The moisture effect on the fatigue crack growth of glass particle and fiber reinforced epoxies with strong and weak bonding conditions: Part 2. A microscopic study on toughening mechanism

The toughening mechanisms in three types of glass-filled epoxy composites subject to fatigue loading were studied by several different microscopy techniques. The observed toughening mechanisms were then related to the macroscopic fatigue crack propagation behavior. The role of matrix-reinforcement adhesion was systematically investigated. Scanning electron microscopy (SEM) and transmission optical microscopy (OM) studies revealed that toughening mechanisms such as microcracking, crack pinning, shear yielding, fiber bridging, and fiber dedonding and pull-out were dependent on the surface treatment of the reinforcements as well as the shape of reinforcements. The nature of the toughening mechanisms observed agreed with the fatigue crack propagation behavior predicted by crack tip shielding concepts. Interestingly, matrix shear yielding turned out to be the prevalent toughening mechanism in those mechanisms subjected to moisture exposure. In dry as-molded composites, a careful investigation involving SEM, OM, fluorescent microscopy, and atomic force microscopy (AFM) indicated that the particular micromechanisms observed were indeed microcracks, and not micro-shear bands as had been suggested by other researchers.