Abstract
A gradient-enhanced ductile damage model at finite strains is presented, and its parameters are identified so as to match the behaviour of DP800. Within the micromorphic framework, a multi-surface model coupling isotropic Lemaitre-type damage to von Mises plasticity with nonlinear isotropic hardening is developed. In analogy to the effective stress entering the yield criterion, an effective damage driving force—increasing with increasing plastic strains—entering the damage dissipation potential is proposed. After an outline of the basic model properties, the setup of the (micro)tensile experiment is discussed and the importance of including unloading for a parameter identification with a material model including damage is emphasised. Optimal parameters, based on an objective function including measured forces and the displacement field obtained from digital image correlation, are identified. The response of the proposed model is compared to a tensile experiment of a specimen with a different geometry as a first approach to validate the identified parameters.
Highlights
The design of metal forming processes requires increasingly more accurate models to predict material behaviour in order to use more of the forming capability of the material and to more accurately predict safety margins, saving costs and energy
The present contribution started with a brief summary of the finite element implementation of the gradientenhanced damage formulation in finite strains
By making use of an additional field variable—denoted nonlocal damage—and by coupling it to a local damage variable, a regularised damage formulation could be achieved by taking gradients of the non-local damage field into account
Summary
The design of metal forming processes requires increasingly more accurate models to predict material behaviour in order to use more of the forming capability of the material and to more accurately predict safety margins, saving costs and energy. The gradient-enhanced theory was recently extended to incorporate non-constant regularisation parameters [45,61,65]; i.e. the gradient contribution in the free Helmholtz energy is weighted by a term dependent on history variables, e.g. the damage state itself Another focus lies on the regularisation of anisotropic damage formulations, see, e.g., [1,29]. The modelling approach pursued within this contribution aims at an accurate prediction of damage and plasticity preceding macroscopic failure in order to establish a material model which can be used to simulate forming processes and which is able to quantify a safety margin To this end, a finite strain formulation of gradient-enhanced ductile damage conceptually following the small-strain contribution [24] is adopted.
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