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Three Stages of Fatigue Failure

Three Stages of Fatigue Failure. Crack Initiation Crack Propagation oscillating stress… crack grows, stops growing, grows, stops growing… with crack growth due to tensile stresses Fracture sudden, brittle-like failure. Identifying Fatigue Fractures. beachmarks. Three Theories.

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Three Stages of Fatigue Failure

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  1. Three Stages of Fatigue Failure • Crack Initiation • Crack Propagation • oscillating stress… crack grows, stops growing, grows, stops growing… with crack growth due to tensile stresses • Fracture • sudden, brittle-like failure

  2. Identifying Fatigue Fractures beachmarks

  3. Three Theories

  4. ships, planes, vehicle chassis Low vs. High Cycle >103 cycles, high cycle fatigue car crank shaft – manufacturing equipment @ 100 rpm – ~2.5 E8 Rev/105 miles 1.25 E8 Rev/year <103 cycles, low cycle fatigue

  5. Types of Fatigue Loading Fully Reversed Repeated Fluctuating stress range amplitude ratio alternating component mean component stress ratio

  6. Testing Fatigue Properties • Rotating Beam – most data is from this type • Axial • lower or higher? Why? • Cantilever • Torsion

  7. Fully Reversed Empirical Data An S-N Curve Wrought Steel

  8. Fully Reversed Empirical Data Aluminum

  9. Endurance Limit A stress level below which a material can be cycle infinitely without failure Many materials have an endurance limit: low-strength carbon and alloy steels, some stainless steels, irons, molybdenum alloys, titanium alloys, and some polymers Many other materials DO NOT have an endurance limit: aluminum, magnesium, copper, nickel alloys, some stainless steels, high-strength carbon and alloy steels for these, we use a FATIGUE STRENGTH defined for a certain number of cycles (5E8 is typical)

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