Fatigue Failures

tom2005

Metal fatigue is caused by repeated cycling of of the load.  It is a progressive localized damage due to fluctuating stresses and strains on the material.  Metal fatigue cracks initiate and propagate in regions where the strain is most severe.   
The process of fatigue consists of three stages:

Initial crack initiation  
Progressive crack growth across the part
Final sudden fracture of the remaining cross section

Schematic of S-N Curve, showing increase in fatigue life with decreasing stresses.


Stress Ratio
The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax).

  If the stresses are fully reversed, then R = -1.

  If the stresses are partially reversed, R = a negative number less than 1.

  If the stress is cycled between a maximum stress and no load, R = zero.

  If the stress is cycled between two tensile stresses, R = a positive number less than 1.

Variations in the stress ratios can significantly affect fatigue life.  The presence of a mean stress component has a substantial effect on fatigue failure.  When a tensile mean stress is added to the alternating stresses, a component will  fail at lower alternating stress than it does under a fully reversed stress.





Preventing Fatigue Failure
The most effective method of improving fatigue performance is improvements in design:

  Eliminate or reduce stress raisers by streamlining the part

  Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes

  Prevent the development of surface discontinuities during processing.  

  Reduce or eliminate tensile residual stresses caused by manufacturing.

  Improve the details of fabrication and fastening procedures

Fatigue Failure Analysis
Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength.  This can result in an unexpected and catastrophic failure in use.

Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component.  The failure may be due the discontinuity, design, improper maintenance or other causes.  A failure analysis can determine the cause of the failure.