7.47 VII.5.2.2: Component Design for Fatigue: The growth of fatigue cracks in a structural member subjected to an alternating stress field may be described by the following equation (7.14): da - C(llKl)n aiJ - ~ where~ is the derivative of crack half length, a, with respect to the number of load cycles, N; C and n are empirical constants; R is the ratio of the minimum stress to maximum stress for a load cycle; and llK 1 is the change in stress intensity factor observed over a load cycle. Recall that the hull is constructed of 4130 steel and is assumed to experience an alternating stress superimposed over the mean stress level with an amplitude equal to 10% of the mean stress level. Justification for the assumed alternating stress is provided in Appendix VVI.B on dynamic loads. Given these conditions: And, C -8 .627xl0 2. 25 R O. 9 1-R 0.1 It is now possible to integrate the above equation to determine the relation between crack length and number of load cycles. Such an integration must be done in two parts: first from an assumed initial surface flaw to the point where the crack just penetrates the material thickness and then as a surface flaw with an initial length equal to 2t. In the first case the value of llK 1 is given by the surface flaw relation:
RkJQdWJsaXNoZXIy MTU5NjU0Mg==