| The carbon fiber fragmentation process is examined under conditions of tensile and compression loading. An empirical model that does not require extensive information about the matrix and fiber constitutive properties is developed to describe the fragmentation process. This model is then used to provide a quantitative comparison of fiber failure initiation stresses, saturation rates, and saturation crack densities. The data from the single-fiber fragmentation tensile test are further evaluated using a variety of analytical methods, including conventional Weibull statistical methods, Weibull-Poisson statistics, in situ determination of Weibull parameters, error minimization techniques, and a one-zone Bessel-Fourier series stress analysis. These analytical methods are compared with respect to their ability to predict the crack density behaviors of a group of six different carbon fibers.; A real-time study of the effects of aging on the properties of the matrix, the fibers, and especially, the interphase is described. The effects are analyzed and compared to unaged specimen behavior. In situ determination of the Weibull parameters for the aged specimens is also described, and these results are compared to the results for the unaged specimens.; Stress birefringence data collected via photomicrography are presented. This information is applied to six carbon fiber types tested under conditions of tensile loading utilizing the "Lambda technique"; calculated values of the interphase shear stress (ISS) are compared to other shear stress values reported in the literature. The cooperative effects of adjacent fiber fragments are also assessed for two of six fiber types using precracked fibers with isolated fiber fragment ends; the equations of the Lambda technique are modified to allow determination of an ISS at the isolated fiber fragment end.; Finally, the dependence of the linear (one-dimensional) size of the stress-transfer zone (STZ) on fiber fragment length is examined. The matrix stress is treated as a controlled variable and is kept constant during measurement of all STZ lengths. These experimental results are overlaid on a set of stress contours predicted by a one-zone Bessel-Fourier series stress analysis to assess the contributions of residual stress to the perturbation stresses at a crack tip or fiber fragment end. |
