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ASTM D638 Tensile Properties of Plastics

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.

Objectives & characteristic values Running the test Specimens and specimen dimensions Environmental conditions Accuracy requirements Testing systems FAQs Plastics brochure

Objective & characteristic values of tensile tests to ASTM D638

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 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

Aging tests

The tensile test to ASTM D638 provides a good basis for demonstrating the change in the mechanical characteristic values of a polymer following aging, heat or media storage, or weathering. Here, the characteristic values of the tensile test are determined in the newly molded state, as well as after defined aging or weathering periods.

How is the ASTM D638 tensile test performed?

Performing the tensile test to ASTM D638

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 when using testXpert.

ASTM D638 in 6 steps with testXpert

ASTM D638 describes different specimen shapes and the accuracy with which specimen dimensions must be measured. It also specifies the tool separation distance, the initial gauge length, as well as permitted tolerances for the measurement. The tensile test to ASTM D638 is used to determine various types of characteristic values, including the tensile modulus, yield points, and strain at break. A specified pre-load ensures reproducible results.

Both theASTM D638 and the ISO 527-1/ISO 527-2 standards 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.

ASTM D638: start testing immediately with testXpert

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

ASTM D638 specimen dimensions / specimen types

Specimens used for tests according to ASTM D638 are classified by type:

  • ASTM D638 Type I - 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.
  • ASTM D638 Type II - Provided that by using the test specimen Type I a fracture cannot be produced in the narrow parallel part of the specimen, ASTM D638 recommends the use of a test specimen Type II, in which the width of the narrow parallel section is significantly reduced.
  • ASTM D638 Type III - If specimens with material thicknesses of more than 7 mm can be obtained through mechanical processing, then specimen Type III is 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.
  • ASTM D638 Type IV - The test specimen Type IV is ideal for testing of very soft polymers (e.g., rubber) and is used when comparing soft and more stiff polymers.
  • ASTM D638 Type V - 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.

L0 Gauge length
L Grip-to-grip separation
l1Length of the narrow parallel section/inner diameter
l2 Distance between the wide, parallel sections
l3 Total length / outer diameter
b2 Specimen width in shoulder area
b1 Specimen width in gauge length area
h Specimen thickness

ASTM D638 specimen dimensions and specimen types
Standard Specimen type Note l3
mm
l1
mm
b2
mm
b1
mm
h
mm
L0
mm
L
mm
ASTM D638 Type I Preferred specimen for
rigid plastics
≥165 57±0.5 19+6.4 13±0.5 3.2±0.4 50±0.25 115±5
ASTM D638 Type II Preferred when type I does not
break in the narrow cross-section
≥183 57±0.5 19+6.4 6±0.5 3.2±0.4 50±0.25 135±5
ASTM D638 Type III For specimen thickness greater than 7mm
(rigid and soft plastics)
≥246 57±0.5 29+6.4 19±0.5 7 ... 14 50±0.25 115±5
ASTM D638 Type IV For comparison of rigid and
soft plastics
≥115 33±0.5 19+6.4 6±0.05 3.2±0.4 25±0.13 65±5
ASTM D638 Type V Small specimens from components ≥63.5 9.53 9.53+3.1 3.18±0.5 3.2±0.4 7.62 25.4±5

 

Conditioning and ambient conditions defined in ASTM D638

  • 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.

ASTM D638 tensile test at low or elevated temperatures

Test on plastics at -80 °C to +250 °C in the temperature chamber

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.

Testing machine accuracy requirements according to ASTM D638

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.

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Additional tests for characterization of molding materials

Tensile tests on plastics
ISO 527-1, ISO 527-2
to Tensile tests on plastics
Tensile tests at high strain rates
SAE J2749, ISO 18989, ISO 18872, VDA Recommendation 287
Crash simulation
to Tensile tests at high strain rates

Frequently asked questions regarding ASTM D638

The most common ASTM standard for tensile tests on plastics is ASTM D638. This standard is used to determine tensile properties of plastic materials for control, specification, qualitative characterization and research and development purposes. It defines the testing equipment, specimen shape and dimensions and the test procedure to be followed. Material properties determined using ASTM D638 include tensile strength, specimen elongation, strain values, the modulus of elasticity, and the option to determine Poisson’s ratio at room temperature.

For more detailed information, please refer to the full standard: ASTM D638

The tensile strength of plastics is essentially determined by measuring the stress that is required to cause a plastic specimen to break or to yield. The standard, therefore, makes a differentiation between the yield point and the break resistance. This defining characteristic of plastic materials is commonly used in research and development and for quality control purposes. The tensile strength of plastics is determined using a tensile testing machine, which pulls the specimen apart until it breaks, while critical values are measured and calculated. Most commonly these tests are performed according to standard ASTM D638, which specifies the shape and dimensions of the specimen to be tested, provides machine specifications, and describes the test procedure.

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.

The ASTM D638 standard covers the determination of tensile properties including the tensile strength of unreinforced and reinforced plastics. Other common standards used to test tensile strength are ASTM E8/ASTM E8M for metals, ASTM D412 for rubber and elastomers, and ASTM D3039 fiber-reinforced composites

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