The tensile test on plastics according to ASTM D638 helps determine essential mechanical properties, including tensile stress, strain, tensile modulus, tensile strength, tensile strength at yield and tensile strength at break. The ASTM D638 standard is not identical to its counterparts ISO 527-1 and ISO 527-2. These standards differ in many aspects, including the shape and dimensions of the specimens, the definition of test results, and the test procedure itself. While ASTM D638 presents a pragmatic characterization of tensile properties, the guiding principle of the ISO 527 standard is the high level of reproducibility of test results across laboratories, companies and national borders.
In the ASTM D638 standard test method for tensile properties of plastics, essential mechanical properties of a molding material or a specimen machined from a defined area on a component are determined.
Typical characteristic values are:
- Tensile stress: force related to the initial cross section of the specimen
- Strain: change in gauge length in relation to the initial gauge-length
- Tensile modulus: gradient of the curve in the stress-strain diagram
- Yield point: stress and strain at the curve plot point at which the gradient is zero
- Point of break: stress and strain at the moment of specimen break
- Poisson's ratio: negative ratio of transverse strain to axial strain
The tensile test provides a good basis for demonstrating the change in the mechanical characteristic values of a polymer following aging, heat or medium aging, or weathering. For this, the characteristic values of the tensile test are determined in the newly molded state, as well as after defined aging or weathering periods.
- The preferred test specimen is the dumbbell Type I, which with a thickness of 3.2 mm (1/8 inch) and a gauge length of 50 mm (2 inches) is still relatively similar to common component material thicknesses and at the same time allows for a good level of strain measurement accuracy.
- Provided that by using the Type I test specimen a fracture cannot be produced in the narrow parallel part of the specimen, ASTM D638 recommends the use of a Type II test specimen, in which the width of the narrow parallel section is significantly reduced.
- If only a small amount of material is available, or if machining the specimen from a component does not allow for a larger specimen, then a Type V specimen is used, which is reduced in every dimension relative to Type I.
- If specimens with material thicknesses of more than 7 mm can be machined, then Type III specimens are used. Here, the width of the narrow parallel section, as well as the shoulder width and the overall length are increased so that the specimen thickness remains less than the width. For sheet thickness of more than 14 mm, the thickness is adjusted through mechanical processing.
- The Type IV test specimen is ideal for testing of very soft polymers (e.g. rubber) and is used when comparing soft and more stiff polymers.
- Observing defined conditioning and ambient conditions with regard to temperature and humidity is of great importance for the comparability of test results.
- Specifications for the conditioning duration can usually be found in the material standards for the plastic being tested. Furthermore, specimens used in tests on molding materials must be kept in a standard atmosphere (standardized temperature and humidity conditions) for at least 16 hours prior to the test.
- If tests are performed in a standard atmosphere, this refers to a defined normal climate that is specified in ASTM D618.
Temperate atmosphere: 23 ± 2 °C, 50 ± 10% r.F.
Sub-tropical atmosphere: 27 ± 2 °C, 65 ± 10% r.F.
- The tolerances correspond to class 2. The tolerances are halved for class 1.
- Room temperature usually refers to a somewhat wider temperature range, between 18 °C and 28 °C.
- Tests can also be performed at high or low temperatures, however, different requirements may be specified.
Force and extension are the two fundamental values measured by a testing machine. Within the framework of a periodic calibration when compared to a measuring instrument traceable to a national standard, evidence has shown that these measured values achieve a level of accuracy defined in the test standard across defined measuring ranges.
Force measurement (ASTM E4)
The ASTM D638 standard requires a measurement accuracy of ±1% for the measured value. This requirement is verified with a calibration to ASTM E4 for the entire measurement range of the load cell. The quality of a force measuring system is therefore particularly defined by the width of the measuring range over which the accuracy can be achieved. ZwickRoell testing machines can achieve the measurement accuracy required in ASTM E4 starting at 1/1000 of their measurement range. This means you can measure the modulus values and tensile stress of many materials with the same test arrangement and without having to reconfigure the arrangement.
