Development of Diagnostic Expert System for Environmentally Assisted Cracking (EXENAC) and Importance Evaluation of Rules in Inference

The expert system, that diagnoses the sensitivity of environmentally assisted cracking (EXENAC; EXpert system for ENvironmentally Assisted Cracking), has been developed using the 0PS83 programming language. This system is composed of three independent subsystems, which respectively diagnose the sensitivity of i) stress corrosion cracking of austenitic stainless steels in a low- or high-concentration chloride solution and/or high-temperature water, ii) hydrogen embrittlement of high-strength steels in various aqueous solutions and gaseous environments, and iii) hydrogen attack of steels in high-temperature high-pressure hydrogen gases. The knowledge base covers the rules relating to environments, materials, loading conditions, heat treatments, material processing including welding and their interaction as well as fracture surface morphologies. The knowledge base consists of production rules, which have certainty factors to represent the uncertainty (contribution) of each rule to environmentally assisted cracking. The adoption of certainty factors enabled the uncertain but widely applicable knowledge to be built in a knowledge base, which in turn raised the flexibility of diagnoses made by the expert system. Applying the developed expert system to the case histories the diagnoses were accurately made. We discussed the characteristics of the individual knowledge base, related diagnosis results, and the suitable knowledge base structures for diagnoses in the area of environmentally assisted cracking. It is desirable that the inference is made through the plural affirmative and negative rules being executed. We also proposed the quantitative measure to evaluate the importance of each rule in the diagnosis. By this measure, not only the importance of knowledge in the diagnoses of the environmentally assisted cracking which occurs in service operation is evaluated but also the lack of knowledge can be made clear, which points out which field of investigation is necessary in the future.