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Interphase strengths of carbon reinforced vinyl ester composites are investigated by a, microindentation. technique as a function of moisture exposure and fatigue cycles. This data will provide the basis for understanding the mechanics of stress transfer in the composite when subjected to similar environmental and mechanical loading conditions. These interphase properties will be incorporated into a micromechanical model for the tensile strength of the composites. Initial experimental results show a 30% and 27% reduction in interfacial shear strength, IFSS, in samples sized with a standard epoxy material due to fatigue and moisture degradation, respectively. Residual tensile strengths of the bulk composites reduced 30% due to fatigue and 16% due to moisture. The micromechanical modeling showed that these large changes in IFSS resulted in very little change in the predicted tensile strength, suggesting that other damage mechanisms may play more important roles in the tensile strength reduction due to moisture and fatigue. The over-prediction of the tensile strength of unaged material indicates the need for a tensile strength model that incorporates damage present upon processing in virgin material.

The goal of this effort was to investigate the effect of temperature and environment (particularly temperature cycling) on the behavior of polymer composite materials typical of that used in infrastructure applications as well as in aerospace applications. To this end, freeze-thaw tests were conducted on a glass/vinyl ester composite using a DSC to verify the presences of freezable water within the composite material. Additionally, freeze-thaw conditioning of composites was conducted according to the protocol specified in ASTM C 666. Tensile test data encompassing strength, stiffness, and strain-to-failure for "as-received", moisture saturated, and freeze-thaw conditioned material is presented as well as saturation moisture uptake data. To examine the role of temperature cycling on an aerospace composite, durability studies were carried out on a woven (5 harness satin) graphite/epoxy system targeted at applications in areas of the aircraft industry where operating environment is of concern. Fatigue and residual strength testing was carried out on the composite material in question with the objective of developing a residual strength based life prediction model in which the effects of environment are considered. In this case, the approach called for the evaluation of tensile strength degradation during fatigue cycling in specified environments. The results could then be combined to predict the fatigue life of the material under periodically changing environments. The study was duplicated with material previously aged (unstressed) under hygrothermal cycling to study the effects of environment alone on the composite.

Composites reinforced with ultra-high molecular weight polyethylene (UHMWPE) fibers are known to perform well under ballistic impact, while being lightweight. In this study, the response of Spectra Shield (a proprietary UHMWPE fiber composite manufactured by Honeywell) is examined. The aim of this study is to determine the modes of failure of this material subjected to dynamic hard-body impact. Dynamic impact tests are performed with a single stage gas gun on 0, 902s panels of Spectra Shield II SR-3124 using hemispherically capped and flat cylindrical impactors (16.7 g mass) with incident velocities between 50 m/s and 300 m/s. Panel failure modes are compared through the range of velocities tested. Damage progression during the impact event is observed using high speed photography. Additionally, quasi-static punch tests are performed and the results compared with those from the dynamic event.

In this paper, we examine the fatigue behavior of a Nextel 610 reinforced alumina-yttria composite at room and elevated temperatures. Our main interest is in the prediction of elevated temperature fatigue behavior from simpler and/or shorter duration tests. We consider a residual strength based approach which involves a stepwise integration of the material stress-rupture behavior. This analysis is found to accurately predict the elevated temperature fatigue behavior.

This paper describes the development of a ceramic matrix composite component design tool that integrates the life prediction methodology, MRLife, with a finite element code CSTEM. The life prediction methodology is based on damage mechanics using residual strength as the damage metric and the critical element concept. Previous work has shown that such an approach is able to give accurate predictions of fatigue lives under combined mechanical and thermal loads. The development of the design tool based on this technique involves modifying the finite element program to account for damage in the component and the linking of the lifing code to it. At the component level, damage results in the redistribution of stresses while at the element level the same damage causes changes in the local stress state and changes the remaining strength in the component to propagate failure. The use of the design tool is described by means of a case study involving the life prediction of a composite notched plate at high temperature and cold grips under an axial fatigue load at high temperature.