Charles S. Barrett

Stress Measurement in Graphite/Epoxy Composites By X-Ray Diffraction from Fillers

The purpose of this work was to measure stresses in graphite/epoxy composites by diffracting X-rays from crystalline filler particles embedded in unidirectional laminates before curing. Particles used were Ag, Nb and CdO, having sharp diffraction peaks at large diffraction angles, θ. The diffraction peaks shift linearly with applied stress in the fiber direction and have stress sensitivities of (2.6, 3.9 and 1.9) X 10-4 deg 2θ/MPa for Ag, Nb and CdO respectively. Elastic strains in filler particles measured by X-rays are proportional to the corresponding composite strains in agreement with the model of H. T. Hahn. Residual strains and stresses in filler particles were also obtained.

Residual Stresses in Resin Matrix Composites

By embedding crystalline filler particles in resin matrix laminates during layup, strains that are transferred to the particles were measured by X-ray diffraction. In tensile tests of unidirectional graphite-fiber/epoxy laminates with Al particles between the first and second plies the X-ray strains increased linearly and reversibly with applied stress up to stress levels that initiated yielding in the filler. Residual stresses in the particles resulting from curing were found to be 5, -34 and -53 MPa in fiber, transverse and thickness directions, respectively, in a specimen dried 7 days at 50°C. Residual stresses in the resin were computed from tensile data and the residual stress data from the particles; neglecting transverse stresses, the residual stress in the fiber direction in the resin was computed to be 8.1 MPa (1.2 ksi). Differential thermal contraction from 177°C to 21°C of matrix and fibers in the absence of particles would lead to a prediction of 25 MPa (3.6 ksi); the former computed value for the filled composite was smaller than this presumably in part because of the inhibition of the contraction of the matrix by the closely spaced particles in the layer between the plies. The difference between the residual stresses in the lateral and thickness directions is also ascribed to this particle interaction. Residual stresses in Al particles of a quasi-isotropic (0, +60, −60)s laminate were not reduced by annealing either in the ambient or in a desiccator at temperatures between 50°C and 175°C; after annealing one hr at 175°C they were 42 and 40 MPa along 0° and 90° directions in the plane of the specimen, respectively, and −29 MPa normal to this plane. Diffraction angles were strongly influenced by moisture content, suggesting the method could be developed as a non-destructive test for moisture content. In quasi-isotropic specimens residual stresses parallel to the surface were tensile when the specimens were dry but were reduced to zero by holding about 150 hrs in 100% relative humidity at 50°C. Substantial stresses remain after 490 hrs at 50°C and 50% relative humidity. There was evidence that the stresses depend to some extent on the moisture history of the specimen. Correlations between the X-ray data and moisture diffusion data were made.

Detection of Moisture in Graphite/Epoxy Laminates by X-Ray Diffraction

The objective of this study was to determine if X-ray diffraction could be utilized to detect moisture non-destructively in graphite/epoxy laminates. CuKα 1 X-rays were diffracted from 333 + 511 planes of Al particles em bedded between the first and second plies of [0 ± 60] s lamiantes during layup. Diffracted peak positions were quite sensitive to environmental moisture, decreasing 0.624 ° ± .015 °2θ on going from a completely dry to a completely wet state at 50°C. The changes were reversible. Correlations be tween in-plane, residual particle strains and both average and local moisture content of the laminate were obtained. Annealing effects were investigated.

Stresses in an Adhesive Bond at an Adhesive/Adherend Interface Under Load

Abstract Triaxial stresses were determined by X-ray diffraction immediately adjacent to the adhesive/adherend interface of a single lap adhesive bond while under a tensile load. One adherend was a Be strip that was relatively transparent to the X-rays; the X-ray beam passed through this and the layer of FM-73M adhesive to diffract from the surface of the other adherend which was of 6061 aluminium alloy suitably annealed. The thicknesses of the Be and Al were made such that their stiffness in tension was matched. Measured stresses were compared with stresses calculated using the Texgap-2D finite element code for a nominally identical joint and at a depth of 0.033 mm into the Al adherend which coincided with the average depth from which the X-ray data were obtained. The comparison showed a general agreement in trends and magnitudes except at the extremities of the bond. In particular the measured peel stress was found to be substantially larger at one extremity than the calculated peel stress. Possible caus...