What is Fatigue life: Definition and 13 Discussions

In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
Fatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue. In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. Most materials seem to experience some sort of fatigue-related failure such as composites, plastics and ceramics.To aid in predicting the fatigue life of a component, fatigue tests are carried out using coupons to measure the rate of crack growth by applying constant amplitude cyclic loading and averaging the measured growth of a crack over thousands of cycles. However, there are also a number of special cases that need to be considered where the rate of crack growth is significantly different compared to that obtained from constant amplitude testing. Such as: the reduced rate of growth that occurs for small loads near the threshold or after the application of an overload; and the increased rate of crack growth associated with short cracks or after the application of an underload.If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals. The stress values that cause fatigue damage are typically much less than the yield strength strength of the material.

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  1. Pipsqueakalchemist

    Engineering (material science) Fatigue life prediction using integral

    So for this question, I understand the math but just wanted to be clear on a few things. So I had this question on my midterm but instead of tensile and compressive stresses, it was tensile and tensile stress. I initially thought that the delta sigma in the integral was the maximum stress so in...
  2. FEAnalyst

    Fatigue life of a simple beam

    Hi, some time ago I wondered if it's possible to calculate fatigue life (number of cycles to failure) for complex models analytically, based on static FEA results: https://www.physicsforums.com/threads/fatigue-life-hand-calculations-based-on-static-fea-results.994264/ Unfortunately, it seems...
  3. FEAnalyst

    Fatigue life hand calculations based on static FEA results

    Hi, advanced FEA programs allow their users to perform fatigue analyses. However, there are still many programs where such simulations can't be performed. I've heard that in such case one can use results obtained from regular linear static FEA to calculate fatigue life manually. Theoretically...
  4. S

    Fatigue of a Rotor: Predicting Life Expectancy with Variable Cyclic Stresses

    I have a fast rotating rotor spinning over 2 axis (over its main axis and over an external axis - the two axis are perpendicular to each other). Easiest to imagine this is like a windturbine yawing as it follows the wind direction.Each of the two rotations obviously causes a distinct centripetal...
  5. S

    Fatigue Life of 0.10mm SS304 Sheet | Sathish Rosario

    Dear All, I would like to calculate fatigue life for the following: A SS430F metal component is repeatedly hitting at 0.10mm SS304 sheet with 1 Kg load from 0.20 mm distance. Assume, Area of hitting: 5.5mm X 16mm = 88 Sq.mm. Hitting load = 1 Kg Tensile strength of 0.10mm SS304 sheet is 515...
  6. Saumya Kar

    Von Mises stress Vs Principle stress

    Hello Folks, May be I am bring up the old topic again., but I've lost enough sleep over this topic. I understand that we use Von-Mises criteria for ductile material failure by comparing to yield limit and max principal stress is used to check failure for brittle materials. My question is bit...
  7. W

    How to calculate fatigue life in the creep realm?

    I am looking for some information on how to calculate fatigue life for materials at temperatures elevated into the creep realm. ASME Sec 8 Div 2 describes how to do this for carbon steel below 700F and stainless below 800F, but I am interested in temperatures > 1000F. I am performing an FEA...
  8. A

    Effect of resonance on Fatigue Life

    I'm doing a research project which aims to develop a mathematical correlation for fatigue life prediction at resonance frequency. An experimental setup is also to be created which would be used to carry out the experiment, cyclic loading cycles until failure. For this a Cussons linear vibrator...
  9. R

    Calculating Fatigue Life for Variable Stress in Cast Aluminum

    Hello How can I calculate the fatigue life of a variable stress between 0 and 90 MPa in cast aluminum? Do I use the mean stress and compare it with the wohler curve? what is the logic of using the mean stress? Why not the maximum stress? Do I have to use the Goodman or Gerber diagram...
  10. R

    Aluminum consumed fatigue life

    How to determine in a cast aluminum part its consumed fatigue life? Lets imagine I get a cast aluminum part that looks good, but I have no idea for how many km it has been used (automotive). Which kind of non destructive tests or methods could I use to determine the consumed fatigue life, and...
  11. Z

    Improve Fatigue Life with Shot Peening: Compressive Stress Benefits

    How does shot peening help improve fatigue life, or rather, how does the compressive surface stress imposed help improve fatigue life? How does materials having high elastic moduli and large grain size prevent creep?
  12. A

    Calculate Fatigue Life of Custom Bolt

    Hi All, We have manufactured a custom bolt (similar to standard bolt). Now we need to calculate the fatigue life of the bolt. Could you please guide me in calculating the fatigue life? Thanks, Ana
  13. C

    How can I estimate the fatigue life of a component subjected to a rotating cam?

    I am working on a physics problem in a project to estimate fatigue life of a component. There is a cam rotating with an offset of 10mm rotating at an rpm of 230. The component is a cantilever fixed at one end is getting hit by this cam while it is rotating. The cam is at the other end of...