ME 388 – Applied Instrumentation Laboratory Fatigue Lab

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

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

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

Properties (from 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 N = 5  10 8

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

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

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

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

Failure Surface

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

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

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

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

Specimen

Experimental Apparatus

Calculations

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