Micro-computed tomographic imaging of void damage in a hot-rolled complex phase sheet steel

Interrupted hole tension tests were carried out on four samples of hot-rolled complex phase Advanced High Strength Steel (AHSS) sheet (UTS=780 MPa) to investigate the evolution of void damage at the edge of an expanding hole. Optical microscopy and micro-computed tomographic (microCT) imaging of plastically deformed hole tension specimens indicated that voids nucleate at TiN particles and grow primarily in the direction of tensile loading, developing ellipsoidal shapes as plastic strain accumulates. Larger, cubic particles are associated with larger voids in one of the four sheets under study, culminating in a single, large coalescence event at high plastic strains. Several smaller coalescence events are observed in the other three sheets containing small, irregularly-shaped TiN particles. Coalescence of closely spaced voids was found to occur through necking of the inter-void ligament while void sheet formation was the primary mode of coalescence for large voids spaced farther apart and at an angle of ~45° between the loading and through-thickness directions. A peak in the void volume fraction was observed approximately 0.6 mm from the hole edge as a result of the competing influences of plastic strain and triaxiality.