THE USE OF MOIRE INTERFEROMETRY AS AN AID TO STANDARD TEST-METHOD DEVELOPMENT FOR TEXTILE COMPOSITE-MATERIALS

The viability of advanced textile composites as an efficient aircraft material is currently being addressed in the NASA Advanced Composites Technology (ACT) Program. One of the expected milestones of the progra m is to develop standard test methods for these complex material syste ms. Current test methods for laminated composites may not be optimum f or textile composites, since the architecture of the textile induces n on-uniform deformation characteristics on the scab of the smallest rep eating unit of the architecture. The smallest repeating unit, also cal led the unit cell, is often larger than the strain gages used for test ing of tape composites. As a result, extending laminated composite tes t practices to textiles can often lead to pronounced scatter in materi al property measurements. It has been speculated that the fiber archit ectures produce significant surface strain non-uniformities, however, the magnitudes were not well understood. Moire interferometry, charact erized by full-field information high-displacement sensitivity, and hi gh spatial resolution, is well suited to document the surface strain o n textile composites. Studies at the NASA Langley Research Center on a variety of textile architectures including 2D braids and 3D weaves, h as evidenced the merits of using moire interferometry to guide in test method development for textile composites. Moire interferometry was u sed to support tensile testing by validating instrumentation practices and documenting damage mechanisms. It was used to validate shear test methods by mapping the full-field deformation of shear specimens and to validate open-hole tension experiments to determine the strain conc entration and compare them to numeric predictions. It was used for thr ough-the-thickness tensile strength test method development, to verify capabilities for testing of both 2D and 3D material systems. For all of these examples, moire interferometry provided vision so that test m ethods could be developed with less speculation and more documentation .