Cracking Behavior in Nodular Graphite Cast Iron under Non-Proportional Cyclic Loading

The cracking behavior of ferritic nodular cast iron subjected to multiaxial non-proportional cyclic loading was studied. Crack length increased logarithmically during cycling, with a slop (in mu m/cycle) as high as the phase shift angle (8) was closing to 90 degrees. A non-symmetrical behavior in terms of fatigue life and crack growth relative to delta = 90 was observed. The application of analytical method revealed that at all times of the fatigue cycle, the amplitude of principal stresses is much higher for 8 90 degrees than for 8 90 degrees than for 8 90 degrees. Microscopic studies revealed that fatigue cracks did not have strong preferential directions for high-phase angle loads due to the continues rotation of principal directions, so their propagation did not stop.