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 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 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
Bearing
Set screw
ø5.00
Cantilever arm
Applied load + self loading weight
125.7 mm
Experimental Apparatus
3
1
D
C
B
4
A
2
Calculations
125.7  P  r
2
b =
(N/mm
)
I
d 4
I = 64
1
  applied  b
N

a


0.81 2ultimate
a
 endurance
b
 0.9 ultimate 
1
log 
X  endurance 
X
1
log[ N reference ]
2
8.40
Log (Applied stress)
8.35
8.30
8.25
y = -0.0977x + 8.8369
R2 = 1
8.20
8.15
8.10
8.05
8.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
Log (Cycles to Failure)
7.00
8.00
9.00
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