Testing Plastic Tensile Samples

Equipment

[Image: Setup for testing plastic tensile samples, including the materials testing apparatus, a plastic sample, and calipers.]

Introduction

The behavior of plastics when deformed in tension is generally like that of metals. We see elastic and plastic deformation, yielding, and strain hardening, but the differences are notable. Often, we might not see a linear-elastic behavior at the start of the test and so Young's Modulus would not represent the plastic's stiffness. Yielding is defined differently, and it may coincide with the tensile strength. Plastics also tend to be more sensitive to the strain rate than metals and this must be considered when conducting the test and when reporting the results. In this lab you will tensile test two types of plastic and measure the stiffness of the materials using several different methods.

Theory

When a plastic sample is loaded in tension it will elongate. If pulled far enough it will fail, breaking into two pieces. Between the point where it was initially loaded and it failed, it will generally exhibit the following types of behavior, as shown in Figure 2.

Elastic Deformation

This deformation is temporary and is recovered as soon as the load is removed. The sample returns to its original size. Depending on the type of plastic, some time-dependent elastic and plastic deformation (anelasticity and creep) may accompany the initial elastic deformation of the specimen.

Yielding

This marks the end of the initial elastic region and the start of plastic deformation and in some cases the onset of necking.

[Graph: Typical stress-strain curve for plastic. Stress (MPa) on y-axis, Strain on x-axis. The curve shows distinct regions: Elastic Region, Yielding, Softening, Cold Drawing, Hardening, and Fracture. The x-axis ranges from 0.0 to 1.0, and the y-axis ranges from 0 to 80 MPa.]

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Strain Softening

Following yielding some materials will appear to soften (load decreases) as a neck forms (see Fig. 3) and the structure begins the transformation from one of randomly oriented chains and crystallites into a more aligned structure.

Cold Drawing

The crystallites are rotating and being reoriented. Most of this is happening in the zone where the neck is forming.

Strain Hardening

Once the specimen's structure is fully drawn the stress increases again. This new structure is now resisting deformation.

Fracture

The specimen finally breaks.

Setup

1. Connect the Materials Testing Machine to a PASPORT interface or AirLink. In the PASCO Capstone calculator, create the following calculations, add your values for the constants as you determine their values:

   Stress=10-9 * [Force (N)] / (π * (Dia/2)2) with units of MPa

   Dia= with units of m

   Strain=[Position (m)]/L unitless

   L= with units of mm

   speed=60000*derivative(2,[Position (m)],[Time (s)]) mm/Min

   poly fit=A + B*[Strain]+C*[Strain]2 + D*[Strain]3

   A=0 MPa

   B=1 MPa

   C=1 MPa

   D=1 MPa

2. Use calipers (or a micrometer) to measure the diameter of the machined portion of the tensile sample. Edit the value for diameter in line #2 of the calculator.
3. Note that the calculator also has a value for the length of the sample. If you measure the complete machined portion, you should get about 38 mm. However, since there is a radius, the length of the thinner part that is actually stretching, is less. A good average value to use for the length is 35 ± 1 mm.
4. In PASCO Capstone, create a Meter display of Speed, a graph display of Force vs. Position, a Digits display of Force, and a graph display of Stress vs. Strain.
5. Set the sample rate to 20 Hz.

[Image: Diagram illustrating 'Necking' in a tensile sample, showing a localized reduction in cross-sectional area before fracture.]

