Experiment #1: Tensile Test

Objectives:

The objective of this experiment is to measure the mechanical properties of two cold-rolled steel (1018) by performing tensile tests under displacement control. The properties that will be measured are Young's modulus; yield strength, tensile strength, ductility, modulus of resilience and toughness. .

Pre-Lab Reading

Thoroughly study this handout, and review relevant material from your Strength of Materials text. A good source of material property data on the internet is:

Required Equipment

-Tensile test machine

-Extensometer

-Calipers

-Metal samples

Background

Tensile tests are done to determine the stress-strain characteristics of materials. Young's modulus, yield strength, tensile strength, ductility, modulus of resilience and toughness are some of the mechanical properties that can be readily determined from the stress-strain characteristics of ductile materials such as metals and plastics and brittle materials such as ceramics and composites. There are many factors that influence the values obtained for the mechanical properties; therefore, an adequate number of tensile tests should be performed in order to determine the degree of variability of these properties. Accuracy of the test is ensured by careful calibration of the load and displacement measuring devices

MECHANICAL PROPERTIES

Young's modulus, yield strength, tensile strength, ductility, modulus of resilience and toughness are the mechanical properties that can be measured from the stress-strain diagram of a tensile test experiment. A more detailed description of how to calculate these properties from the stress-strain diagram can be found in the data analysis section.

Young's Modulus

The following figure depicts a typical stress-strain diagram obtained from a tensile test under displacement control up to the point of failure. The stress-strain diagram is characterized by several distinct parts. The first part is called the elastic region and is characterized by a relatively straight line. In this region, stress ([sigma]) is proportional to strain ([epsilon]) through the relation [sigma]=E[epsilon]. The slope of this line is called the modulus of elasticity or Young's modulus, (E). This property represents the stiffness of the material and is important in determining the deflection of structures when loaded in the elastic region.

Stress-Strain Diagram.

Yield Strength

The yield strength of a material is the stress level at which yielding begins to occur and is a measure of the material's resistance to plastic deformation. For most structural applications, yielding is undesirable. For some metals and plastics, there is a smooth transition from the elastic region to the plastic region. The transition point is called the proportional limit. If the position of this point cannot be determined precisely from the stress-strain curve, then by convention, it is defined as the point of intersection of a line drawn parallel to the slope of the elastic region departing from some specified strain offset, usually 0.002.

Specimen Deformation along Stress-Strain Curve: A) elastic deformation, B) plastic yielding, C) localized necking, and D) failure.

Tensile Strength

The tensile strength of a material represents the maximum stress that a material can withstand including the effects of plastic deformation. Deformation up to this point is uniformly distributed along the reduced section of the specimen. If further deformation is applied, necking occurs. A neck forms in a region with extensive plastic deformation with a localized reduction in the cross sectional area. Further deformation is confined to the necked region until failure occurs.

Ductility

Ductility is a measure of the extent of plastic deformation that a material can withstand. Materials that can withstand large amount of deformation are called ductile, whereas materials that can withstand little or no plastic deformation are called brittle. Ductility may be defined as either the percent elongation (%EL), or percent area reduction (%AR). In terms of the percent elongation, the ductility is the change in gauge length from unloaded to failure divided by the unloaded gauge length times 100 percent. The ductility of a material plays an important role in manufacturing utilizing forging or extruding processes.

Resilience

The modulus of resilience is defined as the amount of strain energy per unit volume needed to stress a material from its unloaded state to the onset of yielding. It is the area under the stress-strain curve up to the yield stress in units of energy per unit volume. It represents the ability of a material to absorb energy when it is loaded within the elastic region without plastic deformation.

Toughness

Toughness on the other hand is defined as the amount of strain energy per unit volume needed to stress a material from it's unloaded state to failure. It is expressed in the same units as resilience. For a material to have a high toughness, it must have both high strength and high ductility. It is for this reason that ductile materials are often tougher the brittle materials.

Data Analysis

Once the stress-strain diagram has been plotted we can determine the following mechanical properties.
YOUNG'S MODULUS
Young's Modulus represents the slope of the stress-strain curve in the elastic region prior to plastic deformation. Typically, the elastic loading response does not follow a straight line; therefore, a least squares fit approach may be used to determine the approximate slope of this curve. An alternative technique for determining Young's Modulus is to perform an unloading cycle. This approach yields a more linear relationship between stress and strain in the elastic region.

YIELD STRENGTH
The yield strength of aluminum cannot be easily discerned from the stress-strain diagram which has a smooth transition from the elastic to the plastic region. In this case the yield strength is obtained by constructing a line with slope E extending from a strain offset of 0.2 percent of the gauge length up to the stress-strain curve. The point of intersection locates the yield stress and yield strain.

TENSILE STRENGTH and DUCTILITY
The Tensile Strength of aluminum can be readily determined from the stress-strain diagram. It is the maximum tensile stress that the specimen achieved throughout the tensile test and is located at the maximum stress point of the entire stress-strain curve

The ductility of aluminum is another mechanical property that can be easily obtained from the stress-strain diagram. It is the maximum strain in percentage imparted to the specimen prior to failure.

RESILIENCE
The Modulus of Resilience is the area under the stress-strain curve up to the yield stress.

TOUGHNESS
The Toughness is the area under the stress-strain curve up to the point of failure.

Experimental Procedure

Specimen Preparations:

The diameter of each specimen must be measured and recorded. Punch marks must be made at an inch intervals along each sample. These should be measured and recorded after making the punch marks.

Tensile tests under displacement control will be performed on specimens made out of Steel ( or 6061-T6 Aluminum) as shown in the following figure. Deformation is restricted to the middle by a reduced cross section of length L. The cross section of the specimens are circular with major diameter D and a reduced section diameter d.

