Experimental determination of crack growth resistance curves of high-strength steel using thin clamped SENT specimens

Martensitic stainless steels (MSS) are known for their high mechanical strength and moderate corrosion resistance across various environments. However, their martensitic structure imposes limitations on fracture toughness. By employing heat treatments like quenching and partitioning, it becomes feasible to augment the presence of residual austenite in the material. This microstructural change enables the material to better absorb energy during fracture, thereby increasing its fracture toughness. The development of high-strength steels with good fracture toughness could influence the project and design of structural components, potentially resulting in reduced structural thickness. Experimental determination of crack growth resistance curves and fracture toughness for thin high-strength steels is challenging because most standardized methodologies were developed for thicker samples. The American Society for Testing and Materials has published a standard test method for the determination of resistance to stable crack extension under low-constraint conditions (ASTM E2472) in terms of critical crack-tip-opening angle (CTOA, ψ_c) and/or critical opening displacement at the original crack tip (δ₅), involving the use of thin compact tension C(T) and middle-tension M(T) specimens. However, this method requires specific instrumentation, relatively large specimens, and additional experimental devices such as anti-buckling guides. In this context, this paper evaluates the applicability of the elastic unloading compliance technique for determining crack growth resistance curves in terms of J-integral of MSS using relatively small, non-standard thin clamped SENT specimens with thickness of 1 mm. The proposed methodology, based on a combination of BS 8571 standard and the compliance, stress intensity factor, and η_pl factor solutions from the literature, has proven to be suitable for evaluating toughness in thin clamped SENT specimens and could be useful for assessing fracture toughness in high-strength steels of small thickness.