Crack instability in ductile materials analyzed by the method of interpolated ellipses

Abstract

The subject of this work is the description of the experimental "method of interpolated ellipses" (MIE) and its application in fracture mechanics studies. The MIE is a new technique based on the optical monitoring of deformations during the loading processes of hexagonal grids of dots deposited on the surface of the monitored specimen. Loading the specimen deforms a circle on the surface into an ellipse. Each ellipse is interpolated by six neighboring dots of the hexagonal grid. Knowledge of the ellipse parameters directly yields the magnitude and the direction of principal strains on the specimen surface. Principal strain evolution yields knowledge of evolution of the plastic strain field by numerical postprocessing. Experimental results obtained from fracture tests of specimens with varying constraint at the crack tip have proven the evolution of plastic strain field to be a discontinuous process. This has been shown to be especially the case at the decisive time interval just before crack instability occurs.