Influence of Specimen Preparation and Specimen Size on the Transverse Tensile Strength and Scatter of Glass Epoxy Laminates
The influence of specimen polishing, specimen configuration, and specimen size on the transverse tension strength of two glass epoxy materials loaded in three and four point bending was evaluated. Polishing machined edges, and/or tension side failure surfaces, was detrimental to specimen strength characterization instead of yielding a higher, more accurate, strength as a result of removing inherent manufacture and handling flaws. Transverse tension strength was sensitive to span length due to the classical weakest link effect. However, strength was less sensitive to volume changes achieved by increasing specimen width. The Weibull scaling law over-predicted changes in transverse tension strengths in three point bend tests and under-predicted changes in transverse tension strengths in four point bend tests. Furthermore, the Weibull slope varied with specimen configuration, volume, and sample size. Hence, the utility of this scaling law for predicting transverse tension strength is unclear.
Tension-Torsion Behavior of Glass Epoxy Flexbeam Laminates
Quasi-static tension-torsion and tension-torsion fatigue tests were conducted on several candidate S2/F584 glass-epoxy flexbeam layups to establish their torsional strength and fatigue behavior in the presence of an axial load, determine typical damage mechanisms, and provide a database for developing predictive analytical methods. Tension-torsion specimens contained 0° longitudinal plies and 15, 30, or 45° angle plies in seven different stacking sequences. The angle plies were ± 45 woven cloth in one layup and prepreg tape in the other layups. In the tape laminates, fatigue failures began with matrix cracks forming on the edge that initiated delaminations in the interfaces between the angle plies and 0° plies. In the cloth laminates, a series of small stair step delaminations formed between the ± 45 cloth layers and initiated delaminations in the neighboring 0/45 interfaces. Tests indicated that the three layups with only 0° and ± 45° plies had the highest torsional strength in the presence of a constant axial load. Of the three strongest layups, the one in which the ± 45° plies were made of woven cloth fabric instead of prepreg tape was the strongest and had the longest fatigue life.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.