No abstract

As a part of the NASA Composite Technology for Exploration project, eight different AS4 3D orthogonal woven composite panels were manufactured and were subjected to mechanical testing including uniaxial tension along the weaves' warp direction. Each set, with four different resin systems (KCR-IR6070, EP2400, RTM6, and RS-50), included weave architectures designed using 12K and 6K AS4 carbon fiber yarns. For the tension testing conducted at Room Temperature Ambient (RTA) condition, the elastic modulus and strength of these eight panels (as-processed and thermally cycled) were measured and compared while the potential evolution of micro-cracking before and after thermal cycling were monitored via optical microscopy and X-Ray Computed Tomography. The data set also included test results of the as-processed materials at Elevated Temperature Wet (ETW) condition. In the second part of this study, efforts were made to compute elastic constants for AS4 6K/RTM6 and AS4 12K/RTM6 materials by implementing a finite element approach and the Multiscale Generalized Method of Cells (MSGMC) technique developed at NASA Glenn Research Center. Digimat-FE was used to model the weave architectures, assign properties, calculate yarn properties, create the finite element mesh, and compute the elastic properties by applying periodic boundary conditions to finite element models of each repeating unit cell. The required input data for MSGMC was generated using Matlab ® from Digimat exported weave information. Experimental and computational results were compared, and the differences and limitations in correlating to the test data were briefly discussed.

No abstract

The damage growth and failure mechanism in graphite/epoxy composite specimens were characterized by using a modified X-ray nondestructive evaluation ( NDE) technique. The NDE monitoring was conducted in real- time while the fracture specimens were under tensile ramp loading and constant amplitude cyclic loading. Tetrabromoethane ( TBE) was applied as an opaque additive at the tips of a slit in the center of the specimens to enhance the flaw image. Damage initiation, growth and failure mechanism were observed from sequences of X-ray pictures recorded during testing. Limited results indicated that matrix failure appeared to precede delamina tion between plies in the failure mechanism study. The NDE technique allowed the actual stress redistributions in the composite laminates to be observed.

The accuracy of the lamination theory in transient wave propagation problems was studied. In-plane and shear-bending impact experiments were conducted with graphite epoxy angle-ply and cross-ply plates. The measured strain histories of these plates were compared with the theoretical predictions of the lamination theory. The results show that, under the given loading conditions, the lamination theory adequately predicts wave velocities and strains in symmetric angle-ply and cross-ply graphite epoxy plates.