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Determination of In-Plane Shear Properties of UHMWPE Composites for Input Into a Thermoforming Model



Thermoplastic composite materials can be molded into complex curvature shapes via thermoforming, where the material is heated to a pliable temperature and then formed into the desired configuration. For instance, a hemispherical composite part can be thermoformed by punching a male hemispherical tool into a flat stack of heated Ultra-High Molecular Weight Polyethylene (UHMWPE) composite sheets held in place by a binder ring. The quality of the as-manufactured part is dependent on many variables including the punch rate, friction between the composite and tooling, binder ring pressure and the material properties. The in-plane shear constitutive response of the composite is a critical input for the numerical simulation of this process, as it will dictate how easily the material conforms to the prescribed shape. Achieving a hemispherical shaped part with minimal wrinkling is the desired outcome. However, experimentally investigating the optimal set of processing parameters to achieve this outcome is time consuming and costly. The goal is to create a computational model that can accurately simulate the thermoforming process and predict the end product quality. This will allow for rapid evaluation of optimal process parameter combinations, greatly reducing experimentation time and cost. Inplane shear characterization of DSM Dyneema HB 210 is performed at relevant process temperatures between 20°C and 130°C, fiber rotational rates (0.1 /s, 0.7 /s, 7 /s) and pre-processing states (unconsolidated [0/90]2 sheets vs. consolidated [0/90]2 sheets) to provide constitutive material response input for the computational process model. Biased extension tests show that an elevated temperature leads to an increase of the in-plane shear compliance and that rate effects are more pronounced at a lower temperature. Consolidated specimens were found to require more applied force to deform than unconsolidated material. Fiber rotation is upwards of ± 35° for all specimens, regardless of the configuration. LS-DYNA finite element simulations are conducted using select in-plane shear response inputs generated from the experimental tests to demonstrate the effect of the in-plane shear response on the thermoforming process, the as-manufactured part thickness, and in-plane shear distributions.


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