Paul Predecki

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.

Use of Glancing Angle X-Ray Powder Diffractometry to Depth-Profile Phase Transformations During Dissolution of Indomethacin and Theophylline Tablets

AbstractPurpose. The purpose of this study was (i) to develop glancing angle x-ray powder diffractometry (XRD) as a method for profiling phase transformations as a function of tablet depth; and (ii) to apply this technique to (a) study indomethacin crystallization during dissolution of partially amorphous indomethacin tablets and to (b) profile anhydrate → hydrate transformations during dissolution of theophylline tablets. Methods. The intrinsic dissolution rates of indomethacin and theophylline were determined after different pharmaceutical processing steps. Phase transformations during dissolution were evaluated by various techniques. Transformation in the bulk and on the tablet surface was characterized by conventional XRD and scanning electron microscopy, respectively. Glancing angle XRD enabled us to profile these transformations as a function of depth from the tablet surface. Results. Pharmaceutical processing resulted in a decrease in crystallinity of both indomethacin and theophylline. When placed in contact with the dissolution medium, while indomethacin recrystallized, theophylline anhydrate rapidly converted to theophylline monohydrate. Due to intimate contact with the dissolution medium, drug transformation occurred to a greater extent at or near the tablet surface. Glancing angle XRD enabled us to depth profile the extent of phase transformations as a function of the distance from the tablet surface. The processed sample (both indomethacin and theophylline) transformed more rapidly than did the corresponding unprocessed drug. Several challenges associated with the glancing angle technique, that is, the effects of sorbed water, phase transformations during the experimental timescale, and the influence of phase transformation on penetration depth, were addressed. Conclusions. Increased solubility, and consequently dissolution rate, is one of the potential advantages of metastable phases. This advantage is negated if, during dissolution, the metastable to stable transformation rate ≫ dissolution rate. Glancing angle XRD enabled us to quantify and thereby profile phase transformations as a function of compact depth. The technique has potential utility in monitoring surface reactions, both chemical decomposition and physical transformations, in pharmaceutical systems.

Determination of Interlaminar Residual Thermal Stresses in a Woven 8HS Graphite/PMR-15 Composite Using X-Ray Diffraction Measurements

This work is a continuation of the research recently presented in [1] and [2] on the determination of residual thermal stresses in graphite/polyimide composites with and without externally applied bending loads. In the previous work [1, 2] a combined experimental and numerical methodology for the determination of the residual stresses in unidirectional graphite/PMR-15 composites based on X-ray diffraction (XRD) measurements of residual strains in embedded aluminum (Al) and silver (Ag) inclusions has been presented. In this research, the previously developed approach has been applied to evaluate the residual thermal interlaminar stresses in an 8 harness satin (8HS) woven graphite/PMR-15 composite. First, residual thermal stresses have been measured by XRD in aluminum inclusions embedded between the first and second plies of a four-ply 8HS woven graphite/PMR-15 composite. The measurements have been conducted with the composite specimens subjected to four-point bending deformations. Second, viscoelastic computations of interlaminar residual stresses in the composite have been performed using classical laminated plate theory (CLPT) following the manufacturing procedure. Third, the residual strains and stresses in the inclusions have been numerically predicted using the viscoelastic Eshelby model for multiple spherical inclusions. Finally, the interlaminar residual stresses in the composite have been extracted from the XRD strains in the Al inclusions, again using the viscoelastic Eshelby model, and subsequently compared with the residual stresses from the CLPT. It has been shown in this study that the residual interlaminar thermal stresses can be accurately determined not only in unidirectional graphite/polyimide systems as presented in [1] and [2], but also in woven graphite polymer matrix composites.

Determination of depth profiles from X-ray diffraction data

A direct method is described for determining depth profiles ( z -profiles) of diffraction data from experimentally determined τ -profiles, where z is the depth beneath the sample surface and τ is the 1/ e penetration depth of the X-ray beam. With certain assumptions, the relation between these two profile functions can be expressed in the form of a Laplace transform. The criteria for fitting experimental τ -data to functions which can be utilized by the method are described. The method was applied to two τ -data sets taken from the literature: (1) of residual strain in an A1 thin film and (2) of residual stress in a surface ground A1 2 O 3 /5vol% TiC composite. For each data set, it was found that the z -profiles obtained were of two types: oscillatory and nonoscillatory. The nonoscillatory profiles appeared to be qualitatively consistent for a given data set. The oscillatory profiles were considered to be not physically realistic. For the data sets considered, the nonoscillatory z -profiles were found to lie consistently above the corresponding τ -profiles, and to approach the τ -profiles at large z , as expected from the relation between the two.

Observations of Deformation in Spherulitic Polyethylene

A thin film marker technique is described for determining quantitatively from replicas the nonhomogeneous deformation modes operative in spherulitic polyethylene. Experiments were conducted at temperatures from −20° to 120°C and strain rates of the order of 0.04 sec−1. At −20°C, pronounced deformation at spherulite boundaries is observed. This results in stress concentrations which cause the accompanying nonhomogeneous deformation of the spherulites themselves to occur. At 70°C, corresponding to the α relaxation, the deformation has become almost entirely homogeneous within lamellae. At 120°C, corresponding to the α′ relaxation, lamellar boundary slip is clearly observed in addition to homogeneous deformation.

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.

Evidence of a Negative Resistance Characteristic in Circuits Consisting of Thin Mylar Films Connected in Series with High Resistances

Evidence of a negative‐resistance characteristic in a circuit consisting of a thin film of Mylar, in series with a high resistance, at high dc fields is reported. This characteristic is a result of a reversible electrical breakdown of Mylar films at a temperature‐dependent high dc field, which precedes an irreversible thermal breakdown. Without an adequate series resistance these two types of breakdown occur essentially simultaneously and in this event the negative‐resistance characteristic cannot be observed.

Computer Models of Polyethylene Spherulites

In order to account for the morphology observed in surface replicas of polyethylene spherulites, two possible mathematical models are proposed. These are based on the requirement that lamellae with a given twist angle about the radius vector of the lamella form continuous surfaces. It was found possible to arrange the lamellae in such a way that the locus of all points of given lamellar twist within the spherulite was a continuous surface of three‐dimensional spiral form. A computer was used to plot out the ring structure to be expected on sections through the models, and these were compared with the actual ring structures observed on surface replicas of polyethylene spherulites. On all sections not passing through the spherulite center, both models show a double‐armed spiral form in keeping with observed structures. Sections passing through the centers of the models do not always give double‐armed spirals, depending on the model.

Galvanic Corrosion of High Temperature Low Sag Aluminum Conductor Composite Core and Conventional Aluminum Conductor Steel Reinforced Overhead High Voltage Conductors

A High-Temperature Low-Sag Aluminum Conductor Composite Core (ACCC) bare overhead transmission line conductor utilizing a load bearing unidirectional carbon and glass fiber reinforced epoxy composite rod was evaluated for potential galvanic corrosion problems. A series of corrosion tests were performed in 0.5 M NaCl aqueous solution at room temperature, and at 85 °C. The corrosion performance of the ACCC conductor was compared to a conventional Aluminum Conductor Steel Reinforced (ACSR) conductor. The bi-metallic ACSR design suffers inherently from galvanic corrosion, while the ACCC design does not develop galvanic corrosion unless its fiberglass composite galvanic corrosion barrier is compromised. Even with a severely compromised barrier in the ACCC conductor, the measured galvanic corrosion rate of the aluminum in the ACCC conductor was much lower than the galvanic corrosion rate measured in the ACSR conductor.