This paper presents the results of an experimental study conducted on a class of three-dimensional braided graphite-epoxy composites as well as the results of a modeling study to relate the microstructural variables such as volume fraction of fibers and braid angles to the anisotropic elastic moduli and strengths of these composites. In the experimental program, composite samples with a range of volume fractions (0.2-0.45) and a range of braid angles (0°-30°) were produced and tested in simple compression in both longitudinal and transverse directions. Such data is currently unavailable in literature on a single material system. Extreme care was taken in processing to minimize porosity and non-homogenieties in microstructure, and in proper strain gaging to ensure the reliability and reproducibility of our measurements. In the modeling study, the measured elastic moduli were compared against the currently employed isotrain and isostress models for these materials. At the present time, it is widely reported in literature that the predictions of isostrain model are fairly accurate. This study revealed that the isostrain model predictions can be in significant error and that a weighted average of the isostrain and isostress model predictions yielded the best comparisons against the experiments. For the strength of the composite in the longitudinal and transverse directions, correlations expressing these properties as a function of volume fraction and braid angle were motivated by micromechanics and experimental observations. The predictive capability of these correlations for the selected class of 3-D braided composites was found to be reasonable.

ABSTRACT--Correction for machine compliance is an important step in analyzing the data obtained in many mechanical testing procedures. The difficulties associated with compliance correction, as they apply to the simple compression mode of testing, are explored in this paper. The commonly employed approach is to extend the procedure suggested in the ASTM standards for testing high modulus, single-filament materials, which implicitly assumes that the machine behaves as a linear spring with a constant compliance factor. It is shown in this paper that this approach results in different values for the machine compliance factor for different materials. The nonuniqueness of the machine compliance factor is attributed to the inherent nonlinearity of the machine compliance, i.e., the nonlinear dependence of the nonsample displacement on the applied load. Through a set of mechanical tests on a range of materials, it has been demonstrated that it is necessary to characterize this nonlinear compliance relationship for the machine to obtain accurate and consistent measurements.

SYNOPSISThis paper describes the development of a new crosslinked poly(methyl methacrylateacrylic acid) copolymer for potential applications in bone implants. This copolymer, comprising hydrophobic and hydrophilic components, has been designed to provide small amounts of controllable swelling strains at saturation when exposed to an aqueous saline environment. The volume fraction of the hydrophobic methyl methacrylate monomer to the hydrophilic acrylic acid monomer strongly influenced the swelling behavior of the copolymer. Two different cross-linking agents, allyl methacrylate and diethylene glycol dimethacrylate, were evaluated for their effectiveness in cross-linking and limiting the saturated swelling levels. The influences of the amounts of crosslinking agents and other processing parameters on the swelling behavior of the copolymer were studied using differential scanning calorimetry measurements, solubility tests, and swelling measurements in saline solutions. These measurements provided a good understanding of the structure of the copolymer, the effectiveness of the crosslinkers, the swelling mechanisms in this system, and the factors that strongly affect the swelling weight gain levels in this copolymer.

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A new swelling copolymeric material suitable for self-anchoring bone implants was introduced in part I of this two-part article. The main goal in the second part of the study was to investigate the in vitro fixation characteristics of these novel implants in bone using push-out mechanical testing. Specifically, we examined the various factors that influence the in vitro fixation levels achieved by these anchors and identified a range of copolymer compositions that provide good fixation characteristics for these implants. The factors studied included the copolymer composition, presence of AS-4 carbon fiber reinforcement, and the time of implantation (in an environment of saline solution). The push-out tests were conducted on smooth cylindrical plugs of the swelling materials that were implanted in bovine cortical bone. The bone-implant system was then immersed in saline solution for various periods of time ranging from 1 to 28 days prior to push-out testing. The refixation characteristics of the implants were also investigated in this study by performing repeated push-out tests on a single implant without completely dislodging the implant from the bone. Holding strengths comparable and often exceeding many current orthopedic fixation techniques were obtained (push-out load exceeding 1000 N and shear strength exceeding 7 MPa) with the implant having 80/20 to 70/30 methyl methacrylate/acrylic acid ratios. Furthermore, more than 80% of the ultimate holding strength could be achieved within 7 days of implantation at ambient temperature for the 80/20 composite implants. Excellent refixation properties were demonstrated in which the implant regained its full holding strength in the bone immediately after an initial failure. These results indicate great potential for the possible use of these implants for orthopedic applications such as suture anchoring and internal fracture fixations.