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Fatigue of Materials

Fatigue of Materials. Fatigue. Definition: Damage accumulated through the application of repeated stress cycles Variable amplitude loadings cause different levels of fatigue Fatigue is cumulative through the life of an engineering element. Factors Affecting Fatigue Life. Loading Conditions

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Fatigue of Materials

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  1. Fatigue of Materials

  2. Fatigue • Definition: Damage accumulated through the application of repeated stress cycles • Variable amplitude loadings cause different levels of fatigue • Fatigue is cumulative through the life of an engineering element

  3. Factors Affecting Fatigue Life • Loading Conditions • Type of stress • Stress amplitude, mean value • Condition of Specimen/Structural Member • Stress concentrations • Surface finish • Material • Thermal history (e.g. grain size in metals) • Environmental conditions • Temperature • Corrosion effects

  4. Loading Characteristics

  5. Effect of Mean Stresses sult sa sa sm>0 sm=0 Mean Stresses reduce the stress range No mean stress

  6. Stress Amplitude vs. Mean • Goodman Relationship: lower the mean stresses, the greater the allowable stress amplitude for the same life. sa sf sm su

  7. Example: Goodman Diagram • If f=u/2, m=u/2, • What is the max and min  that can be applied? sa smin= su/4 smax= 3su/4 su/2=sf su/4=sf sm su su/2=sm

  8. Stress vs. Number of Cycles • S-N Diagram Lower mean stress

  9. Miner’s Rule • Damage from variable loadings is related to the life consumed by number of cycles at each particular STRESS RANGE. The summation of life consumed at each stress range must be less than 1 to avoid failure. where: ni = number of repetitions applied at si Nfi = number of repetitions to cause failure at a stress range, si (ni < Nfi) S ni/Nfi 1

  10. s 5 3 Log N 500 10,000 Example problem - Miner’s Rule • Ni/Nfi= 900/10,000 + 50/500 = 0.09 + 0.10 = 0.19 0.19 < 1.0  OK Used 19% of fatigue life, 81% remains

  11. 1. Beam Fatigue 2. Tension- Compression V M 2c M smax = Mc/I Fatigue tests 3. Others Loading Patterns: 1. Reverse stresses, + to - 2. Alternate zero to some maximum 3. Alternate above some base value

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