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Three Stages of Fatigue Failure

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Presentation on theme: "Three Stages of Fatigue Failure"— Presentation transcript:

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

3 Three Theories Stress-Life Strain-Life LEFM (Fracture Mechanics)
stress-based, for high-cycle fatigue, aims to prevent crack initiation Strain-Life useful when yielding begins (i.e., during crack initiation), for low-cycle fatigue LEFM (Fracture Mechanics) best model of crack propagation, for low-cycle fatigue

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

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

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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