engineering

MECH3315 Mechanics Laboratory

Mechanical Engineering

University of New Haven

Torsion Testing During this experiment a specimen is loaded into a torsion testing machine and tested to failure. A shear

stress-shear strain diagram is plotted for the specimen, which characterizes the torsional behavior of the sample

and material. Mechanical properties such as torsional yield strength, shear modulus and maximum shear strength

are determined using the shear stress-shear strain diagram.

Outcomes:

 Determination of the torsional properties of a ductile steel test sample using a standard torsion test.

 Observation the behavior of the material under torsion and report observations.

 Study the fracture surface characteristics and report observations.

 Gain familiarity with torsion testing procedures and testing equipment.

The specific items to be determined are:

1. Shearing Proportional Limit: the point where the shear stress and shear strain are no longer linear, note that some initial chuck tightening might cause a shift in the data while still in the linear range.)

2. Yield Shear Stress using 0.04 radians/meter of gage length. This is determined in the same manner as the tensile yield strength where the 0.2% offset is replaced by an offset based on the length of

the sample.

3. Probable Tensile Strength: Ultimate and Shear have a rough relation where Shear Strength at failure = 0.45 to 0.65 of the ultimate tensile strength.

4. Experimental Shear Modulus- G, determined with the slope of the shear stress vs shear strain in the linear region.

5. Maximum Elastic Shear Strain (useful for torsion spring design) 6. Maximum Shear Strain to failure 7. Type and character of fracture

Equipment:

1. Tinius Olsen Lo-Torq Bench Model Torsion Testing Machine with 10,000 NM torsion load cell and 200 deg resistive encoder. Model 290 display.

2. Steel Specimen 3. Micrometer Caliper & ruler 4. Output device for recording data (printer or computer)

Equipment (Description) The Tinius Olsen Lo-Torq Torsion Testing Machine is a bench model unit designed to perform torsion

tests on specimens held between a loading and weighing chuck. The standard machine is capable of applying

torsional loads up to 10,000 inch pounds, or 1000 Newton-meters on specimens ranging from approximately 5/16

to 1—1/2 inch in diameter. The distance between the chucks can be adjusted up to approximately 20 inches. The

torsional loads can be applied and indicated in both direction of twist.

The loading chuck is driven by a combination gear reduction unit and variable speed D.C. motor utilizing

solid state controlling circuitry. The standard speed is from 5 to 360 degrees per minute. The angle of twist can be

read from a scale attached to the loading chuck, or indicated on the Model 290 Display with an optional single

Strain Pickup System on the loading chuck or a two Strain Pickup Differential System on both the loading and

weighing chucks. For convenience, the system sends load and angle via a serial port to the attached computer. A

program is written to assist collection of data. Follow the instructions on the program to operate.

The amount of torque applied to the specimen is sensed by a Strain Gauge Torsional Load Cell mounted

directly behind the weighing chuck. The sensed torque, which is converted to an electrical signal by resistance

strain gages bonded within the load cell, is indicated on the Model 290 Display. The standard machine with the

Model 290 Display is provided with four load ranges of 500, 1000, 5000 & 10,000 inch pound-force in U.S.

Customary Units; 50, 100, 500 & 1000 Newton meters in SI Units.

Be careful to:

 Select at least two bar samples to test (two different of two the same).

 Measure the diameter of the bars multiple times for uncertainty.

 Determine the length desired between the lathe dogs and mark with a permanent marker.

 Mount the bar into the test rig using the lathe dogs. The three thumb screws should be loose to allow for independent rotation from the grips. Mount the bar to the sliding grip, making sure to grab at least 1.5-2.0

inches of the bar. Any less than an inch and the grips will start to slip. Gently slide the bar into the other

grip. This may take some initial muscle. Rotate the grip with angle tape to zero and adjust the gear rotation

to zero the angle readout.

 The encoder seems to be having issues when run in the direction of increasing angle so run backwards or counter clockwise.

 Follow the normal procedures but additionally, prior to actual testing, load the sample up to 10-15 degrees then unload until the torque reading goes back to zero. This should tighten up the chucks and give a more

consistent reading. This also changes the starting angle so you can ignore the embossed angles on the

inside grip. All the angle measurements will be from the encoder and will need to be corrected for the

offset.

 Don’t test to failure, test very slowly until the torque begins to level off and shows little increase. At this point the test is essentially over (perhaps 100 degrees). Keep running until the angle reading stops working

then save the data. If you wish to break the sample, simply increase the angle rate and let it run. You can

manually record the maximum torque at breaking.

Normal Testing Procedure:

1. Turn on the computer and start the torsion test LabVIEW program on the desktop. Read the instructions on

the VI before starting the test. The program only collects the data, it does not operate the motor on the

machine.

