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Tuesday, 16 December 2014

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
Se= S’ x C notches X C size x C load x C surface

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