IS IT IMPORTANT?

• Absolutely! An understanding of fatigue effects is CRITICAL to the design of devices and structures that may be subjected to cyclic loading during their lifetime.


• Fatigue effects often cause failures which remain undetected until a catastrophic fracture occurs, frequently at a very inopportune time and without warning.


• These failures drive up warranty costs and cause considerable customer dissatisfaction. The worst case is the potential for devastating financial losses and even endangering lives.


• Therefore, the prediction of fatigue behavior can no longer be considered optional.

Fatigue failure

• Estimated to cause 90% of all failures of metallic structures


• Fatigue failure is brittle-like (relatively little plastic deformation)- even in normal ductile materials


• Fatigue failure is sudden and catastrophic


• Associated with cyclic or fluctuating stresses


• Failure can occur at stresses considerably lower than tensile or yield strengths of materials under a static load


• Cyclic stresses causing fatigue may be:
  –Axial: tension or compression
  –Flexural: bending
  –Torsion: twisting
  

3 distinct stages in a fatigue failure

1. Crack initiation
   •Near stress concentration
   •90% of the fatigue life is taken up trying to initiate a crack


2. Crack propagation
   •Incremental advance of the crack with each stress cycles


3. Final catastrophic failure
   •Remaining area (ligament) cannot bear the load

Fatigue appearance

• Fatigue striations
  –Evidence of crack propagation with each stress cycle
  –Not seen with unaided eye – requires electron microscope

• Beachmarks
  –Macroscopic surface feature representing literally 1000’s of striation