ASTM D638 defines the standard procedure for testing the tensile strength of plastic materials using a Universal Testing Machine (UTM). The process involves pulling a standardized “dogbone” shaped specimen until it fails, while synchronized sensors capture the relationship between load and displacement.
The Testing Procedure
A compliant ASTM D638 test requires specific equipment configurations to ensure the data is accurate and repeatable:
Specimen Preparation: Plastics are molded or machined into one of five standard “dogbone” shapes (Types I through V) depending on the material’s thickness and availability.
Test Speed: The standard requires a constant rate of crosshead movement, typically ranging from 5mm/min to 500mm/min, depending on the material specification.
Grips and Alignment: Precise alignment is required to ensure the force is applied purely along the axial plane. TRI provides wedge or pneumatic grips specifically designed to prevent specimen slippage during the test.
Precision Instrumentation
To meet the high-accuracy requirements of ASTM D638, the testing system must be equipped with specialized sensors:
Load Cells: High-resolution load cells capture the force applied to the specimen.
Extensometers: For accurate modulus and elongation data, TRI recommends the use of a clip-on or video extensometer to measure strain directly on the specimen’s gauge length, rather than relying solely on crosshead travel.
Newton Controller: The controller manages the test speed and data acquisition, providing the high-speed sampling necessary to identify the exact moment of yield and failure.