Nondestructive testing method of residual stresses in metallic materials based on Knoop indentation technique

In the process of manufacturing and shaping critical mechanical structures, the inevitable result is the creation of non-uniform residual stresses (RS). These residual stresses significantly affect the mechanical properties, fatigue strength, and resistance to stress corrosion cracking in the structures, ultimately posing risks to their safety and operational lifespan. Furthermore, obtaining residual stress for equipment in service continues to be a challenging endeavour. Hence, the development of an in-situ testing method for measuring residual stresses is crucial to ensure the safe operation and extended lifespan of structures. In this study, we used finite element analysis to thoroughly examine the connection between residual stresses, indentation loading curves, and material mechanical properties. We then developed a biaxial residual stress testing method suitable applicable to part of mechanical properties, utilising the Knoop indentation technique. The reliability of method is experimentally validated using nuclear pipe material 316 L and 304 stainless steels. The results indicate that this method enables the transformation of biaxial residual stresses into two orthogonal uniaxial residual stresses. By conducting a pair of orthogonal indentation tests, biaxial residual stresses can be accurately determined, with errors mostly within a 16% margin. This method fulfils the criteria for non-destructive testing in engineering, providing ease of use and in-situ applicability.