productivity and the associated iron demands during the 2.5 months bloom period ranged between 14.8 and 42.6 μmol Fe m−2 (see Methods). Together, these observations and calculations highlight that an important iron source or mechanism was required in order to lead to such high levels of POC accumulation inside the open ocean bloom.We calculated the iron necessary to accumulate POC levels up to 20 g C ma in March 2019 follows the deep bathymetric contours at 3500 and 4000 m depth , was larger than the 23-year long mean Chl a + 1 standard deviation over that area  was not significantly different (p > 0.05) between years with and years without an open ocean bloom, and typically initiated on January 15th ±4 days, ended on March 1st ± 3 days, and lasted 44.5 ± 4 days on average.In order to put the observed bloom in a wider context, we inspected satellite-derived ocean color to detect the presence of the open ocean bloom from September 1997 to December 2019 (see “Methods”). Since the surface area of the bloom as observed from satellite-derived Chl pth Fig. , we defia concentration, obtained by averaging de-seasoned anomalies during time periods when the bloom was present shows a large area of positive Chl a anomaly in the open ocean oriented along a northeast-southwest pathway, extending from 55° S to 30° E to 10° W to 70° S .A composite map of satellite-derived Chl ° S Fig.  and impl° S Fig. . Such wiean Fig. . A weak 28.Furthermore, at times when the open ocean bloom takes place, the stronger easterlies push sea ice south, towards the coast. This leads to a negative anomaly in sea-ice concentration in the open ocean west of Astrid Ridge and the opposite positive anomaly in sea-ice concentration along the coast between Astrid Ridge and the Fimbul Ice Shelf Tongue Fig. . By pushThe timing of the wind and sea ice conditions are important for the open ocean bloom. In this region, zonal winds typically increase from January onward, peaking in May . We found that no significant differences existed in preceding winter (i.e. September) maximum ML depths between years with (mean of 124.9 ± 6.3 m) and years without (mean of 113.1 ± 13.3 m) an open ocean bloom observed/(3He/4He)atmospheric − 1) [%]) is a tracer of primordial helium originating from the Earth’s mantle and elevated δ3He ratios (10–12%) are commonly used to detect plumes downstream of hydrothermal vents30. Two sections of δ3He from the Global Ocean Data Analysis Project (GLODAP-2) across the Weddell Gyre, around 0° and 30° E (see “Methods”), and an additional section of δ3He from a recent expedition along 6° E  support hydrothermalism as a major iron source for the open ocean bloom we describe here  between 500 and 100 m depth in the northern part of the section around 50–55° S 29, even though entrainment anomalies are not linked to bloom anomalies in the present study  (between −0.07 at the surface and 0.4‰ at 1000 m depth) from samples collected by a CTD-rosette cast at the northern edge of the bloom (see Methods), suggesting that sea ice meltwater is similarly not a major source of iron for this open ocean bloom38.Considering other known iron sources, atmospheric deposition of iron plays a minor role at these latitudesChaetoceros dichaeta, which represented 39–67% of the total phytoplankton abundance as evidenced from surface underway samples (intake at 4 m depth) and between 31 and 47% of the total phytoplankton abundance within the upper mixed layer, i.e., down to 40 m depth as evidenced from CTD stations . The second most abundant phytoplankton species in this blooming assemblage was the centric diatom Dactyliosolen antarcticus, but it only accounted for a maximum of 10% of total phytoplankton abundance on any occasion.In addition to high biomass, the taxonomical composition of this open ocean bloom has important biogeochemical and ecological consequences. The bloom was dominated by the centric diatom Chaeotoceros dichaeta is a large (~10 to 50 µm) chain-forming diatom species with prominent spines. It can dominate phytoplankton blooms in the coastal and the sea ice zone around Antarctica, particularly in summer, whereas, in frontal regions, it can be dominant in the fall39. Due to their prominent spines, members of the Chaetoceros genus, including C. dichaeta, easily form rapidly sinking aggregates when undergoing mass mortality and are considered to be important carbon exporters40. The large bloom of C. dichaeta we observed over deep waters in the eastern Weddell Gyre was actively contributing to deep carbon export as shown by the large optical spikes created by particles and phytodetritus (i.e Chl a) below 100 m and throughout the bloom area 42 and efficiently export carbon to the deep sea and the sea floor. A similar event could be shown during the EIFEX fertilization experiment which took place north of the ACC, northeast of the Weddell Gyre, where C. dichaeta, contributed significantly to carbon export 24 days after bloom fertilization20. In this instance, half of the bloom biomass sank below 1000 m, and a phytodetritus layer composed of intact cells of C. dichaeta was observed on the seafloor at > 3000 m depth, suggesting that blooms of C. dichaeta contribute significantly to deep carbon sequestration in the Southern Ocean.Euphausia superba) was mostly observed  in two areas: in the vicinity of the open ocean bloom we describe here  outside the open ocean bloom and throughout our entire survey across the Weddell Gyre. In addition, we observed high numbers of humpback whales foraging in the area of the open ocean bloom , but did not report similarly high local abundance numbers like the ones we observed in the bloom area. As shown in previous studies48, the distribution of flying seabirds and baleen whales reflects the predictable distribution of their main prey, the Antarctic krill. Krill is a key prey item for numerous Antarctic species because of the extraordinary size of their swarms49 and their overall dominance in the Southern Ocean12, making it an easily accessible and profitable prey. It is, therefore, very likely that the large number of flying seabirds and baleen whales we observed in the open ocean bloom area were attracted by the presence of Antarctic krill.In addition to Antarctic krill, we observed very high abundances and densities of Antarctic and snow petrels . This possibly brings new insight into the foraging strategy of Antarctic petrels along the Dronning Maud Land coast and highlights this open ocean bloom as their possible favored feeding ground during the breeding season.Dronning Maud Land hosts a large population of seabirds, and in particular half of the World’s Antarctic petrel population, which represents four to seven million breeding Antarctic petrels54. Therefore, our observations along with the results from previous studies highlight this region as an important foraging area for Antarctic wildlife. Although unpredictable, these re-occurring open ocean blooms are a possible key feature of the Weddell Gyre marine ecosystem.Our study provides further evidence that humpback whales feed in the open waters of the eastern Weddell Gyre. These findings are consistent with two recent acoustic studies showing that humpback whales utilize this area during the typical open ocean bloom period: humpback whale vocalizations were recorded from March to May at Maud Rise and almost throughout the year near Ekström Ice Shelf with peak occurrence in February to May55 or for Antarctic petrels that are raising and fledging chicks in February–March51.The evidence we report here points towards the importance of this open ocean bloom for the eastern Weddell Gyre ecosystem; particularly the characteristics directly observed at sea (i.e. high phytoplankton carbon biomass and abundant krill), its recurrence during favorable atmospheric and oceanographic conditions , the likely additional sources of iron sustaining it , and its seemingly long-term significance for the biological carbon pump and top trophic level consumers . The timing of the wind and sea ice conditions are important for the open ocean bloom which typically happens in late summer, i.e., most often in February. Such a productive bloom late in the summer is significant for example for krill that need to prepare for the long austral winter56 due to poleward intensifying winds and increases in meltwater over the last two decades, which could have large consequences on the fate of this open ocean bloom. Given its seemingly importance for the Weddell Sea, the driving forces and the fate of this key ecosystem feature warrant future studies.The next legitimate question concerns the fate of this open ocean bloom and its associated ecosystem in the context of climate change. A recent study described major changes in the Southern Ocean physical and biogeochemical propertiesN = 5) as well as from a SBE 32 carrousel water sampler connected to a CTD (conductivity-temperature-depth) SBE911+ system which was deployed to calibrate the glider sensors at the glider deployment station . Seawater was sampled to measure the concentrations of chlorophyll a , Particulate Organic Carbon and Nitrogen (POC and PON), the phytoplankton species composition, phytoplankton physiology, and stable oxygen isotopic ratio (δ18O)).Seawater was collected from the ship’s scientific seawater intake at 4 m depth throughout the large phytoplankton bloom (a was measured with reverse-phase HPLC including a Agilent Technologies Microsorb-MV3 C8 column (4.6 × 100 mm), a Waters Alliance HPLC System and a photodiode array detector (2996). HPLC-grade solvents (Merck) and the EMPOWER software were used in the analysis. More details can be found in Tran et al.57.Water samples (1 L) for HPLC analyses were filtered on 25‐mm Whatman 0.7 um GF/F glass fiber filters  under low vacuum pressure and then stored at −80 °C until analysis ashore. Chlorophyll Water samples (1 L) for POC analyses were filtered on pre-combusted 25‐mm Whatman 0.7 um GF/F glass fiber filters  under low vacuum pressure, dried at 60 °C and stored at room temperature in PALL filter slides until analyses ashore. 