High Strain Flow Stress Characteristics of Marine Structural Steel beyond Onset of Necking

摘要

Flow stress beyond onset of necking is necessary for simulations of ship-to-ship collisions, ship-to-rock strandings, and hydrocarbon explosions of oil and gas platforms. But it is known that flow stress beyond necking is no more obeying power law of plastic flow because uniaxial stress state at necked section is changed to triaxial one and flow stress becomes nonlinear in logarithmic scales of flows stress and plastic strain. In order to experimentally verify flow stress versus plastic strain in large plastic strain regime, average true stress are obtained from a series of tensile tests for round bar specimens. Logarithmic plastic strain data are also obtained by measuring reduction of cross section using DICAs (digital image correlation analyses). The constructed average flow stress data versus logarithmic plastic data, which are fully experimental ones, are compared with estimated flow stress based on three constitutive equations (Hollomon, Ludwik, and Swift). It is proved that Bridgman stress correction reduces the experimental average flow stress data, and thus it is between experimental one and estimated one by Hollomon. Parametric study by changing hardening exponents, a best fit value of the hardening exponent with experimental ones is presented. The correction formula is proposed based on best fit value of the hardening exponent with experimental ones. Predicted flow stress using the new formula well forecasts other experimental results on notched specimen tensile tests by Choung (2011; 2013).