J-Integral Approach to Fatigue Crack Propagation in Vulcanized Natural Rubber

Abstract

Fatigue tests of vulcanized natural rubber were conducted under displacement-controlled conditions. The specimens were of three types: smooth specimen, specimen with an artificial surface defect of 0.2 mm in diameter, and specimen with an artificial through-thickness defect of 2 mm in length. In smooth specimens, a fatigue crack started from natural defects of about 0.15 mm in size, and grew as an internal crack, and then became a through-thickness crack. The spring constant of the specimen decreased slowly in the range of about 80% of the total life (Stage A), followed by a rapid decrease (Stage B). The transition from Stage A to B took place when a crack penetrated through the thickness. The rate of fatigue crack propagation is expressed as a second-power function of the J-integral range both for surface cracks and through-thickness cracks. The J integral was evaluated from the crack-center opening displacement.