Carbon Fiber Composites Under Combined Environmental- Compressive Loading Leveraging Nonparametric Full-Field Techniques

Carbon fiber composites are often used in defense applications where structures undergo loading at high strain rates, as well as experience seawater exposure and other extreme environments. While the absorption of seawater has been shown to degrade material over time, competing effects of water absorption in the matrix and corrosion attacking the interfaces and their influence on the bulk damage response, particularly under dynamic loading conditions, are not entirely understood. As such, this study investigates the short-term exposure of both saltwater and distilled water on the compressive response of unidirectional carbon fiber epoxy matrix composite under uniform quasi-static (10−3 ?−1) conditions using a load frame, and dynamic (103 ?−1) conditions using a Kolsky (split-Hopkinson) bar. Environmentallyconditioned specimens were soaked for a period of three weeks. Due to the orthotropic nature of the unidirectional laminates, experiments were performed in both the longitudinal, 0o, and transverse, 90o, loading orientations with respect to the fiber direction. However, maintaining force equilibrium during Kolsky experiments in the transverse direction is particularly challenging, and often not possible without major modifications or specialized bar systems. Here we present a novel, non-parametric inverse identification technique that does not rely on the strict assumption of stress equilibrium to achieve accurate deformation mapping over the surface of the sample in real time. The information is derived from full-field high-speed digital image correlation (DIC) via surface acceleration measurements, in addition to utilizing a boundary stress measurement on the transmitted bar. Thus, using full-field metrology in a ‘stress-gauge’ approach allows for the stress-strain response along the transverse direction to be fully, and more accurately measured. For the longitudinal orientation, the ultimate stress and maximum strain at failure significantly decreased for the conditioned samples loaded quasi-statically. Under high-rate compression, the failure strain significantly increased for the conditioned samples with very small variation in ultimate stress. These results are compared with the transverse orientation where samples exposed to water showed an increase strain at failure in the low strain rate regime.