Abstract
When subjected to shear loading of sufficiently high rate, many materials do not fail by cracks, propagating at an angle of 70° with respect to the ligament, but by adiabatic shear bands, which extend nearly straight in the direction of the ligament. Work is reported on investigations for determining the dependence of the impact shear fracture toughness as a function of loading rate, in particular in the regime of failure mode transition from cracks to adiabatic shear bands. For achieving high rate shear conditions of loading, edge cracked specimens are asymmetrically impacted at the cracked edge by a projectile accelerated by an air gun. The resulting mode-II stress intensity factors and the times of onset of failure are determined by a specially developed strain gauge measuring technique. Results on shear fracture toughnesses with increasing loading rate are reported for two structural materials, a 1% chromium steel and a high strength aluminum alloy. Whereas decreasing fracture toughnesses are observed with increasing loading rate when failure occurs by tensile cracks, the fracture toughness increases with loading rate when failure occurs by adiabatic shear bands.
Highlights
In the regime of linear elastic or small scale yielding fracture mechanics, cracks that are subjected to quasistatic shear mode-II conditions of loading fail by the initiation of tensile mode-I cracks, propagating of an angle of about 70° with respect to the ligament
Shear mode-II fracture toughnesses K*IId are measured as a function of loading rate
The steel 42 CrMo 4 shows the following behaviour: for failure by tensile cracks at low loading rates the impact fracture toughness decreases with loading rate; for failure by adiabatic shear bands at higher loading rates the fracture toughness increases with loading rate
Summary
In the regime of linear elastic or small scale yielding fracture mechanics, cracks that are subjected to quasistatic shear mode-II conditions of loading fail by the initiation of tensile mode-I cracks, propagating of an angle of about 70° with respect to the ligament. When the loading is applied dynamically, e.g. by an impact event, a failure mode transition [1,2] is observed when the loading rate exceeds a certain limit value: failure occurs by an adiabatic shear band propagating almost in the direction of the ligament. These adiabatic shear bands absorb more energy for propagation than cracks. The stress intensity factor KII derived from the measured strain gauge signal at the moment of instability, i.e. at the time of onset of failure by a crack or by an adiabatic shear band, defines the impact mode-II fracture toughness K*IId
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