TECHNOLOGY AREA(S): Space Platforms
OBJECTIVE: Develop detailed testing methodologies and procedures, enabling the fundamental understanding of high strain composite structural components for critical space strain energy driven deployable architectures.
DESCRIPTION: High strain composites (HSC) have recently gained regard as a viable enabling technology within the aerospace structures community as a means to fold space and aero structures with high reliability, stiffness, dimensional stability, and low cost. HSC's currently serve as a favorable prospect in evolving deployable solar array, reflector, and instrument boom architectures. A significant challenge to widespread adoption of HSCs is the lack of established constitutive mechanics to describe behaviors observed in thin composites subjected to high strain flexural deformations”the primary load case in HSC applications. In regards to carbon fiber reinforced polymers (CFRP), fiber failure in bending appears to occur at elevated strain levels contrary to values determined by traditional uniaxial testing approaches. It is also evident from stress-strain plots that there is an appreciable nonlinear tensile stiffening and compressive softening behavior present in the specimen when large strains are induced. The problem is that currently available test standards (i.e., ASTM) used to measure bending stiffness, strain, and failure onset of high strain composites are limited; typically resulting in lower material capacities non-representative of those observed in thin HSC's in bending. In addition, the fundamental nonlinear composite failure mechanics are not fully understood. The challenge lies in understanding the increased capacity seen in these thin composite flexures in bending; why do thin composites fail at elevated strains in bending? It is postulated that the heightened strains seen in thin flexures are attributed to tension mechanics causing an increase in local shear stiffness that stabilizes the compressive fibers by the adjacent tensile fibers. As a result, this prevents a compressive micro-buckling failure mode commonly observed in thicker composite samples [1-3]. Therefore new test protocols are needed. It is desirable to develop a complete empirical and analytical protocol to characterize the composite mechanics of a thin laminate system in a high strain flexural loading regime pertinent to HSC applications. Specifically, the determination of critical parameters for understanding failure and stiffness at the unidirectional lamina level is desired. Such knowledge is critical for members of industry to design optimized HSC driven structural architectures. Expected work includes development of detailed test methods including both fixtures and procedures, creating and validating post-testing data analysis tools using commercial codes, and a streamlined lamina and laminate characterization workflow that leads to an industry recognized test standard intended for widespread use by the aerospace community.
PHASE I: Phase I work should identify the testing approaches of interest and prove its feasibility in high strain flexural application to CFRP and GFRP composite unidirectional lamina and multi-directional laminates.
PHASE II: Refine and validate the testing methods from Phase I by conducting a complete testing and analysis campaign with focus on developing comprehensive industry recognized testing standards.
PHASE III: Identify space industry recognized testing entities and transition previously developed methodologies.
1. Thomas W. Murphey, William Francis, Bruce Davis, and Juan M. Mejia-Ariza. "High Strain Composites", 2nd AIAA Spacecraft Structures Conference, AIAA SciTech, (AIAA 2015-0942).
2. Thomas W. Murphey, Michael E. Peterson, and Mikhail M. Grigoriev. "Large Strain Four-Point Bending of Thin Unidirectional Composites", Journal of Spacecraft and Rockets, Vol. 52, No. 3 (2015), pp. 882-895.
3. Michael E. Peterson and Thomas W. Murphey, "High Strain Flexural Characterization of Thin CFRP Unidirectional Composite Lamina", 31st ASC Technical Conference, 2016.
KEYWORDS: Composite, High Strain, Testing, Deployable Structure