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, 6th edition

• Metals Handbook, Vol.2, 10th edition, ASM International

• Holmon, Experimental Methods for Engineers, 6th edition

• www.matls.com

AA 6061 – T6

• Most common structural AlMgSi alloy

• Temper designation indicates thermal solutionizing and aging treatment to achieve strength

• See www.matls.com for properties

Properties (from www.matls.com)

• Density = 2700 kg/m3

• 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 = 5 108

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 strainN = 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 103 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

125.7 mm

Applied load + self loading weight

Bearing

Cantilever arm

ø5.00

Set screw

Experimental Apparatus

1

2

3

4

DC

B A

Calculations

b = 1257. P r

I (N/mm2)

I = d4

64

N

aapplied b

1

a ultimate

endurance

0 81 2.

endurance

ultimate9.0log

X

1b

]Nlog[2

1X reference

y = -0.0977x + 8.8369

R2 = 1

8.00

8.05

8.10

8.15

8.20

8.25

8.30

8.35

8.40

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Log (Cycles to Failure)

Log

(A

pplie

d st

ress

)

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 = 5 108

• Redo for r = 2.45 mm