How to Perform a Tensile Strength Test on Plastics According to ASTM D638
Written by Erica Lawrence
ASTM D638 is the most common testing standard for determining the tensile properties of reinforced and non-reinforced plastics. With the use of plastics being at an all-time high, it is critical that manufacturers be able to properly gauge the mechanical strength of their materials. This guide is designed to introduce you to the basic elements of an ASTM D638 plastic tensile test, including an overview of the equipment, software, and samples needed. However, anyone planning to conduct ASTM D638 testing should not consider this guide an adequate substitute for reading the full standard.
What Does it Measure?
ASTM D638 is performed by applying a tensile force to a sample specimen and measuring various properties of the specimen under stress. It is conducted on a universal testing machine (also called a tensile testing machine) at tensile rates ranging from 1 to 500 mm/min until the specimen fails (yields or breaks). Though ASTM D638 measures many different tensile properties, the following are the most common:
- Tensile strength – the amount of force that can be applied to a plastic before it yields (stretches irreparably) or breaks.
- Tensile modulus – how much a material can deform (stretch) in response to stress before it yields. Modulus is a measurement of the material’s stiffness.
- Elongation – the increase in gauge length after break divided by the original gauge length. Greater elongation indicates higher ductility.
- Poisson’s Ratio - a measurement of the relationship between how far a material is stretched and how thin it gets during the stretching process.
Is ASTM D638 the Right Standard for You?
There are many different test methods for various types of plastics. ASTM D638 only applies to rigid plastic samples between 1.00 mm and 14 mm in thickness. If your sample is a sheet or film less than 1.00 mm thick, it should be tested to ASTM D882. While it provides similar results to ISO 527-2, ASTM D638 is not considered technically equivalent due to differences in specimen size and test requirements. While some large multinational manufacturers test to both ASTM D638 and ISO 527-2, most of our customers demonstrate a preference for one standard or the other based on their geographic location. North American manufacturers usually test to ASTM D638 while those in Europe and Asia primarily test to ISO 527-2. Customers in China equally test to ASTM D638 and ISO 527-2. All of these test methods can be found in Bluehill® Universal’s Applications Modules, which are pre-configured method templates for the most popular ASTM and ISO standards.
Materials Testing System
Most ASTM D638 testing is performed on a table top universal testing machine such as Instron's 6800 Series. A 5 kN or 10 kN (1125 or 2250 lbf) system is most common, but as reinforced plastics and composites increase in strength, higher capacity units – such as 30 kN or 50 kN systems - may be required.