With the increased use of plastics, it is essential for manufacturers to accurately assess the mechanical strength of their materials. ASTM D638 is the most common standard for determining the tensile properties of reinforced and unreinforced plastics. This guide is designed to introduce you to the basics of ASTM D638 tensile testing for plastics, including an overview of the equipment, software, and samples needed. However, anyone planning to conduct ASTM D638 tests should not consider this guide a suitable substitute for reading the full standard.
What Does ASTM D638 Measure?
Tensile Strength Tensile strength is the amount of force that can be applied to a plastic material before it yields (stretches irreparably) or breaks. This parameter is crucial for understanding how much stress a material can withstand before failing.
Tensile Modulus The tensile modulus indicates how much a material can deform in response to stress before yielding. The modulus is a measure of the material’s stiffness, representing its ability to resist deformation.
Elongation Elongation is the increase in the gauge length after breaking divided by the original gauge length. Greater elongation indicates greater ductility, meaning the material can plastically deform without breaking.
Poisson’s Ratio Poisson’s ratio measures the relationship between how much a material is stretched and how much it thins during the stretching process. This parameter is important for understanding the three-dimensional behavior of the material under stress.
Applicability of ASTM D638
Reinforced and Unreinforced Plastics ASTM D638 applies to rigid plastic samples with thicknesses between 1.00 mm and 14 mm. It is ideal for testing both reinforced and unreinforced materials, providing a comprehensive view of their mechanical properties.
Differences with Other Standards (ASTM D882 and ISO 527-2) If your sample is a sheet or film less than 1.00 mm thick, it should be tested with ASTM D882. Although it provides similar results to those expected from ISO 527-2, ASTM D638 is not considered technically equivalent due to differences in specimen dimensions and test requirements.
Geographical Preferences North American manufacturers usually test according to ASTM D638, while European and Asian manufacturers primarily test according to ISO 527-2. Customers in China also test according to both standards. The choice between these standards often depends on geographical location and specific manufacturer preferences.
Required Equipment
Universal Testing Machine Most ASTM D638 tests are performed on a benchtop universal testing machine. These machines are designed to apply a controlled tensile force to the samples, measuring various properties during the process.
Types of Samples
Type I Samples Type I samples, with a thickness of 3.2 mm, are the most common and are generally created through injection molding. They have a total length of 165 mm and a width of 13 mm, with a gauge length of 50 mm.
Type IV and V Samples When the material is limited, many laboratories use Type IV or V samples. Type IV samples have dimensions similar to those required for ASTM D412 Die C cutting, while Type V samples are the smallest, with a gauge length of only 0.3 inches.
“Dogbone” Shapes Flat samples are typically molded, die-cut, or machined into a “dogbone” shape to ensure that the break occurs in the center of the sample rather than in the clamping areas.
Sample Preparation
Measuring Samples All samples must be measured before testing in accordance with ASTM D5947. Most typical micrometers should be suitable for making these measurements, ensuring precision and uniformity.
Using Micrometers For the testing system to display stress measurements rather than just force measurements, operators will be asked to enter the cross-sectional area (or thickness and width) of the specimen, as stress is calculated by dividing the applied force by the cross-sectional area of the specimen.
Batch Variation Die-cut or machined samples must be measured individually, but operators using injection-molded samples need only measure a single sample from a batch sample, provided the variation in that batch sample is less than 1%.
Sample Alignment
Importance of Alignment To test correctly, the samples must be held perpendicular to the faces of the jaw and not tilted at an angle. Misalignment of the samples can cause significant variation in the results.
Sample Alignment Devices One way to prevent misalignment is to use a jaw face that is close to the same width as the sample, making it relatively easy to visually adjust the alignment.
Prevention of Compression Forces When the grips are tightened on the plastic samples in preparation for performing a test, unwanted compression forces are often applied. It is important that they are not balanced after inserting the sample, as this will cause deviation in the results.
Performing the Test
Force Application Procedure ASTM D638 is performed by applying a tensile force to a specimen and measuring various properties of it under stress. The force is applied in a controlled and uniform manner until the specimen yields (yields or breaks).
Test Speed The test is conducted at a speed between 1 and 500 mm/min, depending on the specifications of the sample and the material.
Result Analysis
Interpreting Strength Data After performing the test, the collected strength data must be interpreted to determine the tensile strength of the material.
Calculating Tensile Modulus The tensile modulus is calculated based on the deformation of the material under stress, providing a measure of its stiffness.
Evaluating Elongation and Poisson’s Ratio Elongation and Poisson’s ratio are evaluated to better understand the behavior of the material during deformation.
Practical Tips
Common Errors to Avoid Avoiding common errors such as sample misalignment or applying unwanted compression forces can significantly improve the accuracy of the results.
Tips for Accurate Results Ensure that all samples are prepared and measured correctly, and use alignment devices to maintain consistency between tests.
Related Standards
ASTM D5947 This standard specifies procedures for measuring samples before testing, ensuring the accuracy of sample dimensions.
Other Relevant ASTM Standards There are several other ASTM standards that may be relevant depending on the type of material and test specifications.
Comparison with ISO Standards Although ASTM D638 and ISO 527-2 provide similar results, differences in specimen dimensions and test requirements make it necessary to choose the most appropriate standard based on specific needs.
Frequently Asked Questions (FAQ)
What is the difference between ASTM D638 and ISO 527-2? ASTM D638 and ISO 527-2 are both standards for tensile testing of plastics, but they differ in specimen dimensions and test requirements.
Why is Poisson’s ratio important? Poisson’s ratio measures the three-dimensional behavior of the material during stretching, providing crucial information about lateral deformation relative to longitudinal deformation.
What are the most common samples for ASTM D638? Type I samples are the most common, with a thickness of 3.2 mm, generally created through injection molding.
How does sample alignment affect test results? Sample misalignment can cause significant variations in results, making correct alignment essential for accurate tests.What errors should be avoided during testing? Avoiding common errors such as sample misalignment and applying unwanted compression forces can significantly improve the accuracy of the results.
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