Methodology for Characterizing the Forming Limits of Highly Aligned Discontinuous Fiber Composite Laminates

Abstract

Highly aligned discontinuous fiber (ADF) composite laminates have demonstrated potential to stretch form biaxially to complex geometry while retaining aerospace grade properties. This work utilizes Tailored Universal Feedstock for Forming (TuFF) [1], an ADF format that uses 3mm IM7 fibers and PEKK 7002 thermoplastic resin converted to unidirectional prepreg and multiaxial blanks. To lay the groundwork for the forming process-structure-property relations, a novel methodology is developed for defining the forming limits of an ADF composite lamina by constructing a forming limit diagram (FLD). The ADF-FLD defines the strain mode forming limits based on lamina fiber orientation and will be defined with respect to a broader set of failure criteria—which can be material uniformity, consolidation quality, or specific mechanical properties. To demonstrate the methodology, ADF TuFF blanks were stretch formed at various strain levels and strain modes (longitudinal and transverse plane strain). Surface strain on stretch formed samples was measured by photogrammetry. Preliminary failure criteria were established based on acceptable variability in strain measurements to reflect the uniformity of material deformation. This unique approach to defining forming limits will be used in studying material and process variables in forming ADF composites, as well as providing part geometry and forming process design requirements.