Fatigue-life curve-4130 Steel
From material science viewpoint, fatigue is caused by the application of repeated loads. Fatigue weakens the material. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Generally, the nominal maximum stress that cause this kind of damage in a material may be much lower than the strength of the material normally known as the ultimate tensile stress limit or the yield stress limit. Fatigue happens when a material is under the influence of repeated loading and unloading conditions. If these loads are above a certain threshold, microscopic cracks will start to form at the stress concentrators like the surface, persistent slip bands (PSBs), and grain interfaces. At some point a crack will reach a critical size, the crack will then propogate abruptly resulting in fracture.
Fatigue life is defined by the total number of stress cycles required to cause failure. Engineers have used any of three methods to determine the fatigue life of a material: the stress-life method, the strain-life method, and the linear-elastic fracture mechanics method. Fatigue is a surface driven failure. This surface can be both internal as well as external, since defects are internal.
Fatigue life curve-4130 Steel
There is a wide application area of 4130 in the petroleum and gas industry. Owing to this use, it can be exposed to high temperatures and also severe cyclic loading conditions. The fatigue curve below show the influence of thermal treatment on high temperature behavior of this steel. These tests are carried out at a temperature of with a strain range of 0.8% to 1.5%. Figures 1 and 2 below show the cyclic stress response of 4130 steel.
An S-N curve, also known as a Wohler curve, is determined from the result of subjecting the material to a range of fatigue tests at variable stress levels. An example of such tests is shoen in figure 3. In this test, , and therefore the stress ratio . The variable in figure 3 is the stress amplitude .
Figure 4 displays the predicted S-N curve for a mildly notched specimen of SAE 4130. The details of the test can be found in .
Figure 5 shows the result of a cumulative fatugue damage in AISI steel. The details of the study can be found in the works of Jeelani and Musial 
Figures 6, 7, and 8 show the rest of the X-ray diffraction study conducted in Shot-peened and fatigued 4130 steel by Esquivel and Evans 
- ↑ http://en.wikipedia.org/wiki/Fatigue_%28material%29
- ↑ Kim, W.H; Laird, C. (1978). Crack Nucleation and State I Propagation in High Strain Fatigue- II Mechanism. Acta Metallurgica. pp. 789–799.
- ↑ Joseph E. Shigley, Charles R. Mischke, and Richard G. Budynas.
- ↑ ICME class lectures. Dr. Horstemeyer, Spring 2015
- ↑ 5.0 5.1 Bultel, H., and Vogt, J-B., "Influence of heat treatment on fatigue behavior of 4130 AISI steel",Procedia Engineering 2(2010) 917-924.
- ↑ 6.0 6.1 6.2 Fatigue of Structures and Materials, J. Schiive. []
- ↑ 7.0 7.1 Jeelani, S., Musial, M., A study of cumulative fatigue damage in AISI 4130 steel. Jour. of Mat. Sci. 21 (1986) 2109-2113.
- ↑ 8.0 8.1 8.2 8.3 Esquivel, A. L., Evans, K. R.,X-ray Diffraction Study of Residual Macrostresses in Shot-peened and Fatigued 4130 Steel,Experimental Mechanics, pp. 496-502