80 μL of 10% v/v hydrochloric acid was added to each filter to remove any inorganic carbon before the filters were folded into aluminum capsules. POC concentrations were then measured with a C/N analyzer Europa Scientific, ANCA-MS 20-20 15N/13C mass spectrometer at the Tvarminne Zoologica Station, Finland. POC values were corrected for dry filter blanks.58.Samples for phytoplankton species counts by microscopy (190 mL) were filled into 200 mL brown glass bottles and fixed with 25% glutaraldehyde and 20% hexamethylenetetramine-buffered formalin solution at final concentrations of 0.1 and 1%, respectively, and thereafter stored cool and dark until analyses ashore. For analysis, 10–50 mL Utermöhl sedimentation chambers  were used for settling sub-samples for 48 h and Nikon Ti-S inverted light microscope for identification and counting by the Utermöhl methodTM fast repetition rate fluorometer (FRRf) integrated with a FastActTM laboratory system. After collection, samples were dark acclimated at in situ temperatures for 30 min before measurement. The active Chl a measurements consisted of a single turnover protocol with a saturation sequence  and a relaxation sequence . The sequence interval was 100 ms which was repeated 32 times resulting in a total acquisition time of 3.2 s. The power of the excitation LED (λ450 nm) was adjusted between samples to saturate the observed transients following the manufacturer specifications. Each sample underwent a fluorescence light curve which measured the active Chl a fluorescence at 15 light levels between 0–1500 μE m−2 s−1, with an optimized duration per light level consisting of 12 acquisitions per light level, except for the first light level (10 μE m−2 s−1) which consisted of double the number of acquisitions (24). All samples were blank corrected using carefully prepared 0.2 μm filtrates59.Measurements of phytoplankton photophysiology were performed using a Chelsea Scientific Instruments FastOcean18O analyses, samples were collected in 25 mL Wheaton bottles. The caps were sealed with Parafilm® and the bottles were stored in +4 °C and dark conditions until analysis in the laboratory. The analysis of δ18O was done with a Thermo Fisher Scientific Delta V Advantage mass spectrometer with Gasbench II. The δ18O data were standardized to Vienna Standard Mean Ocean Water (VSMOW). The reproducibility of the replicate analyses was ±0.04‰.For δ60 using online pre-concentration methods. All samples were measured in duplicate. The detection limit of Fe was 0.08 nM.For the analyses of Dissolved labile (dFe) and total acid leachable (TaLFe) iron, water samples were collected at 12 stations with 5 L acid-cleaned Teflon-lined GO-FLO bottles deployed on a 500 m aramid rope (6 mm-vpg industri), using a dedicated winch and Teflon coated messengers. The samples were collected at 8 discrete depths . After water collection, the GO-FLO bottles were double bagged and taken to a makeshift clean room, placed on a rack under clean air blowing from a Class-100 laminar flow hood and drained into acid washed 125 mL low-density polyethylene (LDPE) Nalgene bottles. Water for TaLFe determination was collected directly into the LDPE without filtration. For dFe samples, water was filtered through Sartorius filters (0.45 + 0.2 μm pore size filtration) using acid-washed Tygon tubes. The dFe fractions were defined operationally by the 0.2 μm nominal pore size. During filtration, an additional HEPA air-filter cartridge  was connected to the pressure relief air-vent valve of the GO-FLO bottles to ensure that the air in contact with the sample during the filtration was clean. All samples for dFe and TaLFe were acidified to pH 1.7–1.8 with ~3 M double quartz distilled HNO3 (UP HNO3). The acidified water samples were stored (> 1 year) until analysis at Stellenbosch University  as described in Samanta et al.http://norgliders.gfi.uib.no/), profiling continuously between the surface and 1000 m depth and measuring a suite of parameters that includes conductivity , temperature, depth (CTD), dissolved oxygen, Chl a, and optical backscattering by particles as a proxy for POC. The Seaglider Chl a fluorescence and backscattering signals were calibrated against the Chl a and POC concentrations measured during a calibration CTD cast shortly after the Seaglider deployment (2.5 h). The Seaglider Chl a fluorescence and backscattering signals were processed following the glider toolbox described by Gregor et al.61. In situ dark counts were calculated from the 95th percentile between 400 and 600 m and removed from the data. A 3-point rolling mean was subtracted from each profile to de-spike fluorescence and backscattering profiles. The mixed layer depth (ML) for each profile was computed as the depth to which temperature exceeded 0.04 °C of the surface temperature, which was a good indicator of the bottom of the ML based on visual inspection of the profiles and general Temperature and Salinity properties in the area15.An autonomous Seaglider (SG563) was deployed and recovered during the cruise, collecting vertical profiles during 31 dives along a 55 km transect across the bloom region between March 12 and March 15, 2019 Fig. . The glia concentration maps are extracted from the Ocean and Land Colour Instrument (OLCI) Level-2 full resolution Near Real Time (NRT) product, obtained by the EUMETCast broadcast system. The OLCI sensors on board ESA Sentinel-3A and B satellites launched in February 2016 respectively April 2018, and have large swath widths (~1270 km) covering large regions with high temporal resolution . The OC4ME algorithm which uses a polynomial approach of a maximum band ratio algorithm of 4 reflectances at 443, 490 and 510 nm over the 560 nm was extracted directly from OLCI Level 2 products and gives the Chl a pigment concentration in [mg/m3]. The cloud free chlorophyll orbit tiles are binned and averaged on a daily base to obtain one image per day. This leads to a very good resolution, coverage in reasonable timeframe and meets the requirements of monitoring water dynamics also on smaller spatial scales.The Chl https://oceandata.sci.gsfc.nasa.gov/SeaWiFS/) and MODIS  level 3 Chl a data processed with the default chlorophyll algorithm (chlor_a) which employs the standard OC3/OC4 (OCx) band ratio algorithm merged with the color index (CI) in 8-day and 9 km resolution.The analysis of satellite-derived ocean color was complemented using SeaWiFs , with prior processing using the NASA team algorithms62.Daily sea ice concentration (1997–2020) were obtained from the NASA’s Nimbus‐7 Scanning Multichannel Microwave Radiometer (SMMR) and Defense Meteorological Satellite Program (DMSP)‐F13, ‐F17, and ‐F18 Special Sensor Microwave/Imager (SSM/I). Data with a spatial resolution of 25 km were provided by the National Snow and Ice Data Centre, University of Colorado in Boulder, CO  using the mean ratio of Fe:C (4.3 μmol:mol) generated for the Sub-Antarctic Zone of the Southern Atlantic Ocean64.To calculate Net Primary Productivity (NPP) in the area where we detected the bloom, we used a 2.25-degree bounding box around the position of the station where we deployed the glider and calculated the mean monthly NPP from satellite data products . We further converted NPP from all methods into Fe demand  SBE911+ system. For helium analyses, water samples were collected into copper tubes, which were clamped off after sampling and later analyzed at the IUP Bremen noble gas mass spectrometry lab HELIS. Samples were processed initially with an ultra-high vacuum gas extraction system. For analysis of the noble gas isotopes  the extracted gas is transferred into a fully automated mass spectrometric system equipped with a two-stage cryogenic trap system. The system is calibrated using atmospheric air standards on a regular basis and measurements are made for blanks and linearity. For details, see Sültenfuß et al.21 from September 2014 to December 2020. Data were collected and made freely available by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project funded by the National Science Foundation, Division of Polar Programs (NSF PLR −1425989 and OPP-1936222), supplemented by NASA, and by the International Argo Program and the NOAA programs that contribute to it . The Argo Program is part of the Global Ocean Observing System. We considered the profiles where the integrated POC in the upper 100 m was above 15 g C m−2. In addition, to avoid comparing our data with episodes of eddy-subduction pump which can send high concentration of organic matter in one short-time process, we removed 15 profiles where the integrated POC in the upper 100 m was above 15 g C m−2 but more than 90% of the POC was found below the surface mixed layer.We used all SOCCOM BGC-Argo floats profiles that measured upper ocean POC concentration in the Southern Ocean3 . We then averaged the obtained mixed layer depths to monthly means.We calculated the mixed layer depth in the area of the bloom between 2002 and 2020 by using all Argo floats that drifted through the area within 4 to 8E and −69 to −67S, i.e., total of 1024 profiles of temperature, salinity and depth. Furthermore, we removed profiles earlier than 2007 due to large seasonal gaps. From all profiles, we calculated the mixed layer depth as the depth where the potential density exceeds its 10 m value by 0.03 km/m68. During the campaign, we used two transducers: one on the drop-keel (3 m from the hull when down) and one hull-mounted. The latter was used in ice-covered areas, with low impact on the detection of krill swarms15. The density of krill was calculated as nautical area scattering coefficient (NASC)69.The abundance of krill was studied with a Simrad EK80 research echosounder with six frequencies. For this study, we focused on the 38 kHz frequency. It was scrutinized with the Large Scale Survey System (LSSS) software version 2.5.02 mouth-opening and 3 × 3 mm diamond shaped mesh (7 mm stretched) from mouth to rear. Towing speed was normally 2.5–3 knots. The velocity of the trawl through water and depth of the trawl were monitored by a depth sensor (SCANMAR) attached to the headline. From each trawl, a subsample of approximately 150 individuals of Euphausia superba was taken, and the length of the individual krill was measured (±1 mm) from the anterior margin of the eye to tip of telson excluding the setae. Sex and maturity stages of E. superba were determined using the classification methods according to Makarov and Denis70.Krill swarms were sampled with a Macroplankton trawl at two locations within the bloom region , during daytime and decent weather conditions . Observations were only recorded during ships transits. Marine mammals were primarily recorded from 270-0° (0° being the bow of the ship), on the port side of the ship. Sea birds were both recorded continuously, counting every bird within 300 m in a sector from the bow (0°) of the ship to 90° in 10 min periods, and with point counts every 30 min, identifying ship’s followers behind the ship. As many birds follow the ship for some time, this means double counting the same individuals happens as birds that are counted during a 10-minute period will be counted again during the next 10-min period if they are still present. This method is therefore not for population estimates, but more suitable for density assessment.52. The utilization distribution maps (or heat maps) were created with the kernelUD function of package adehabitatHR in R software version 4.0.2, using the href (reference bandwidth) smoothing parameter (https://www.rdocumentation.org/packages/adehabitatHR/versions/0.4.19/topics/kernelUD).Breeding adults were captured on theirs nest during late incubation and mid chick rearing (from early January to mid-February) and instrumented with Global Positioning System (GPS) loggers  just before leaving on a foraging trip. The GPS units weighed approx. 20 g (ca. 3% of bird body mass) and were taped to tail feathers. Birds were recaptured upon return to their nest to retrieve the GPS units and download the data. GPSs recorded the locations of the birds along their foraging trip at 5 to 30 min intervals. In total, data were available from 161 tracks collected in January/February 2012, 2013, 2014, 2016 and 2018. Additional details about the procedure are given inhttps://www.eoas.ubc.ca/~rich/map.html in November 2021.Data analyses were conducted with Matlab R2019b. All maps were built on QGIS (v. 3.20.0) and using the m_map package as downloaded from Further information on research design is available in the Supplementary InformationPeer Review FileReporting Summary


