Effect of number of fatigue cycles on fatigue data: CFRP (prepreg and wet-layup)

In carrying out fatigue delamination propagation tests to assess the propagation rate versus a function of the energy release rate, the question arises as to the necessary number of cycles required to properly characterize the behavior. Tests were performed elsewhere on two different carbon/epoxy, multidirectional woven composite laminates containing a delamination using constant amplitude cycling by means of displacement control for various cycle ratios. The aim was to carry out the tests for 3.0x106$$ 3.0\times 1{0}<^>6 $$ cycles. Most specimens complied with this requirement. In this study, the fatigue data are analyzed including the first 0.5x106$$ 0.5\times 1{0}<^>6 $$ cycles, the first 1.0x106$$ 1.0\times 1{0}<^>6 $$ cycles, the first 1.5x106$$ 1.5\times 1{0}<^>6 $$ cycles, and 3.0x106$$ 3.0\times 1{0}<^>6 $$ cycles. It was seen that although the delamination length versus the cycle number could be quite similar between different cycle ranges, the Paris relation constants could be significantly different. It is suggested to carry out tests for as many cycles as possible. Double cantilever beam (DCB) specimens fabricated from multidirectional woven material were considered.Two material systems were examined.Different cycle ranges were used: 0.5 x 106, 1.0 x 106, 1.5 x 106, and 3.0 x 106.It was found that more reliable results were obtained for longer cycle ranges.