Plastics – including those used in consumer goods, construction materials, cars, and airplanes – must withstand immense pressure before breaking. To guard against premature failure, those who manufacture and assess the quality of plastic components rely on tensile testing. These tests use universal testing machines and motorized test stands and wedge grips to determine the force required to break a plastic specimen and evaluate its performance under stress.
Different industries and materials have their own acceptable testing force limits, defined by standards like ASTM D638. This commonly used standard has been adopted across industries to test the tensile properties and durability of reinforced and unreinforced plastics from 1 mm to 14 mm thick.
Here’s what you need to know about ASTM D638, including the testing procedure and required equipment.
What Is ASTM D638?
ASTM D638 testing can be used to measure the strength, elongation, and stiffness of plastics. R&D professionals test to the standard to determine which materials to use in production. Quality control professionals test to the standard to ensure products are safe to ship to consumers and channel partners.
ASTM D638 Specimen Preparation
Five specimen types allowed under ASTM D638: Types I, II, III, IV, and V. The length, width, and thickness of each type vary, but all specimens must be flat and dumbbell- or dogbone-shaped. There are three primary ways to create the signature shape: through injection molding, machining, or cutting with a specialized die cutter.
ASTM D638 Testing Equipment
ASTM D638 testing requires the following equipment:
- Motorized test stand: Pulls the specimen and collects data about the plastic’s tensile properties
- Wedge grips: Hold the specimen in place during testing
- Extensometer: Tests the stretchiness of the plastic (aka the modulus of elasticity)
- ASTM D638 sample cutter die: Creates a dogbone-shaped specimen that meets ASTM D638 specifications (only needed if the specimen is created from a larger, pre-formed component)
ASTM D638 Testing Procedure
Step 1: Prepare the Specimen
Create the sample by cutting or molding your material into one of the approved dumbbell types. Ensure your specimen meets the length and width requirements that best suit your material’s thickness and rigidity.
Step 2: Set Up the Test
Mount the specimen in the grips of your testing machine and attach the extensometer. Set the test speed according to the material’s specification; these typically range from 1 to 500 mm/min.
Step 3: Run the Test & Collect Results
Pull the specimen at the specified rate until failure. The system will record force and displacement data throughout the test cycle.
How to Get Started
If you are a researcher or developer testing new plastic materials, a manufacturer creating plastic components, or a quality control professional who needs to test a product before it goes to market, you will likely need to test to ASTM D638. If you don’t yet have the equipment to test, look for a reputable manufacturer or distributor who supplies a wide range of testing materials – including force testers, wedges, grips, sample die cutters, digital and mechanical force gauges, and extensometers – and can answer questions you may have about testing setup.