FACTORS AFFECTING THE INTRINSIC CHARACTER OF THE CRACK GROWTH KINETICS CURVE IN STRESS CORROSION CRACKING: A REVIEW

摘要

Analysis of stress corrosion cracking (SCC) is performed with emphasis on the validity of the fracture mechanics approach, whose keystone is the crack growth kinetics curve, i.e., the plot "crack growth rate V vs. stress intensity factor K", as an intrinsic characteristic of a system {material-environment}. The uniqueness of the v(K)-curve as an attribute of a given material-environment couple forms the backbone of the approach and guarantees the soundness of the applications. However, ample experimental evidence does exist of the non-uniqueness of the v(K)-curves. Apart from K, crack growth rate depends on a family of variables related to the pre-SCC loading history, geometry and testing/service routine. The origins of the uncertainty of the fracture mechanics characterisation of SCC are analysed in general terms. In parallel, hydrogen assisted cracking is addressed in explicit terms as an important particular case. Two kinds of factors responsible for the essential weakness of the fracture mechanics approach to SCC are revealed: (1) crack tip plasticity which suppresses the K-dominance of the near tip stress-strain field depending on the history of crack formation/extension even under small scale yielding within an elastic K-controlled crack tip zone; (2) time-dependent reactions between the material and the environment which affect the tip shape and nearby plasticity. Suggestions to consolidate the customary fracture mechanics approach are outlined. A rigorous treatment of SCC in terms of local values of governing parameters just at the crack tip is emphasised. In addition, a safe approach is recommended for design against SCC based on the idea of "the worst crack tip state" intrinsic for each material-environment system.