In recent times, particularly in applications used to build various structures for construction purposes or machines, solid sections have been gradually replaced by hollow sections due to their attractive features such as being light weight and having high specific strength. In the present investigation, an attempt was made to investigate, in detail, the flexural capability of aluminium hollow tubes (AHTs) with square cross-sections. The objective of the investigation was to study the influence of stress concentration on the flexural behaviour of the hollow tube. The stress concentration factor considered in this investigation was holes of various cross-sections and quantities. Three-point bending tests with concentrated loads were conducted on specimens of a hollow tube with different stress concentrations such as circular holes, multiple circular holes, square holes and perforations. The load was applied manually during the bending test with appropriate increments. The bending test was carried out on specimens with support spans of 110, 130, 170 and 200 mm. The output measures of the study were maximum bending load, deflection and flexural stiffness. The output measures were analysed in detail in order to recommend the type and nature of stress concentration in a hollow tube applied to structural applications to ensure the safest workability. The flexural stability of the tube was analysed by experimental and numerical procedures, and the results were validated using an analytical approach. It was found that the results of all the approaches complement each other with a low significance of error. AHTs with a circular hole, multiple circular holes and perforations were observed to have better flexural stability than other AHTs such as AHTs with square hole and plain AHTs. In recent times, most of the engineering applications in the automobile, aerospace, structural member, etc., industries have consistently replaced conventional bulk materials with high-performance and high specific-strength materials such as super alloys and composites and are slowly replacing the solid sections with hollow sections, which not only minimizes the amount of material used in construction but also increases their strength and stability by several times, meeting the desired requirements for which it is intended for. Conventional bulky materials such as steel and iron were replaced by aluminium alloys, magnesium alloys, stainless steel, composite materials, etc., which offer superior specific strength and are economical too. The structural members used in engineering applications are available in many sections, either in the solid or hollow types. Hollow sections have more specific strength than that solid sections. Hollow sections are lighter than solid sections and, therefore, mostly preferred in many structural applications. Square and circular hollow tubes have many applications in the area of multidirectional loading due to their uniform geometry along two or more cross-sectional axes. This, in turn, has uniform strength across the complete structure, which makes them good choices for many structural applications. In spite of heat treatment and super finishing of the materials used in the structural applications, certain machining operations such as drilling, threading, riveting, etc., are mandatory in the case of assembling the structures. These secondary machining operations include the concentration of heavy stress at localized spots around the discontinuities such as holes, notches, etc., that exist in the members for fasteners or any assembly work ,2,3. ThiA hollow section used for a structure is a type of construction used in many structural applications. It can be circular, square, rectangular, or any other section based on the need. Square and circular hollow tubes have many applications in the area of multidirectional loading due to the uniform geometry along two or more cross-sectional axes. They have uniform strength across the complete structure, which makes them good choices for many structures ,6,7,8. TThe flexural behaviour of hollow tubes was investigated. The material used in the investigation was commercially available aluminium hollow tubes of square cross-section with dimensions of 20 × 20 × 1.5. The aluminium used in the hollow tube was Al 6061. The composition and properties of AA 6061 are shown in The flexural capability of the aluminium hollow tubes (AHTs) was investigated in detail by conducting three-point flexural tests in a Universal tester machine of 20 kN capacity . A concentrated load was applied at the mid span of the specimen, perpendicular to the axis of the tube. The analysis of flexural capability of AHTs was carried out as a plain specimen and specimen with stress risers as well. Holes of different shapes and types were introduced to the AHTs in order to investigate their influence on the flexural capability of the specimen. Five different types of specimens were used in this flexural study. Plain AHTs, AHTs with a through square hole at the midspan on one of the lateral faces with dimension 10 mm, AHTs with through circular hole at the midspan on one of the lateral faces with dimension 5 mm, AHTs with multiple circular through holes at the midspan equally spaced on one of the lateral faces with dimensions 10 mm and 5 mm, and AHTs with perforations of multiple through holes of diameter 10 mm and 5 mm on adjacent lateral faces equally spaced throughout the length. Each test was repeated three times to obtain the results more accurately. The experiment was performed at normal room temperature, and the load applied during the test was manual. The output of the flexural test was taken through a data acquisition system (DAQ), attached to the machine and displayed. The performance measures considered in the flexural test were bending load, deflection and flexural stiffness. The critical bending load capacity and deflection were measured at the midspan of the specimen. The critical bending load was considered an indicator of the maximum resistance offered by the specimen against bending with minimum deflection. The flexural stiffness was calculated using the empirical relation between the critical bending load and the deflection. It was the measure of resistance against deformation, which was greatly influenced by the geometrical properties of the specimen and the loading characteristics. The schematic diagram of flexural test setup is shown in The flexural performance measures such as maximum bending load, maximum deflection and flexural stiffness, which were determined experimentally, are shown in Flexural stiffness was observed to be maximum for AHTs with circular holes and perforations compared to that of AHTs with square holes and plain AHTs as well. This was due to the fact that creating additional holes on the specimen on either side reduced the effect of stress concentration, which in turn increased the stiffness of the specimen. The area moment of inertia of the specimen about minimum cross-sectional area was reduced for the specimens where the stress concentration effect was reduced by additional holes. This in turn increased the resistance to deformability or the flexural stiffness of the specimen under loading. The highest flexural stiffness in the range of 16 to 18 kN/mm was observed for AHTs with square holes. In order to ensure the consistency and accuracy of the results obtained through experimental approach, a numerical analysis using ANSYS software was made with constraints and boundary conditions similar to those of the conditions and assumptions made in the experimental approach. The results of the numerical study are listed in (i)The support span aspect ratio (S/t) significantly influenced the flexural behaviour of plain AHTs and AHTs with holes of different quantities and geometries.(ii)Following the support span aspect ratio, the type of discontinuity in terms of shape, size and quantity significantly influenced the flexural stability of AHTs. This may be due to the fact that these discontinuities acted as stress risers and influenced the flexural capacity of the structure to a larger extent.(iii)AHTs with a circular hole, multiple circular holes and perforations were observed to have better flexural stability than that of other AHTs such as AHTs with square holes and plain AHTs. This may be attributed to the fact that the sharp corners in the holes are the sources of crack initiation and propagation, which in turn lead to the failure of the specimen under loading.(iv)Reducing the effect of stress concentration abruptly increased the flexural behaviour of AHTs by offering better flexural resistance.(v)The results obtained through experimental and numerical approaches complemented each other with utmost accuracy and consistency. The assumptions and constraints made during the analysis were very close and complemented each other.An attempt was made successfully to study the flexural stability of aluminium hollow tube (AHTs) with different discontinuities in the form of holes of different geometries such as circular holes, square holes, multiple circular holes and perforations. The aspect ratio support span to thickness of the tube (S/t) was considered as the process parameter during the flexural test of AHTs. The study was performed in two aspects, experimental and numerical, and fruitful outcomes and conclusions were drawn from the study and listed below.