Figure 2. Specimen Geometry

MTS Set Up Procedure:

  1. Follow start-up procedure
  2. Make sure that the power & water are turned on to the hydraulic pump. (Main power switch is located in the closet)
  3. Start the Fast Track Console, start  programs instronFast Track Console. you may need to minimize the test log screen. Right click to set Specimen protect off.

Don’t calibrate the displacement.

  1. Start the Wave Maker Runtime program.

(StartprogramsInstron WaveMaker Runtime

  1. Run the Wave Maker Editor to setup the test sequence. (Start ProgramsInstronwaveMaker Runtime)This is the same program you can use to program the MTS for almost any testing cycle.

  1. Click on the data storage tab and change the filename to a unique name for your group that should include the date (ME315groupa-test1 feb-24-05). Press the save button and exit the editor. Use the CE_tensiletest_1.blk as a template for the tension test. Click on wave form make sure that Mode: position, shape: relative ramp, Endpoint :0.5229, Rate:0.002 . Filesave- Exit
  1. Turn on the MTS hydraulics by pressing the | then the || buttons mounted on the main rail leaving a full minute between pressing each button. If any odd noises occur, immediately press the 0 to shut down.
  1. Using the up & down arrows on the black hand control of the MTS, adjust the lower grip so that the displacement track reads close to (0.000). Use the thumbwheel on the black handle to fine-tune the position. This setting is not really necessary since the initial offset can be subtracted out later.
  2. Insert the specimen in lower grip. Leave upeer grip open
  3. Install Extensometer onto specimen
  4. Pull pin out of the extensometer
  5. Closer upper grip
  6. Note: Tighten the grip by gently loosening (CCW) the knob then gently tightening to finger tight. (CAUTION: THESE ARE NEEDLE VALVES THAT CAN EASILY BE DAMAGED WITH TOO MUCH TORQUE!!! KEEP YOUR FINGERS CLEAR OF THE GRIPS, THE CLAMPING FORCE CAN EASILY BREAK BONES!!)
  7. Right click on load icon and select balance
  8. Right Click on the Extensometer Icon and select balance
  9. Make sure that your safety goggles are on then tighten the upper grip onto the specimen. A compressive load 1,000 lbf is not an unusual result of this step! Use caution.
  1. To eliminate the preload, use the set point icon ( the arrow shown above) from the Fast Track Console.
  2. Change the mode top Load and the target value to 0 (enter). Use a 5 sec ramp and then press the start button. The preload will be reduced to <10 lbf with some small oscillations. Close the set point window.
  3. Press the yellow play button and select the CE_tensiletest_1 file. Click OK and run the test.
  4. After the specimen breaks, press the stop button but do not turn off the hydraulics.
  5. Make sure the system is in displacement control and open the upper then lower grips to remove the test specimen.
  6. Measure distance between punch marks and final diameter at the location of failure
  7. Repeat procedure for additional specimen
  8. Turn off the hydraulics.
  9. copy the test file from the desktop/test data directory to a floppy.
  10. Note: After each test, note the nature of the plastic deformation in each material (i.e. necking, brittle fracture, extensive plastic flow). Estimate the estimate values for the final gage length by placing two halves of each broken sample together and measuring the distance between scribe lines.

Experiment Report and Analysis

Give careful thought to a list of questions that you feel are import to the results of this lab and that define a theme for your report. Annotate your report as directed. Be sure to include uncertainty analysis as appropriate. Include in the reduction of data the quantities described below. Be sure to include error bars on all experimental data points that appear in plots. Make sure to comment on the physical source of any particularly “bad” measurements. In your discussion please address differences that are observed in stress strain curves for the three metals.

REPORT:

The report outline found on the Blackboard should be used for all formal reports handed in for ME315.

REPORT REQUIREMENTS:

For each of the two materials tested;

1. Determine and tabulate the following properties:

a. Proportional Limit

b. Yield Strength

c. Ultimate Strength

d. Modulus of Elasticity

e. Percent elongation for each 1" gage length between punch marks (including the segment containing the failure)

f. Percent reduction in area

g. Modulus of Resilience

h. Modulus of Toughness

i. True Fracture Strength

2. Compare b,c, and d to published values by calculating the percentage error.

3. Discuss possible reasons for the discrepancies in (2).

4. Provide stress versus strain plot, appropriately labeled, for all three specimens

tested. (Refer to Appendix A for example).

5. Discuss the consistency of the percent elongation measurements using different gage

sections. Comment on the possible reasons for discrepancies obtained for a given

specimen.

6. Briefly summarize, in words, the similarities and differences in material properties for the

two materials tested. When observed, present relationships between various

material properties for the two materials tested (example: increasing Modulus of

Toughness for the three materials was accompanied by increasing percent reduction

in area and decreasing Modulus of Resilience).

QUESTIONS:

1. Chances are that the specimens failed somewhere other than directly in the middle. What determines where a specimen fails?

2. For the steel specimen compare the stress in the bar at rupture, as computed from the area

at the break, with the ultimate strength. Why isn't the actual area of the fractured crosssection

a suitable basis for defining strength?

3. Why is it often difficult to evaluate the elastic limit?

4. What is the effect of poor alignment of the specimen? Why does a specimen that is properly aligned provide a more accurate estimate of the tensile strength compared to the estimate provided from results from a test where the specimen was not accurately aligned?

5. Why would a stress-strain diagram be preferable to a load-elongation diagram for presenting the results of a tension test?

6. Why is it important to know the gage length when using percent elongation information?

7. Explain why the percent elongation in a one inch gage section may exceed that of an 8

inch gage section.

8. Can the elongation of a specimen be determined accurately by measuring the

movement of the test machine cross head? Why?