Forming limit measurements for random-fiber mats

In resin transfer molding, the fabrication of the fiber preform is critical to both part quality and production efficiency. The deformation induced by preforming introduces porosity and permeability variations, which affect mold filling, and may result in defects (tears and wrinkles) in the preform. In this study we develop an experimental method for determining the formability of thermoformable random-fiber mats, and provide a mathematical framework for expressing the results. We characterize formability of random-fiber mats using a forming limit diagram, similar to the diagrams used for sheet metal. This diagram maps the combinations of principal stretch ratios λ1 and λ2 that cause the material to fail. The maximum load during constant-rate extension provides the failure criterion in forming limit tests. The experiments combine uniaxial and constrained tests. The uniaxial tests reveal behavior when the sample is free to contract in the lateral direction, and also help determine the strains that cause wrinkling in the sheet. The constrained tests give data when lateral contraction is prohibited. Biaxial data is acquired by conducting a sequence of two constrained tests on a single sample, rotating the sample by 90° between the tests. Forming limit data is presented for CertainTeed's U750 thermoformable random-fiber mat. The results are expressed mathematically in a formability function W (λ1, λ2), in which the forming limits are incorporated through a quadratic penalty function. This type of data can be used in an ideal forming theory to determine the feasibility of forming any given shape from a random-fiber mat.