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Procedure: "Seating" the Test Sample and Setting Pre-Load

1. Note: The following is written for using a test sample (see Figure 1), but you should also use this method when performing a compliance calibration of the Materials Tester using the Calibration Rod. In the following procedure, you will load the system to introduce a small pre-load.
2. Install the acrylic test sample as shown in Figure 1. Make sure that the knurled cap is loose, not creating a force on the load cell.
3. Click on Record. Tighten the knurled cap. Note that the digits display shows the force on the load cell.
4. Slowly turn the crank clockwise, increasing the force to about 10 N. Note that the position and force data are being plotted on the graph.
5. Click on Stop, and do NOT change the crank position. Since the sensor will auto-zero the next time you start recording data, this puts a 10 N pre-load on the sample which results in better data. You should use this same method when performing any compliance calibration of the Material Tester.
6. You can use the Delete Run menu (Controls tool bar) to delete your practice runs. Proceed to the next page to collect your actual data runs.
7. You can use the Calibration routine (in the Tools Palette) to perform a compliance calibration at any time. You should calibrate the Materials Tester over the same range you expect to use in testing samples. In this lab, for example, a max range of 1000 N is sufficient. When you store your calibration, create a name that includes the max force used, and record that value in the lab as well. Compliance calibration called "Calibration 1000" was over a range of 1000 N and used a pre-load of 10 N.

Procedure: Breaking the Sample

1. You will deform the acrylic tensile sample, pulling it apart until it breaks. Try to turn the crank at a steady rate of about 20 to 30 mm/Min. Make sure the safety shield is in place.
2. Click on Record. Turn the crank clockwise, stretching the sample. Continue cranking until the sample breaks. Click on Stop.
3. Replace the broken sample with your second acrylic sample. Repeat the steps from the previous page to properly seat the sample.
4. For this run, turn the crank very slowly, less than 5 mm/Min. Click on Record. Turn the crank until the sample breaks. Click on Stop.

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5. Repeat using the polyethylene sample. This material will stretch a long way, so it is best to use a fast rate. Continue cranking at least until the necked region that forms is 2 cm long. You probably will not be able to break the sample.
6. Rename your runs Acrylic fast, Acrylic slow, and Polyethylene.

Analysis: Linear Elastic Behavior

1. For plastics that exhibit linear-elastic behavior, Young's Modulus (E) is the property that describes the stiffness of the material. It is measured as the slope of the linear portion of the stress-strain curve.
2. Confirm that the equations for Stress and Strain in the calculator are being done correctly.
3. For your two acrylic samples, Measure the slope in the linear portion of the Stress vs. Strain graph to find Young's Modulus for the material. What is the uncertainty in your measurements?
4. How do the two values compare? What effect did the strain rate have on the modulus and on the tensile strength?
5. How do your values compare to those listed in reference data tables for the material?

Analysis: Nonlinear Elastic Behavior

1. For plastics that do not exhibit linear-elastic behavior, the following moduli can be used to describe the stiffness of the material: Initial Modulus is the slope of the stress-strain curve at the very start of the test. Secant Modulus is the slope in the stress-strain curve from the origin to a specified strain. Tangent Modulus is slope in the stress-strain curve at a specified strain: For example, 2% of strain can be used. Chord Modulus is the slope in the stress-strain curve measured between two specified strains. Since the actual data tends to be fairly noisy, the general approach is to fit a polynomial curve to the data, and then make all measurements from that fit.
2. Use the highlighter to select only the data up to about 5% strain.
3. Fit a 4-term polynomial to your polyethylene test sample data. Use the Curve Fit Editor in the Tools Palette. You can change the number of terms, and lock A=0.
4. Open the Calculator window and edit the coefficients for the poly fit. You will make your remaining measurements on this curve fit model, not on your actual data.
5. In PASCO Capstone, create a graph display of Poly fit vs. Strain.
6. Lock the scale to only show the curve up to 5% strain.

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7. Use the Slope tool to measure Initial Modulus.
8. Use the Slope tool to measure Tangent Modulus at 2% strain.
9. Use rise over run to measure the Secant Modulus at 2% strain.
10. How do your values compare to those listed in reference data tables for the material?
11. Measure and record the Tensile Strength of the polyethylene material. Compare your value to those listed in reference data tables for the material. Compare the value found for this material to other materials tested.
12. Use Text annotations to mark the following regions of your polyethylene graph: Yielding, Softening, Cold Drawing, Necking.
13. What are the differences in the two plastics?