Effect of Young's modulus on fatigue crack propagation under variable loading

In order to predict the most reliable fatigue life under variable loading conditions such as the service environment we have developed a new fatigue test method, and by using it the fatigue crack propagation properties were studied for SM50B steel, Ti-6Al-4V alloy and 5083 Al alloy. The test results showed that the acceleration of the crack propagation rate occurred on all alloys especially at low Delta K regions when the intermittent compressive overload was applied. The maximum acceleration of crack propagation was dependent on the alloy. When the intermittent compressive overload is applied, the crack propagation rate becomes proportional to the second power of Delta K and the Delta K-th decreased. The crack propagation rate under intermittent compressive overloading conditions is expressed as da/dN = C(Delta K-2 - Delta K-th(2)) (C=10/E-2) independent of alloys. This crack propagation behavior is easily derived from the theory if CTOD is the critical factor for the fatigue crack propagation. These accelerations of the crack propagation rate under intermittent compressive overloading conditions were discussed with the deformation mode at the crack tip.