<p> Modeling a material in its manufacturing process saves resources by optimizing the process conditions without using the material in trial-and-error tests. A finite element material model is in development to predict the deformation of highly-aligned discontinuous fiber prepregs in thermoforming. Unlike other high-performance composite prepregs, this material can stretch even in the fiber direction, due to the short fibers flowing and repositioning in the sheet. During stretching, this material exhibits highly-anisotropic, viscous behavior with a dominant viscosity in the fiber direction, as well as a unique stress-strain response called strain softening in which the material viscosity reduces with increasing strain. Both of these behaviors are inherently unstable: the material can stretch with little resistance in the transverse direction, and the more it stretches longitudinally, the easier it can stretch further due to strain softening. An equation was developed that describes this material’s extension behavior and implemented into the finite element solver AniForm. The parameters that describe the stretching behavior were derived from axial extension tests performed on TuFF made of IM7 carbon fibers and 977-3 thermoset resin under one set of process conditions (temperature, strain rate). The model was verified by matching the experimental data in longitudinal extension. The model was then validated by accurately predicting the extension-dominated deformation of a 4-ply laminate in gas-pressure bladder molding through a die similar to realistic part forming. The strain state on the surface ply was tracked in-situ using a DIC setup during forming and the results were compared with the AniForm simulation in which the constitutive equation that described the highly-aligned discontinuous fiber prepreg behavior was implemented.</p>