In this tensile test, essential mechanical properties of molding material are determined. These characteristic values are mostly used for comparison purposes.
The characteristic values are:
- Tensile stress: force applied to the initial cross-section of the specimen
- Strain: change in gauge length with reference 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
Both ISO 527-1/-2 and ASTM D638 define the test methods for tensile tests. The two standards are technically equivalent but do not provide completely comparable results, because specimen shapes, test speeds and method of result determination differ in some respects.
In the standardized tensile test, results are based on a defined specimen pull-off speed on the specimen. However, the loads on a component or structure in actual service may lie within a very wide range of the deformation rate. Due to the viscoelastic properties of polymers, mechanical properties different from those measured on a standardized test specimen normally result under altered strain rates. For this reason the characteristic values determined in a tensile test are only of limited suitability for component design, but represent a very reliable basis for material comparisons.
Aging tests: 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.
Specimen shapes for tests on molding materials
The overriding goal of testing molding materials is to achieve a high degree of reproducibility. This requires limiting the number of specimen types.
- Specimens are usually produced by injection molding. Type 1A specimens as defined in ISO 527-2 are used; in ISO 3167 these are designated as Specimen Type A and are additionally restricted to a specified thickness of 4mm. These specimens are also designated as Type A1 in ISO 20753.
- Injection molded specimens display decreasing orientation as the distance from the feed point increases, leading to non-constant mechanical property curves along the length of the specimen, and therefore frequently resulting in specimen break on the side away from the gate.
- The preferred gauge length for the specimen is 75mm, or alternatively 50mm.
- Alternatively, Type 1B specimens are permitted; these are designated as Type B in ISO 3167 and as Type A2 in ISO 20753.
- They are generally machined from pressed or injection molded sheets. The orientations of the polymer normally differ significantly from those in injection molded specimens. Comparability of results obtained using different specimen shapes is not guaranteed.
- A gauge length of 50 mm is specified for Type 1B specimens due to the larger radius resulting in a shorter parallel area.
Specimens for aging tests, media aging tests, and weathering tests
- A small cross-section is advantageous for all aging procedures which progress from the surface of the specimen.
- Often only the maximum tensile stress is used for assessment of this behavior. The use of extensometers is not necessary, and thin, waisted specimens can be used.
- ISO 527 offers Types CP and CW for this purpose; these are borrowed from impact tensile standard ISO 8256.
- 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.
- A standard atmosphere for testing refers to a defined standard atmosphere as specified in ISO 291 or ASTM D1349.
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 at high or low temperatures are also possible, for which differing requirements can be specified.
- Determining the specimen dimensions can result in a relatively high number of stress value errors. When a specimen is subjected to a tensile load, the measurement error is reflected linearly by the stress result. When a specimen is subjected to a flexure load, the specimen thickness measurement error has a quadratic effect.
- In addition to the reading accuracy of the measuring equipment, the size and form of the contact element and the applied surface pressing during measurement also play an important role.
- Furthermore, the cross-section of the specimen often differs from an ideal rectangular form. This could be angular errors resulting from mechanical processing or sink marks and minor draft angles in injection molded specimens.
- Many test standards refer to ISO 16012 and/or ASTM D5947 for defining the requirements and methods of dimension measurement. Sometimes, individual test standards contain additional specifications.
- For example, a caliper is normally used to measure the overall length of hard plastics larger than 10 mm. Since surface pressing during measurement cannot be checked, measurement accuracy is rather low even if the resolution of the caliper is high.
- The thickness and width of the specimen is normally determined by a micrometer screw with ratchet. The contact surface is flat and circular with a diameter of 6.35 mm. The ratchet limits the measurement force to 5–15 N.
- In automated systems, the thickness and width is determined by a cross-section measuring device. This device holds the specimen during measurement and determines the dimensions with four digital measuring transducers, a defined measurement force, and sensor feet.
- For soft plastics and films, it is imperative that the measurement force is strictly observed. To ensure this, digital thickness measuring instruments with dead weight supports must be used.
Testing machines measure two fundamental values: force and extension. As part of periodic calibration when compared to a measuring instrument based on national standards, evidence has shown that these measured values achieve a level of accuracy defined in the test standard across defined measuring ranges.
Force measurement (ISO 7500-1, ASTM E4)
Most test standards require a measurement accuracy of 1% for the measured value. This requirement is categorized as Class 1 in the ISO environment. Almost all modern testing machines today achieve Class 1 accuracy, or even Class 0.5 with tolerances that are halved. Decisive is therefore the measurement range in which a test machine achieves the specified class accuracy. Various ZwickRoell testing machines achieve Class 1 at a little as 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.
Extension measurement (ISO 9513, ASTM E83)
Along with a defined relative (in percentage) error, the class specifications for measuring extension also include a specification for an absolute error, which occurs when measuring smaller extensions.
ISO and ASTM differ here significantly.
- While ISO tolerances refer to the extension, reference is made directly to strain in ASTM.
- Furthermore, the requirements for smaller strains are defined more narrowly in ISO than in the corresponding ASTM class.
- Depending on the gauge length used, this sometimes results in significant differences by definition, in particular when measuring small extensions.
Special considerations for measurement of a tensile modulus
- As seen in the table above, the accuracy requirements for the strain range of the tensile modulus in ISO Class 1 are ±3 µm. This means that a deviation of up to 6 µm may exist between measurements at the beginning and end of the modulus range. This would result in a correspondingly large measurement error.
- To solve this problem, an additional requirement for measuring the tensile modulus was added to ISO 527-1. This additional requirement states that the measurement path between the beginning and end of modulus determination must be measured with an accuracy of 1%.
The testXpert III testing software is 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/-2 and ASTM D638 define test methods for tensile tests. The two standards are technically equivalent but do not provide completely comparable results, because specimen shapes, test speeds and method of result determination differ in some respects. With testXpert III, ZwickRoell offers a prepared 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.
Flexure test (ISO 178, ASTM D790)
- Flexure tests are performed at loading rates similar to those for tensile tests and therefore provide similar material characteristics.
- A major advantage of the flexure test is the relatively easy measurement of low specimen strains. For this reason the flexure test has been the preferred test for modulus measurement for a long time.
- However, since high-accuracy, easy-to-use extensometers are now available the significance of this advantage plays less of a role.
- Inherent to the method, the flexure test is more accurate in characterizing the material condition on the surface of the specimen. If strong orientations are present in the material, the result will be differences in measured values when compared to the tensile test.
- The calculation methods applied in the standards are subject to a measurement error which increases as specimen deflection becomes greater. For this reason the flexure test, unlike the tensile test, can be used only for low specimen strains.
Creep test under tensile loading (ISO 899-1)
The creep test is performed under constant tensile load. The loading rate is practically zero. The change in strain is shown as a creep curve.More on creep testing ISO 899-1
Impact tensile test (ISO 8256, ASTM D1822)
- This test offers a simple method for determining a tensile property at a high loading rate using a pendulum impact tester.
- With a conventional pendulum impact tester, only energy values can be determined, however, and the pull-off speeds are generally limited to around 3.8 m/s. An instrumented pendulum impact tester can determine additional characteristic values, such as maximum tensile force.
High-speed tensile test (ISO 18872)
The high-speed tensile test can be performed using drop weight testers or hydraulic high-speed tensile testing machines. Pull-off speeds up to 20 m/s are achieved. In addition, the use of direct extension measurement on the specimen is possible, allowing informative stress-strain diagrams to be generated. The high-speed tensile test also provides valuable parameters for crash simulations.More on high-speed tensile tests