Crack-growth phenomena in fatigued poly(methylmethacrylate)

The conditions under which crack-growth occurs from a natural crack induced in one side of a tensile specimen of poly(methylmethacryiate), when subject to a cyclic tensile load, at 20±3° C and 166 c/min, have been examined. The fracture surface can show three distinct regions, viz. a fissured rough growth area, a smooth growth area in which crack profiles extend across the width of the specimen, and a final area where catastrophic rupture has occurred. Under the conditions used, the fatigue life is mainly determined by the rate of growth of the crack through the rough-growth region. The number of cycles to failure has been determined for various values of the maximum bulk stress applied per cycle and of the initial crack length. The results appear to be consistent with a relationship put forward by Thomas, based on a tearing energy concept, which successfully predicts the dynamic cut-growth behaviour of certain natural and synthetic (SBR) gum rubbers. The conditions for the onset of catastrophic failure appear to be in accord with the Griffith criterion for brittle fracture.