TESTING OF MATERIALS IN TENSION
Object: The object of this experiment is to measure the tensile properties of two polymeric materials, steel and aluminum at a constant strain rate on the Tension testing machine.
For structural applications of materials such as bridges, pressure vessels, ships, and automobiles, the tensile properties of the metal material set the criteria for a safe design.
Polymeric materials are being used more and more in structural applications, particularly in automobiles and pressure vessels. New applications emerge as designers become aware of the differences in the properties of metals and polymers and take full advantage of them. The analyses of structures using metals or plastics require that the data be available.
The tensile properties of a material are obtained by pulling a specimen of known geometry apart at a fixed rate of straining until it breaks or stretches to the machines limit. It is useful to define the load per unit area (stress) as a parameter rather than load to avoid the confusion that would arise from the fact that the load and the change in length are dependent on the cross-sectional area and original length of the specimen.
The stress, however, changes during the test for two reasons: the load increases and the cross-sectional area decreases as the specimen gets longer.
Therefore, the stress can be calculated by two formulae which are distinguished as engineering stress and true stress, respectively.
(1) s= P/Ao= Engineering Stress (lbs/in2 or psi)
P = load (lbs)
Ao= original cross-sectional area (in2)
(2) sT= P/Ai = True Stress
Ai = instantaneous cross-sectional area (in2)
Likewise, the elongation is normalized per unit length of specimen and is called strain. The strain may be based on the original length or the instantaneous length such that
(3) e=(lf - lo)/ lo = Dl / lo = Engineering Strain
lf= final gage length (in)
lo= original gage length (in)
(4) eT= ln ( li / lo ) = ln (1 +e) = True Strain
li = instantaneous gage length (in)
ln = natural logarithm
For a small elongation the engineering strain is very close to the true strain when l=1.2 lo, then e= 0.2 and eT= ln 1.2 = 0.182. The engineering stress is related to the true stress by
(5) sT= s(1 + e)
Hence the true stress would be 20% higher in the case above where the specimen is 20% longer than the original length.
As the relative elongation increases, the true strain will become significantly less than the engineering strain while the true stress becomes much greater than the engineering stress. When l= 4.0 lo then e = 3.0 but the true strain =ln 4.0 = 1.39. Thus, the true strain is less than 1/2 of the engineering strain. The true stress (sT) = s(1+ 3.0) = 4s, or the true stress is 4 times the engineering stress.
Tensile Test Nomenclature
The tensile test data are characterized by terminology shown in Figure 4-1.
Figure 4-1: Engineering Stress-Strain Curve
The material test curves have a region where the deformation caused by the stress is elastic, or not permanent. This means when the stress is removed the specimen returns to its original length. At stresses greater than a certain value a portion of the strain becomes permanent, or plastic. The stress required to cause a 0.2% plastic strain, or off-set, is called the yield stress.
Ductility is measured as % elongation, representing the ability to deform in the plastic range
Tensile Testing Machines: table-supported (1000 lb. capacity) and floor-mounted (10,000 lb. capacity)
Operating instructions for the various Tensile Test Machines are contained in Appendices G and H which should be read thoroughly before operating the machines.
The Instron machine (1,000lb) contains a chart which moves as a function of time The load is measured by a load cell and is read across the chart depending on the scale, 0-100 lb, 0-500 lb., etc. The speed of the chart can be set to run at a fixed speed (inches/minute) or proportional time (ratio to the extension rate 1:1, 2:1, etc.). The crosshead of the Instron is set to move at fixed rates either up or down between two limit switches. The cross-head rate of motion is controlled by the machines microprocessor (See Appendix G).
The United machine (20,000lb) is controlled and records the data by means of its computer controller.
You will be provided with one specimen each of high density and low density polyethylene, steel, and aluminum. The metal samples are tested on the 10,000 lb. machine, while the polyethylene samples are tested on the 1,000 lb. machine.
Use a strain rate of 2 inches per minute for the plastomers, and 0.1 inches per minute for the metals. The chart length represents the sum of the plastic and elastic deformations.
