Threshold stress estimation in hydrogen induced cracking by Small Punch tests based on the application of the incremental step loading technique

Abstract

The small punch test consists on punching a plane small specimen until it breaks. This technique, born in the 80?s, should be considered when evaluating mechanical properties in situations where materials are in shortage. In recent works, it has been used to estimate the mechanical properties of steels in aggressive environments, where characterizations usually consist on the determination of the threshold stress to avoid subcritical cracking by means of constant loading tests, which is a slow technique, and sometimes presents a considerable dispersion in the results. The standard ASTM F1624 solves these problems, by applying constant loads gradually increased, called loading steps, until the sample fails. In the present work, it is proposed to apply the incremental step loading technique from ASTM F1624 adapted to the Small Punch Test (SPT). As a novel approach, modifications on the steps durations for SPT are proposed according with the sample thickness, allowing to obtain the threshold stress in aggressive environments within a few days, by using at least 3 samples. The proposed methodology is applied to a set of two steels, of medium and high-strength, in hydrogen embrittlement environments under three different levels of cathodic polarization in an acid electrolyte. As a reference, cylindrical tensile specimens were subjected to conventional standard tests in accordance with ASTM F1624. The correlation between the threshold stresses, obtained according to ASTM F1624, and the threshold loads, obtained by the Small Punch proposal, is presented and analyzed. Finally, from the aforementioned correlation, a threshold stress estimation based just on Small Punch tests is proposed.