During the high speed propagation of cracks, large temperature increases occur at the crack tip due to the intense dissipation of plastic work there. This increased temperature may have a significant effect of the material’s dynamic fracture toughness. An experimental investigation of the temperature fields at the tip of dynamically propagating cracks in 4340 steel was performed using a focused array of high speed, infrared detectors. Temperature fields were measured for cracks growing at speeds from 700m/s to 1900m/s. Maximum temperature increases were as high as 465°C. The temperature fields were differentiated to determine the plastic work rate distribution at the crack tip and to estimate the plastic strain rate. Effects of crack tip heating on dynamic fracture toughness are discussed.

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