I wonder if necking can occur during strain-controlled low-cycle fatigue deformation (R=-1)? I found that the fracture area is smaller than the initial area after low-cycle fatigue deformation in a polycrystalline superalloy. Can this be called necking? What is the mechanism of necking if the total strain amplitude is controlled?
The fracture surface is as follows. The area is about 27.5 mm2, which is smaller than the initial area (33.18 mm2 ).
Dear Luqing Cui,
To the best of my knowledge, in the fatigue fracture surface, the area corresponding to the initiation region, propagation region and final fracture is dependent on various parameters such as the toughness of materials, fracture toughness, loading mode, max to min stress ratio, applied stress to yield stress ratio, temperature and so on. At first glance, in my opinion, the shown overview fractograph of the fracture surface is not representing the occurrence of necking. As you know, at the low-cycle fatigue regime, the crack initiation stage takes the small number of cycle of whole fatigue life and occurs at the smaller number of cycle in respect to propagation stage. In this case, it is obvious that area of the crack propagation region is considerably larger than crack initiation region. Also, the final fracture region has a smaller area than the crack initiation. This observation demonstrates the acceptable fatigue performance and the serviceability of the alloy in the mentioned conditions. In other words, the material has displayed a good fracture toughness during the fatigue straining. Also, in the case studies, the ratio of the initiation area to final fracture area is considered as an important parameter to evaluate the fatigue performance.
In summary, at the low-cycle fatigue regime, the area of fatigue crack propagation region must larger than fatigue crack initiation region. This strict viewpoint would not be exist for comparison of the area of initiation and final fracture region in the fracture surface.
I hope I have helped.
Regards
depends on the amplitude of the strain imposed. Conventionally, for a tension-compression strain-controlled fatigue test, the imposed strain amplitude will never exceed the amount of strain required to see necking. Otherwise, the gauge of the fatigue sample would be easily subject to buckling during the compression. Hope it helps
Hi Cui,
Basically Stree ratio -1 is the high amplitude range while you're testing also getting more displacement/strain on the sample. So you have to find the nicking so difficult in this condition. Your marked location is the sample was pre cracking of fracture. And one more thing is the strain control sample getting the both elastic and plastic deformation. So to risk to find nicking on this strain controlled test.
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