True stress-strain curve extraction from ion-irradiated materials via small tensile, small punch and nanoindentation tests: method development and accuracy/consistency verification

Ion irradiation has been widely used to emulate material degradation behavior in reactor environments. Due to the limited depth of ion-irradiated layers, post-irradiation mechanical properties are mostly characterized by miniature test techniques, such as small tensile, small punch and nanoindentation tests. Although limited, the penetration depth of high-energy ions is possible to be 'large' enough to cover many grains. In such cases, it is possible to extract 'bulk' stress-strain curves from the ion-irradiated volumes, since they are now capable of reflecting bulk material behavior. One of the widely used extraction approaches is inverse finite element method (iFEM). However, the extracted curves by iFEM may contain significant errors in such cases due to some adverse effects, e.g. large scatter in the tensile curves for small tensile test, and indentation size effect (ISE) and strain rate effect (SRE) for nanoindentation test. In order to overcome these negative influences, we proposed different solutions for each test technique. For small tensile test, digital image correlation (DIC) was used to improve the accuracy, by directly obtaining the local true strain in the weakest section, and thereby the local true stress. For nanoindentation test, a two-step correction-conversion method was proposed to refine the iFEM-extracted curves. The three sets of test and data-processing schemes were quantitatively evaluated to check their accuracy and consistency for both pristine and ion-irradiated materials. The extractions from the pristine material show that, all the schemes are capable of providing true stress-strain curves with good accuracy, as they exhibit little deviation from those by the standard specimen. After ion irradiation, although the mechanical behavior has changed drastically, these schemes still exhibit good consistency among the extracted curves. These findings verify the accuracy and consistency of the three approaches with the new developments to obtain 'bulk' true stress-strain curves from the ion-irradiated materials.