MIME 3330 Mechanics Laboratory
LAB 5: ROTATING BENDING FATIGUE
Introduction
In this experiment, the finite life fatigue behavior of a smooth cylindrical specimen as
shown in Figure 1 will be studied in rotating bending. The cyclic loading is applied to
the test specimens with a Model RBF-300 Rotating Beam Fatigue Machine manufactured
by Fatigue Dynamics, Inc. of Walled Lake, Michigan. A schematic drawing of the
machine is shown in Figure 2. The Rotating Bending Fatigue Machine consists of a 1/2
HP motor that rotates at a variable speed between 500 rpm and 10,000 rpm. The
specimens are loaded in bending with a weight and mechanism that applies bending
moments from 0 to 300 in-lbf. The specimens are fixed in the machine with collets that
hold the specimen so the motor can rotate the specimen. The amount of moment applied
to the specimen is set by positioning the weight along a graduated beam as shown in
Figure 2.
Figure 1 Specimen Geometry
Objectives
At the end of the experiment, the student should know:
1. How a rotating bending fatigue machine is used to generate fatigue data.
2. How the data is used to generate an SN curve on a log-log plot.
3. How to develop an empirical fatigue equation from the SN plot.
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Figure 2 Schematic of Rotating Bending Fatigue Machine
Procedure
One set of fatigue data will be generated for this experiment for the entire class. Each
group assigned to a laboratory period will perform one fatigue test according to a
schedule that gives applied moment and motor speed for the test to be run by each
laboratory section. Results from all the groups will be summarized and tabulated by the
teaching assistants and a copy of this summary will be available the day after the
experiment was performed. A hardcopy of the data will be posted on the door of NI 4065
and an Excel spreadsheet will be posted in the course website.
Each group should:
1. Measure and record the minimum specimen diameter with a caliper, making sure
that the surface of the specimen is not scratched during the measurement
process.
2. Observe the specimen surface for smoothness/polish quality, scratches or other
imperfections.
3. Turn off the power at the main switch on the end of the machine where the
motor is located.
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(Steps 3 – 12) are taken directly from the RBF-300 Operating Instructions
4. Loosen the lock screw fixing the weight to the calibrated beam and move the weight
to the zero position at the extreme left end of the beam.
5. Loosen the nuts holding the safety bar at the end of the load arm and swing the bar
free of the load arm.
6. Pull the safety guard straight upward free from the phenolic block base. The guard is
retained only by a friction fit.
7. Swing the load arm up and to the right so that a specimen may be inserted into the
drive spindle collet. Position the load arm to prevent contact with the free end of the
specimen.
The specimen should be pushed into the collets until the specimen contacts the
bottom of the collet. The loading system has been designed to apply the correct
moment to the center of the reduced section based on the design shown in Figure 1.
8. Tighten the drive spindle collet onto the specimen. The collet must be tightened
sufficiently to prevent any relative movement between the collet and the specimen
which could cause fretting corrosion.
9. Insert the free end of the specimen into the load arm collet observing the same
procedures and precautions noted above for the drive spindle.
While tightening the load arm collet with the wrenches, particular care should
be taken to insure that pure torsional wrenching is used and that no bending
forces are imparted to the specimen. The aluminum specimen with a 0.24 inch
diameter is easily bent during the tightening process.
10. Set the counter to “zero”.
Turn on the main power switch that is located on the end of the machine below the
motor.
11. Turn the speed control knob counterclockwise to the zero position. Back off the
cutoff switch adjusting screw on the beam as required to prevent the switch from
tripping by the movement of the load arm as it comes up to speed.
With the fingers of the right hand, grasp the load arm bearing housing do damp out
any resonances and slowly rotate the speed control knob clockwise to bring the
machine up to the desired speed.
The speed may be readily determined from a counter/timer relationship. Two zeroes
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must be added to the indicated reading of the counter for the actual spindle count.
12. After the spindle speed has been adjusted to its desired rate, slowly move the weight
along the calibrated beam to the required bending moment setting.
While adjusting the position of the poise weight, watch for interference between the
cutoff switch adjusting screw and the switch guard.
