Onset-of-growth behavior of mode II delamination in a carbon fiber composite under spectrum fatigue loads

End notched flexure (ENF) test specimens of unidirectional IMA/M21 carbon fiber composite were fabricated using standard autoclave process. A teflon insert was used to simulate a delamination at the mid plane. Three point bend tests were conducted under a modified mini-FALSTAFF spectrum load sequence with various reference strain energy release rate, Gref to determine the onset-of-growth of mode II delamination. The onset life was identified as 5% increase in compliance during the fatigue test. Onset fatigue life was observed to increase with decreasing, Gref. Further, the onset-of-growth was predicted using empirical models. For this purpose, the mode II fracture toughness, GIIc and the constant amplitude fatigue data, Gmax–Nonset curves at two different stress ratios were determined and used to construct a constant onset life diagram (COLD). Following the general procedure of damage estimation, the onset-of-growth was predicted using the COLD and compared with experimental results.