2. If degrees of twist are to be read from the Scale on the loading chuck, drive the chuck to 0° on the scale by

use of the SELECTOTWIST Controls. This can be done by pressing the red button to shut down the

motor, selecting the direction of rotation (note the scale on the chuck is taped on upside down). Then rotate

the rotation rate dial to a small number and press the green start button. YOU MUST PRESS THE STOP

BUTTON BEFORE CHANGING DIRECTION OR YOU MAY DAMAGE THE MOTOR!

3. If degrees of twist are to be read using Strain Pickup(s), drive the loading chuck to 0 on the scale by use of

the SELECTOTWIST Controls.

4. Mark the steel bar using a permanent marker one inch from each end and carefully measure the length of

the specimen. Repeat length measurement as many times as needed for statistical sampling purposes.

5. Set up the Model 290 Display to the proper configuration for the test to be conducted. Make sure the 200

degree rotation is selected and the proper units and maximum torque reading are also selected. If FIXED

Ranging is selected, the operator must also select which range to be used for each channel.

6. Mount the specimen in the self—tightening chucks of the machine:

a. Slide the weighing chuck along the bed of the machine and open the jaws of the chucks to permit

insertion of the specimen.

IMPORTANT: The bolt mechanism that clamps the weighing chuck assembly to the ways on the

bed of the machine should be sufficiently tightened to prevent the weighing chuck assembly from

lifting, yet loose enough to allow for horizontal movement that might be necessary as the specimen

changes length during testing.

b. Insert the specimen into the loading chuck with the lathe dogs and slide the chuck assembly to the

right until it is stopped by the specimen ends bearing against the inside of the chucks. Back off the

chuck about a 1/2 inch, and reposition the specimen midway between the chucks, and lightly

tighten the jaws alternately so that the reference lines on the pins of each grip are concentric with

the reference rings on the face plates of the chucks. NOTE: Do not over tighten the self-tightening

chuck jaws on the specimen, as this will prevent the self-tightening feature of the jaws from

operating properly. The gripping inserts in the jaws are designed to pivot in the direction of the

twist increasing the amount of clamp force to the amount of load being applied. It is also important

that the gripping inserts are well lubricated.

c. Tighten the lathe dogs so any slipping is not measured in by the angle indicators. The lathe dogs

only measure the angle change between.

7. Begin the test by starting the LABVIE code and waiting for “collecting data”. Next set the speed of

rotation and the direction of twist using the SELECTOTWIST Controls. The speed can be adjusted during

the test if required. If the direction is switched during a test, the SELECTOTWIST Controls will go to

STOP. When changing directions and going through zero load, the specimen will not begin loading in the

opposite direction until the gripping inserts have pivoted to the opposite side. NOTE: A good

rule-of-thumb is to select a speed that permits the applied torque to be read at any time during the test and

produces many data points in the 0-30 degree range.

8. Remember that the LabVIEW program is only collecting data at fixed intervals, you are operating the

motor.

9. As the test progresses you will quickly exceed the elastic limit of the bar and the torque values will level

off. At 200 degree or so the angle encoder will stop operating and you will need to manually keep track of

rotations using the scale on the chuck and note the torque measurements as they change slightly. Write

these values down by hand since the computer can no longer read angles at this point.

10. If the specimen should happen to fail, STOP the drive. Press the [READ] key on the Model 290 to display

the maximum value(s) reached during the test. Remove the specimen and return the loading chuck to the

00 starting position for the next test. If the capacity of the machine or the selected Stress Range when in

FIXED Ranging, the drive will automatically STOP. If only one of the lower Stress Ranges were

exceeded, simply select a higher range and continue the test. If the capacity of the machine was exceeded,

the operator must reverse the direction and run the loading chuck until zero torque is applied to remove the

specimen.

Some Simple Torsion Theory

Please refer to your engineering materials and mechanics of materials texts for additional references.

Shear stress for a rotating constant cross-section is equal to:

𝜏 = 𝑇𝑐

𝐽

Where T is the torque, c is the distance from the centroid (radius for the surface of a circular shaft) and J is the

polar moment of inertia of the cross-section. This relation is valid in both the linear and nonlinear region.

The shear strain is equal to:

𝛾 = 𝑐𝜃

𝐿

Where L is the length over which the rotations theta are measured. When referring to the linear region of the shear

stress vs. shear strain curve,

𝜏 = 𝐺𝛾

Analysis/Results (be sure to include)

 Plot a torque-angle & shear stress-shear strain diagram for the test and compute all properties.

 Compare the results with published data and Von Mises failure by shear (www.matweb.com & others).

 Discuss the results obtained.

 Include and assessment of the uncertainty

Discussion (address these questions within the discussion section as well as the test results)

 How does the experimental shear modulus compare with the published data for this material? This test is notorious for how difficult it is to reproduce the shear modulus.

 Why do you think that published values for shearing yield stress are so uncommon? Are there any factors that might have a significant effect on the results from a torsion test?