Study of fatigue crack growth retardation due to overloads in polymethylmethacrylate

Abstract

Retardation of the fatigue crack growth after overloading was investigated in conjunction with the craze deformation at the fatigue crack tip in polymethylmethacrylate. The craze deformation was measured by optical interference and analysed numerically with reference to a previously proposed craze model. In the base line loading, the craze stress concentrates at the crack tip with the applied load and, hence, the non-uniform stress distribution is attained at the maximum load. The overload alters this stress distribution. Just after overloading, the crack tip stress does not reach the previous level, even at the same maximum load. The reduced crack tip stress correlates well with the retarded duration after the overload. It is concluded, therefore, that the craze stress reduction at the crack tip is the cause of crack growth retardation.