ASTM D638 Tensile Properties of Plastics
The ASTM D638 standard describes the test method for determination of the tensile properties of reinforced and unreinforced plastics. It helps determine essential mechanical properties, including tensile stress, strain, tensile modulus, tensile strength, tensile strength at yield and tensile strength at break.
ASTM D638 contains information about specimen shape and specimen sizes, test procedures including environmental conditions as well as accuracy requirements for the corresponding testing machines and extensometers. The following information provides a comprehensive overview. However, to ensure that the tensile test is carried out in compliance with the ASTM D638 standard, it is essential to reference the full standard.
While ASTM D638 and its ISO counterpart ISO 527-1 / ISO 527-2 are technically equivalent, they do not delivery fully comparable results, since the specimen shapes, the test procedure and determination of the results differ in a few aspects. The plastics tensile test on films and sheets with a thickness of less than 1 mm are described in the ASTM D882 or ISO 527-3 standards.
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Measuring the specimen before testing according to ASTM D638
In addition to the specimen shapes, ASTM D638 also describes the accuracy with which the specimen dimensions must be measured.
- The width of the plastic specimens can be determined using a caliper, a micrometer or a cross-section measuring device
- The thickness of plastic specimens is measure with a micrometer or a cross-section measuring device
- The measuring force is between 5 and 15 N, the measuring surfaces are circular and flat, usually with a diameter of 6.35 mm (6.5 mm). Other contact elements are possible
- The measurement is carried out in the center of the specimen and within the gauge length. Injection-molded specimens are measured within 5 mm of the center of the gauge length.
- A thickness measurement error of 0.1 mm produces and error of 2.5%!
Testing machines for ASTM D638
For the performance of tensile tests according to ASTM D638, our universal testing machine series, including the zwickiLine, ProLine and AllroundLine series can be used. The typical testing machine has a nominal load of 10 kN. This allows tensile strengths of just under 250 Mpa to be measured with the type I specimen, which is usually sufficient for unreinforced plastics. Lower forces are adapted by selecting the corresponding load cell. For larger forces, we offer load frames with high nominal loads.
The ASTM D638 standard requires a measurement accuracy of ±1% for the measured value. This requirement is verified with a calibration according 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.
Specimen grips for tests to ASTM D638
When testing according to ASTM D638, the specimens must be held securely in the tensile tester without slipping, while ensuring that they are not damaged by excessive clamping force.
Different operating principles apply to the specimen grips:
- Close at the touch of a button
- Allow a very direct and slip-free transmission of the crosshead speed into the specimen and thus ensure a very constant and reproducible strain rate during the test
- Precisely adjustable gripping force
- Wide range of applications for soft and hard polymers
- Recommended if high test result reproducibility is required, if very different materials are to be tested or if a large number of tests have to be carried out every day.
- Required in connection with automated specimen feeding
- Manual closing by screwing motion
- Very direct and slip-free transmission of the crosshead speed to the specimen, resulting in very constant and reproducible strain rate during the test
- Wide application range from semi-rigid to rigid plastics
- Suitable for tests with high demands on the reproducibility of test results
- Manual closing via lever movement
- Ideal for very frequent testing
- During the tensile test, the gripping force is applied by the crosshead movement via a wedge effect reducing the strain rate on the specimen, which in turn can have an effect on the test result
- Suitable for internal QA tasks with low requirements for reproducibility with measured values from other laboratories
- Combination of screw and wedge principle; the jaws are applied manually via a screw movement and apply the gripping force during they test via wedge action
- Used when different specimens up to over 30 mm thick need to be tested without changing the wedge jaws
ZwickRoell extensometers for ASTM D638
Depending on your requirements, ZwickRoell offers various extensometers, most of which meet accuracy class B-2 to ASTM E83:
- Non-contact videoXtens for optical non-contact measurement with high resolution and accuracy. The videoXtens has a wide measurement range which reaches >100% from modulus measurement to specimen strains. Attachment onto the specimen is automatic. You have the option of integrating a transverse strain measurement and use temperature control devices.