Shortcomings to tibial-side fixation have been reported as causes of failureafter anterior cruciate ligament reconstruction. Adjustable-loop suspensorydevices have become popular; however, no comparison with hybrid fixation exists to our knowledge.The purpose of this study was to compare the biomechanical properties ofadjustable loop devices (ALDs) in full-tunnel and closed-socketconfigurations in relation to hybrid fixation. We hypothesized that primarystability of fixation by a tibial ALD will not be inferior to hybridfixation.Controlled laboratory study.Tibial fixation of a quadrupled tendon graft was biomechanically investigatedin a porcine tibia–bovine tendon model using 5 techniques (n = 6 specimenseach). The tested constructs included hybrid fixation with a corticalfixation button and interference screw (group 1), single cortical fixationwith the full-tunnel technique using an open-suture strand button (group 2)or an ALD (group 3), or closed-socket fixation using 2 different types ofALDs (groups 4 and 5). Each specimen was evaluated using a materials testingmachine (1000 cycles from 50-250 N and pull to failure). Force at failure,cyclic displacement, stiffness, and ability to pretension the graft duringinsertion were compared among the groups.P < .001); however, no difference was observed ingroups 3, 4, and 5 as compared with group 1. The remaining tension on theconstruct after fixation was significantly higher in groups 3 and 4 vsgroups 1, 2, and 5 ,irrespective of whether a full-tunnel or closed-socket approach wasused.No differences in ultimate load to failure were found between the ALDconstructs  and hybrid fixation (group 1). Cyclicdisplacement was significantly higher in group 2 vs all other groups(Tibial anterior cruciate ligament graft fixation with knotless ALDs achievedcomparable results with hybrid fixation in the full-tunnel and closed-sockettechniques. The retention of graft tension appears to be biomechanicallymore relevant than tunnel type.The study findings emphasize the importance of the tension at which fixationis performed. A quadrupled semitendinosus graft provides a sufficient length in most cases and,in general, is superior in diameter as compared with a 4-strand semitendinosus-gracilisgraft. One reason for short- and long-term failure is the fixation method, particularlyon the tibial side. Despite the superiority of hybrid fixation with combined intratunnel  and extratunnel  devices, single interference screw fixation at the tibia is a common practice.In anterior cruciate ligament (ACL) reconstruction, a 4-strand hamstring autograft is acommonly used graft option. Nevertheless, many surgeons hesitate to adopt the closed-socket technique withconcerns of the lacking aperture fixation, which has been suspected to cause elongationand delayed graft-to-bone healing. Although some studies have revealed superior graft incorporation related to theclosed-socket technique, others have shown tunnel widening and assumed a “bungee” or “windshield wiper”effect to be responsible for increased graft motion and reduced graft-to-bone healing.With a closed-socket technique , the use of aninterference screw is not practiced. Several potential advantages of closed-socket ACLreconstruction have been described, such as bone preservation attributed to the drillingof closed-socket tunnels, lower postoperative pain scores, and the ability to retightenthe construct using adjustable loop suspensory fixation devices. there has been no study comparing these constructs, to our knowledge. In thecurrent study, we used a porcine tibia–bovine tendon model to compare adjustable loopdevices (ALDs) with hybrid full-tunnel fixation at the tibia. We hypothesized that thebiomechanical properties of tibial fixation with ALDs  will not differ significantly from hybrid fixation,irrespective of conducting the full-tunnel or closed-socket technique. In a secondaryanalysis, we assessed the relationship of construct tension before testing to cyclicdisplacement after preconditioning.While it appears from a biomechanical point of view that suspensory closed-socket devicesare not inferior to single interference screw fixation, A total of 30 fresh-frozen porcine knees and bovine extensor digitorumtendons were obtained from the local butcher. Six grafts were tested for eachfixation method. All procedures were performed by 2 orthopaedic senior registrars(E.A. and A.H.) of the hospital’s knee team.Five fixation methods were investigated in a commonly used porcine tibia–bovinetendon in vitro model .1,15,17The full-tunnel technique was used in groups 1, 2, and 3, testing 3 fixation methods.Group 1 consisted of a hybrid fixation using a 9-mm bioabsorbablepoly interference screw  and a cortical fixation device . An isolated singlecortical suspensory fixation (Endotack) was tested in group 2. In group 3, an ALDbased on a modified sling lock mechanism was used. As a larger surface is necessary to cover the naturally larger bone tunnelentrance in the full-tunnel technique, a custom-made extension button was machinedfor this study .In groups 4 and 5, a closed-socket tunnel configuration was applied . Fixation in group 4 was performed with the same ALD as ingroup 3 without using the additional extension button. In group 5, an ALD based onthe finger trap mechanism was tested .After thaw and removal of the femur and all soft tissue from the tibias, the bonetunnels were drilled. For groups 1, 2, and 3, full tunnels  were drilled from the anteromedial tibial surface to the anatomicporcine ACL footprint using a target guide, a guide wire, and a cannulated 8-mmbone drill. To create closed-socket tunnels for groups 4 and 5, a 30-mm socketwith a diameter of 8 mm was drilled over the guide wire using a retrograde drillstarting from the ACL footprint. The remaining 10 mm was completed with acannulated 3.5-mm bone drill. All tendons were prepared on a graft preparationboard and sized to a length of 240 mm, and tendon ends were baseball-stylewhipstitched in both directions over the last 2 cm with No. 2 FiberWire sutures(Arthrex).The tendon grafts of group 1 and 2 were quadrupled and distally enlaced with aNo. 6 Ethibond loop (Ethicon Inc). For the ALD in groups 3 to 5, sutures of bothgraft ends were knotted with 7 surgeon’s knots and manually tightened to achievecontinuous loop grafts that were quadrupled . This alx-ytable with the mounted tibias allowed unconstrained travel in thex and y directions. The femoral side wasrepresented by a steel hook over which the quadrupled graft was folded. Thesteel hook was connected to a load cell and the uniaxial drive of the tensionmachine.Before graft insertion, tibias were mounted on a universal materials testingmachine  . Each tiBefore insertion, the diameter of the quadrupled graft was measured with agraft-sizing block resulting in an either 8- or 9-mm graft. Bone tunnels andsockets were augmented with a 9-mm dilator for 9-mm grafts. Quadrupled grafts ofthe full tunnel in groups 1 to 3 were pulled from distal to proximal through thebone tunnel with a passing suture and set in the hook proximally. Intratunnelfixation of group 1 was achieved with a 9-mm–diameter, 23-mm–length interferencescrew  that was inserted over a guide wire while as muchtension as manually possible was applied on the distal suture ends. Corticalfixation of groups 1 and 2 was performed by passing the sutures through anEndotack suspensory button and knotting the sutures 7 times. In group 3, thegraft loop was also pulled through the bone tunnel from distal to proximal andfixed cortically on the tibia using the sling lock device and the custom-madeextension button. Manual tension was applied to tighten the sling lock mechanismbefore cutting the sutures 10 mm distal of the blocking mechanism. In each group, graft fixation was performed with the maximal tension thatwas manually achievable with the fixation mechanism.The grafts in groups 4 and 5 were inserted from the articular side through thebone tunnel with a passing suture until the grafts reached a depth of 25 mm inthe bone socket. The implant-graft constructs were then tensioned with theadjustable loop systems. While suture ends of group 4 were cut 10 mm distal fromthe modified sling lock mechanism, 5 surgeons’ knots were applied on the sutureends of group 5, as recent studies reported better mechanical properties afterknot tying with a button based on the finger trap mechanism.Biomechanical testing was performed in 3 phases. In the first phase, theconstructs were preconditioned with 10 cycles between 10 and 50 N, representinga phase of low stress to the construct. In the second phase, cyclic testing from50 to 250 N was initiated . An ultimatepull-to-failure test was conducted at a speed of 20 mm/min in the third phase.Maximum tensioning forces (in newtons) on the graft were recorded during andafter insertion, and the tension for each construct was assessed after corticalfixation and before testing. The displacement after preconditioning wasmeasured, representing the initial laxity just after surgery. Cyclicdisplacement (in millimeters), stiffness (in newtons per millimeter), andultimate failure force (in newtons) were determined per the