Fatigue crack growth mechanisms in steels

Abstract

Fatigue crack growth behavior of structural steels is examined by using the Unified Approach developed by the authors. In this approach, fatigue requires two-load parameters involving maximum stress intensity, K max, and stress intensity amplitude, △ K. For a fatigue crack to grow, both K max and △ K must exceed their respective threshold values. Similarly, for any other crack growth rate, two limiting values, K max∗ and △ K∗ are required to enforce the growth rate. The variation of these two critical values forms the crack growth trajectory map, which is defined by plotting △ K∗ vs. K max∗ as a function of crack growth rate. In this trajectory map, the line defined by △K ∗=K max∗ represents pure fatigue crack growth behavior induced by cyclic strains. It is shown that this line provides a reference norm for defining deviations in the trajectory resulting from environmental and/or monotonic fracture modes superimposed on fatigue. Using this approach, we have examined the crack growth behavior of many structural steels. The trajectory maps of these steels show deviations due to superimposed environmental effects. These effects vary with grain size, yield strength, microstructure, and chemistry. In addition, for a given material, changes in the trajectory paths occur because of changing crack growth mechanisms. Analysis of material behavior using trajectory maps provides a clear understanding of the relative magnitude of cyclic and environmental damage and how the microstructure, chemistry, and crack tip plasticity affect fatigue crack growth behavior of steels.