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Process zone models for fracture of fiber-reinforced concrete, metal-particle reinforced glass, and polymer crazes have good experimental verification. This is because the microfracture process zone is often macroscopic in size or easily identifiable or both. Similar models have been proposed for metals and ceramic microstructures. This paper addresses how the microfracture process zone develops in such microstructures. Specifically, the microcrack evolution process may be controlled by chemistry and microstructure as well as localized stress distributions. The importance of this is that the microcrack distribution and the energy dissipation process in the remaining ligaments behind the advancing crack front control the R-curve and final fracture instability. Examples of R-curve behavior in ductile fracture and semibrittle composites are shown, and a model for brittle fracture of steel is proposed. The latter shows that a semi-cohesive process zone of variable size and strength may represent an approach to brittle fracture where weakest-link models are not applicable. Predictions of fracture toughness for ferrite/pearlite steels as a function of test temperature and grain size are obtained with such an approach.
fracture toughness, process zones, R-curves, microstructure, ductile ligaments, composites, brittle fracture, cleavage
Professor, University of Minnesota, Minneapolis, MN