Continuous measurement of crack growth in plastics

Abstract

Fracture mechanics testing is frequently required to provide measurements of crack length to calculate the stress intensity factor. Hence, it is important to know the length of a crack in a specimen when fracture tests or fatigue tests are being conducted, under monotonic or cyclic loading. The conventional measurement of a crack is usually conducted optically with a travelling microscope. The disadvantage of this measurement is the need for constant attention by the operator, thus requiring laborious effort. To overcome this obstacle there have been two major developments in the crack length measurement of plastics. One is to make use of electrically conductive grids in which the crack breaks the grid iin a stepwise manner as it propagates [1, 2]. This technique has been used successfully in the measurement of fatigue crack growth [3, 4], but is not suitable for the measurement of the nearthreshold fatigue crack growth due to the very low resolution whic]h leads to impractically long testing times. The other technique is continuous measurement using the electrical resistance change as the crack propagates in a rectangular film coated on a specimen. This technique was used by Stalder et al. [5] to measure the crack velocity. However, the accuracy of this technique is limited due to the geometry of the gauge, i.e. the linearity between the electrical resistance and the crack length is dependent on the aspect ratio of the gauge. In this letter an equation relating the electrical resistance to the crack length independently of the aspect ratio of a rectangular gauge is derived for the continuous measurement of crack length. We employed the "three-resistor concept" shown in Fig. 1. The resistance without a crack between the electrodes is