Elasticandplastic strain fatiguelifeisshort

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Unformatted text preview: ength using a cantilever load applied to a rotating beam • A test apparatus can be designed to duplicate stress conditions stress level, time, frequency, stress pattern, etc as shown below: • A specimen is put under a stress cycle at a relatively large maximum stress amplitude • The number of cycles to failure are counted • The Stress VS. Number of cycles are plotted • Fatigue limit No fatigue Limit • High stress smaller number of cycles the material is capable of sustaining before fracture • Some metals have a limiting stress level called an endurance or fatigue limit: – The largest value of fluctuating stress that will not cause failure for an infinite amount of cycles – Examples: Titanium and some iron based alloys • Fatigue Strength: stress level at which failure will occur for some specified number of cycles • Fatigue Life, Nf : the number of cycles to cause failure at a specified stress level • • – High loads – Elastic and plastic strain – Fatigue life is short Less than 104 to 105 cycles – Lower stress levels – Elastic strain – Fatigue life is long Greater than 104 to 105 cycles • Fatigue failure has three distinct steps: 1 2 3 Crack initiation – crack forms at a point with high stress concentration Crack propagation – crack increases with each stress cycle Failure – occurs rapidly • • Crack propagation in bisphenon‐A polycarbonate and dimethylbisphenolcyclohexane polycarbonate. Click to Play Cracks normally nucleate on the surface of a component at some point of stress concentration Nucleation sites include: – – – – – Surface scratches Sharp fillets Keyways Threads dents Beachmarks striations • • – – Case hardened lock – surface hardness and fatigue life are improved • – Thermal fatigu...
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