Crack propagation in CFRP laminates under mode I monotonic and fatigue loads: A methodological study

Delamination in composite materials is often accompanied by fiber bridging that increases resistance to crack growth. Here, delamination of two unidirectional carbon/epoxy systems was studied under monotonic and displacement-controlled fatigue loading. One material (CP004) presented a random distribution of fiber clusters and matrix-rich zones while the other (SE-84) had an ordered microstructure. The results showed that monotonic and fatigue delamination of CP004 presented similarities, with smooth crack growth and significant bridging. The SE-84 showed no bridging but matrix cohesive failure related to stick-slip growth under monotonic load and adhesive/cohesive failure with limited fiber bridging in fatigue. Data from embedded strain sensors showed that bridging under monotonic and fatigue were similar in CP004. The identified traction separation relation under monotonic loads was used in a cohesive model to approximate the traction separation at lower subcritical displacement, corresponding to crack arrest in fatigue. The results demonstrated that monotonic and fatigue delamination may have important similarities (or differences) depending on the microstructure and material properties.