Analyses of Failure Mechanisms and Residual Stresses in Graphite/Polyimide Composites Subjected to Shear Dominated Biaxial Loads

This research contributes to the understanding of macro- and micro-failure mechanisms in woven fabric polyimide matrix composites based on medium and high modulus graphite fibers tested under biaxial, shear dominated stress conditions over a temperature range of -50 C to 315 C. The goal of this research is also to provide a testing methodology for determining residual stress distributions in unidirectional, cross/ply and fabric graphite/polyimide composites using the concept of embedded metallic inclusions and X-ray diffraction (XRD) measurements

Statistical Mechanics of Vacancy and Interstitial Strings in Hexagonal Columnar Crystals

Columnar crystals contain defects in the form of vacancy/interstitial loops or strings of vacancies and interstitials bounded by column ``heads'' and ``tails''. These defect strings are oriented by the columnar lattice and can change size and shape by movement of the ends and forming kinks along the length. Hence an analysis in terms of directed living polymers is appropriate to study their size and shape distribution, volume fraction, etc. If the entropy of transverse fluctuations overcomes the string line tension in the crystalline phase, a string proliferation transition occurs, leading to a supersolid phase. We estimate the wandering entropy and examine the behaviour in the transition regime. We also calculate numerically the line tension of various species of vacancies and interstitials in a triangular lattice for power-law potentials as well as for a modified Bessel function interaction between columns as occurs in the case of flux lines in type-II superconductors or long polyelectrolytes in an ionic solution. We find that the centered interstitial is the lowest energy defect for a very wide range of interactions; the symmetric vacancy is preferred only for extremely short interaction ranges.Comment: 22 pages (revtex), 15 figures (encapsulated postscript

X-Ray Stress Measurement in Graphite/Epoxy Composites

Small amounts (\u3c 2 mg/cm2) of Al and Nb filler powders were incorporated between the first and second plies of 6-ply unidirectional graphite/epoxy laminates. Cu Kα1 X-rays were diffracted from specific crystallographic planes in these fillers: 333 + 511 for Al and 411 + 330 for Nb, giving peaks in the back reflection region. The peak positions shifted linearly with stress applied to the laminates in the fiber direction and had stress sensitivities of 8.52 and 3.92 x 10-4 deg 2θ/MPa for Al and Nb respectively. Elastic strains in the filler particles measured by X-rays were found to be proportional to the corresponding composite strains measured by strain gages, in agreement with the model of H. T. Hahn. Residual strains and stresses in filler particles were also measured

A methodology development for the study of near surface stress gradients

A modification of the geometry used in the sin2 [psi] technique of X-ray diffraction is described. A modified equation for residual stress determination, including geometric adapted Fij, is presented. This method allows near surface stress gradients determination and is called pseudo-grazing incidence method. The limits of the new technique were first tested on different powder materials with X-ray radiation produced by conventional tubes and by a synchrotron radiation source. The technique was finally applied for the determination of a residual stress profile in a polished molybdenum surface before and after the deposition of a PVD chromium film.http://www.sciencedirect.com/science/article/B6TXD-40GHPVT-C/1/82a45e55041866281a13e3fc1bf8cc4

X-Ray Stress Measurement in Graphite/Epoxy Composites

Small amounts (1 X-rays were diffracted from specific crystallographic planes in these fillers: 333 + 511 for Al and 411 + 330 for Nb, giving peaks in the back reflection region. The peak positions shifted linearly with stress applied to the laminates in the fiber direction and had stress sensitivities of 8.52 and 3.92 x 10-4 deg 2θ/MPa for Al and Nb respectively. Elastic strains in the filler particles measured by X-rays were found to be proportional to the corresponding composite strains measured by strain gages, in agreement with the model of H. T. Hahn. Residual strains and stresses in filler particles were also measured.</p

X-Ray Stress Measurement in Graphite/Epoxy Composites

Small amounts (1 X-rays were diffracted from specific crystallographic planes in these fillers: 333 + 511 for Al and 411 + 330 for Nb, giving peaks in the back reflection region. The peak positions shifted linearly with stress applied to the laminates in the fiber direction and had stress sensitivities of 8.52 and 3.92 x 10-4 deg 2θ/MPa for Al and Nb respectively. Elastic strains in the filler particles measured by X-rays were found to be proportional to the corresponding composite strains measured by strain gages, in agreement with the model of H. T. Hahn. Residual strains and stresses in filler particles were also measured.</p

Measuring Triaxial Stresses in Embedded Particles by Diffraction

The method presented In 1976 for measuring stresses in polymeric materials including fiber-reinforced composites (1,2) yielded directly only one or two of the principal stresses and (elastic) strains in the embedded crystalline particles. The method is now extended so as to yield determinations of all three principal stresses σ1, σ2, σ3 and strains, ε1, ε2, ε3, The method applies to both residual and applied stresses and strains.High angle diffractometry such as is used in ordinary X-ray stress determinations in metal objects is used, with suitable particles being embedded in a homogeneous plastic or reinforced composite before curing.</jats:p