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 0.2mm in diameter, and specimen with an artificial through-thickness defect of 2mm in length. In smooth specimens, a fatigue crack started from natural defects of about 0.15mm 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. Fracture surfaces for three types of specimens were observed by scanning electron microscope (SEM). In smooth specimens and specimens with an artificial surface defect, fatigue cracks propagated in the shape of a concentric circle centering on natural or artificial defects. Fracture surfaces neighboring natural or artificial defects were very flat. After a crack penetrated through the thickness, the fracture surface became rough as the crack propagated. In specimens with an artificial through-thickness, the same tendency was also obtained.
