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ME 388 – Applied Instrumentation Laboratory Fatigue Lab

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Objectives Learn fatigue testing procedures –Wohler machine (Rotating cantilever beam machine) –R.R. Moore (Rotating beam machine) Evaluate fatigue behavior of AA 6061-T6 –Generate S-N diagram –Determine endurance limit Observe surface characteristics of fatigue failure

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References Shigley and Mischke, Mechanical Engineering Design, 6 th edition Metals Handbook, Vol.2, 10 th edition, ASM International Holmon, Experimental Methods for Engineers, 6 th edition www.matls.com

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AA 6061 – T6 Most common structural AlMgSi alloy Temper designation indicates thermal solutionizing and aging treatment to achieve strength See www.matls.com for propertieswww.matls.com

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Properties (from www.matls.com)www.matls.com Density = 2700 kg/m 3 Yield strength = 275 MPa Tensile strength = 310 MPa Elongation = 12% Young’s Modulus = 69 GPa Poisson’s ratio = 0.33 Fatigue strength = 95 MPa @ N = 5 10 8

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Fatigue failure Fracture by cyclic stressing or straining The amplitude of or for fatigue failure may be well below those for static failures Fatigue process –Initiation of small cracks during “early” cycles –Propagation of cracks during subsequent cycles –Fracture

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Factors affecting fatigue Surface finish (amount and direction) Stress concentration or raisers Internal metal defects (voids, cracks, inclusions) Temperature Size Miscellaneous

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Effect of Geometry Effect of geometry (i.e., a notch) is a “constraint” that favors higher stresses Small cracks reduce area producing a higher stress Stress concentration at the tip of small fissures provides a much greater influence Actual stress can be several orders of magnitude larger than the applied stress

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Progress of fatigue failure From R.A. Higgins, Engineering Metallurgy

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Failure Surface

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Fatigue data Plotted on S-N diagram S = stress or strain N = number of cycles Fatigue is a statistical phenomenon with significant scatter Ferrous alloys typically show a distinct fatigue limit, below which failure does not occur (roughly UTS/2) Many non-ferrous alloys do not have a distinct fatigue limit

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Ferrous vs. non ferrous alloys From R.A. Huggins, Engineering Metallurgy

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Steel vs. Aluminum alloy From Manufacturing Processes for Engineering Materials, Kalpakjian

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High cycle fatigue Greater than 10 3 cycles or more Sensitive to surface quality May involve little large scale plastic flow, characteristic of brittle fracture Local crack propagation may involve a wide variety of ductile and brittle phenomena

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Specimen

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Experimental Apparatus

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Calculations

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Lab Analysis and Report Determine weight for each stress level Predict N for each trial Calculate mean and standard deviation for each data set Perform Chauvenet’s criteria analyses Plot bending stress vs. mean cycles to failure showing one standard deviation Extrapolate endurance limit for N = 5 10 8 Redo for r = 2.45 mm

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