Tensile testing is one of the most common mechanical tests carried out to measure and verify the properties of plastic and rubber materials. Properties measured include the ultimate tensile strength, yield strength, elongation, percent elongation, Modulus of Elasticity, and more.
During a tensile test, the test specimen is pulled and stretched. Most tests require the specimen to be pulled apart completely to calculate the force capacity at failure. Units of force calculated is expressed in lbf (imperial) or kN (metric) and this property is noted as the tensile strength at break or the ultimate tensile strength. In addition to force calculations, the amount of stretch in the specimen is another property that provides information on the material characterization and on product verification. The length of the specimen will increase with the applied tensile force. How much the sample stretches before failure is expressed as the elongation of the material. Depending on the followed format, the unit of elongation is often in inches (imperial) or in millimeters (metric).
Noting the percent elongation may be a better expression of results for certain materials. Percent elongation takes into account the initial length of the specimen in addition to the final length after it is stretched. Several hundred percent elongation for rubber and elastomers are common as they are high-stretch materials. Percent elongation of rigid plastics, on the other hand, is often under five percent.
Modulus of Elasticity or Tensile Modulus (E) measures the stiffness, it is calculated based on the ratio of the normal stress component in phase with the normal strain, to the normal strain. The modulus of rigid plastics is often high as more stress is required to produce a given amount of strain.
Poisson’s Ratio measures the relationship between how far the specimen stretches and how its thickness changes during the process.
Note that some measurements such as the measurement of stress and strain at yield applies only to some thermoplastic rubbers and certain other compounds.
Tensile Test Setup
A typical tensile test setup includes a tensile testing machine and two grips with the appropriate jaw surfaces. This is shown on the image on the right, from Engineering Archives. To measure the elongation of the specimen, a displacement transducer or an extensometer is often added to the setup. The testing machine should be connected to an indicator or a controller to acquire data. Setups with software and closed-loop controls provide more repeatable and accurate testing, thus are often preferred by quality control departments and testing laboratories.
During a test, the specimen is mounted and secured in the tensile grips. Tensile grips may be manually tightened or may have pneumatics to open/close the grip jaws. For rubber specimens, eccentric roller grips are commonly used. Click here for more information.
Test specimens need to be aligned properly in between the grip jaws to ensure consistency for each test. Standard test methods may specify the required distance between grips when running the described test method. Some methods also specify the required gauge length. Note that distance between grips and the gauge length are not necessarily the same value.
Once the specimen is mounted in the grip jaws, an optional axial extensometer can be clipped on the specimen. The image to the right shows Epsilon Technology clip-on axial extensometer. Extensometers must be selected by the required gauge length and the expected travel range. The travel range is often expressed as a percentage and must be calculated based on the initial length and the expected final length of the specimen. Long-travel extensometers offer a travel range around 1,000 mm (40 in) to accommodate such samples and are preferred for rubber and elastomers that exhibit high elongation. For Poisson’s ratio measurements and the measurement of the change in the width of the specimen, a transverse extensometer is required in addition to the axial extensometer.
Full testing systems will come with a controller that ensures variables such as the testing speed is held constant throughout the test. Closed-loop controllers operate with a feedback loop that allows the controller to send and receive feedback to/from the machine, so that if there has been an adjustment in the speed, the controller will revert it back to the desired value. Without the closed-loop feedback system, the operator will assume that the machine is running at the set speed and the calculated test statistics will be based on an inaccurate speed value.
Testing equipment suppliers offer software to automate the setup of tests and the calculation of results. Some software have preset ASTM/ISO test methods that the operator can select from the menu. The screenshot to the right is from a Cal-Cert customer software menu. Often certain fields will be left blank for the operators to enter values such as company logo, specimen ID, specimen geometry and dimensions, load thresholds, etc. This allows for customization and ensures that the test reports can include all the information needed per each specific company’s quality requirements system. Automation in testing helps reduce human factors related errors while making it possible to continue to run tests and obtain results while the operators are attending to other tasks.
Tensile Test Standards
Common tensile test standard methods for plastics include ASTM D638 Standard Test Method for Tensile Properties of Plastics and ISO 527 Plastics – Determination of tensile properties.
Common tensile test standards for rubber materials include ASTM D412 Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension and ISO 37 Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties.