Wade C. Jackson

The use of moiré interferometry as an aid to standard test-method development for textile composite materials

Abstract 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. Moire 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 moire interferometry to guide in test method development for textile composites. Moire 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, moire interferometry provided vision so that test methods could be developed with less speculation and more documentation.

An Interlaminar Tensile Strength Specimen

Abstract : This paper describes a technique to determine interlaminar tension strength, sigma 3c, of a fiber reinforced composite material using a curved beam. The specimen was a unidirectional curved beam, bent 90 deg, with straight arms. Attached to each arm was a hinged loading mechanism which was held by the grips of a tensile testing machine. Geometry effects of the specimen, including the effects of loading arm length, inner radius, thickness, and width, were studied. The data sets fell into two categories: low strength corresponding to a macroscopic flaw related failure and high strength corresponding to a microscopic flaw related failure. From the data available, the loading arm length had no effect on sigma 3c. The inner radius was not expected to have a significant effect on sigma 3c, but this conclusion could not be confirmed because of differences in laminate quality for each curve geometry. The thicker specimens had the lowest value of sigma 3c because Of poor laminate quality. Width was found to affect the value of sigma 3c only slightly. The wider specimens generally had a slightly lower strength since more material was under high stress, and hence, had a larger probability of containing a significant flaw.

Damage prediction in cross-plied curved composite laminates

Analytical and experimental work is detailed which is required to predict delamination onset and growth in a curved cross plied composite laminate subjected to static and fatigue loads. The composite used was AS4/3501/6, graphite/epoxy. Analytically, a closed form stress analysis and 2-D and 3-D finite element analyses were conducted to determine the stress distribution in an undamaged curved laminate. The finite element analysis was also used to determine values of strain energy release rate at a delamination emanating from a matrix crack in a 90 deg ply. Experimentally, transverse tensile strength and fatigue life were determined from flat 90 deg coupons. The interlaminar tensile strength and fatigue life were determined from double cantilevered beam specimens. Cross plied curved laminates were tested statically and in fatigue to give a comparison to the analytical predictions. A comparison of the fracture mechanics life prediction technique and the strength based prediction technique is given.

Through-the-thickness tensile strength of textile composites

A series of tests was run to characterize the through-the-thickness tensile strength of a variety of composites that included two-dimensional (2D) braids, 2D and three-dimensional (3D) weaves, and prepreg tapes. A new test method based on a curved beam was evaluated. The through-the-thickness deformations were characterized using moire interferometry. Failures were significantly different between the 2D materials and the 3D weaves. The 2D materials delaminated between layers due to out-of-plane tensile stresses. The strength of the 2D textile composites did not increase relative to the tapes. The 3D weaves failed due to radial cracks that initiated between the surface yarns because of the circumferential stresses along the inner radius. A circumferential crack similar to the 2D materials produced the final failure. Final failure in the 3D weaves occurred at a lower bending moment than in the other materials. The early failures were caused by radial crack formation rather than a low through-the-thickness strength.