Abstract
An unusual fractographic effect has been observed in phenol-formaldehyde polymers. This consists of a regular array of tracks and features running in the direction of crack propagation. Similar effects have been observed in other highly crosslinked organic networks viz. polyesters, epoxy resins, and polydimethacrylates. Effects which may be related have been observed in some thermoplastics, such as polymethyl methacrylate and a polycarbonate, and appear to be enhanced when fracture is caused by cyclic loading. The fractographic effect can be modeled by Preston's mechanism of intersecting crack propagation but, additionally, needs to invoke localized plastic deformation. In networks prepared by polymerization of ethylene glycol dimethacrylate, both the "track-feature" effect and flexural strength can be increased by inclusion of as little as 1% polymethyl methacrylate which, it is supposed, serves to increase localized plastic deformation.
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