Fix the weight to the beam by tightening the lock screw and quickly reset the counter
to zero without stopping the machine.
The intent in this procedure, as well as moving the weight to the desired moment
setting after the machine has been brought up to speed, is to minimize any overload
condition on the specimen if the machine passes through a critical (resonant) speed.
Also, it is important to select a non-resonant test speed and to hold the load bearing
housing with fingers during any speed changes to dampen vibration when passing
through critical speeds.
13. After the specimen breaks, the machine will automatically shut off.
14. Even though the limit switch has cut the power. Turn off the main power to the
machine.
15. Record the final number of cycles that the specimen endured. Note that the counter
increments by 1 for every 100 cycles, so the number of cycles read from the counter
should be multiplied by 100.
16. Before removing the specimen, move the load spindle so the crack can be clearly
seen.
17. Examine the fracture surface of specimens that have been complete broken to see
evidence of fatigue failure, failure location and plane orientation,
smoothness/roughness, defects, etc.
Data and Observation
Include the following items:
A table of moments, measured test specimen diameters, and measured values of cycles to
failure for the entire class
Observations made during the experiment especially the broken cross-section
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Analysis and Results
Include the following items in your report with equations and explanations given for all
quantities requested.
1. A table that lists the ultimate tensile stress, yield strength, and fatigue strength for
both 2024-T4 aluminum and 6061-T6 aluminum. (Be sure to include a reference
pointer here and a reference in the reference section.)
2. A table that lists the computed stress amplitude for each moment. The calculation will
utilize the measured test specimen diameter.
3. A semi-log plot with the linear y axis showing the computed stress amplitude and the
x axis showing the logarithm of Nf, the number of cycles to failure.
4. A log-log plot of stress amplitude versus cycles to failure. Draw a best-fit straight
line and label it on the graph.
5. Use the best fit line drawn in the previous step to calculate the a and b coefficients in
the SN equation given by S f = a N b .
6. Using the web or handbook find a published value of the fatigue strength for T2024T4 and 6061-T6 aluminum and note the number of cycles to failure at the endurance
limit. Use the S f = a N b equation determined in step 5 to calculate the acceptable
stress level for the same number of cycles used in the published value.
Discussion and Conclusions
Discuss the topics in the following list.
1. Explain the effect of semi log plots and log-log plots as used in items 3 and 4 of the
results and analysis section.
2. From the best fit line obtained in steps 4 and 5 of the analysis section discuss the
calculation of endurance limit for the test specimen.
3. Compare the published and experimentally determined values for the fatigue strength.
Based on this, state whether you think the specimens used in this experiment were
made from 2024-T4 aluminum and 6061-T6 aluminum.
Appendix
Include the following items in the appendix:
1. The original data sheet showing the diameter, cycles to failure and any
observations made during the experiment.
2. A copy of the spreadsheet with cycles to failure and corresponding moments
generated by the entire class.
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LAB REPORT GRADE SHEET: Rotating Bending Fatigue
STUDENT’S NAME ___________________________________
LAB SECTION:
GROUP:
DATE OF EXPERIMENT:
Title Page………………………………………………………………………
(3)
___________
Executive Summary……………………………………………………………
(5)
___________
Objective……………………………………………………………………….
(5)
___________
Procedure………………………………………………………………………
(7)
___________
Results and Analysis………………………………………………………
(40)
___________
Table of material properties for aluminums (T4, and T6)
Table of specimen diameters, stress amplitudes, and Nf
Semi-log plot of computed stress versus log of Nf
Log-log plot of computed stress versus Nf,
Coefficients a, and b in the SN equation
Endurance limit for the test specimen
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Discussion and Conclusions
(15)
Differences between semi log and
Log-log plots
Discussion related to items 2, and 3 in the discussion part
of the handout
___________
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Original Data Sheets (in appendix)…………………………………………..
(5)
___________
Neatness (entire report)………………………………………………………
(10)
___________
Spelling and Grammar (entire report)……………………………………….
(10)
___________
TOTAL………………………………………………….
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(100)
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