- makroXtens for mechanical probing measurement with very high resolution and accuracy. The makroXtens has a wide measurement range which reaches >100% from modulus measurement to specimen strains. Attachment onto the specimen is automatic. Measurements can be carried out up to specimen break without removing the sensor arms. You have the option of integrating a transverse strain measurement and use temperature chambers.
- The multiXtens combines the measurement of the tensile modulus with the measurement of high strains up to specimen break. It also has a very large travel range of up to 740 mm. It can be used when testing very different materials, e.g. brittle plastics and elastomers with very large specimen elongations.
- Manually attaching clip-on extensometer for mechanically probing measurement with very high resolution and accuracy. The extensometers are attached to the specimen by the operator before the test and are manually removed again once the maximum measurement displacement has been reached or before the specimen breaks. Depending on the version, you have access to measuring ranges from approx. 2 mm to around 25 mm.
- Mechanical longstroke extensometer for measuring medium to large specimen strains, suitable for elongation at yield and elongation at break. The specimen is attached automatically, and measurement up to specimen break can be carried out without removing the sensor arms.
- Optical longstroke extensometer for measuring medium to large specimen strains by tracking reflective gauge marks, suitable for elongation at yield and elongation at break. Attachment is carried out automatically, as is the measurement of the marker distance; the measurement is carried out until the specimen breaks.
ASTM D638 tensile test at low or elevated temperatures
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.
For tensile tests on plastics according to ASTM D638 at low or elevated temperatures, ZwickRoell also provides a temperature chamber. The temperature range can be adjusted from -80 °C bis +360 °C. 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, allowing you to run your tests more cost effectively.
More tests in the same time frame: increased specimen throughput in the plastics lab
For tensile tests according to ASTM D638, an automated solution may prove to be worthwhile starting at 10 specimens per day. Our roboTest N testing assistant assumes simple pick-n-place tasks and provides cost-efficient support in the test lab.
For laboratories with high testing requirements, we recommend our robotic testing system roboTest L. Our workhorse for plastics testing in large quantities can accommodate 450 specimens. It autonomously performs the specimen cross-section measurement and tensile tests according to ASTM D638. After the test, separate disposal grippers remove the specimen remains from the specimen grips.
But other specimen shapes or other test methods are also no problem for our automated testing systems. The automated testing system roboTest R is more complex and allows you to connect a second or third testing machine, as well as additional devices such as a centering station or a temperature chamber that ranges from -80 °C to +360 °C. This not only increases specimen throughput, it also offers you the option to perform different tests in parallel for fast and reliable test results.
Test procedure to ASTM D638: start testing immediately with testXpert
Performing the test to ASTM D638 with our testXpert testing software:
The tensile test is one of the most frequently performed tests when it comes to mechanical testing of plastics and polymers. The standard-compliant procedure is described in detail in the ASTM D638 standard. Step-by-step instructions eliminate the need to read all of the standard pages and show you how easy it is to perform the test with 100% compliance.
Simple and efficient testing to ASTM D638 also guarantees fast onboarding of new employees:
- All characteristic values and parameters defined in the ASTM D638 standard are already saved in the ASTM D638 standard test program. Standard compliance is guaranteed and you avoid the time consuming task of studying the standard.
- Users are guided through the test step-by-step. The only tasks they see on their screen are those that apply to their specific responsibilities, which is made possible through the user management feature. This ensures fast onboarding for new employees. All users reliably perform the test according to ASTM D638—no step is forgotten.
- Maximum testing efficiency is achieved through the connection of peripheral devices: When the specimen dimensions from the micrometer are sent directly to the testing software, you save time and avoid input errors.
testXpert testing software Running the test in 6 steps with testXpert
Frequently asked questions regarding ASTM D638
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 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.
