Evaluation of fracture toughness test under hydrogenation condition and microstructural aspects using unloading compliance technique

Unloading compliance technique in fracture toughness tests according to ASTM E1820 standard is used to obtain the resistance curve (R-curve) of metallic materials in air tests. Even in this situation, there are aspects inherent to work hardening around the crack-tip that could affect the unloading compliance technique and that are not well discussed. Another important aspect is the application of this methodology in conditions involving corrosive environments. Indeed, the presence of hydrogen around the crack-tip can promote significant changes in the strain and stress around the process zone, leading to the phenomenon called subcritical crack growth. This phenomenon can affect the compliance increasing the divergence between the crack sizes obtained by the unloading compliance technique and the correct crack sizes measured after the test. The aim of this paper is to analyse the influence of subcritical crack growth and to present a methodology to correct crack sizes and the influence of work hardening during fracture.