1986 Volume 50 Issue 4 Pages 369-373
Low carbon steel (SM 41) was cyclically pre-loaded under a low stress amplitude in tension-compression to 1, 3, 5, 10, 15 and 30% of fatigue life. Then the effect of cyclic pre-loading on the monotonic tensile properties and Charpy impact values was investigated. The microstructures were also observed by transmission electron microscopy. The results are as follows:
(1) Grains containing bundle and/or isolated dislocation structures were observed in the specimens cyclically pre-loaded to 5 to 15% of fatigue life. Dislocation network structures were also observed in some grains pre-loaded to 30% of fatigue life. In the case of cyclic loading under a low stress amplitude, the dislocation structure was quite different among individual grains in a specimen. These results suggest that the deformation was inhomogeneous among grains in contrast with monotonic tensile pre-deformation.
(2) All of the cyclically pre-loaded specimens showed apparent upper yield points and Lüders strain during a monotonic tensile test. These phenomena are probably caused by a dynamic strain aging of isolated dislocations during cyclic pre-loading.
(3) The increase in tensile strength of the specimens, cyclically pre-loaded up to 10% of fatigue life, is caused by the work hardening due to bundle dislocation structures, and conversly, the decrease in tensile strength of the specimens cyclically pre-loaded to 15 and 30% is caused by a certain dynamic recovery during cyclic pre-loading.
(4) The maximum impact value (Imax) decreased and the temperature at the onset of ductility (Td) increased with increasing amount of cyclic pre-loading. These results suggest that cydic pre-loading always makes the specimens brittle.
(5) The transition temperature (Tc) decreased with increasing amount of cyclic pre-loading. However, the decrease in Tc does not relate to the increase in toughness.
