Forming limits of highly aligned discontinuous fiber composite laminates

Highly aligned discontinuous fiber (ADF) reinforced composites can retain mechanical performance comparable to unidirectional continuous fiber laminates, with the advantage of biaxial extensibility in forming processes. To characterize and define formability for this class of material, this work develops a novel forming limit diagram (FLD) for ADF composites, for which failure modes and forming limits can vary by material orientation. Multiaxial laminates were stretch formed and consolidated into a series of cylindrical molds with a double diaphragm process. By orienting the surface plies (0, 45, or 90 degrees) relative to the major strain direction, longitudinal and transverse plane strain modes as well as biaxial strain were imposed. The strain tensor was measured by photogrammetry of an isometric dot pattern. Rather than localized tearing (necking), spatial variability in strain grew proportionally and with similar magnitude for longitudinal and transverse forming strain. The longitudinal and transverse strain measured from each experiment were used to populate the a novel aligned discontinuous fiber forming limit diagram (ADF-FLD) which evaluates the strain level and mode relative to material orientation. The methodology established correlates material performance of a unidirectional ADF lamina by defining first ply failure of a lamina in a multiaxial ADF laminate. To establish failure criteria, average strain level and variability within a gauge section were used to compute the expected formed thickness and thickness variability. Applying the failure criteria that ply thickness must have less than 10% variability, the experimentally determined forming limit curve maintains that thickness reduction must be less than 37% for all strain modes.