recordedload-displacement curve. A priori power calculations were performed for cyclic displacement basedon pilot and literature data. Effect sizes for this experiment were anticipated to be large (Cohend ≥ 3); therefore, 5 samples per group seemed to beadequate to yield sufficient statistical power (P 0.98).For cyclic tests without complete unloading, a sample size of 6 per group wasadapted from previous literature.Standard software  was used for statisticalanalysis. Kolmogorov-Smirnov testing was used to test whether the values wereadequately fit by Gaussian distributions. Retained tension, stiffness, cyclicdisplacement, and ultimate tensile strength were compared among fixation devicesusing 1-way analysis of variance, and a Tukey multiple-comparison test was usedfor post hoc testing (α = .05). Data that were nonnormally distributed werecompared using Kruskal-Wallis analysis of variance.P = .013). Regarding cyclicdisplacement, similar values were found in all groups except in group 2 (full CFD),in which the displacement was significantly higher than all other groups(P < .001). In accordance, stiffness was lowest in group 2(full CFD) and differed significantly from group 1 ; this was the only significant difference in stiffness.P < .02for all comparisons) (R2 = 0.85) between those factors (The remaining tension on the construct after graft fixation was significantly higherwhen fixation was performed with ALDs using the modified sling lock mechanism(groups 3 and 4) as compared with groups 1, 2, and 5 (arisons) , Table 2 factors .The most important findings of our study were first that tibial ACL graft fixationusing a knotless ALD (groups 3-5) was mechanically comparable with hybrid fixation(group 1) regarding total load to failure, stiffness, and cyclic elongation.Furthermore, ALDs (groups 3-5) provided comparable fixation properties with hybridfixation (group 1) irrespective of a full-tunnel or closed-socket technique.Conversely, solitary cortical fixation with an open-suture strand button (group 2)was inferior. Finally, analysis of performance after cyclical loading showed thatretained graft pretension depended strongly on the device and related technique,favoring devices that limit slippage during loading. In the present study, no difference was found with hybrid fixation (group 1)when single extracortical fixation was performed with an ALD (groups 3-5). Moreover,it appears that tibial fixation with ALDs can compete with hybrid fixationindependent of the performed technique, since a significant difference did not occurin the full-tunnel technique (group 3) or the closed-socket technique (group 4 and5) when these ALDs were applied.Preexisting literature has demonstrated superiority of tibial hybrid fixation ascompared with single interference screw and single cortical suspensory fixation. However, this inferiority of single cortical suspensory fixation does notapply to the tested ALDs (groups 3-5).A solitary cortical suspensory fixation with an open-suture strand button in thefull-tunnel technique (group 2) seems to be insufficient, showing significantlyinferior fixation properties as compared with the tested ALDs (groups 3-5) and thehybrid fixation (group 1). This finding stands in accordance with former studiesthat have reported better fixation properties with hybrid fixation in a full-tunnel technique. The fixation mechanism is based on a double-pulley system allowing thesurgeon to pretension the graft with 4 times the manual pulling force. Here asmaller button designed for a closed-socket fixation was adapted within a largerhousing to accommodate use in a full-tunnel technique. This housing was sized toaccord with the CFD (Endotack button) tested in group 2 and is not available on themarket; however, the device is comparable with other commercially available productsthat help enlarge the surface of ALDs in a suspensory full-tunnel technique. Cyclicdisplacement with this double-pulley ALD in the full-tunnel technique (group 3) wassignificantly lower than the open-suture strand button (group 2) and did not differfrom hybrid fixation (group 1) or the ALDs with the closed-socket technique (groups4 and 5).In the present study, an ALD based on a modified sling lock mechanism was tested in afull-tunnel technique (group 3).Regarding final failure of fixation constructs, it appeared that fixation with theclosed-socket technique is prone to bone–button interface failure, whereas the mostcommon mode of failure with the full-tunnel technique was suture rupture. In theclosed-socket technique, small buttons can be applied, which might be beneficial forslim patients. However, the smaller buttons come with higher pressure on thecortical surface and should be used carefully when bone quality is suspect. In thefull-tunnel technique, stronger suture material could increase the maximum load atfailure, yet the mean maximum load at failure in a worst-case scenario was >800 Nin all but group 2 (full CFD). As such, ultimate failure of tibial fixation seems tobe a minor concern vs cyclic displacement that occurs owing to repetitivestress. Improved ability to retain tension through use of an ALD is generallyconsistent with the stronger fixation of these devices as compared with theopen-suture strand button. Our data suggest that performance differences related toa surgeon’s choice between the closed-socket and full-tunnel techniques diminishwhen one uses full-tunnel devices that effectively retain graft tension. Moreover,the characteristic of tensioning is a general advantage of ALDs that is not reservedfor tibial fixation, and surgeons preferring a tibial interference screw mayconsider using an ALD for femoral fixation to have the opportunity to retension thegraft from proximal after tibial fixation.On the premise that fixation quality is reflected as retained graft pretension, weobserved a clear nonlinear relationship between retained tension and creepdisplacement upon cyclical loading based on analysis across all tested constructs.The nonlinear relationship can be explained by the “stretching out” or “uncrimping”of crimped tendon fibrils, which occurs from mechanically loading the tendon up to2% strain. This region is responsible for a nonlinear stress/strain curve becausethe slope of the toe region is not linear.; however, less is known regarding how much pretension on a hamstring graftshould be optimally applied for ACL reconstruction. The findings of the present study elucidate the relationship between fixationslippage during cyclic loading and the expected degree of tension loss. This is inalignment with Yasuda et al, who conducted a prospective clinical study with 70 patients and concludedthat the initial relatively high tension in the graft (80 N) decreases postoperativelooseness in the knee. Other research groups recommend an optimal graft pretensionof 90 N. Noyes et al strongly suggested the need for increased load and graft cycling duringimplantation to remove residual elongation after ACL reconstruction.It has already been shown that reconstruction stiffness is dependent on the initiallyapplied graft tension Nevertheless, weak tibial fixation is frequently cited as a potential causeof graft failure, and the development of tibial-side fixation devices remains an active field.Our data suggest that regardless of the applied technique, retained fixation isreflective on minimizing residual elongation. We speculate that it may play a rolein clinical outcome, and this should be investigated in further clinicalstudies.Despite the expected benefits of adequately retaining graft tension, there areconcerns about overtightening an ACL reconstruction and how this may result in jointstiffness and/or elevated tibiofemoral cartilage stress. Furthermore, this is a time-zero biomechanical study and does not exploreany postoperative issues, such as graft incorporation or tunnel widening. Inaddition, the tested constructs were a combination of bone, fixation device,sutures, and tendon. We did not differentiate how local displacements cumulatedto the overall mechanical response. However, graft preparation and suturing werestandardized using 2 techniques: a continuous loop technique for fixation withan ALD (groups 3-5) and a quadrupling technique for fixation with the knottingbutton (groups 1 and 2). While clinical studies will be required to validate theclinical relevance of the experimental conclusions that we draw, these data doprovide interesting insight and add unique perspective to the wide body ofexisting experimental literature.Of course, there are several limitations to consider when judging the relevanceof these findings. First, the study was designed as a biomechanical in vitromodel using porcine tibias and bovine tendons. The findings cannot be applied ina clinical setting without caution; yet, these specimens are commonly used forbiomechanical ACL testing, as their biomechanical properties are similar tohuman bone and tendon, respectively.Tibial fixation of quadrupled ACL grafts with knotless ALDs achieves comparableresults with hybrid fixation in the full-tunnel and closed-socket techniques. Thefindings of the present study emphasize the importance of the tension at whichfixation is performed.


