No abstract

The effect of sea water on carbon/epoxy cross ply specimens is studied by consideration of fatigue data and failure modes. Tests were conducted using dry and saturated coupons fatigued in air, as well as pre-saturated coupons fatigued while immersed in sea water. The saturated coupons fatigued in air had the longest fatigue life, which was attributed to stress relief from sorption induced swelling. Saturated coupons fatigued in an immersed environment exhibited the shortest fatigue life, and also experienced significant deliminations prior to failure. Water trapped inside the transverse cracks during the load cycle provides a physical mechanism explaining this behavior. Analytical and numerical analyses are presented which show how moisture inside the transverse cracks can alter the coupon stress state and enhance delimination.

No abstract

Temperature and humidity play a significant role in the mechanical behavior and long-term durability of polymer matrix composites (PMCs). Several standard test methods including some accelerated test methods involving higher temperatures and pressures are available for environmental qualification of these PMCs. While the effect of temperature on humidity diffusion has been studied extensively, the effect of pressure has received less attention. This study seeks to interrogate the effect of pressure on diffusion. This effect was observed by building test chambers designed to maintain a constant relative humidity of 80 % at 60 • C at three different pressures (0.1014 MPa, 0.5171 MPa, and 1.0342 MPa). A liquid water immersion test chamber at 60 • C was also considered. It was observed that the time to saturation and the saturation moisture content increased with increasing chamber pressure. Immersion in liquid water provided the upper bound for the maximum moisture saturation and a lower bound for the time to saturation.

Braided preforms of carbon fiber are under consideration for structural applications because of the potential for automated fabrication and possible improvement in through-the-thickness properties, although some loss of in-plane properties is expected. In the present investigation, the biaxial compressive strength properties of triaxial braided cylinders are measured experimentally, and the effects of four different sets of braid architectures are considered. The results show that fiber direction strain can correlate the results of biaxial tensile and compression failure tests. The braid parameters are found to have an effect on measured strengths, primarily through the introduction of defects into the fiber paths. The axial compression strength was found to be sensitive to waviness in the fiber path that was produced by uneven braiding coverage. Braid direction compression and tension strength is significantly lower than axial strength, and is also affected by braid parameters.