The mechanisms of ductile and brittle fracture in the fracture toughness tests of structural low-carbon steel at low and ambient temperatures were discussed based on the plastic strain measured at the crack tip and beneath the fracture surface with the X-ray diffraction technique. The plastic strain in the close vicinity of pre-fatigue cracks increased proportionally with J integral and the crack-tip opening displacement during the process of ductile fracture at an ambient temperature. The critical strain at the crack tip corresponding to the point of crack nucleation was much smaller than the fracture strain obtained in simple tension testing. The plastic work for fracture was calculated from the distribution of plastic strain measured beneath the fracture surface. In the case of ductile fracture, the plastic work done in the fracture process zone was about 10 percent of JIc. On the other hand, in the case of cleavage fracture, the plastic fracture work was nearly equal to the toughness value, when the influence of X-ray penetration on the measured distribution was corrected.