The DRX kinetics models, including DRX volume fraction and grain size models, and the meso-scale cellular automaton (CA) models, were established based on the obtained true stress–strain curves and microstructure observation results. In order to dynamically reveal DRX behaviors and grain morphology evolution, a multi-field and multi-scale coupling finite element (FE) model for the hot compression process was developed by embedding the solved DRX kinetics models and CA models. Results show that the DRX volume fraction and grain size increase with temperature increasing and strain rate decreasing. The DRX grains are easier to nucleate at the initial grain boundaries. As strain increases, DRX grains grow up by devouring the matrix grains until DRX occurs completely. The microstructures after compression are composed of equiaxed DRX grains. Finally, the comparisons of grain size between experimental results and simulation results were performed. The mean relative errors between experimental results and predicted results from DRX kinetics models, and between experimental results and predicted results from CA models, were evaluated as 6.5% and 6.0%, respectively. It proves that the developed FE model can well describe the microstructure evolution in the hot deformation process of SAE 5137H steel.Describing the dynamic recrystallization (DRX) behaviors and grain evolution mechanisms in the hot forming process contributes to controlling microstructures and enhancing mechanical properties of materials. Here, the isothermal compression experiments for SAE 5137H steel were conducted under temperatures of 1123–1483 K and strain rates of 0.01–10 s SAE 5137H, as a medium-carbon alloy steel, is widely applied in automobiles, railways, and ships due to its superior toughness and high strength . The hotUp to now, much research has been carried out on developing DRX kinetics models to describe the correspondence between DRX behaviors and deformation parameters, and furthermore these models were implanted into finite element (FE) models to predict the dynamic evolution of microstructures in the hot forming process of materials. Quan et al.  establisWith the rapid development of image simulation technology, the cellular automaton (CA) method has been extensively applied in the topographic simulation of microstructures to uncover the meso-scale grain evolution mechanisms in DRX, DRV, grain growth, etc. In 1998, Goetz et al.  simulate−1. Based on the results of compression experiments and microstructure observation, the DRX kinetics models including DRX volume fraction and grain size models, and the meso-scale CA model were constructed. Then, these solved models were embedded into a hot compression FE model to numerically describe the microstructure evolution in the hot deformation process of SAE 5137H steel. The established DRX kinetics models revealed the distributions of DRX volume fraction and grain size, and the meso-scale CA models uncovered the evolution process of grain morphology. Finally, the comparisons of grain size between the experimental and simulation results indicate that the simulation results have a great agreement with experimental ones. This work contributes to the design and optimization of processing parameters in the hot forming process of SAE 5137H steel for obtaining the desired microstructures. This work aims to uncover the DRX behaviors and grain evolution mechanisms in the hot forming process of SAE 5137H steel from the microscopic scale and mesoscopic scale. Here, a series of isothermal compression experiments were conducted over the deformation temperature range of 1123–1483 K and the strain rate range of 0.01–10 sΦ10 mm × 12 mm were cut from a rolled billet for isothermal compression tests conducted on a Gleeble 3500 thermal simulator. These tests were performed under five different deformation temperatures of 1123 K, 1213 K, 1303 K, 1393 K, and 1483 K, and four diverse strain rates of 0.01 s−1, 0.1 s−1, 1 s−1, and 10 s−1. The processing routes were simplified as The studied material in the present work was SAE 5137H steel whose chemical composition was given in In order to characterize the deformed microstructures, the compressed specimens were cut in half along their compression axials, and the schematic illustration of the section was shown in Usually, the critical strain e curves ,23. By ta is a material constant. The peak strain is the function of deformation activation energy T, strain rate R is the universal gas constant taken as 8.31 J mol−1 K−1; a1, n1 and m1 are material constants. As the effect of initial grain size has been overlooked in this work, i.e., n1 = 0, Equation (2) can be replaced by Equation (3):The critical strain tion (1) ,25:(1)εcAccording to the true stress–strain curves  and the Take a natural logarithm on both sides of Equation (3) and obtain Equation (4):m1 and m1 and a1 was calculated as 0.03811. Therefore, the models of critical strain for SAE 5137H steel were obtained as Equations (5) and (6):From Equation (4), it can be derived that a2, n2, and m2 are material constants; Q2 is the activation energy for recrystallization. Here, the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation was employed to describe the evolution of DRX volume fraction in the hot forming process for SAE 5137H steel, and its formulation can be expressed as Equation (7) :(7)Xdrx=It is widely accepted that the DRX volume fraction can also be evaluated from true stress–strain curves, and its calculation formula can be described as Equation (9) ,27:(9)Xd−1.By combining the critical stress Take natural logarithms on both sides of Equation (8) and obtain as Equation (11):n2 = 0, it can be easily obtained from Equation (11) that m2 and Q2 were evaluated as 0.1376 and 23,059.65, respectively. By substituting the calculated results of m2 and Q2 into Equation (8), the value of a2 was calculated as 0.03432. As the effect of initial grain size has been overlooked in this work, i.e., Taking a natural logarithm on both sides of Equation (7) yields Equation (12):Based on Equation (12), it is derived that Therefore, the DRX kinetics models for SAE 5137H steel can be summarized as Equation (13):−1. Apparently, the microstructures after compression strongly depend on temperature and strain rate. With the increase of strain rate, there are abundant equiaxed and refined grains formed in the microstructures. In addition, the microstructures become more homogeneous with temperature increasing. The observation results indicate that DRX has occurred during the isothermal compression process of SAE 5137H steel. Furthermore, the average grain size of microstructures in Q3 is deformation activation energy; a3, n3 and m3 are material constants. In this work, the Sellars model was used to characterize the evolution of grain size during the hot deformation process of SAE 5137H steel, in which DRX grain size is strongly associated with initial grain size, temperature, and strain rate, as described in Equation (14) :(14)ddrxTaking the natural logarithm on both sides of Equation (14) yields Equation (15):n3 = 0, the material constants m3 and Q3 can be derived from Equation (15) that m3 and Q3 were evaluated as –0.10067 and –61735.26, respectively. By substituting the calculated results of m3 and Q3 into Equation (14), the value of a3 was figured out as 8594.31. As the effect of initial grain size has been overlooked in this work, i.e., Therefore, the grain size model of DRX for SAE 5137H steel can be summarized as Equation (16):t is time; l0 is the initial height of workpiece.It is well-accepted that the resistance heating isothermal compression can be summarized as an electrical-thermal-mechanical multi-field issue ,29. In o−1, the distributions of DRX volume fraction at different true strains were demonstrated in When the specimen deformed at 1303 K and 0.1 s−1 and different deformation temperatures of 1123K, 1213 K, 1303 K, 1393 K, and 1483 K. It is evident that, for a fixed true strain, the distribution of DRX volume fraction at any given temperature is similar to each other. From the center region to the outer edge, DRX volume fraction gradually decreases, and its maximum value appears at the center region of the deformed specimen. As true strain increases from 0.223 to 0.916, the DRX volume fraction in all region of specimen increases. By comparing the distribution of DRX volume fraction under different temperatures, it is shown that the volume fraction of DRX grains increases with increasing temperature. In addition, it is worth noting that, at higher temperature, DRX volume fraction at the center region of specimen has reached the maximal value at a relatively small true strain. This is owing to the fact that higher temperature can provide more activation energy for the nucleation of DRX grains. Hence, DRX occurs more easily and more completely at elevated temperature. The distributions of grain size at the end of hot compression (the true strain of 0.916) under the constant strain rate of 0.1 s−1 and different temperatures of 1123K, 1213 K, 1303 K, 1393 K, and 1483 K were demonstrated in −1, 0.1 s−1, 1 s−1, and 10 s−1. Comparing −1 and 10 s−1. Especially, for the center region of specimen, the DRX volume fraction at the strain rate of 0.01 s−1 is much higher than that in the strain rate of 10 s−1. In addition, this difference is decreased when the true strain exceeds 0.511, since DRX occurs completely with the continuous increase of strain. The comparisons of the distributions of DRX volume fraction under different strain rates indicate that DRX volume fraction increases with strain rate decreasing, and, meanwhile, for a fixed true strain, the center region of specimen with relatively high DRX volume fraction also increases as well. This attributes to the fact that the lower strain rate can provide enough time for the DRX process, and then DRX occurs more completely. −1, 0.1 s−1, 1 s−1, and 10 s−1, respectively. It is noted that the distributions of grain size in the deformed specimens exhibit the similar tendency under different strain rates, and all distributions are inhomogeneous. The standard deviation (SD) of grain size distribution increases as the strain rate increases, which implies that the degree of deformation inhomogeneity increases with the strain rate increasing. Under the strain rate of 0.01 s−1, 0.1 s−1, 1 s−1, and 10 s−1, the values of average grain size  and Equation (20), respectively:Q is apparent activation energy; m is strain-rate sensitivity. During a hot deformation process, the evolution of dislocation density is associated with WH and softening behaviors. WH causes the increase of dislocation density, while DRV and DRX softening behaviors result in the decrease of