Quasi-non-destructive Small Punch and Hole Drilling tests extended to the assessment of both material properties and residual stresses

Small punch (SP) tests and Hole Drilling (HD) tests are at present frequently applied in diverse industrial contexts, the former for mechanical characterization of structural metals and the latter for the determination of parameters governing residual stresses near the surfaces. Both experiments are standardized in national and international codes, can be performed “in situ” and are regarded as “quasi-non-destructive” because almost negligible are the damages caused in the tested structural components. The innovative developments to be presented in this communication can be outlined as follows. (a) The extraction of specimens for SP tests is exploited for identification of parameters governing the residual stresses and the consequent displacements around the generated cavity are measured by Digital Image Correlation (DIC) cameras, employable to other purposes as well. A finite element (FE) model elaborated once for all and employed for back analysis leading from measured displacements to the residual stress parameters by exploiting the material properties estimated by a preceding backanalysis resting on experimental data measured by the SP test carried out as second experiment on the extracted specimens. (b) An usual HD test is performed; by DIC (no longer strain gauges) displacements are measured due to consequent changes of residual stresses; an indenter is employed for indentation (IND) either on the hole bottom or on the hole wedges (depending on the chosen geometry) and the digitalized “indentation curves” are used as input of inverse analysis based on test simulation by FE modelling; the material properties (elasticity and plasticity) quantified by the so assessed parameters are employed for another backanalysis which is based on HD test simulation by another computational FE model and which leads from DIC measurements of displacements earlier generated by the HD test to the sought residual stress parameters. Both diagnostic methods (a) and (b) are almost non destructive; the FE models and the inverse analyses (centered on “discrepancy minimization” by mathematical programming or genetic algorithms) can rest on software “a priori” elaborated once-for-all; the experimental instruments, except the SP one, can be portable and employed “in situ”, The above circumstances make economically advantageous the here proposed novelties in the methodology of structural diagnosis on metallic plant components.