Stepwise modelling method for post necking characterisation of anisotropic sheet metal

Abstract

Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin-Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking

A Simple Procedure for the Post-Necking Stress-Strain Curves of Anisotropic Sheet Metals

“…Another inverse identification procedure is the virtual fields method, in which it is necessary to formulate some preliminary assumptions about the formulation of the target material model to calibrate, in order to satisfy the principle of virtual work [12][13][14]. Another example is the numerical stepwise modelling method (SMM), proposed by Marth et al [15,16] to characterize the tensile behaviour of thin sheet metals, in which the material flow curve is discretized by a continuous piecewise linear function, where each angular coefficient is calculated iteratively, step by step. A mixed indirect/direct approach has been proposed by Maček et al [17] to calibrate a complex material model for 304 austenitic stainless steel, making use of data from simple uniaxial tests and a novel heterogeneous strain field tensile test.…”

Evaluation of Crashworthiness Using High-Speed Imaging, 3D Digital Image Correlation, and Finite Element Analysis

“…Typically, material models for these conditions are calibrated using extensive inverse modelling, which may be a complicated and timeconsuming process. Recently, a direct method, the stepwise modelling method (SMM), to obtain the flow stress past initial yielding to final fracture using 2D digital image correlation has been published [20][21][22]. The method was initially developed for isotropic materials but was later established for anisotropic materials as well [23].…”

“…Recently, a direct method, the stepwise modelling method (SMM), to obtain the flow stress past initial yielding to final fracture using 2D digital image correlation has been published [20][21][22]. The method was initially developed for isotropic materials but was later established for anisotropic materials as well [23]. The method works for different stress states and gives the evolution of the triaxiality as a function of the equivalent plastic strain.…”

“…The method works for different stress states and gives the evolution of the triaxiality as a function of the equivalent plastic strain. SMM has been successfully used to characterise the hardening behaviour [21] and to calibrate different fracture loci [24].…”

“…Two AHSS grades are selected, a 1180 MPa third-generation TRIP AHSS grade for cold-stamping applications and a 1000 MPa PHS steel grade for hot-stamping applications, and the constitutive and fracture behaviour are investigated at different strain rates and stress states. An isotropic version of a previously published SMM [20,21] is utilised to calibrate a viscoplastic hardening model and an empirically based fracture locus implemented in the commercial FE code LS-DYNA ® . In order to validate the crashworthiness simulations, the results from the FE analyses are then compared to the experimental results from the crash box tests using high-speed imaging and 3D-DIC.…”