Posted: February 19th, 2015

List some of observed changes in the stress-strain curve of high density polyethylene (HDPE) at the onset of “necking” and after “necking”.

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MSE 250 LABORATORY EXPERIMENT #5 – TENSILE TESTING of POLYMERS
Introduction

The most common method of determining the mechanical properties of structural materials is with the tensile test.  In this test, a state of uniaxial tension is obtained so that the stress and strain on a sample can be easily calculated from the sample length and cross-sectional area.  The stress at the start of plastic deformation (yield strength-YS), the stress at the onset of necking (ultimate tensile strength-UTS), the uniform elongation (e_u), and the total elongation at failure or strain to fracture (e_f or e_t), can all be quantitatively determined in a tensile test. Tensile testing is performed on a sample of a standard “dog-bone” shape. By pulling on the sample until fracture, this test provides load vs. elongation data, which are converted into stress-strain data using specimen geometry. These data are very useful, as they give us a source for values including ultimate tensile stress (UTS), Young’s modulus (E), yield stress (YS), % elongation, etc.

Engineering stress-engineering strain (s₀-e₀) curves provide useful information for design purposes and are relatively easy to obtain.  However, the s₀-e₀ curves do not account for changes in the geometry of the sample with deformation.  True stress-true strain curves account for changes in cross-sectional area and gage length with deformation and thus the stress and strain are obtained for the instantaneous geometry. The data obtained provide a more fundamental insight into material deformation behavior.  These engineering curves are more difficult to obtain, but they can be obtained from engineering curves as long as the deformation is uniform (up to the onset of necking or the UTS).  True stress-true strain data provide more realistic information for predicting material behavior in forming processes.

The mechanical properties of polymer composites are dependent on the orientation of polymer chains and/or any reinforcements. In particular, for composites containing continuous fibers, the strongest orientation will be parallel to the fiber axis, while the weakest orientation will be perpendicular to this axis.  The elongation to failure and Young’s modulus will also be dependent on the orientation of the fiber axis with respect to the tensile axis.

Objectives

• To generate engineering stress-strain curves and true stress-strain curves for dog-bone samples tested in uniaxial tension using the Instron mechanical testing machine in the MSE 250 laboratory.
• To determine the tensile properties of a number of polymer-based materials and compare these values across samples.

Materials and Strain Rates

HDPE (Tg ~ – 90°C)                            (4 strain rates, 2, 5, 20 and 40 in/min)

PC (Tg ~ 147°C)                                  (2 strain rates, 2 and 40 in/min)

PMMA (Tg ~ 105°C)                         (2 strain rates, 0.2 and 2 in/min)

Procedure

• Each lab group will receive different tensile test specimens. For each sample, determine the cross-sectional area and gage length (for the reduced section).
• Obtain an engineering stress-engineering strain curve for each sample. The instructor will demonstrate the use of the test frame and will assist with the testing.
• Print out the engineering stress-strain (s₀ e₀) curves for each sample and save the raw data as ASCII file. You can open the data file in Excel.  Keep track of the data and the sample geometry in your notebook, as some of this information may not be saved on the spreadsheet.

Lab Analysis

• Generate tensile curves in terms of engineering and true stresses and strains (s₀ e₀ and s vs. e) for all materials. Plot all curves for a given material on a single plot.
• Using the tensile curves, find the yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (s_f and e_f) for each material. Compare these values with literature data (refer to Callister).
• What are possible sources of error in your measurements?
• Sketch and compare the fracture surface/fracture region for all of the different samples.
• Is there anything unusual about your stress-strain curves or do they behave as expected? Compare and contrast differences from sample to sample.  Compare your results against published data and/or data in your textbook.

Questions to consider in your analysis

Q1:     List the dimensions that are critical for tensile testing.

Q2:     Calculate the % elongation and % area reduction at fracture for PC and PMMA.

Q3:     What is the effect of strain rate on the tensile behavior of high density polyethylene (HDPE)?

Q4:     List some of observed changes in the stress-strain curve of high density polyethylene (HDPE) at the onset of “necking” and after “necking”.

Q5:     Describe the failure surface of any two samples from your tests.

Q6:     What is the strain rate effect on polymer stress-strain curves?