PREDICTING THE EFFECTS OF CRACK-TIP CONSTRAINT ON MATERIAL RESISTANCECURVES USING DUCTILE DAMAGE THEORY

In recent years much interest has been focused on the geometry depende nce of the resistance to stable ductile crack growth of engineering ma terials, and in particular, in explaining this in terms of ''constrain t'' effects. This paper describes the results of work using the Rousse lier ductile damage model in finite element studies to simulate the gr owth and coalescence of voids, and hence the mechanics of ductile crac k growth, to predict the effect of constraint on resistance to fractur e. Using the modified boundary layer solution, where constraint is con trolled by the application of remote displacements, it was possible to simulate resistance curves for different constraint conditions. This has produced a ''net'' of resistance curves, within which the curve fo r any specimen geometry can be found from a knowledge of the crack tip constraint for that specimen. This has been tested by comparing the r esults with those obtained from two specimens for which the constraint conditions are known. Good agreement has been achieved. The results s how that, although constraint has very little effect on conditions at the crack tip at initiation of crack growth, beyond that constraint pl ays an important part in defining the resistance curve. For low constr aint geometries there is a very large loss in crack tip constraint whi ch results in a large increase in the slope of the resistance curve. O n the other hand, high constraint geometries exhibit very little depen dence on crack tip constraint.