High Strain Flexural Characterization of Thin CFRP Unidirectional Composite Lamina

Abstract

Thin composite flexures possess unique characteristics desirable to the aerospace deployables community. These high strain composite (HSC) laminates have been found to exhibit elevated strength and deformation capacities prior to failure in comparison to those observed in thicker traditional composite structures. For this reason, HSCs are regarded as a prime candidate for strain energy driven deployable space structures for their ability to endure high levels of stress and strain enforced during the stowage process. Means of quantifying this phenomenon have been met with difficulty due to current limitations in testing methodologies; typically resulting in lower material capacities non-representative of those observed in thin HSCs in bending. In attempts to describe this occurrence, a nonlinear fiber based constitutive material model is used along with high strain lamina flexural mechanics. A workflow is laid out, detailing the various test approaches employed to determine the essential material constants (composite constituent moduli, tensile/compressive nonlinear parameters, etc.) required to identify the failure modes at play. Such testing methods include ASTM tensile testing, optical micrographing, Large Deformation Four Point Bending, and Platen Flexure. To demonstrate the validity of this approach, the entire process is performed for an IM10 carbon fiber/PMT-F7 aerospace toughened epoxy based unidirectional composite. Matrix modulus, fiber initial modulus, tensile and compressive nonlinear parameters were found to be 3.53 GPa, 284.2 GPa, 23.6, and 34.0 respectively. These values were utilized to fit Platen Flexure test data to determine ultimate stress-strain values at representative failure conditions. The IM10/PMT-F7 unidirectional displayed capability in achieving tensile strain levels up to 25% greater than manufacturers stated limits (2.0% observed vs 1.6% reported). Compressive strains calculated at failure also showed notable values, approaching close to 3% at its best.