Understanding of the failure mechanism of stress corrosion cracking by SEM analysis

Stress corrosion cracking (SCC) results from strongly coupled electro-chemical and thermo-mechanical processes, and this cracking is sensitive to material composition and morphology. There are four stages of SCC such as initiation, individual SC crack propagation, many crack interactions and clusters of crack formation, and finally crack or cluster instability and dynamic growth leading to the ultimate failure. In this paper the mechanism of SCC is investigated by the observation of SCC surface using a scanning electron microscope (SEM). Thermodynamic model of SCC propagation and statistical modeling of SCC initiation based on chemical degradation has been proposed before by the authors. The model predicts a change of the mechanisms of crack growth from chemically driven to mechanical stress control propagation. This prediction is validated by fractographic observations. It provides important information on the change of SCC propagation mechanism by the variation of micro-morphology and striation on the fracture surface. The duration of chemically driven stage of SCC and transition to stress controlled propagation depend on temperature and stress.