The use of moiré 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 program is to develop standard test methods for these complex material systems. Current test methods for laminated composites may not be optimum for textile composites, since the architecture of the textile induces non-uniform deformation characteristics on the scale of the smallest repeating unit of the architecture. The smallest repeating unit, also called the unit cell, is often larger than the strain gages used for testing of tape composites. As a result, extending laminated composite test practices to textiles can often lead to pronounced scatter in material property measurements. It has been speculated that the fiber architectures produce significant surface strain non-uniformities, however, the magnitudes were not well understood. Moiré interferometry, characterized by full-field information high-displacement sensitivity, and high spatial resolution, is well suited to document the surface strain on textile composites. Studies at the NASA Langley Research Center on a variety of textile architectures including 2D braids and 3D weaves, has evidenced the merits of using moiré interferometry to guide in test method development for textile composites. Moiré interferometry was used 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 concentration and compare them to numeric predictions. It was used for through-the-thickness tensile strength test method development, to verify capabilities for testing of both 2D and 3D material systems. For all of these examples, moiré interferometry provided vision so that test methods could be developed with less speculation and more documentation.