Micromechanical modelling of ductile fracture in pipeline steel using a bifurcation-enriched porous plasticity model

Sondre Bergo,Tore Børvik,Odd Sture Hopperstad,David Morin

doi:10.1007/s10704-020-00495-7

Abstract

In this paper, we investigate the possibility of predicting ductile fracture of pipeline steel by using the Gurson–Tvergaard–Needleman (GTN) model where the onset of void coalescence is determined based on in situ bifurcation analyses. To this end, three variants of the GTN model, one of which includes in situ bifurcation, are calibrated for a pipeline steel grade X65 using uniaxial and notch tension tests. Then plane-strain tension tests and Kahn tear tests of the same material are used for assessment of the credibility of the three models. Explicit finite element simulations are carried out for all tests using the three variants of the GTN model, and the results are compared to the experimental data. The capability of the simulation models to capture onset of fracture and crack propagation in the pipeline steel is evaluated. It is found that the use of in situ bifurcation as a criterion for onset of void coalescence in each element makes the GTN model easier to calibrate with less free parameters, all the while obtaining similar or even better predictions as other widely used formulations of the GTN model over a wide range of different stress states.

Highlights

It is well established that ductile fracture results from the nucleation, growth and coalescence of voids and flaws driven by plastic flow of the matrix material (Anderson 2005)
It follows that strain localization can be captured by porous plasticity models, like the GTN model, or coupled damage models since such models are able to describe strain softening as a result of damage evolution
When calibrating the GTN models, the predicted sudden slope change in the response curve was taken as the point of crack initiation for the R2 and R0.8 specimens

Summary

Introduction

It is well established that ductile fracture results from the nucleation, growth and coalescence of voids and flaws driven by plastic flow of the matrix material (Anderson 2005) Based on this knowledge, Gurson (1977) established a micromechanical model to represent the yield condition for a porous material with spherical microvoids based on an upper bound analysis. A softening mechanism must be present in the constitutive equations of the material in order to trigger strain localization (Rudnicki and Rice 1975). It follows that strain localization can be captured by porous plasticity models, like the GTN model, or coupled damage models since such models are able to describe strain softening as a result of damage evolution. The two approaches have been reviewed and summarized by e.g. Rice (1976), Needleman and Rice (1978), Yamamoto (1978) and Morin et al (2018)

Objectives

Results

Conclusion