Strain measurement (ASTM E83)
Values to be measured include the crosshead travel, or the change in clamping distance and the directly measured specimen elongation.
For measurement of the crosshead travel, ASTM D638 requires an accuracy of ± 10% relative to the respective measured value. The crosshead travel is especially used for determination of the nominal strain at break, so that the distances to be measured are relatively large.
The tensile modulus is determined with direct measuring extensometers. The permitted error in strain determination is specified at 0.0002 [mm/mm]. This corresponds with a strain error of 0.02% or—at a gauge length of 50 mm—a permitted displacement error of 0.01 mm.
For the measurement of yield points and small elongations up to approximately 20%, ASTM D638 requires compliance with class C according to the calibration standard ASTM E83. For elongations up to 10% this means a set maximum permitted error value in the strain determination of 0.001 [mm/mm], which corresponds to 0.1% elongation. For the measurement of higher elongations, the relative error of the respective measured value may not exceed ± 1%.
For the measurement of elongations higher than 20%, ASTM D638 permits a relative measurement error of ± 20%, which can be achieved even with relatively simple extensometers.
Our testXpert III testing softwareis the ideal solution for every test requirement. The software is the result of close cooperation with software users from the materials testing industry and includes numerous functions to support you in your day-to-day operations. Both ISO 527-1/ISO 527-2 and ASTM D638 define test methods for tensile tests. The two standards are technically equivalent but do not provide fully comparable results, because specimen shapes, test speeds and the method of result determination differ in some respects. With testXpert III, ZwickRoell offers a pre-configured standard test program that is guaranteed to fulfill the requirements of ISO 527-1 and ASTM D638. All settings for performing tests to ISO 527 have already been preset. Many other details are also available in testXpert.
Tensile test on plastics in the new ZwickRoell temperature chamber. The temperature range can be adjusted from -80 °C to +250 °C (-112 °F to 480 °F). The fully integrated system ensures efficient, reliable and easy operation during the performance of tensile tests. With the door-in-door solution, nitrogen consumption and ice formation are significantly reduced while testing at cold temperatures, so that the tests can be performed more cost effectively.
The ASTM D638 standard describes tensile testing on plastics. It is applied to measure tensile properties including the tensile modulus, yield stress, yield strain, tensile strength and strain at break. These properties are important for the determination and comparison of plastics and plastic compounds, as well as for quality assurance purposes.
For brittle plastics and plastics, which do not exhibit a yield point, strain at break is measured directly with a mechanical or non-contact measuring extensometer. In the case of thermoplastics that do present a yield point, the nominal strain at break is measured from the travel of the testing machine’s pulling grip.
Tensile strength is the highest tensile stress that a specimen can reach during a tensile test. This tensile strength can occur at a yield point, in which case it is referred to as tensile strength at the yield point. If the tensile strength occurs shortly before failure of the specimen, it is referred to as tensile strength at break.
The ASTM D882 standard is used for tensile tests on plastic films and sheet material with a thickness up to 1 mm. For this, there are special specimens and a test method that addresses the particular requirements for testing of film.
Long fiber reinforced plastics such as GRC or CRC are measured in the tensile test to ASTM D3039. These materials can have very different properties depending on the direction and structure of the laminate or fabric. They, therefore, often require special specimens and test methods.
The ASTM E132 standard specifies a suitable test method to measure Poisson’s ratio at ambient temperature. In the case of unreinforced plastics, however, a fixed value for Poisson’s ratio can usually be used for calculation purposes, since it is largely constant.
The standard is the measurement of the specimens in a conditioned state according to method A of ASTM D618. The ASTM D638 standard, however, allows for deviations in conditioning and measurement requirements so that tests can also be performed at low or elevated temperatures according to this standard.