How to Calculate Fatigue Life 

If you are unaware of how to calculate fatigue life, look into the article written below to understand the process.

Fatigue Life Estimation – Strain-Based Approach: Constant Amplitude

We have only looked into stress-based algorithms up to this point. Strain-based or low-cycle fatigue typically results from extreme loading, including bolt loading, temperature loading, and several others.

Your material drifting into the plastic zone can be attributed to various circumstances. For instance, some materials may experience a higher strain rate when you start your car than other materials, which forces them to shift to the plastic zone and causes what is known as “truck cranking.”

The number of times you start your car or put it in the park would be lower than the vibrational number of cycles. Even if your material were to enter the plastic zone, it would only do so for a small number of times—roughly the same number of times you would start your car.

However, vibration cycles would be in the millions when your wagon travels down the road and encounters obstacles or other environmental factors. As a result, the material would likely only endure for 5000 cycles in the plastic zone rather than forever. Strain-based methods, therefore, deal with high-strain zones.

Here, as shown in the formula below, we emphasise plastic strain rather than taking into account stress-fatigue calculations.

  • The Δεp/2 is the plastic strain amplitude,
  • The ε’f is the fatigue ductility coefficient,
  • c is the fatigue ductility exponent (-0.5 to -0.7 for most metals)

Instead of using the stress amplitude σa to characterise the loading, we use the plastic strain amplitude Δεp/2.

To establish how many cycles a material can withstand, calculate the Δεp and utilise the Coffin-Manson curves (found in fatigue handbooks).

The graph’s longest uppermost curve is made up of both low-cycle and high-cycle fatigue. If you want to develop a general fatigue-life calculation, please use the following format:

Static/Fatigue Life Estimation: In the Presence of Crack Fracture Mechanics – Residual Life Estimation

The mechanics of cracks and fractures are covered in this section.

The life of the component is controlled by ΔK, which is Δ(Stress Intensity Factor) if there is a crack inside the product’s body.

ΔK = Aσ√πa

  • A is a geometrical constant
  • σ is the stress level at the tip
  • a is the crack length

The material’s fatigue life is controlled by the stress intensity factor rather than the absolute strain and stress.

The formula for crack growth is as follows:

This is known as Paris’ Law.

As shown in the curve diagram above, the rate of increase starts spontaneously before stabilising at the point when Paris’ Law applies.