1. ME 418 Lab D1 Fatigue testing
Unyime (Uy) Bassey Umoh
address –
Office # - 1B70
September 2013

2. Objectives To understand the fatigue behaviour of 1018 steel
To understand treatments to improve the fatigue strength of 1018 steel
To understand the operation of 4 fatigue machines
Krouse
RR. Moore
Budd
Warner-Swasey
To complete 2 fatigue tests on a standard specimen using a Krouse rotating beam machine
Without treatment
With treatment
ME 418 Lab D

3. Laboratory Timeline Go through concepts of fatigue and equations
Carry out calculations relevant towards the Krouse test
Observe operation of Krouse machine and carry out test
Observe operation of 4 fatigue testing machines
Carry out research challenge
Record information from Krouse test and sketch sample identifying crack initiation
ME 418 Lab D

4. Logbook checklist Objectives Background information Lab calculations
Information for 4 machines
Sketch of Diagrams
Method of loading
Types of stresses produced
Brief description of operation
Experiment
Number of cycles at failure
Sketch of failed specimen
Treatment
Procedure
Justification, and
References
Future discussion points
ME 418 Lab D

5. Theory – concepts
Fatigue Failure: Progressive Failure process of a material due to repeated cyclic loading. Loading induces cyclic stresses, which initiates cracks and cause them to be propagated until failure occurs (Collins 43)
Stress-time patterns
Completely reversed (Zero-mean) cyclic stresses*
Nonzero-mean cyclic stresses
Random stresses
ME 418 Lab D
*Stresses are induced by the loads applied

6. Theory – concepts
Fatigue (endurance) Limit: The stress level under which an infinite number of cycles can be sustained without failure of the material.
Response of Ferrous materials and Titanium
Denoted as Sf for actual machine part and Sf’ for small polished specimen.
ME 418 Lab D

7. Theory – concepts Sf
ME 418 Lab D
Figure 1: S-N curves showing response to cyclic loading

8. Theory – concepts
Fatigue strength: The stress level at which a material will fail when loaded by N number of cycles.
Response of Non-ferrous materials
Denoted as SN for actual machine part and SN’ for small polished specimen
ME 418 Lab D

9. Theory – concepts SN
ME 418 Lab D N
Figure 1: S-N curves showing response to cyclic loading

10. Theory – concepts
ME 418 Lab D
Figure 1: S-N curves for Ferrous and non-ferrous materials (Taken without permission from Collins, J. A. Mechanical Design of Machine Elements and Machines: a Failure Prevention Perspective. New York, NY: Wiley, Print)

11. SN = 1/6 (S106 - S1).log(N/106) + S106 Theory – equations
*Logarithmic relationship - A straight line between S1 and S106 on a semi-log plot (i.e. SN vs. log[N]):
SN = 1/6 (S106 - S1).log(N/106) + S106
ME 418 Lab D
* - This relationship can be obtained for the estimation of an S-N Curve for materials, if the curve is assumed to be a straight line in a semi-log plot (i.e. SN vs. log[N]). The resulting equation can be derived on your own based on this estimation. Note that the equation itself is not established in literature, but the approach of estimating the curve as a straight line is established in literature and can be seen in texts. Refer to Chapter 2 of “Mechanical design of machine elements and machines” by Jack Collins to see more about this estimation.

12. Theory – equations
*Power relationship – A straight line between S103 and S106 on a log-log plot (i.e. log[SN] vs Log[N]):
SN = S106 . (N/106)(-1/3log[S103/S106])
ME 418 Lab D
* Log-log plots for ferrous S-N curves is convenient as it provides a straight line relationship. Again, the equations can be derived based on these straight line relationships. More about this can be seen in Chapter 8 of Juvinall and Marshek 2000.

13. Theory – equations Where: S1 = SUT SN’ = ZSUT *S103 = 0.9SUT
S106 = SN’CLCSCGCT CR
and,
CL = Load factor
CS = Surface factor
CG = Gradient factor
CT = Temperature factor
CR = Reliability factor
SN’ = Endurance limit for a mirror polished specimen
Z = Endurance ratio (Estimated as 0.5)*
ME 418 Lab D
*Equation obtained for S103 and Z seen for rotating bending scenario (See Juvinall and Masherk 2000 for more information). The equations above were obtained based on observation of large amount of empirical data.
Also, C factors can be seen in:
1. Juvinal and Marsherk 2000 – Fundamentals of Machine Component Design - Table 8.1, page 316; Figure 8.13
Referred to as Marin factors as seen in:
1. Shigley’s Mechanical Engineering Design 2008 – Pages
2. Jack A. Collins 2003 – Mechanical design of Machine Elements and Machines – Table 2.2 pages ; Figure 2.21 page 53

14. Logbook checklist Objectives Background information Lab calculations
Information for 4 machines
Sketch of Diagrams
Method of loading
Types of stresses produced
Brief description of operation
Experiment
Number of cycles at failure
Sketch of failed specimen
Treatment
Procedure
Justification, and
References
Future discussion points
ME 418 Lab D

15. calculations (Power relationship)
Determine: 1) SN NGroup = To be Provided, DGroup = To be Measured, and: SUT = 114 ksi CL = 1 CS = 0.74 CG = 1 CT = 1 CR = 1 Z = 0.5
ME 418 Lab D

16. calculations 2) Estimated time for test, assuming 5000rpm 3) Moment, M
ME 418 Lab D

17. Logbook checklist Objectives Background information Lab calculations
Information for 4 machines
Sketch of Diagrams
Method of loading
Types of stresses produced
Brief description of operation
Experiment
Number of cycles at failure
Sketch of failed specimen
Treatment
Procedure
Justification, and
References
Future discussion points
ME 418 Lab D

18. Research Challenge Literature search and Brain storming
Treatment of same specimen to Increase fatigue strength showing
Specimen treatment
Temperature
Time
Justification
Mechanical properties
Micro-structural considerations
Note references
ME 418 Lab D

19. After lab to do Treatment
Submit treatment procedure within 2 school days
Complete treatment within 1 week of acceptance
Submit specimen according to schedule posted in 2C26
Rob peace –
Include all treatment results that will be sent out in your logbook
Compare strength of heat treated sample with non-heat treated sample
Plot S-N relationship from all groups data
Assuming power relationship, determine experimental endurance ratio and ultimate strength of the steel
Compare S-N for theoretical and experimental
ME 418 Lab D

20. Logbook checklist Objectives Background information Lab calculations
Information for 4 machines
Sketch of Diagrams
Method of loading
Types of stresses produced
Brief description of operation
Experiment
Number of cycles at failure
Sketch of failed specimen
Treatment
Procedure
Justification, and
References
Future discussion points
ME 418 Lab D