On the Experiences of Adding a Complex User-Material-Model to LS-DYNA

LSTC’s DYNA codes (both SMP and MPP) have been extensively used at AWE over a period of many years since they represent state-of-the art computational analysis capability which is required for simulating most of our complex problems. Occasionally there is a requirement to enhance existing code capabilities. An example is a need to improve the simulation of the constitutive behaviours exhibited by Polymer-Bonded Explosives (PBXs). Most polymer-bonded explosives are dual-phase composites consisting of an explosive crystalline filler material bonded in an elastically softer polymer matrix, which results in a very complex heterogeneous material. This heterogeneity, and the non-linear properties of the matrix, can lead to very complicated constitutive responses being generated when a PBX is loaded. PBXs possess unequal properties in tension and compression, and show marked strain-rate and temperature dependency. In the explosives arena, it is generally the energetic response of PBXs that forms the subject of in-depth studies, particularly from a viewpoint of safety assessments, e.g. accidental insults such as shock loading or fragment impact. The treatment of PBXs as structural, load bearing materials is less well investigated, for the primary purpose of a PBX is to act as an energetic source, rather than as a constructional material. However, there can be occasions when the structural response of a PBX needs to be well understood and controlled, and this new material model, known as GPM (Generic Polymer Material), was developed for this purpose. The GPM model is a damage-based material model that can simulate some of the complex constitutive responses displayed by PBXs, with a particular emphasis on creep and viscous response. This paper will briefly describe the formulation and application of this model, but the focus will be on the implementation of this material model into the LS-DYNA codes as a user-material-model. This was a non-trivial exercise and its implementation required a good deal of effort, together with some internal code changes by LSTC to their codes in order for the model to function correctly, since the user-material-model interface was found to be inadequate to accommodate a complex, non- linear material model’s requirements. Our experiences will be of potential benefit to other users who might find themselves in a similar situation.