Gradient‐enhanced damage growth modeling of ductile fracture

Abstract

We present a gradient-enhanced damage model for ductile fracture modeling, describing the degraded material response coupled to temperature. Continuum thermodynamics is used to represent components of the energy dissipation as induced by the effective material response, thermal effects, and damage evolution. The viscoplastic Johnson-Cook constitutive model serves as prototype for the effective material. The continuum damage evolution of Lemaitre type is focusing the degradation of the shear response, eventually leading to ductile shear failure. A novel feature is the damage-driving dissipation rate, allowing for elastic and plastic components separated by a global damage threshold for accumulation of inelastic damage-driving energy. In the application to a dynamic split-Hopkinson test and two quasi-static tensile tests, the gradient damage model is compared with the corresponding local model. For isothermal conditions, the examples show that both damage models exhibit mesh convergent behavior when using the global damage threshold. K E Y W O R D Scontinuum damage, damage-driving energy, ductile fracture, gradient damage, mesh convergence INTRODUCTIONFinite element modeling and simulation of machining processes relates strongly to the ability of the constitutive model to represent localized shear and chip form in the vicinity of the cutting tool. [1][2][3] These phenomena can be modeled using thermoviscoplasticity combined with continuum damage to describe ductile failure for the representation of serrated chip formation. 4 The major ductile failure scenario is that failure initiates with void nucleation/microcrack development that coalesce, first on the microstructural level, eventually forming a macroscopic crack or a diffuse damage zone on the macroscale. In many engineering fields, for example, machining and lightweight design, ductile failure is an important phenomenon to account for. It is of vital importance to efficiently and accurately control the fracture process. To achieve this, finite element based continuum damage modeling needs to correctly predict important features such as stress state, damage/initiation/propagation, load carrying capacity of the structure associated with the damage patterns.From the key work of Gurson, 5 ductile damage is attributed to growth of microvoids, whose evolution is formulated in (local) Gurson-type plasticity models. Various formulations of the Gurson-based void growth have been considered, for existing and nucleated voids including the morphology of the voids, for example, Needleman and Tvergaard, 6 Pardoen and Hutchinson. 7 In the FE-application, the void formation is assumed to coalesce into fracture surfacesThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.