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Lecture28 - ME 382 Lecture 28 EFFECT OF VARIABLE STRESSES...

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ME 382 Lecture 28 24/iii/06 1 E FFECT OF VARIABLE STRESSES ON FATIGUE LIFE Loading history may not be uniform To estimate approximate life-times use Palmgren 1920 1 2 4 3 4 ! Miner 1945 1 2 3 Rule : N i N f i i " = 1 (often called Miner’s rule) where, N i = # cycles in load block i N fi = # cycles to failure for constant cycling of type in load block i Note : All these fatigue laws are empirical . They do not always work, and should be used with caution! Examples of fatigue calculations Example 1 A loading sequence consists of Block 1. 20 cycles ( σ m = 0 MPa, σ a = 200 MPa) Block 2. 5 cycles ( σ m = 0 MPa, σ a = 250 MPa) How many times can this be repeated until failure using a 2024 Al alloy for which ! a N f 0.102 = 839 MPa and σ Y = 303 MPa Step (i): Block 1: σ m = 0 OK to use Basquin’s Law σ max = 200 MPa; σ min = -200 MPa Check for yield: ˜ ! H < ! Y no yield Block 2: σ m = 0 OK to use Basquin’s Law σ max = 250 MPa; σ min = -250 MPa Check for yield: ˜ ! H < ! Y no yield
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ME 382 Lecture 28 24/iii/06 2 Step (ii): N f 1 0.102 = 839/200 N f 2 0.102 = 839/250 N f 1 = 1.27 ! 10 6 cycles N
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