Fractographic Study of Fiber Composites.

The fracture surface resulting from a crack propagating in a plane parallel to the axes of fibers in uncoated and flexible interlayered unidirectional glass composites was studied by scanning electron microscopy (SEM). The study was conducted on specimens with 1 mm diameter glass rods embedded in DGEBA epoxy resin. In the uncoated glass specimen, the microscopic study clearly demonstrated that the crack creates a debonding at the glass-matrix interface before reaching the glass. It then propagates along the interface and induces secondary cracks and parabolic marks on the polymer side of the interface. After passing over the top of the glass rod, the crack generates a ripple texture on the polymer side of the interface. Finally, the crack reentered the matrix from a small patch of polymer slightly better adhered to the glass surface. The mechanism of fracture energy absorption when a transverse crack propagates in a fiber composite is proposed based on these morphologies. In the flexible interlayered specimen, glass rods were coated with a flexible epoxy resin. A separation occurred within the flexible interlayer ahead of the crack tip. Smaller and denser parabolic marks were observed on the inner surface of the interlayer at the glass-polymer interface. The crack on the opposite side of the matrix was reinitiated inside the rigid matrix near the interlayer, not at the glass-polymer interface. A stress analysis by finite element methods was conducted to present the stress contour of a transversely loaded fiber composite with a crack near the fiber. The fracture surface of a borosilicate glass was studied by high magnification SEM. The occurrence of a tiny basic longitudinal texture was revealed only by tilting the specimen normal away from the incident beam toward the detector by 60$\\sp\\circ$ to 70$\\sp\\circ.$ This texture was explained by the crack fingering hypothesis, which is based on the meniscus instability of the propagating crack front. Other features on the glass surface, including steps, welts, arrays of skewed cracks and gouged texture were all explicable by this hypothesis. The deformability of glass was shown by the ridges and grooves of the basic longitudinal texture and the elongation of the welt. (Abstract shortened with permission of author.)Ph.D.Materials scienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161446/1/8712188.pd