J-R curve determination using the load separation parameter S pb method for ductile polymers

Abstract

It is common practice to measure the fracture toughness of ductile polymers by the incremental crack growth resistance curve, where J is plotted as a function of crack extension. This article examines the applicability of a simple alternative methodology which has the inherent advantage of inferring the crack extension by comparing two experimental load displacement records obtained from a precracked and a blunt notched specimen. This methodology is based on the variable separability property, and on the assumptions that the load is independent of the crack growing history and it falls in correspondence with stable crack propagation. The separation parameter, S p b , is defined as the load ratio of a precracked and a blunt notched specimen. Its constancy identifies which points of the load displacement record have the initial crack length, while its variation is related to the onset of crack extension. From the S pb expression it is simple to obtain an equation from which to estimate the crack length using some predetermined calibration points. The methodology has been assessed in several commercial grade polymers, ABS, MIPS and thermally treated PP. Good agreement was found between the standard multiple specimen technique and the S pb method. In addition, a comparison was made with the single specimen normalization method.