Bending fatigue of hybrid composite rods
The flexural fatigue behavior of hybrid composite rods comprised of unidirectional carbon and glass fibers was investigated. Damage was evaluated by monitoring stiffness loss as a function of cycles, and bending fatigue failure was defined in terms of strength retention. The acoustic emission technique and microscopic examination were used to characterize damage progression and failure mechanisms. The number of cycles to failure depended on applied stress level, and a two-parameter Weibull analysis was used to incorporate probability of failure to the S–N curve. Damage initiated and propagated as a result of matrix cracking and fiber bundle failures within the GF shell. Bending fatigue damage only initiated when the hybrid was exposed to a deflection in excess of 42% of flexural strength, which does not occur in actual conductor field use. Damage reached a saturation point along the GF/CF interface because of the stress concentration that existed between the two material systems, resulting in asymptotic behavior of the stiffness loss. Because damage did not extend into the CF core, static mechanical properties were retained to ∼85% or more.
Long-Term Environmental Fatigue of Pultruded Glass-Fiber- Reinforced Composites Under Flexural Loading
Abstract Pultruded glass-fiber-reinforced vinyl ester composite coupons were subjected to four-point-bend fatigue in various environments to study long-term durability for infrastructure applications. Several groups of specimens were aged in water or in salt solutions containing mass fractions of either 5% NaCl or 10% NaCl for up to 6570 h. Both as-received and aged specimens were then cyclically tested in air or while immersed in water or in salt solution. For specimens cyclically loaded at or above 45% of the average flexural strength of the dry coupons, no substantial difference in fatigue life was observed among all the specimen groups. For samples cyclically loaded at 30% of the dry flexural strength, however, all specimens tested in air survived beyond 107 cycles while all those tested in water environments did not. It is found that long-term environmental fatigue behavior is not controlled by the quantity of water absorbed; rather, it is governed by a combination of both load and fluid environment. No difference in fatigue life was found for specimens aged in different fluid environments at room temperature prior to fatigue testing. Relative to these samples, however, a significant difference was seen for specimens aged in water at 75°C for 2400 h prior to cyclic test at load levels above 30% of the dry flexural strength. When tested at 30% of the dry flexural strength the differences were within the experimental uncertainty. Microscopic examination of the fatigue specimens revealed evidence of a degraded fiber/matrix interphase region in those specimens where environmental exposure caused premature failure so this is believed to be a controlling factor in the environmental performance of the glass composite.
Characterization of Aqueous Dispersions and Gels Made of Sodium Caseinate and Basil Seed Gum: Phase Behavior, Rheology, and Microstructure
The interactions between sodium caseinate (NaCas) and basil seed gum (BSG) in the presence of calcium chloride (CaCl 2 ) were investigated. The phase behavior of the mixed aqueous dispersions and their gels revealed a homogeneous mixture, obtained at the higher concentrations of both CaCl 2 and BSG. The Herschel-Bulkley model sufficiently fitted the flow behavior of the mixture solution data. Apparent viscosity increased significantly ( p  < 0.05) by increasing the concentration of BSG, where the addition of CaCl 2 had no significant effect on the viscosity of the samples ( p  > 0.05). Furthermore, there was an increase in thixotropy due to the higher concentrations of BSG and CaCl 2 . Based on the frequency sweep test, at the low frequencies, a more gel-like behavior was observed in the case of the higher concentrations of either BSG or CaCl 2 . The rheological and SEM data suggested that the stronger structure of NaCas-BSG gel in the presence of the higher concentrations of CaCl 2 was related to the induction of complex formation between the two biopolymers.