In order to verify validity and applicability of GTN model, one of the popular micromechanics yield model, for fracture simulations, punch tests are carried out for round plate specimens machined from steel of JIS G3131 SPHC. Incremental tensile coupon tests are also conducted in order to obtain plastic mechanical properties of the material. Material constants required for GTN model are identified through parametric numerical simulations for coupon tests. It is confirmed that simulated punch force vs. indentation curves using GTN model with identified material constants show good agreement with punch test results. It is also verified that fractured shapes are almost similar with experimental ones where fracture occurs along the circumferential.
It is essential to understand plasticity and fracture behaviors of structural steels for rational design and quantitative damage estimation against ALS(Accidental Limit State) such as ship-to-ship collisions, ship-to-rock groundings or explosions in FPSOs. Reminding that fracture is the final stage of irreversible plastic deformation process, strain hardening properties as well as initial yield stress are required for large strain problem accompanying fracture of material. However, zero or single slope plastic hardening provided from mill sheets is still frequently employed in many studies which have been focused on structural re-arrangements of bow, side or bottom structures to reduce damage extents. Most of marine structural steels such as classification steels for ship structures or API steels for offshore structures are categorized into ductile material of which plastic deformation process up to fracture typically shows three micromechanical stages : nucleation, growth and coalescence of voids. Nucleation of voids usually implies debonding of inclusions or 2nd phase particles from continuum, called steel matrix. On the other hand, growth and coalescence of voids means enlargements of nucleated voids and weakening of ligarments between enlarged voids, respectively.
