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.
High-temperature strength and storage modulus in unidirectional hybrid composites
Abstract The mechanical properties of pultruded rods of unidirectional hybrid glass/carbon-epoxy composites were evaluated with respect to retention of high-temperature properties up to 250 °C. Two prototype matrices were produced using developmental epoxy formulations with glass transition temperatures (Tg) of 176 and 202 °C, respectively. The two prototype composites showed diminished tensile strength at high temperatures, but with different temperature thresholds. The high-Tg composite retained full strength to 170 °C, while the lower-Tg composite retained full strength to 80 °C. Matrix softening and loss of fiber–matrix adhesion was a major factor affecting the strength reduction observed at high temperatures. The storage modulus, measured by dynamic mechanical analysis (DMA), showed temperature dependence nearly identical to the tensile strength for both composites. The correlation between storage modulus and tensile strength was analyzed in terms of the effect of temperature on the shear modulus of the matrices. The storage modulus behavior is presented as a basis for projecting the strength-temperature dependence of unidirectional composites.
The Assessment of Fatigue Damage in CFRP Laminates
Results of an investigation into damage in an un-notched carbon fibre-epoxy composite laminate subjected to tensile fatigue loading are presented. The progress of damage, such as cracking in off-axis plies and delamination, was monitored using optical microscopy scanning electron microscope, x-radiography and ultrasonic C-scan.