Residual stresses estimation by “small punch” experiment

Semi-non-destructive Small Punch (SP) tests, at present frequently employed for mechanical characterization of structural metals in plant components [1], are here employed also for assessment of stresses. The innovative procedure proposed herein belongs to the structural inverse analysis methodology (see e.g. [2]) and can be outline d as follows. The sample removal from an in-service component for SP tests is exploited as external action altering the residual stress state possibly present in the location considered. Full-field measurements by Digital Image Correlation of consequent displacements in the surrounding surface are input for inverse analysis. The inverse analysis leading to deterministic estimation of the plane stress tensor is based on: computer simulations by finite element modelling of the sample removal test, Proper Orthogonal Decomposition (POD) [3] for model reduction apt to simplify the computational operations, discrepancy function minimization with employment of the elasticity parameters provided by the subsequent SP test. Such minimization is performed by a Trust Region Algorithm made fast and economical through preliminary POD. The purpose and practical advantage of the proposed method consist of no more need of Hole Drilling or other tests [4], additional to SP tests with further “damage”, for residual stress estimation and mechanical characterization of the material: namely, the back-analysis procedure proposed herein implies exploitation of present standardized SP test for estimations of both stresses and elastic-plastic properties of materials.