Interfaces in polymer-based carbon fiber composites are critical regions that are most susceptible to delamination under static and impact loads. The interface strength and fracture toughness can be improved significantly by structural micro-reinforcements using polymer additive manufacturing (PAM). A micromechanics embedded macro-model is developed for the evaluation and extraction of interface properties of laminated coupons with printed polymer reinforcements (PPRs) at ply interfaces to efficiently tailor the interface properties. Given the microstructure of the printed reinforcements, an automatic cohesive insertion module is developed to simulate the improved interface fracture toughness resulting from a wavy crack path steering around PPRs. The applicability and accuracy of the developed modeling approach are demonstrated using ENF coupons with PPRs fabricated and tested. The validated modeling approach is then applied to explore the effect of geometric configurations of PPRs through double layer printing on the enhanced Mode II energy release rate. The developed numerical model provides a digital testing tool for tailoring interface properties, optimization of PPRs, and extraction of enhanced interface properties for delamination failure prediction of a large scale composite structure.

The adverse effects of sea water environment on the fatigue life of woven carbon fiber/vinyl ester composites are established at room temperature in view of long-term survivability of offshore structures. It is observed that the influence of sea water saturation on the fatigue life is more pronounced when the maximum cyclic displacement approaches maximum quasi-static deflection, that is, the reduction in the number of cycles to failure are comparable between dry and sea water saturated samples at lower strain ranges (37% at 0.46% strain), but are drastically different at higher strain ranges (90% at 0.62% strain). Key damage modes that manifest during the fatigue loading is also identified, and a non-linear model is established for predicting low cycle fatigue life of these composites in dry and sea water saturated conditions.

Fiber reinforced ceramic matrix composites (FRCMCs) have been shown to retain exceptional thermal and mechanical properties at high temperatures in inert atmosphere. However, FRCMCs like carbon fiber reinforced carbon ceramic matrix composites (C/C CMCs) degrade rapidly at temperatures as low as 450^oC in oxidizing environments due to the conversion of solid carbon to gaseous oxides of carbon. With a rising demand for composite material structures to perform in extreme environmental conditions involving high temperature gradients, the prediction of oxidation damage is crucial in designing such structures. In this paper, a multi-scale finite element analysis framework is presented to study the oxidation behavior of ceramic matrix composites.