Role of K{sub max} on fatigue crack growth

Fatigue crack growth involves two driving forces, {Delta}K and K{sub max} with corresponding two thresholds. K{sub max} depends not only on the applied stress but also on the internal stresses that the crack tip encounters in its path; while {Delta}K for the most part is independent of the internal stresses. Unfortunately, the failure to consider the K{sub max} requirement in fatigue crack growth analysis has led to several incorrect physical descriptions of the growth and in modeling for life prediction. Major issues are in the area of apparent anomalous crack growth behavior of short cracks, and retardation or acceleration effects under overloads and underloads. The authors show that the consideration of K{sub max} and its threshold, and its dependence on internal stresses helps to provide a correct perspective of the fatigue crack growth phenomenon and a clear physical description. In this paper, the authors show how the internal stresses affect fatigue crack growth through K{sub max}, and why the understanding of the role of K{sub max} is important for the development of better physical models of fatigue damage.