A new approach for ctod evaluation in slow crack growth situations

In ductile materials slow crack growth often follows crack initiation, and the R-curve analysis characterizes both the crack initiation and crack growth behaviour of a material. The existing standards determine the crack tip opening displacement (CTOD) from the total clip gauge output by separating it into elastic and plastic components. The present work outlines the need for further separation of the plastic part of CMOD into the displacements due to slow crack growth and true plastic yielding. While the CTOD values for the elastic and true plastic yielding parts may be inferred according to the present methods, a new and simplified approach is proposed to infer crack growth CTOD from the corresponding displacement. The crack growth displacements may be obtained by using unloading compliance variation with crack growth. The present work is conducted on two materials, namely a steel and a 7004 grade aluminium alloy. The crack growth displacement in aluminium is found to be three times that in steel for a given crack growth. Geometrical as well as analytical comparison between the CTOD values obtained by the standard method and the proposed method is made. The comparison reveals that the existing method overestimates the CTOD value as reported by others. However, as the crack growth continues, the difference in CTOD values is found to diminish.