investigated is well described by: 1/K/sup 2//sub Id/=-A(ln v/sub o//v/sub c/)T+B where A, B, and v/sub o/ are experimental constants. A dynamic value of K/sub Id/ was 70% of K/sub Ic/ at the same temperature, although in the temperature and crack velocity range investigated the fracture surface energy varies by a factor of 6. The temperature T/sub T/=B/Aln(v/sub o//v/sub c/) for which 1/K/sup 2//sub Id/=0 is similar to Charpy impact transition temperature values when v/sub c/ = 3x10/sup -3/ ..sqrt..(E/p). If the plane-strain stress condition could be maintained, then T/sub T/ would define a brittle-ductile transition temperature for dynamic plane-strain fracture toughness. The constants A and B are interpreted by understanding the plastic energy dissipated by a moving crack.
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