Laboratory and Classroom Study of Low Cycle Fatigue

1. Laboratory and Classroom Study of Low Cycle Fatigue Rochester Institute of Technology Mechanical Engineering Department Rochester, NY 14623-5605 M. Kasemer E.A. DeBartolo S. Boedo American Society of Engineering Education Annual Conference and Exposition Atlanta, GA June 24, 2013

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

3. 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)

4. Why is LCF Important?

5. 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.

6. 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!

7. 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.

8. 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

9. 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!)

10. 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)

11. 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

12. 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…

13. Fatigue Testing Fatigue test stage Gripped sample Failure is always an option!

14. Independent Study Results: HCF Assume HCF applies: Experimental data compared with stress-life model. Un-conservative, exactly where we expected!

15. Independent Study Results: LCF Assume LCF applies: Experimental data compared with strain-life model. Much better!

16. 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

17. Classroom Implementation

18. Classroom Implementation Results HCF Model LCF Model

19. “What?!?”

20. “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%)

21. “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

22. Results from Fall 2012 Class “Was the addition of the fatigue test as a course topic a positive change?” Yes: 21 No: 6 Did not answer: 7

23. 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

24. References Acknowledgements Steel stock was purchased and samples were machined by the RIT Machine shop staff. Their help is much appreciated! • Sepahpour, B., and Chang, S.-R., 2005, “Low Cycle and Finite Life Fatigue Experiment,” Proceedings of the 2005 ASEE Annual Conference and Exposition, ASEE. • Hagigat, C.K., 2005, “Using Commercially Available Finite Element Software for Fatigue Analysis,” Proceedings of the 2005 ASEE Annual Conference and Exposition, ASEE. • Bannantine, J. A., Comer, J. J., and Handrock, J. L., 1990, Fundamentals of Metal Fatigue Analysis, Prentice Hall, Englewood Cliffs, NY.

26. Test Data Independent Study Data Classroom Implementation Data

27. Fracture Mechanics Experiment Three “crack” (sharp notch) configurations on one sample: Center crack, 2a Single edge crack, 2a Double edge crack, a & a Which will fail first…?