1. Measure the thickness of the specimens. Make identifying marks (1, 2, 3, etc.) on each specimen.
2. The original crosshead distance will be 2.00 to 3.00 inches (gage length). Check this with a metal ruler, measuring the distance from the top of the lower clamps to the bottom of the upper ones for the polymers and measuring the length of the reduced area for the metal samples.
3. Follow the General Operating Instructions in Appendix G or H for Zeroing, Balancing, and Calibration as needed. (Ask your instructor if the machined needs to be zeroed balanced or calibrated before making any changes, the machine may already be set up for your use).
4. Install the specimen as directed in Appendix G or H. For the manual lock clamps, tighten the clamps very securely.
5. The order and settings for this experiment are:
Sample Range Setting
A. Low Density PE (200 lbs. full scale )
B. High Density PE (500 lbs. full scale )
C. Steel (5,000 lbs. full scale)
D. Aluminum (5,000 lbs. full scale)
6. Be sure to mark gage length with an ink pen after installing the specimen in the clamps. Mark as close to the clamp edge as possible.
8. Tensile Test:
Original specimen: length, width, thickness, gage length
At fracture: length (from graph)
After fracture: gage length (from graph)
Head and Chart Speeds
Full Scale Load (from RANGE setting)
Measure the overall length and gage length to +.1 inch.
Measure the width to +.02 inch (Use a dial caliper).
Measure the thickness to +.002 inches (use a dial caliper).
Measure the original thickness and width in several places and average.
Measure the final width and thickness in several locations to find the minimum
Glossary of Terms
Understanding the following terms will aid in understanding this experiment:
Ductility. The ability of a material to be permanently deformed without breaking when a force is applied.
Elastic deformation. Deformation of the material that is recovered when the applied load is removed. This temporary deformation is associated with the stretching of atomic bonds.
% Elongation. The total percent increase in the length of a specimen during a tensile test.
Engineering strain. Increase in sample length at a given load divided by the original (stress-free) length.
Engineering stress. The applied load, or force, divided by the original cross-sectional area of the material.
Engineering stress-strain curve. A plot of the Engineering stress versus the Engineering strain.
Hooke's law. The linear relationship between stress and strain in the elastic portion of the stress-strain curve.
Modulus of elasticity. Young's modulus, or the slope of the stress-strain curve in the elastic region.
Necking. Local deformation of a tensile specimen. Necking begins at the tensile point.
Offset yield strength. A yield strength obtained graphically that describes the stress that gives no more than a specified amount of plastic deformation.
Plastic deformation. Permanent deformation of the material when a load is applied, then removed.
% Reduction in area. The total percent decrease in the cross-sectional area of a specimen during the tensile test.
Tensile strength. The maximum engineering stress experienced by a material during a tensile test (ultimate tensile strength).
Tensile test. Measures the response of a material to a slowly applied uniaxial force. The yield strength, tensile strength, modulus of elasticity, and ductility are obtained.
True strain. The actual strain produced when a load is applied to a material.
True stress. The load divided by the actual area at that load in a tensile test.
Yield strength. The stress applied to a material that just causes permanent plastic deformation.
Prepare a memo report on the results of the tests. The report should contain engineering stress-engineering strain diagrams, and true stress-true strain diagrams from tensile tests for each material. Three of these should be graphed using a computer. A spreadsheet program, such as Excel, should be used to manipulate the data in order to produce the stress-strain diagrams. Instructions for graphing with Excel can be found in the Appendices of this manual. PC clones are available in the materials lab as well as several work rooms around campus. Label (by hand) diagrams to show Young's Modulus, Yield Stress, Ultimate Tensile Strength, and Total Strain (Include values). Include these values in your report and discuss them with respect to published values. Plot the fourth diagram by hand on a high quality (like K & E) graph paper with about 20 divisions to the inch. Label this diagram in the same manner as instructed above. Discuss your four plots. Discuss errors in this experiment and their sources.
McClinock, Mechanical Behavior of Materials
Dieter, Mechanical Metallurgy
Nielsen, Mechanical Properties of Polymers
Schmitz, Testing of Polymers
Van Vlack, Elements of Materials Science and Engineering, Chapter 1 and 6