Overview of Suitable Methodologies for Threshold Stress Determination by Small Punch in Aggressive Environments

The Small Punch Test is a recently standardized technique successfully employed for estimating the tensile, creep and fracture properties of metallic materials, and is considered as one of the most suitable options for evaluating mechanical properties when materials are in shortage. Several advances have been made in order to apply this methodology for mechanical characterizations in aggressive environments. The first tests were carried out pre-embrittling the samples in environment and then testing them in air. Subsequent methodologies proposed tests in environment after pre-embrittling the samples in order not to lose part of the embrittling capacity while testing; in these scenarios the punch rate has an enormous influence on environmental characterizations. The most recent works propose to implement in the Small Punch Test the step loading methodology collected in ASTM F1624 standard, which solves these problems by applying steps at a constant load which gradually increases until the sample fails. In this work, guidelines for the application of Small Punch Tests to determine the threshold load in aggressive environment are given, based on tests results under hydrogen embrittlement scenarios. A range of punch rates for constant punch displacement is provided, together with the suitable step times when applying the step loading technique, and a correlation to estimate the threshold stress based on Small Punch samples tested with this novel technique.