The Effect of Stress Amplitude on Fatigue Processes of Annealed and Cold-Rolled Low-Carbon Steels

Abstract

The specimens of 0.01%C annealed and 15% cold-rolled steels were fatigued under completely reversed plane bending. The effect of stress amplitude on fatigue process of these materials was investigated by using the X-ray microbeam diffraction technique, optical microscopy and electron microscopy. The results are summarized as follows:(1) The stress cycles at slip band formation Ns and the crack initiation Nc were measured of both the materials by means of an optical microscope. The ratios of Ns and of Nc to the number of stress cycles at fracture Nf were obtained of the annealed material asNs:Nc:Nf=0.02∼0.04:0.40:1, and of the cold-rolled material they are expressed asNs:Nc:Nf=0.002∼0.003:0.26:1.The critical stress amplitude of slip band formation during fatigue in annealed materials is nearly equal to the frictional stress.(2) The amount of crystal deformation during fatigue was dependent on the applied stress amplitude. In the case of the annealed material, the micro-lattice-strain Δd/d at the half of the total fatigue life was related to the stress amplitude σn in the following equation, α·σn=9.3+2.0×104(Δd/d), where α is the stress concentration factor of the specimens 1.03.The excess dislocation density D is also a function of stress amplitude. The function is approximated as follows:α·σn=11.0+5.8×10-4√D, when σn is less than 22kg/mm2.(3) For the cold-rolled material, total misoriention β at the half of the total fatigue life increased when stress amplitude was higher than the yield stress. Subgrain size decreased during fatigue and the amount of its decrease was larger when σn was higher.(4) Fatigue cracks were nucleated through the linking of pores formed along the subboundaries in the annealed material. A similar mechanism of fatigue crack initiation by pore linking was verified for low stress amplitude fatigue in the case of the cold-rolled material. On the other hand, the formation of pores was not observed in high stress amplitude fatigue.