Abstract
Near-threshold growth of delamination fatigue cracks was investigated in unidirectional laminates made from Ciba Geigy prepregs 914C both under mode I and mixed mode of I and II. Double cantilever beam specimens (DCB) were used for mode I tests. Cracked lap shear specimens (CLS) were used for mixed mode tests. The stress ratio, R, of the minimum to maximum stress was kept constant in the tests, and the constant value was between 0.1 and 0.5. Cyclic load was shed automatically by keeping maximum displacement constant. In the region of rates above about 1-5×10-10m/cycle, the crack growth rate is expressed as a power function of fracture mechanics parameters. Below this region, there exists the growth threshold for both mode I and mixed mode. When compared at the same value of fracture mechanics parameters, the crack growth rate for mixed mode is higher than that for mode I. The values of crack growth threshold under mixed mode were smaller than those under mode I. The influence of the stress ratio becomes smaller when the crack growth rate is correlated to the energy release rate range than when the crack growth rate is correlated to the stress intensity range or the maximum energy release rate. The relative contributions of the maximum stress and of the cyclic stress on the crack growth rate are discussed on the bases of fractographic observation and mechanism consideration. The contribution of the maximum stress was high.
