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BACKGROUND
Fatigue is defined as the condition of a material where it cracks or fails because of the repeated cyclic stresses which are applied below the ultimate strength. A crack usually nucleated at a microscopic level while loading on a structure which propagates in the next step and then leads to the failure of the specimen.1 This type of failure occurs suddenly with a catastrophic result. According to a research of microscopic investigation in the 20th century, the initiation of nucleation of fatigue cracks occurs at a very early stage of the cycle. With grain size crack, it starts with a slip band which leads to the formation of slip steps which gets oxidised in the presence of oxygen and prevents slip reversal.
Fatigue strength can be defined as the maximum cyclic stress or the amplitude range that can be applied to a material without causing its failure. It can be understood by an example of the copper alloy whose fatigue strength is 100,000,000 cycles and applying stress above this limit could lead to sudden catastrophic failure of the material. Fatigue cycle diagram or S-N diagram is used to determine the fatigue limit or endurance limit of the elements. Fatigue strength is affected by various factors including environmental factors, the effect of roughness, concentrations, notch sensitivity.2
Fatigue failure occurs in three stages:
- Crack Initiation
- Crack Propagation
- Final Rupture.
Fatigue testing machine consists of a 3-phase motor with 2800 rpm speed, and we can efficiently perform two experiment simultaneously by using it. In this machine, the behaviour of loading specimen is either cyclically or axially.
The central objective of this experiment is to determine the fatigue strength of the materials..
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REQUIREMENTS
Materials: Specimen with correct design
Vernier Caliper
Dead Weight
Wrench for tightening.
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PROCEDURE
First of all, make the surface of the specimen smooth by polishing and if any surface defects or any scratch found, then reject the sample. After selecting the specimen of mild steel, measure its dimension and fit it in a sample holder in such manner that it passes through the opening of the rod on which loads are seated. Now, keep the desired loads on the loaded seat and note down the time of failure during the fatigue test, when it occurs. Note down the looking of the fracture surface in each case.
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CONCLUSION
The process of finding fatigue strength of material through fatigue testing machine is determined. Fatigue test result varies depending upon the history of the parent material, testing machine, operations performed during testing, etc.3
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REFERENCES
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M. E. Yukitaka Murakami, “Quantitative evaluation of fatigue strength of metals containing various small defects or cracks,” Engineering Fracture Mechanics, vol. 17, no. 1, pp. 1-15, 1983.
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M. E. Y. Murakami, “Effects of defects, inclusions and inhomogeneities on fatigue strength,” International Journal of Fatigue, vol. 16, no. 3, pp. 163-182, 1994.
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C. B. Q.Y. Wang, “Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength,” International Journal of Fatigue, vol. 24, no. 12, pp. 1269-1274, 2002.
