Evaluation of Validity Criteria for Subsize Compact Tension Specimens Using a Bending Modified J-A2 Solution

Abstract

Many existing pieces of plant equipment do not have original impact test data or suffer from a loss in toughness over time. As a result, the ability to remove material for testing can be of significant value. Often, extraction of sufficient material for conducting fracture toughness testing necessitates weld repair to fill in the sample location. Some equipment may also require stress relief. Use of subsize test specimens that minimizes the amount of material removed can permit local thin area acceptance criteria to be met and avoids weld repair in most cases. However, for subsize specimens that are substantially smaller than conventional geometries, it is important to examine the limits within which a valid J integral can be measured when tests display significant amount of ductility. Standards like ASTM E1820-20 provide restrictions on the maximum J integral (Jmax) based on specimen thickness (B), uncracked ligament length (bo) as well as maximum crack growth Δa. These limits ensure the stress-strain fields at the crack tip are well described by J, a condition referred to as J dominance. In the first phase of this project, detailed 3D FEA analyses of subsize compact tension C(T) specimens (commonly called mini-CT) have been performed to explore the criteria necessary to obtain a valid measurement of J. This is generally considered to be a function of the materials hardening exponent, mode of loading, and the specimen dimensions. One methodology to examine if J controlled crack growth exists is to evaluate the crack tip stress field, and compare this to the theoretical HRR solution. In the case of the mini-CT, due to limitations associated with the small specimen size and extent of yielding across the uncracked ligament, the two parameter J-A2 solution was used to extend the range of J dominance. Additional modification of J-A2 is made by accounting for the effect of global bending on crack tip opening stress (J-A2-M) following the procedures developed by Zhu et al. [1] and Chao et al. [2] for SEN(B) specimens. The results of this study indicate that Jmax for the mini-CT may be given as Jmax<(Bn,bo)σo5 for typical pressure vessel steels. Where Bn is the specimen net thickness, bo is the ligament length and σo is the yield stress. However, test results as well as consideration for plastic collapse suggest that while a valid measurement of initiation J (Ji) may be possible, the extent of J controlled crack growth is more restrictive than the ASTM E 1820-20 limit of Δa = 0.25bo, and may be closer to 0.08 bo.