MATERIAL FAILURE UNDER CREEP AND FATIGUE LOADING

Let us discuss the investigating method of material failure under creep and fatigue loading. Creep and fatigue failure is divided in three steps

1. Crack starting
2. Propagation of crack
3. Final fracture

Investigating method - S-N curve

Fatigue failure inspection with the help of S-N curve

For estimating product service life

Endurance limit of material

This is the stress below which fatigue failure will not be occur for any number of cycles

Endurance strength of material

This is the stress below which fatigue failure will not be occur for a specified number of cycles.

Formula that we use to calculate the life

N= N₀ [S/S₀] ^(1/b)

Where

N – Required no. of cycle for failure

S₀= endurance limit

b= Basquin slope

S= stress amplitude

Second approaches to fatigue loading inspection

Suppose a flaw free structure

1. Initiation of flaw dominant
2. Safety factors to account for flaws

 Suppose presence of flaws of known type

1. Fracture mechanics for prediction
2. Measurement/ monitoring required

Factors affecting Fatigue Life

A lot of factors are considered to improve the service life and these are as following

1. Defects in material
2. Surface roughness
3. Misalignment and uncertainty in assembly
4. Loading type
5. Effect of environment
6. Improper maintenance
7. Surface treatment
8. Fretting and wearing
9. Thermal effect
10. Component size
11. Effect of notches
12. Surface finishing

After considering all factors they have provided one more formula to calculate the service life of product

S_e= S’ x C _notches X C _size x C _load x C _surface

_{Also Discuss:}  

Effect of corrosion

Shaft alignment procedure

Application of hydraulic accumulator

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