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R•I•T Mechanical Engineering
Laboratory and Classroom Study of Low Cycle Fatigue
Rochester Institute of TechnologyMechanical Engineering Department
Rochester, NY 14623-5605
M. KasemerE.A. DeBartolo
S. Boedo
American Society of Engineering EducationAnnual Conference and Exposition
Atlanta, GAJune 24, 2013
R•I•T Mechanical Engineering
Outline
• Motivation• Course background• Activity design• Project results• Class implementation• Summary and future work
R•I•T Mechanical Engineering
Motivation
• Low Cycle Fatigue (LCF) theory and Fracture Mechanics: important, but often not covered in traditional Mechanical Engineering curriculum– High Cycle Fatigue (HCF) often taught as part of machine
element design courses– Static failure theories often taught in strength of materials
courses– Flawed assumptions about failure model can have serious
consequences.
• Some documented efforts to include LCF theory in undergraduate curriculum (Sepahpour and Chang, Hagigat)
R•I•T Mechanical Engineering
Why is LCF Important?
Δϵe/2
Δϵp/2
Δϵ/2
Transition Life
Reversals to Failure, 2Nf (reversals
Stra
in A
mpl
itud
e (i
n/in
)
R•I•T Mechanical Engineering
Why is LCF Important at RIT?
• Our students do…– Work for aircraft industry– Work for automotive industry– Work in manufacturing– Work in biomedical engineering
• Our students do not (only)…– Find the factor of safety on infinite life for a rotating
shaft with a circular cross-section.
R•I•T Mechanical Engineering
RIT Course Background
• Design of Machine Elements– 10 week (quarter system) course – Load and stress analysis (2 weeks)– Deflection and stiffness (2 weeks)– Static (stress-based) failure theories (1 week)– Fatigue (stress-life) (3 weeks)– 4 Case studies (throughout quarter, 2 weeks)
Note: phasing out machine elements in preparation for conversion to semesters!
R•I•T Mechanical Engineering
Case Studies
• Added to the course in Fall 2011. • Each involves the design & analysis of a mechanical
system. – Examples include the design of cable bar bracket, a bearing test
rig, and a microphone stand.
• Socratic method: the instructor posed a question, the student provided an answer, followed by another question from the instructor.
• Intended to simulate design practice in the workplace or natural cross-disciplinary design team interactions.
R•I•T Mechanical Engineering
Course Needs/Constraints
• Needs:– Illustrate the limitations of the HCF prediction
methods covered in class– Illustrate the importance of understanding the problem
at hand before applying a model
• Constraints:– Does not add material to an already-full course– Does not require significant additional resources
R•I•T Mechanical Engineering
Solution Approach
• Student Project: Fatigue and Fracture Mechanics Experiment Design– 5th year undergraduate student– Create a set of experiments that can be used to show
the importance of LCF– Create a set of experiments that can be used to show
the importance of fracture mechanics
(Bonus student learning through Independent Study!)
R•I•T Mechanical Engineering
Specimen Design
• Constraints:– Test stand grip capacity
(f0.39 – f0.63 in)– Load capacity (±22kip)– Sample length (2-6 in)– Test duration (< 2 hr)– Distinct LCF and HCF
behavior
• Sample:– 1018 Steel– ASTM standard design– f0.5 in (grip)– f0.25 in (gage)
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Mechanical Characterization
• Tensile tests done to determine as-received properties:• E = 28,700 ksi
• Su = 92 ksi
• Sy = 7 ksi
• %RA = 40%
• Calculated values (Banantine)• Se’ = 46 ksi and Se = 24.8 ksi
• ef’ = 0.51
• sf’ = 142 ksi
• c = -0.5• b = -0.1074
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Test Conditions
• Instron 8801 servo-hydraulic fatigue test system • Load control, fully reversed, 10Hz
– Relatively short LCF tests– Manageable HCF tests
• 3 HCF tests– Failure expected in > 50 min
• 6 LCF tests– Failure expected in < 50 min
• Independent Study Results…
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Fatigue test stage Gripped sample
Failure is always an option!
Fatigue Testing
R•I•T Mechanical Engineering
100 1000 10000 100000 10000001E+04
1E+05
Experimental
Theoretical (stress-life)
Transition Life
Life to Failure, N (cycles)
Alt
erna
ting
Str
ess,
S (p
si)
Independent Study Results: HCF
Assume HCF applies:Experimental data compared with stress-life model.
