Models for strain path independent necking prediction in LS-DYNA

Failure in sheet metal can be caused by one of, or a combination of, the following mechanisms: ductille fracture, shear fracture and plastic instability (necking). Ductile fracture is causrd by the initiation, growth and coalescence of voids in the material during plastic straining, commonly referred to as damage growth. Micro defects can also lead to through-thickness shear fracture in the sheet metal. There are several difIerent scenarios involving the a_ove mechanisms leading to material fracture. In ductile materials fracture is normally preceded by the formation of a neck in which the strains localizes after further loading. This neck has the width of the order of the sheet thickness. After the initiation of the neck the strains and the damage inside the neck start to grow rapidly. and finally the material breaks accoding to one of the mechanisms described above. The incipient necking is in this case also referred to as "plastic instability" as the phenomenon is solely dependent on the plastic properties of the material. It should be observed that the incipient necking phenomenon can be captured by a plane stress shell FE model if the elements are small enough, i.e. of the order of the sheet thickness. After the formation of the neck the stress state in the neck turns into a 3D one, and a highly refined 3D FE- model is rEquired to be able to simulate the post-necking behaviour. Another possible scenario is that damage starts to grow before the formation of a neck. The softening of the material can than speed up the neck formation. In some less ductile materials, e.g. some aluminium alloys and austenitic stainless steels, crack formation can even take place before strain Iocalization. Experience has, however, revealed that incipient necking is the by far the most common failure mode in sheet metal forming applications, as well as in automotive crash applications. The current paper will therefore be focused on the prediction of necking, and especially on methods for handling the strong strain path dependence of this phenomenon. The LS-DYNA code is used by Volvo Can for pertorming car crash simulations. Currently the third- party module CrachFEM from the MaHem company, which is linked to the LS-DYNA code. is used for performing material failure analmes. However. Iately several models for perIorming necking as well as fracture simulations have been implemented in LS-DYNA. It is the object of the current paper to give an overview of these options and to present results from some evaluations of these models.