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

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Outline

• Motivation• Course background• Activity design• Project results• Class implementation• Summary and future work

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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)

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Why is LCF Important?

Δϵe/2

Δϵp/2

Δϵ/2

Transition Life

Reversals to Failure, 2Nf (reversals

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)

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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.

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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!

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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.

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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

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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!)

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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

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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!

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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

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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

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Classroom Implementation

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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

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“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%)

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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%)

• Likely problems with data collection• Test frame down during following quarter for

repairs, so no opportunity to investigate

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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

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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!

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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…?