Dissertation

Defining Forming Limits of Highly Aligned Discontinuous Fiber Composites

Morris, Kyle S

2025

Overview

Carbon fiber reinforced composites have become a material of interest in various high-performance sectors such as aerospace and automotive applications due to their high stiffness to strength ratio, allowing for improved durability and energy saving. Typically, carbon fiber reinforced composites are made with continuous carbon fiber which imposes limitations on manufacturability due to inextensibility of the fibers. Highly aligned discontinuous fiber (ADF) composites have been shown to achieve aerospace-grade properties and have the additional advantage to stretch form biaxially to complex geometries. This thesis focuses on evaluating the formability of ADF composites utilizing Tailored Universal Feedstock for Forming (TuFF) aligned fibers combined with thermoplastic and thermoset resin systems. The primary aim is to establish a comprehensive process-structure-property relationships for ADF composites through novel methodologies for defining forming limits and characterizing material performance. A novel aligned discontinuous fiber forming limit diagram (ADF-FLD) was developed to define formability for this class of material relative to material orientation. Methodologies were developed to construct a forming limit diagram (FLD) for ADF composites, providing a detailed framework for strain mode forming limits based on lamina fiber orientation and a predicted thickness variability in a closed mold and an open mold forming process. To demonstrate this, ADF composite blanks were stretch formed to various strain levels and modes (longitudinal plane strain, transverse plane strain, and biaxial plane strain), while employing both in situ and ex situ techniques to measure deformation. By manipulating surface ply orientations of ADF laminates (0, 45, or 90 degrees) relative to the major strain direction, different strain modes were imposed and measured using photogrammetry post forming and digital image correlation (DIC) for real time strain analysis.First, a method was developed to characterize deformation of multiaxial thermoplastic TuFF laminates in a series of closed molds at various strain levels, using a double diaphragm gas bulge forming process and photogrammetry to analyze strains post forming. A first-order failure definition based on a predicted thickness coefficient of variation relative to average strain was employed to evaluate the forming limits of the material and populate an ADF-FLD, describing the formability of the ADF composite.Additionally, a method was devised to characterize the deformation response of multiaxial thermoset TuFF laminates in diaphragm forming for longitudinal and transverse plane strain. Using a gas bulge method and an Interlaken SP75 highly instrumented forming press, high fidelity strain measurements were obtained via an in situ 3D digital image correlation system. This method allowed for real-time recording of surface strains, allowing for progression of strain variability to be measured continuously. The same failure criterion was applied to the strain data, and an ADFFLD was constructed with repeatable results for each strain mode.Overall, this thesis provides a comprehensive methodology and experimental framework for defining forming limits and optimizing the stretch forming process of ADF composites. The outcomes offer significant insights into the material and process variables, crucial for the design and manufacturing of complex composite parts in aerospace and other high-performance applications.

• Publisher: ProQuest Dissertations & Theses
• Subject: Aerospace engineering / Materials science / Mechanical engineering
• ISBN: 9798290968384