dislocation density. The dislocation density can be expressed by the Laasroui–Jonas model, as shown in Equation (18) ,31,32::From Ref. , the reck, then r = −k. The values of r were given in In Equation (21), it can be derived that Take the natural logarithm on both sides of Equation (20) and give Equation (22):r into Equation (20), the parameters can be calculated by multivariate linear fitting, m = 0.06594, Q = 382,301.89 J·mol−1, r0 = 159.74.Substituting the obtained values of h can be calculated by Equation (23):µ is shear modulus; b is Burgers vector; and α is Taylor factor taken as 1. According to Ref. , the harr and h can be calculated, as listed in h0 can be obtained as 56.33.Substituting the obtained values of K is a constant taken as 6030; Nr is the amount of cells in which DRX occurs; “#rows” and “#columns” are the number of rows and columns of discrete lattice points, respectively.Here, the Goetz recovery model was employed to describe the influence of recovery mechanism on the evolution of dislocation, as expressed in Equation (24) :(24)Nr=#C and l are material constants taken as 0.9 and 200, respectively.It is well-known that dislocation density increases continuously as strain increases. When the dislocation density reaches a critical value, DRX grains first nucleate at the grain boundary. The relationship between nucleation rate ion (25) :(25)n˙=CF can be expressed as Equation (26) [The dislocation density difference between new grains and original grains provides the driving force for grain growth, and the new grains can grow continuously until the dislocation density difference is zero. The driving force ion (26) :(26)F=4πion (27) :(27)γ=γmN is the number of nucleation sites; S is the simulation area.In order to describe the grain morphology evolution process in the hot deformation process of SAE 5137H steel, the solved dislocation evolution models, grain nucleation, and growth models were embedded into the FE model in hot deformation process, and then the FE model embedded with CA models was developed. In this model, a two-dimensional CA model with a simulation area of 800 μm × 800 μm was constructed, and the periodic Moore’s neighboring rule was applied in it. The initial grain morphology was generated through the following steps . Accordi−1, the grain morphology evolution during the hot deformation process of SAE 5137H steel was simulated numerically. Then, the grain evolution mechanisms were uncovered. −1. In For the temperatures of 1123–1483 K and strain rates of 0.01–10 s−1 were simulated by the same way. As analyzed in In order to validate the simulation results, for the center of specimens marked “X” in −1. The main conclusions drawn from this work are as follows:The dynamic recrystallization behaviors and grain evolution mechanisms during the hot forming process of SAE 5137H steel were investigated by hot compression experiments under temperatures of 1123–1483 K and strain rates of 0.01–10 s(1) The DRX kinetics and grain size models in the hot deformation process of SAE 5137H steel were established based on the stress–strain data and microstructure observation results. The solved models are as follows:(2) The multi-field and multi-scale coupling FE model for the hot compression process was developed to describe the DRX behaviors and grain size evolution of SAE 5137H steel. The simulation results show that, with increase of strain, DRX volume fraction increases, and grain size is refined gradually. For a constant strain rate, DRX volume fraction and grain size increase with temperature increasing. For a constant temperature, DRX volume fraction and grain size decrease with strain rate increasing. (3) The CA models of SAE 5137H steel were established and embedded into the FE model to reveal the evolution process of grain morphology in the hot compression process. The results indicate that the recrystallized grains are easier to nucleate at the initial grain boundaries. With strain increasing, the recrystallized grains grow up by devouring the matrix grains until DRX occurs completely. The microstructures after compression are composed of equiaxed DRX grains. (4) The comparisons of grain size between experimental results and simulation results were performed. The mean relative errors between experimental results and predicted results from DRX kinetics models, between experimental results and predicted results from CA models were evaluated as 6.5% and 6.0%, respectively. This result indicates that the developed FE model can well describe the microstructure evolution in the hot forming process of SAE 5137H steel.


Background: Open septorhinoplasty is a common facial plastic surgery procedure that requires extensive planning and knowledge to achieve predictable outcomes. Many patients want to keep their nasal tip characteristics, and the surgeon's task is to reliably meet this expectation and provide stable long-term results. Techniques used to reconstruct nasal tip support include the tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft procedures.Methods: We assessed the 1-year reliability of tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft procedures in maintaining nasal tip rotation and projection in open septorhinoplasty. We conducted a retrospective case series review of septorhinoplasty cases between 2015 and 2019 at the Medical University of South Carolina. Cases with intention to change nasal tip rotation or projection were excluded. Two blinded reviewers analyzed standardized preoperative and 1-year postoperative photographs.Results: Fifty-seven patients fit the inclusion criteria and were included in the analysis. Mean preoperative and postoperative nasal tip rotations and projection ratios were similar . Twenty-six patients underwent a tongue-in-groove procedure, 24 had a caudal septal extension graft, and 7 had a caudal septal replacement graft with preoperative nasal tip rotations of 98.93°, 99.35°, and 96.89°, respectively (P=0.73). At 1 year, patients who received a tongue-in-groove procedure had a significant increase in nasal tip rotation to 101.24° (P=0.013), while patients who received a caudal septal extension graft had a significant decrease in nasal tip rotation to 97.25° (P=0.009). Patients who received a caudal septal replacement graft had no significant change in nasal tip rotation (P=0.117). The preoperative and postoperative projection ratios were not significantly different among the 3 techniques.Conclusion: Tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft are reliable techniques for maintaining nasal tip projection in open septorhinoplasty. In our experience, when attempting to maintain preoperative nasal tip rotation, the tongue-in-groove technique resulted in a significant increase in tip rotation of 2.31°, while the caudal septal extension graft resulted in a significant decrease of 2.1° at 1 year postoperatively. The open septoplasty approach is frequently required for severe septal deviations, caudal deformities, and significantly crooked noses and involves dividing the interdomal ligament to expose the septum. One key step in the procedure is restoring nasal tip rotation and projection. Postoperative loss of nasal tip support is one of the most common faults in septorhinoplasty.2 Many patients want to keep their nasal tip characteristics, and the surgeon's task is to reliably meet this expectation and provide stable long-term results.Controlling nasal tip rotation is a complex aspect of rhinoplasty. Nasal tip rotation, generally, is the relationship between the base of the nose and the face in profile view. Rotation can be determined in 5 ways: the nasolabial angle, the columellar-facial angle, the nostril axis angle compared to the facial plane, the nostril axis angle compared to a line perpendicular to the Frankford horizontal line, and the columellar-facial angle in relation to a line perpendicular to the Frankfurt horizontal line.Techniques used to reconstruct nasal tip support include the tongue-in-groove technique and the caudal septal extension graft. The caudal septal replacement graft is a technique used for a severely deviated or fractured caudal septum in which the native caudal strut cannot be salvaged.3 Evolution of the tongue-in-groove technique has proven successful in modulating nasal tip rotation and projection, as well as treating alar-columellar disharmony.4 For the tongue-in-groove technique, the medial crura are advanced cephaloposteriorly onto the caudal septum; this procedure can be performed in both open and endonasal rhinoplasty.5Tongue-in-groove, a common technique used to reconstruct the nasal tip, was originally described for the treatment of an elongated nose by Rethi in 1934.6 The medial crura are fashioned similarly to the tongue-in-groove technique, but they are secured to a cartilage graft fixed to the caudal septum.Byrd et al first described the caudal septal extension graft in 1997, and this technique has been used to control nasal length and tip rotation, projection, and shape.7 The caudal septal replacement graft is used when significant caudal septal deficiency or deformity requires complete replacement of the caudal strut.Foda introduced the caudal septal replacement graft in 2008 for restoring nasal tip support in cases with caudal septal deficiency.Deciding which technique to perform requires an understanding of the patient's anatomy and the long-term changes that occur over months to years. This study was designed to measure and compare the 1-year changes in nasal tip rotation and projection after tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft open septorhinoplasty.We retrospectively reviewed all open septorhinoplasty procedures performed at the Medical University of South Carolina by the Facial Plastic and Reconstructive Surgery Division from 2015 to 2019 . Patients were excluded if an open septoplasty approach was not performed, if they did not have a 12-month postoperative follow-up visit with standardized photographs, or if intervention or revision surgery took place within the 1-year postoperative period. Seventy-two patients underwent open septorhinoplasty and had 12-month postoperative photos. After excluding cases with intention to change nasal tip rotation and cases with radix grafts or chin augmentation, a total of 57 patients were included for analysis. For these patients, the following data were extracted from the medical record: patient age at time of surgery, sex, history of trauma, prior septal surgery, and technique used to reset the nasal tip. The study was approved by the institutional review board of the senior author's primary institution.Absorbable 5-0 PDS sutures were used to reset the tip for all 3 techniques . In most8 Projection was analyzed by