Fatigue of Copper by Unidirectional Repeated Tensile Load

Abstract

Measurements of deformation and internal friction, and microscopic observations were carried out on annealed OFHC copper specimens when they were under various uniaxially repeated tensile loads. The results obtained were summarized as follows.
The relation between amplitude and the repeated number to failure was nearly identical with the S-N Curve in the ordinary push-pull fatigue test. The ductile fracture occurred under the condition of higher stresses than 13.7 kg/mm², and its fracture strain attained about 50∼60%. On the contrary, the fracture strain under the condition of lower stresses attained about 12∼15%. Local contraction could not be observed under the condition of lower stresses.
Amplitude-independent internal friction which increased at the early stage of loading decreased monotonically with the increase of total plastic strain and then the internal friction increased again before failure by repeated loading.
Persistent slip bands were produced under the lower-stress condition and microcracks to be characterized by fatigue failure also appeared along those slip bands. Development of cell structure under the condition of lower stresses was observed in a lower plastic strain region than that under the condition of higher stresses. These structural changes different from those produced in the structure under the condition of higher stresses or the tensile test.
The changes in internal friction gave good agreement with that of the substructure of dislocations.