Fatigue initiation in fibre metal laminates

It is assumed that fatigue crack initiation in Fibre Metal Laminates is determined by the stress cycles in the metal layers only. It is further assumed that if the stress cycles in the metal layers are known, the fatigue initiation life can be established using S– N data available for the given metal alloy. The internal stresses in the aluminium layers of a Fibre Metal Laminate are different from the applied stresses on the laminate because of differences in stiffness and coefficient of thermal expansion between the metal and fibre layers. The difference in thermal expansion will cause residual stresses during cooling down after the curing process of the laminate. Classical laminate theory can be applied to calculate the internal stresses. Additional elastic considerations have led to accounting for the effect of anisotropy on the stress concentration factor. Validation fatigue tests on Glare 3-3/2-0.3, Glare 4B-3/2-0.3 and monolithic aluminium 2024-T3 showed that the above assumptions proved to be correct. The tests showed also that the effect of off-axis loading could be predicted with the classical laminate theory. Further, is has been shown that exposure to high temperature (70 °C) and humidity (85%) for 3000 h prior to testing has no effect on the fatigue initiation properties of Glare 3-3/2-0.3.