J-T and J-Q characterization of surface crack tip fields in metallic liners under large-scale yielding

Composite overwrapped pressure vessels (COPV) are widely used in aerospace applications. Understanding surface crack behavior in COPV metallic liners is challenging because, (i) the liner experiences initial plastic deformation during manufacturing processes, (ii) the liner may experience cyclic plastic strains during normal operation, and (iii) surface crack behavior may be strongly influenced by the constraint effects of the bonding. Methodology is developed to assess the constraint effects imposed by the backing on the surface crack tip stress fields in the liner.;Near tip stress fields in uniaxial tensile loaded metallic liner specimens are developed using J-T characterization and modified boundary layer (MBL) solutions, where J measures the asymptotic field and T measures constraint. The increased elastic constraint imposed by the backing on the liner results in enhanced validity of J-T characterization.;In addition, presented is a rigorous study of strain hardening effects on near tip stress fields in surface cracks using J-Q MBL reference fields and near tip deformation estimates, where Q is a stress difference constraint parameter. At moderate loads, the radial independence of Q cannot be assured for a low strain hardening material. In ASTM geometric standard development, this effect must be considered before a conservative limit is applied.;These methodologies are applied to sub-scale and full-scale COPV models. J-Q fracture prediction of surface cracks in COPV metallic liners is found to be possible up to large deformations. Surface cracks in bonded geometries produce higher near tip triaxiality with respect to constraint level for large deformations. This indicates that while J-Q and J-T predicted stress fields are valid at large loads in these regions, the nonlinearity may hinder Q as a ductile fracture parameter. The results from this study aim to address the feasibility of using a two-parameter;fracture theory to predict failure and characterize limits of both surface crack specimens and COPV liners. These results and methodologies are of practical value for establishing testing standards for surface crack geometries, development of proof test logic for COPVs and broader establishing of methodologies for assessing near tip dominance, parameterization deformation limits and plastic material property effects in fracture prediction.