Determination of Mechanical Properties of Operating Components Using Instrumented Hardness Testing, Small Punch and Small Size Tensile Testing Techniques

Abstract

To have reliable information about the actual mechanical properties of an operating structure is a key issue for the fitness for service assessment. In many cases either there is no any data available about an ageing structural material or the reliability and validity of the available data is questionable. In most cases, it is not allowed to cut off larger pieces from an operating structure to perform destructive laboratory tests, because it may have significant impact on its integrity. Thus other non-destructive or semi non-destructive techniques are needed. If a small piece of material can be excavated form the component, it is possible to determine the yield stress and ultimate tensile strength of steels using small size specimen test techniques. The Small Punch Test (SPT) is a useful technique in residual life assessment owing to the lower tests evaluation cost compared to destructive methods and has higher life evaluation accuracy than the NDT techniques. In several cases the hardness measurement is the only accepted non-destructive test method, whose results can be used to assess the actual mechanical properties such as yield or tensile stress. Since scattering of the properties calculated directly from the pure hardness value is usually too large, methods have been developed for the evaluation of the mechanical properties from the load-indentation curve using instrumented hardness (IH) testing. The uniaxial small size tensile (SST) test is the most commonly used material testing method. Ideally the results of the tensile test are independent of the size of the specimen, but in practice the size of the grains and the test speed are relevant if it is not possible to sample standard sized specimens from in situ components. Validation of these methods is done using experimental tests and finite element simulations.