Modes I and II interlaminar fracture toughness and fatigue delamination of CF/epoxy laminates with self-same epoxy interleaf

Interlaminar fracture toughness and delamination fatigue crack growth behavior were investigated for carbon fiber (CF)/epoxy laminates with the self-same epoxy interleaf. The matrix epoxy with a thickness of 50 μm was chosen as the interleaf material in order to clarify the effect of resin-rich layer thickness on the delamination fatigue crack growth behavior. Tests under mode I loading were carried out using double cantilever beam specimens. For tests under mode II loading, three-point end notched flexure specimens were used for interlaminar fracture toughness tests, while four-point end notched flexure specimens were used for delamination fatigue tests. The mode I properties (interlaminar fracture toughness and delamination fatigue threshold) of these epoxy-interleaved CFRP laminates were almost identical to those of the laminates without interleaf (base CFRP laminates). The effect of epoxy interleaf was completely different under mode II loading. The mode II interlaminar fracture toughness for the epoxy-interleaved laminates was 1.6 (initial value) and 3.4 (propagation value) times higher than that for the base CFRP laminates. The mode II delamination fatigue threshold of the epoxy-interleaved laminates was 2–2.3 times higher than those of the base CFRP laminates. While the toughness of the interleaf is the key factor under mode I, the thickness of interlayer is the key factor under mode II. The difference in the effect of the self-same epoxy interlayer on the interlaminar fracture properties under modes I and II loadings was discussed on the bases of the fractographic observations and mechanism considerations.