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Fracture mechanics, acoustic emission, and fractography were used to study the process of crack growth in 2-in. thick double cantilever beam specimens of a low-alloy steel over a wide temperature range. KIc values ranged from 33 ksi√in. at -200°C to 240 ksi√in. at +18°C. The acoustic emission rate rose steadily with load up to fast fracture. Changes in the emission amplitude distribution were observed shortly before fracture at one of the warmer temperatures. Emission during load holds was measured and emission waveforms were photographed. Fractography showed a small region of ductile tearing prior to fast fracture at warmer temperatures. A model of the interaction of ductile tearing and cleavage fracture is proposed. Ductile tearing is seen as a process taking time and strongly dependent on temperature. Cleavage is seen as a rapid process averted by ductile flow and tearing. Emissions are believed to be produced by plastic zone growth and by cleavage. The experimental facts can be consistently interpreted through this small set of assumptions.
fracture properties, acoustic emission, crack propagation, fractography, brittle fracture, mechanical properties
Research fellow, Imperial College, London,
Manager, Technical Services, Dunegan/Endevco (Europe), Royston, Herts.