A popular way in applying to identify incompatibilities of a product are non-destructive testing (NDT). However, this research is not allowed to identify the causes of incompatibilities. It concerns solving the decision problems, in which the causes and effects of these incompatibilities are determined. Therefore, the aim of the article is to propose the method to solve causes and effects problems as a part of determining the main cause of problem. This includes generating possible causes of the problem and their careful analysis in order to determine the main cause of this problem. This method is a combination of techniques, i.e. brainstorm (BM), Ishikawa diagram, and DEMATEL method. The method was designed in five main stages, i.e.: determining the aim, selecting the team of experts, determining the causes of problem, identifying the main cause of problem, determining the improvement actions, and determining the improvement actions. The test of the method for frequent incompatibility with non-metallic inclusions on product from AMS6415 steel was conducted. This incompatibility was detected by NDT research (magnetic powder method) in the Podkarpacie enterprise. After using the proposed method for this problem, it was shown that the main causes of its occurrence were errors in plastic working and untrained workers. The originality of the article is a new combination of methods for an analysis of a complex cause-and-effect problems with non-destructive testing.Keywords: AMS6415 steel, non-metallic inclusions, mechanical engineering, DEMATEL method, decision support
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