Integrated material modelling on the crashworthiness of automotive high strength steel sheets

The aim of this study is to investigate the impact of microstructure features on the crashworthiness for automotive high-strength steel sheets by using multiscale modelling approach ondifferent length scales, which provides a toolkit for the further microstructure design to meet the desired improvement of component performance. An extensive experimental program is designed involving various sample geometries that cover a wide range of stress states and tests are performed under quasi-static and high strain rate conditions and up to 2500 s-1 for an automotive dual-phase steel sheet (DP1000). The modified Bai-Wierzbicki (MBW) damage model is extended to a non-local formulation to cope with the simulations for lab and component levels. For the linking between the microstructure and mechanical properties, the representative microstructure model which considers the distributions of grain size, grain shape, crystallographic orientation and misorientation etc., is employed. The bridging between the models at different levels are powered by the virtual experiments and the entire approach is validated by lab-scale experiments and the crash box tests

Modelling of ductile fracture in single point incremental forming using a modified GTN model

Understanding the deformation and failure mechanisms in single point incremental forming (SPIF) is of great importance for achieving improved formability. Furthermore, there will be added benefits for more in depth evaluation of the effect of localised deformation to the fracture mechanism in SPIF. Although extensive research has been carried out in recent years, questions still remain on the shear and particularly its effect to the formability in SPIF processes. In this work, a modified Gurson–Tvergaard-Needleman (GTN) damage model was developed with the consideration of shear to predict ductile fracture in the SPIF process due to void nucleation and coalescence with results compared with original GTN model in SPIF. A combined approach of experimental testing and SPIF processing was used to validate finite element results of the shear modified Gurson–Tvergaard-Needleman damage model. The results showed that the shear modified GTN model improved the modelling accuracy of fracture over the original GTN model under shear loading conditions. Furthermore, the shear plays a role under meridional tensile stress to accelerate fracture propagation in SPIF processes

The problem of sharp notch in microstructured solids governed by dipolar gradient elasticity

In this paper, we deal with the asymptotic problem of a body of infinite extent with a notch (re-entrant corner) under remotely applied plane-strain or anti-plane shear loadings. The problem is formulated within the framework of the Toupin-Mindlin theory of dipolar gradient elasticity. This generalized continuum theory is appropriate to model the response of materials with microstructure. A linear version of the theory results by considering a linear isotropic expression for the strain-energy density that depends on strain-gradient terms, in addition to the standard strain terms appearing in classical elasticity. Through this formulation, a microstructural material constant is introduced, in addition to the standard Lamé constants . The faces of the notch are considered to be traction-free and a boundary-layer approach is followed. The boundary value problem is attacked with the asymptotic Knein-Williams technique. Our analysis leads to an eigenvalue problem, which, along with the restriction of a bounded strain energy, provides the asymptotic fields. The cases of a crack and a half-space are analyzed in detail as limit cases of the general notch (infinite wedge) problem. The results show significant departure from the predictions of the standard fracture mechanics

Recent developments in the study of hydrogen embrittlement at the University of Illinois

This paper summarizes recent work at the University of Illinois on the fundamental mechanisms of hydrogen embrittlement. Our approach combines experimental and theoretical methods. We describe the theoretical work on hydride formation and its application to hydrogen embrittlement of titanium alloys through the stress-induced hydride formation and cleavage mechanism, the localization of shear due to solute hydrogen, and finally, we present experimental evidence that favors the decohesion mechanism of hydrogen embrittlement in a ??-Ti alloy.published or submitted for publicationis peer reviewe

Plane-Strain Problems for a Class of Gradient Elasticity Models-A Stress Function Approach

The plane strain problem is analyzed in detail for a class of isotropic, compressible, linearly elastic materials with a strain energy density function that depends on both the strain tensor epsilon and its spatial gradient a double dagger epsilon. The appropriate Airy stress-functions and double-stress-functions are identified and the corresponding boundary value problem is formulated. The problem of an annulus loaded by an internal and an external pressure is solved

A simplified canonical form algorithm with application to porous metal plasticity

A canonical form for the representation of material constitutive equations within standard finite element codes has been developed which is deceptive in its simplicity. Substitution of different equation systems is trivial and a consistent material Jacobian may be obtained automatically. The process is essentially numerical and does not involve the difficult algebraic manipulation associated with more traditional approaches. It is nevertheless exact because partial derivatives are derived analytically and simple because calculation of these derivatives is the only algebraic manipulation required. The remainder of the process is generic. In this paper, the algorithm is simplified further. A much more detailed and transparent explanation of the key to the method is given, namely calculation of the consistent Jacobian. A trivial modification also extends the method to plane stress. The original algorithm was validated for a simple material model. The new form is used to implement the significantly more difficult modified Gurson model for porous metal plasticitly with hydrostatic yield dependence. It is tested for single element cases involving total collapse and also used to simulate necking of a notched cylindrical bar. Copyright © 2005 John Wiley & Sons, Ltd