measuring the distance from the tip-defining point to the alar-facial groove using a line perpendicular to the facial plane. Although standardized photography was used, minor zoom adjustments required projection to be measured with a ratio. The ratio of projection to nasal height  was used  in relation to the facial plane (line from the sellion to the soft tissue pogonion) . The angwas used .t tests. P values <0.05 were considered significant. Statistical analysis was completed using SAS, version 9.3 (SAS Institute Inc.).Preoperative nasal tip rotations and projections were compared using analysis of variance 1-way tests. The 1-year postoperative nasal tip rotations and projections for the 3 techniques were also compared with analysis of variance 3-way tests. Preoperative and 1-year postoperative nasal tip rotations and projections were compared using paired P=0.62), respectively, and the average preoperative and postoperative projection ratios were 0.55 and 0.56 (P=0.22), respectively . Females had a significant increase in projection ratios at 1 year postoperatively (P=0.02). Patients undergoing revision septorhinoplasty had significantly decreased preoperative projection ratios compared to primary cases (P=0.018), but no significant difference was observed postoperatively. Neither age nor history of trauma had a significant correlation with nasal tip rotation or projection ratio.For all 57 patients included in the analysis, the average preoperative and postoperative nasal tip rotations were 98.86° and 99.5° (ectively . Thirty-P=0.73). Patients who underwent a tongue-in-groove procedure had a significant increase in nasal tip rotation to 101.24° (P=0.013) at 1 year postoperatively, while patients who had a caudal septal extension graft had a significant decrease in nasal tip rotation to 97.25° (P=0.009) at 1 year (P=0.117). Preoperative and postoperative projection ratios were not significantly different among the 3 techniques.Twenty-six patients underwent a tongue-in-groove procedure, 24 had a caudal septal extension graft, and 7 had a caudal septal replacement graft with preoperative nasal tip rotations of 98.93°, 99.35°, and 96.89°, respectively (t 1 year . AlthougP=0.032) and costal cartilage , the preoperative and postoperative nasal tip rotation and projection ratios were not significantly different between the septal and costal cartilage graft groups , postoperative nasal tip rotation and preoperative and postoperative projection ratios were not significantly different between the 2 groups.In the 7 patients who underwent a caudal septal replacement graft, costal cartilage was used in 4 cases, cadaveric cartilage was used in 2, and septal cartilage was used in 1 case. Although preoperative nasal tip rotation was significantly higher in patients who had costal vs cadaveric caudal septal replacement graft (101.1° vs 85.6°, 10 While these techniques can be used to modulate nasal tip rotation and projection, they are often used to restore the nasal tip when the open septoplasty approach is performed. We specifically analyzed cases in which the goal was to restore preoperative nasal tip rotation and projection after correcting the underlying septal and nasal valve deformities.Control and long-term predictability of the nasal tip position are crucial for favorable rhinoplasty results. Because the literature is limited, the goal of this study was to compare 1-year nasal tip rotation and projection between tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft techniques for patients who wished to maintain their preoperative nasal tip position.2 This loss was hypothesized to be attributable to the gravitational forces on the nasal tip, as well as the intrinsic resiliency of the lateral crura. Postoperative nasal tip rotation decreased by an average of 7.9° after 1 year; thus, overcorrection is typically desired.2 Kadakia and Ovchinsky compared absorbable vs nonabsorbable sutures for the tongue-in-groove technique in endonasal rhinoplasty.4 They demonstrated a significant increase in nasal tip rotation at 1 year with permanent sutures that was not present with absorbable PDS sutures.4 In our practice, we use 5-0 PDS sutures for the tongue-in-groove technique, with over-rotation of approximately 8° to 10°.The tongue-in-groove technique is a well-known maneuver in rhinoplasty to control nasal tip rotation and projection and improve nasal tip support. Antunes and Quatela demonstrated a significant loss of nasal tip rotation with the tongue-in-groove technique during the first postoperative year.9 In our study, caudal septal extension grafts were used when the caudal septum was judged to be deficient to maintain the patient's preoperative nasal tip rotation. We did not measure immediate postoperative nasal tip rotation, but 12-month postoperative rotation was significantly decreased with the caudal septal extension graft technique.The caudal septal extension graft is commonly used in rhinoplasty to control nasal length as well as nasal tip rotation and projection. The caudal septal extension graft lengthens the caudal septum and thus allows for a decrease in nasal tip rotation when needed. The caudal septal extension graft has been found to have more stability in nasal tip rotation over time than columellar strut grafts. Although Sawh-Martinez et al demonstrated a trend toward a decrease in nasal tip rotation over time with the caudal septal extension graft vs the columellar strut graft, they found no significant difference between 6-week and 12-month postoperative nasal tip rotation.Patients having the tongue-in-groove procedure had a significant increase in nasal tip rotation of 2.31° at 12 months postoperatively. This increase may be the result of too much overcorrection during the initial maneuver or overutilization of this technique when increased length of the caudal septum is needed. Measurements were performed 1 year postoperatively, so a decrease in nasal tip rotation may possibly occur with time. Conversely, patients having the caudal septal extension graft procedure had a significant decrease in nasal tip rotation of 2.1°. This change is likely attributable to a loss in nasal tip rotation over time because of the same gravitational forces that affect rotation in tongue-in-groove procedures. Absorbable PDS sutures are used for both techniques in our practice; thus, tongue-in-groove and caudal septal extension graft ultimately rely on scarring and fibrosis to maintain nasal tip position. While 1 year is a commonly referenced postoperative timepoint in rhinoplasty, continued changes may still be possible over decades, and studies are necessary to determine the true durability of tongue-in-groove and caudal septal extension graft results.10 The 2 patients in our study who received caudal septal replacement grafts with cadaveric cartilage had no adverse events or significant differences in postoperative nasal tip rotation or projection ratios when compared to patients who received costal cartilage.Our study population included few caudal septal replacement graft cases. When replacing the entire caudal septum, additional variables must be considered that are not present with tongue-in-groove and caudal septal extension graft procedures. The caudal septal replacement graft must be secured to the nasal spine and crest, as well as the dorsal septum, and is typically performed with extended spreader grafts in our practice. The degree and location of the caudal septal deviation determine the extent of septal cartilage that is removed and replaced. Although studies have demonstrated no increased risk of resorption with cadaveric cartilage, in our opinion, costal or septal cartilage is ideal for the caudal septal replacement graft because of improved stability.2 Suture position along the vertical axis of the septum allows adjustment in nasal tip rotation as the tip is restored.11 The caudal septal extension graft is used for increasing nasal tip support in patients with insufficient caudal septal cartilage and/or patients who cannot tolerate an increase in tip rotation. The surgeon should consider slight overcorrection of 2° to 3° with the caudal septal extension graft as it experiences the same gravitational forces as the tongue-in-groove technique. Although we identified a significant change in nasal tip rotation with these techniques, clinically, the changes were small and none of the patients in the study were dissatisfied by the change in rotation.For restoring the nasal tip in open septorhinoplasty, we feel that the tongue-in-groove technique is appropriate for patients with adequate caudal septal length and height and those who can tolerate a slight increase in rotation. Rhinoplasty surgeons typically overcorrect with the tongue-in-groove technique because some loss of nasal tip rotation is expected with time.10 A stable foundation without horizontal mobility is critical with the caudal septum or caudal septal replacement graft securely fixed to the spine. In our practice, we use at least 2 sutures spaced approximately 1 cm apart along the nasal spine and maxillary crest when resetting or replacing the caudal septum. In our experience, the tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft are all reliable techniques for maintaining nasal tip projection at 1 year postoperatively.Projection ratio stability was maintained with all 3 techniques used in the study. When restoring the nasal tip in open septorhinoplasty, suture position along the horizontal axis of the septum can be adjusted to modulate nasal tip projection without changing the rotation.This study has several limitations. Although all cases were performed with the goal of maintaining the patients’ preoperative nasal tip rotations and projections, patient characteristics may have created some bias with the surgeons’ choice of tip reconstruction. Second, immediate postoperative nasal tip rotation and projection were not recorded, and follow-up was only measured for 1 year. Consequently, we could not evaluate changes that happened very early or very late in the postoperative course.Tongue-in-groove, caudal septal extension graft, and caudal septal replacement graft are reliable techniques for maintaining nasal tip projection in open septorhinoplasty. In our experience, despite attempting to maintain preoperative nasal tip rotation, the tongue-in-groove technique resulted in a significant increase in tip rotation of 2.31°, the caudal septal extension graft resulted in a significant decrease of 2.1°, and no change was seen with caudal septal replacement graft 1 year postoperatively. Studies assessing immediate and long-term postoperative rotation are necessary to determine the long-term effect of gravity on these rhinoplasty techniques.