Investigation of Solid and Shell Elements for the Modeling of a Cross-ply Thermoplastic in Thermoforming

Abstract

The feasibility of implementing a thickness-stretch shell element in combination with a user-defined material model in LS-DYNA for completing the simulation of the thermoforming of a cross-ply thermoplastic is investigated. For fabric-reinforced sheets, the primary mode of deformation during the preforming phase of thermoforming manufacture is in-plane shearing of the fabric with varying degrees of shear to conform to the geometric variations over the surface of the preform tool. The decreases in areal coverage that occur with changes in the local shear angle will induce local changes in thickness. During the consolidation phase, multiple preform layers are compressed in a set of matched tools, and the compounding of the thickness variations through the plies can adversely affect the uniformity of the pressure distribution on the composite part in the matched die tooling. Pressure variations over the surface of the part can then lead to incomplete consolidation of the ply stack. Although solutions for implementing thickness changes and contact of the general shell elements have successfully been employed, the challenge of through-plane compressive properties cannot be addressed with that approach. The simulation must accurately capture the thickness changes not only from shear deformation, but also from compression between tooling and other laminate plies. New formulations of solid and shell elements in LS-DYNA may be able to handle the aspect ratios of a general shell element without any compromise in the fidelity of the solution and should be able to model the through-plane compressive properties as well as thickening from shear deformation. A user-defined material model is combined with thickness-stretch shell elements, and LS-DYNA simulations of tension and shear testing of Dyneema HB80 are performed.