EMch_215_Fatigue__H_

EMch_215_Fatigue__H_ - EMch 215 Fatigue and Related...

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1 EMch 215 Fatigue and Related Failures Taken from: www.luk.or.id/activities/failure_analysis.html
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2 Topics Covered in EMch 215 Tensile and Compressive Properties Elastic Behavior of Materials True Stress - True Strain Analysis of Stress Analysis of Strain Yield Safe Design Stress Concentration Factors Viscoelasticity Creep Fracture Fatigue and Related Failures
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3 Fatigue Automobile scrap yards are filled with broken parts- the majority of these components failed at stresses below the yield strength. These failures were not a result of imperfections in the material, but of the phenomena called fatigue. Fatigue is the name given to the: ________________________________________ _________________________________________.
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4 Fatigue HISTORY - The first reported failures occurred in the mid 1800’s with train axles which had the same design as horse- drawn carriage axles. C T σ max σ min σ time σ m S
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5 Fatigue Stress versus Time Behavior: σ TIME σ a = σ ao The majority of engineering failures involve cyclic (or fluctuating) loading of one form or another. Examples of this loading include: -Alternating stresses associated with a rotating shaft, -Pressurizing and de-pressurizing in an aircraft fuselage at takeoff and landing -Load fluctuations affecting the wings during flight. σ a = _______________ σ ao = if σ m = 0 + 0 completely reversed stress amplitude (amplitude on tension side equals that on the compression side)
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6 Fatigue 200 400 600 800 10 4 10 5 10 6 Unnotched Notched The material response parameter that we are looking for is the number of cycles to failure (N) - this is also termed the _______________. The plot of a material’s endurance N as a function of applied stress is called a _____________________________________. Wöhler studied the railway axle failures and found that axle life increased with decreasing stress level. Below a certain stress level the steel axles seemed to have an infinite life. Endurance limit Stress Number of Cycles to Failures (N )
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7 S/N Curves SAFE Dowling, Norman E. Mechanical Behavior of Materials Chapter 9. p. 364. 1999.
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8 Variation in S-N data for ten specimens drawn from the same bar of steel: Statistical Variation
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9 Statistical Variation We can’t ignore statistical variations in fatigue data. Because a lot of scatter is found in the number of cycles to failure a large number of specimens need to be tested all at the same reversed stress amplitude σ ao . 75% Survival Log N NO. OF FAILURES Schematic histogram of cycles to failure determined for similar fatigue specimens loaded with amplitude σ ao 90% survive 50% survive 10% survive
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10 Scatter found in S-N data for an un-notched aluminum alloy. 45 40 35 30 25 7075-76 Al × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × Statistical Variation σ ao Log N
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11 From this type of data we would find: Log N 95% Survive 80% Survive 50% Survive σ ao S-N plot implies an average. Unless otherwise noted, the line on an S-N Curve implies that 50% of the specimens will fail at the stresses and times that the line represents.
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EMch_215_Fatigue__H_ - EMch 215 Fatigue and Related...

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