Crack Shape Development of Two Interacting Surface Elliptical Cracks

The issue of assessing residual life of an aged structure based on damage tolerance concepts attained significance in high technology fields such as aerospace, piping and pressure vessels and nuclear engineering. Computational fracture analysis of these structures in the presence of single or multi-site surface flaws is essential for life estimation and life extension. In this paper development of accurate post-processing technique (Modified Virtual Crack Closure Integral) to estimate strain energy release rates, and simple numerical method to simulate crack shape development in single and multiple interacting cracks (till they merge into single dominant crack) is presented. Crack shape development in single surface elliptical cracks was carried out earlier in literature using 2 degree of freedom model wherein fatigue crack growth is estimated along the major and minor axis of the ellipse and new crack shape was derived by fitting an ellipse to these points. A special three-degree of freedom model is proposed and presented in this paper for interacting and coalescing cracks. The crack shape development was checked with experimental work on coupons with multi-site surface cracks tested under fatigue loading. In safety critical aerospace and thick piping structures this work is significant in predicting the remaining life of aged components with multi-site damage.