Strain Conversion Between Inner and Outer Surfaces of Cracked Thin-Walled Tubes: Quantitative Investigation and Unified Predictive Model

Strain conversion between inner and outer surfaces is critical for the integrity assessment of cracked thin-walled tubes, yet its dependence on crack geometry remains insufficiently clarified. In this study, X46 tubular specimens with surface and through-wall cracks were tested under uniaxial tension, with full-field deformation measured by digital image correlation (DIC) and a validated 3D finite element model (FEM) employed. The effects of crack type, length and orientation on strain concentration and inner–outer strain transfer were systematically investigated. Results show that increasing crack length enhances local strain localization; through-wall cracks induce significantly stronger localization than surface cracks. Small surface cracks cause limited deformation disturbance with failure governed by global necking, whereas longer surface and through-wall cracks lead to crack-dominated fracture. Eventually, a unified inner–outer strain conversion model was established, which showed good internal agreement for the investigated tube geometry, material and loading conditions.