Tensile, Tear, and Elongation Testing
As indicated earlier, many application tests are specific to the application and even the customer. Many of these unique standard tests are modifications of ASTM tests. Some relevant ASTM tests are listed in Table 5.7.
For tensile, tear, and elongation information , the workhorse piece of equipment is an instrumented tensile testing machine. Such machines are made by several purveyors including Instron Corporation and MTS Systems Corporation. These machines are essentially rectangular frames. Samples are positioned within the frame using specially designed fixtures. The fixtures are made to insure that meaningful and reproducible results can be obtained according to precepts of good mechanical engineering and physics. Attached to one end of the fixture is a device called a load cell, which is essentially a device that measures force. Stress is obtained from the force measurement by normalizing to a geometrical factor such as sample cross-sectional area. At the beginning of a test, the top or bottom end of the frame will move relative to the other three sides of the rectangle applying stress to the sample. The stress is recorded by the load cell much as a scale will measure weight upon standing
TABLE 5.7 Conventional polyurethane mechanical tests and AsTM procedures for measurement
Figure 5.20 Illustration of conventional tensile measurement configuration. Stress on the sample can be applied from either top or bottom, and the load cell measuring force can also be above or below the sample depending on the instrument manufacturer.
upon it or hanging a weight on the end of a spring. Figure 5.20 shows a diagram for a standard fixture for tensile measurements.
Representative stress-strain tensile data for a polyurethane elastomer and calculated output is provided in Figure 5.21. The instrument is capable of providing frequent sampling of the load cell reading with time such that the data presentation is smooth and does not show significant loss of resolution due to time averaging. The tangent straight line in the image is a computer-drawn measure of the tangent to the initial part of the stress-strain curve termed the modulus. The slope at different strain intervals can also be captured and compared to the initial readings. In addition to the slope of the curve at any given interval, the data analysis can determine the ultimate elongation, the tensile strength (the maximum stress achieved by the sample), the ultimate strength (the stress achieved at the sample break), the toughness of the sample (the integral under the stress-strain curve), and numerous other tensile parameters.
Polyurethanes are unique in their versatility. They can be used effectively in almost any potential application and that application will usually have a tensile fixture with explicit mechanical engineering so as to provide the most useful and accurate information. Figure 5.22 shows example fixtures designed specifically for application testing. Each fixture takes into account maintenance of correct angles, mechanical compliance, and mass so that the loads to be measured are correctly matched to the load cell capability . In this regard, one would not want to test the compressive properties of a low-density foam specimen with a load cell used for compression of dense elastomers where the modulus can differ by orders of magnitude.
Figure 5.21 Representative stress-strain data for polyurethane elastomer sample and example of data output.
Figure 5.22 Useful fixtures for mechanical testing of polyurethanes. Clockwise from upper left: standard tensile fixture for testing plastics, 90° peel test fixture, cord and yarn fixture, and compression fixture. Images courtesy of InstronR. (See insert for color representation of the Figure .)
Software for obtaining information from stress-strain experiments is generally provided by the instrument vendors. The software available is highly advanced and becoming ever more intuitive in its user interface. It is in the correct use and setup of the individual test from which information is obtained that differentiates the quality of results. To optimize results, one must use the equipment such that the load cell measuring force is in the correct range of the measurement (for instance—not measuring 50 g of force with a 5000 kg load cell), using fixtures that are correctly sized, aligned to the sample and do not damage the sample such that the grips predispose the sample to failure, correct measurement of extension, and performing the experiment at an appropriate extension rate. There are clearly a large number of things to keep in mind, and there is no substitute for training and experience. Figure 5.23 shows a typical tensile testing device in use. The fixtures in this case are those used for measuring the tensile properties of a film. The fixture is optimized to minimize damage to the film while at the same time providing a firm grip on the sample ends.
A common fixture for measurement of elastomer properties is that shown previously in Figures 5.20 and 5.22. In this case, the sample is formed such that the ends are wider, providing additional gripping surface by the hydraulically actuated flat or serrated grips. While the sample geometry is well known and widely used, a common artifact with this geometry occurs when the sample yields and draws material for extension from the wide sample ends. A device called an extensometer (depicted in the diagram) is sometimes employed to provide a true measure of sample extension within the narrow sample zone meant to be studied.
An example of the design concepts in the fixtures is shown in Figure 5.22 showing a fixture designed for testing a pressure-sensitive adhesive-type material. One can see that the fixture is designed to maintain a draw angle of 90° with respect to the adhered surface, and this is maintained by a pulley system that draws the adhered surface an amount equal to the length of adhesive pulled. The adhered surface and the drawing grip have sufficient stiffness that they do not vibrate or distort the signal representing the force of pulling the adhered surfaces. At the same time, the fixture, particularly the grip, is low mass and does not require the use of a large load cell in order to allow a tare or zeroing of the measured load prior to beginning the draw experiment. Similarly, the foam compression fixture is a low mass and high stiffness design allowing for easy centering of the foam sample between the compressing platens.
Figure 5.23 A technician measuring a film sample using a tensile testing machine.
Figure 5.24 Hysteresis loop of a polyurethane elastomer with 45% hard segment.
An additional experiment often applied to polyurethane elastomers and flexible foams is the hysteresis measurement . This is a cyclic loading-unloading experiment in which the sample is extended to a set fraction of gauge length (usually 100 or 300%) and the crosshead lowered back to the original gauge length. It is clear from Figure 5.24 that the sample has reached zero stress prior to the crosshead reaching its original position. This is an indication that the sample does not relax as fast as the crosshead was lowered and may have been permanently distorted. The sample is subsequently extended again. From its distorted state, with a longer length and polymer changes having occurred on the molecular level, the material does not achieve the same stress state on subsequent pulls. In general, it is deemed desirable for the area encompassed within the loop be as small as possible (having low hysteresis), which would suggest a high degree of network connectivity and fast chain relaxation processes. A typical hysteresis experiment will often comprise three or five loops trying to achieve unchanging hysteresis during the stress-strain loop.