The dependence of the fracture toughness of mi steel on temperature and crack velocity

The dependence of the dynamic plane-strain fracture toughness,K _Id, on temperature and crack velocity was measured for propagating cracks in 1020 steel. The dynamics of crack propagation in double-cantilevered specimens was recorded using electroresistivity techniques. The fracture surface energy was found by comparing the crack propagation to solutions of crack motion in wedged-open cantilevered specimens. The_KId behavior was investigated over a range of temperatures from —196° to —50°C and crack velocities of 3 × 10^-3 to 5 × 10^-2 of √E/p. The rate and temperature dependence ofK _Id over the range ofT and υ_c investigated is well described by:1/K _ld ²= υ₀ are experimental constants. A dynamic value ofK _Id was 70 pct ofK _Ic at the same temperature, although in the temperature and crack velocity range investigated the specific fracture surface energy varies by a factor of 6. The temperatureT _T =B/A in(υ _o/υ_c) for which1/K _Id ² = 0 is similar to Charpy impact transition temperature values whenυ _c = 3 × 10^-3√.E/p. If the plane-strain stress condition could be maintained, thenT _T would define a brittle-ductile transition temperature for dynamic plane-strain fracture toughness. The constantsA andB are interpreted by understanding the plastic energy dissipated by a moving crack. Formerly with Brown University, Providence, R. I.