Fracture analysis and constitutive modelling of ship structure steel behaviour regarding explosion

Consistent constitutive modelling of material behaviour and further reliable numerical prediction of the response of structures under severe loading necessitate the knowledge of the microstructural mechanisms at the origin of failure. The present work deals notably with the identification of the microstructural damage mechanisms of a high purity (ferritic-pearlitic) mild steel employed as structural material in military ship building. With this aim in view, an extensive campaign of experiments has been carried out, including interrupted and until fracture tests on smooth and notched, axi-symmetric and plane specimens. Initial and post-mortem microstructures of the samples have been observed using a scanning electron microscope (SEM) in order to reveal the damage mechanism. The latter is double: quasi spherical cavity nucleation and growth inside the soft ferritic matrix and microcracking at the (soft)ferrite-(hard)pearlite interphase. Conditions for initiation and evolution of these two kinds of damage appear as being different as expected. The various steps of diffuse damage, microcracking and macro cracking yielding ultimate failure are also observed. Fractographies obtained from tensile tested samples and explosion loaded plates are also compared. Moreover, the material behaviour has been modelled, describing the salient effects observed experimentally, namely strain and strain rate hardening, and thermal and damage softening. The parameters identification was accomplished using an inverse method based methodology. Finite element numerical simulations involving far-field underwater explosion loading implemented as user subroutine in the FE computation code ABAQUS has also been performed, leading to a satisfying agreement between experimental and numerical results. (C) 2010 Elsevier Ltd. All rights reserved.