Causes of evolution and influence on failure strength of thermal crazing

Abstract

Craze cracking is a typical cracking morphology caused by thermal fatigue loading; however, the reason for its cause by thermal fatigue loading is not fully understood. It is known that craze cracks do not grow deeply. In this study, evolution of craze cracking was simulated by Monte Carlo simulations using three-dimensional finite element analyses. Cracks were modeled using the X-FEM technique in Abaqus in order to consider the interaction between multiple cracks on the crack initiation and growth. Craze cracking was observed only when the growth in the depth direction was arrested. Since cracks kept the same depth for a long duration, many cracks could be initiated and craze cracking occurred due to the equi-biaxial stress state. Next, to investigate the change in failure load due to craze cracking, the limit load of a straight pipe subjected to a tensile, bending or internal pressure loading was analyzed. The simulated craze cracking was assumed to be located inside the pipe. It was revealed that the change in the limit loads due to craze cracking could be predicted well by replacing the craze cracking with an equivalent single crack. Although the replacement rule prescribed in the fitness-for-service code of the Japan Society of Mechanical Engineers predict reasonable limit loads, it did not result in conservative prediction. Conservative limit load could be predicted by setting the equivalent crack length to be the maximum extend of distributed cracks.

Keywords

• crack network
• thermal crazing
• thermal stress
• crack arrest
• limit load
• combination rule