The Effect of Prestrain on Fracture Ductility and Low-Cycle Fatigue Properties of Low Carbon Steel

Abstract

The effect of precompression on the fracture ductility and the low-cycle fatigue properties was investigated with low carbon steel. The fracture ductility in the transverse direction of the longitudinally precompressed material decreased very slowly with precompression in a range of intermediate and high magnitude of precompression, while this decrease was rather rapid at small prestrains. As a result, when subjected to a longitudinal precompression as high as 120%, the fracture ductility of the low carbon steel investigated still retained in the transverse direction a magnitude of about 30%, which was about 1/2.5 of the ductility in the transverse direction of the initially unstrained condition.
The low-cycle fatigue strength of the longitudinal specimen made of the carbon steel highly compressed in the longitudinal direction, the fracture ductility of which was about 55%, was found to be approximately represented by the Manson-Coffin type relation, as was also found in the case of initially unstrained materials, including the point plotted as twice the fracture ductility value at a quarter cycle. On the other hand, the transverse specimen, the fracture ductility of which was about 11%, showed a strange behavior. The plastic strain range sharply dropped at cycles ductility between 10² to 2×10², and little increase in the number of cycles to fracture was observed with further decrease in the plastic strain range. At cycles less than 10², however, a straight line relation in the log-log co-ordinates was observed, although the inclination of the straight line was much lower as compared with that of the foregoing longitudinal specimen and also of the initially unstrained specimens. This strange behavior was interpreted as an instability phenomenon resulting from the cyclic strain softening characteristics of the material subjected to extremely high prestrain.