Plasticity effects in crack propagation

Abstract

This paper describes experimental results of crack propagation studies in part-through crack specimens. Crack extension under cyclic loading is characterized during the loading cycle by the interaction of the crack with a surface wave. The surface wave signal contains information about the absolute crack length, changes in crack length, and the way the crack front opens and closes during cyclic loading. Simultaneously, compliance measurements have been carried out. The results of the experiments are as follows: 1. (1) Direct measurements of the crack depth as a function of the number of load cycles and of different holding times at various loads have been obtained. 2. (2) Going through the load cycles the surface wave signal displays a change in the ‘apparent’ crack depth vs. load. This is interpreted as crack closure at low load levels. 3. (3) Changes in the loading spectrum cause time dependent relaxation and retardation effects which are not only reflected in the crack growth rate but also in the crack closure behavior. The results are discussed in terms of Elber's model which suggests that internal residual strains are responsible for this crack closure.