FATIGUE CRACK-PROPAGATION AND RELATED FAILURE IN MODIFIED, ANHYDRIDE-CURED EPOXY-RESINS

The fatigue crack propagation (FCP) of neat and modified, anhydride-cured epoxy resin (EP) was studied in tensile-tensile mode at ambient temperature. As modifiers, liquid carboxyl-terminated acrylonitrile-butadiene (CTBN) and silicon rubber (SI) dispersions were used. The latter modifier in a defined particle size distribution was produced by a special latex technology, whereas the former developed in situ in the EP by phase separation during curing. The dispersion-type morphologies of the EPs were characterized by using polished sections and viewing them in a scanning electron microscope (SEM). The resulting frequency distribution curves were compared with those analyzed from fatigue fracture surfaces. Probable failure mechanisms were also studied by SEM-fractography. Both modifiers improved the resistance to FCP by shifting the curves to higher stress intensity factor ranges (DELTA-K) in relation to the reference curve determined for the neat EP-matrix. The failure mechanisms, summarized also schematically, differed basically for the various modifiers. According to this, rubber-induced cavitation and shear yielding of the matrix seemed to be dominant for CTBN, which did not affect the principal crack plane. In contrast to this, crack bifurcation, branching and, hence, a forced deviation in the fatigue crack path induced by debonded SI-particles in the EP-matrix were concluded for the SI modifier. The common use of both modifiers yielded a positive synergistic effect due to the superposition of the above failure mechanisms.