Un-conservative, exactly where we expected!
R•I•T Mechanical Engineering
Independent Study Results: LCF
Assume LCF applies:Experimental data compared with strain-life model.
Much better!
100 1000 10000 100000 1000000
0.00033
0.00333
Experimental
Δϵe/2 (in/in)
Δϵp/2 (in/in)
Δϵ/2 (in/in)
Reversals to Failure, 2Nf (reversals)
Stra
in A
mpl
itud
e
R•I•T Mechanical Engineering
Classroom Implementation
• Fall 2012 Quarter• LCF problem introduced as a case study• Fatigue tests conducted within 50 minute class
period.• Half the class in lecture discussing problem with
instructor• Half the class in lab running fatigue tests with TA
R•I•T Mechanical Engineering
Classroom Implementation
R•I•T Mechanical Engineering
Classroom Implementation Results
1000 10000 100000 100000010000
100000
Experimental
Theoretical
Transition Life
Life to Failure, N (cycles)
Alt
erna
ting
Str
ess,
S (p
si)
100 1000 10000 100000 10000000.00010
0.00100
0.01000
0.10000
Experimental
Δϵe/2 (in/in)
Δϵp/2 (in/in)
Δϵ/2 (in/in)
Reversals to Failure, 2Nf (reversals
Stra
in A
mpl
itud
e
HCF Model LCF Model
R•I•T Mechanical Engineering
“What?!?”
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“It’s not a mistake, it’s a learning experience”
• Reviewed all data, back to tensile test to characterize material• 1065 steel ordered, not 1018• E = 70,000 ksi
• Su = 129 ksi (published: 92.1 ksi)
• %RA = 40% (published: 45%)
R•I•T Mechanical Engineering
“It’s not a mistake, it’s a learning experience”
• Reviewed all data, back to tensile test to characterize material• 1065 steel ordered, not 1018• E = 70,000 ksi
• Su = 129 ksi (published: 92.1 ksi)
• %RA = 40% (published: 45%)
• Likely problems with data collection• Test frame down during following quarter for
repairs, so no opportunity to investigate
R•I•T Mechanical Engineering
Infinite Life problem (p=0.348) Fall 2011 Fall 2012 Number of students 42 34 Mean score (max 25) 21.79 21.21 Standard deviation 2.97 2.23Finite Life problem (p=0.000002) Fall 2011 Fall 2012 Number of students 42 34 Mean score (max 25) 22.24 18.00 Standard deviation 3.47 3.73
Results from Fall 2012 Class
“Was the addition of the fatigue test as a course topic a positive change?”Yes: 21No: 6
Did not answer: 7
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Summary and Future Work
• Suspect data: interesting discussion, but may have led to confusion about the activity goal
• Select different materials in future:• 1018 steel (as called for)• Al alloy (to illustrate endurance limit issues)
• Exam questions to measure lab outcome will be more carefully chosen – focus on LCF/HCF distinction
R•I•T Mechanical Engineering
References1. Sepahpour, B., and Chang, S.-R., 2005, “Low Cycle and Finite Life Fatigue
Experiment,” Proceedings of the 2005 ASEE Annual Conference and Exposition, ASEE.
2. Hagigat, C.K., 2005, “Using Commercially Available Finite Element Software for Fatigue Analysis,” Proceedings of the 2005 ASEE Annual Conference and Exposition, ASEE.
3. Bannantine, J. A., Comer, J. J., and Handrock, J. L., 1990, Fundamentals of Metal Fatigue Analysis, Prentice Hall, Englewood Cliffs, NY.
Acknowledgements
Steel stock was purchased and samples were machined by the RIT Machine shop staff. Their help is much appreciated!
R•I•T Mechanical Engineering
R•I•T Mechanical Engineering
Test Data
Force Stress (ksi) Total Strain Cycles3000 61115 0.012171715 3683000 61115 0.012171715 5502800 57041 0.009271549 32902700 55000 0.008064174 29212600 52967 0.007004755 144352500 50930 0.00607303 249432400 48892 0.005258352 653832200 44818 0.003932862 2432572000 40744 0.002941619 410000
IndependentStudyData
ClassroomImplementation
Data
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Fracture Mechanics Experiment
Three “crack” (sharp notch) configurations on one sample:Center crack, 2a
Single edge crack, 2aDouble edge crack, a & a
Which will fail first…?
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