Qualitative Analysis about Effect of Shot Peening on Fatigue Limit of a 300M Steel under the Rotating Bending Condition

Author:  S Peige, Y Mei, L Xiang-bin, R Jilai, W Renzhi
Source:  Conf Proc: ICSP-6, (p.290-295)
Doc ID:  1996076
Year of Publication:  1996
Abstract:  
Rotating bending tests were conducted to determine the fatigue limits of peened and unpeened specimens of a quenched an tempered 300M steel with the up and down method. The fatigue fractograph were observed under a scanning electron microscope. The results show that for unpeened specimans the fatigue limit is 840 MPa and the fatigue cracks always initiate at the specimen surface, while for the peened ones the fatigue limit 1040 MPa and the fatigue cracks initiate in the specimen interior, and the depth of the crack source Z measured on the specimens fractured at a stress a little higher than fatigue lilmit is about 200 um which is larger than the whole depth of the compressive stress Z induced by shot peening. That is to say, the fatigue source of the peened specimen is located in the region with the tensile residual stress beneath the strengthened layer. In our previous works, a concept of internal fatigue limit has been proposed. This concept represents that the critical stress for the initiation of a fatigue crackin the interior of material (internal fatigue limit) is higher than that at the surface of the same material (surface fatigue limit) and their ratio is about 1.35 - 1.40. It is believed that the present experimental result can also be analyzed from this standpoint. The result shows that the ratio of the internal fatigue limit to surface fatigue limit is about 1.39. So the internal fatigue limit concept is still valid under the rotating bending condition.