Evaluation of Environmentally Assisted Cracking of a High Strength Steel Using Elastic-Plastic Fracture Mechanics Techniques

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Abstract
The objective of this investigation was to assess the feasibility of determining the environmentally assisted crack growth characteristics of a high strength 4340 steel during cathodic polarization in seawater using J-integral techniques. A secondary objective was to demonstrate the capability of using these techniques for investigating stress corrosion cracking-hydrogen embrittlement mechanisms in high strength steels. It was concluded that stress corrosion-hydrogen embrittlement crack growth behavior in a high strength steel can be characterized using techniques developed for the study of stable elastic-plastic crack growth. In the case of environmentally assisted cracking, an added dimension results from the rate dependence of the process, giving results which are displacement-rate dependent. Cathodic polarization of 4340 steel specimens in seawater resulted in up to a four-fold decrease in the energy required for fracture initiation (JIc). The extent of this decrease was a strong function of both pre-exposure time and displacement rate. A good correlation was obtained between fracture morphology changes predicted from the material J-R curves and actual fracture surface measurements. The technique should therefore be a useful tool in mechanistic studies of environmentally assisted crack growth in high strength steels.