ASTM D638 – Standard Test Method for Tensile Properties of Plastics

ASTM D638 · ASTM standard

ASTM D638 is the standard test method for the tensile properties of plastics, measured on standardized “dumbbell” (dog-bone) specimens under controlled speed and conditioning. It applies to both unreinforced and reinforced plastics and anchors material qualification, incoming inspection, process control, and lot-to-lot comparison. To match a specimen type, thickness limit, or strain-measurement approach to your material, talk with our team.

What ASTM D638 Covers

The test produces a stress-strain curve and the properties taken from it: tensile strength at yield and at break, elongation, and, with suitable strain measurement, tensile modulus.

How the Test Is Run

D638 follows a straightforward preparation-to-reporting sequence:

Prepare or machine the specified specimen type.
Condition specimens to the required temperature and humidity.
Test at the specified speed using appropriate grips and alignment.
Calculate and report tensile results from the force and extension data.

Where a material specification cites D638 but modifies details such as conditioning or reporting, those requirements take precedence over the generic method.

Reading the Designation

Cited with a year suffix such as ASTM D638-22. Confirm the edition before fixing specimen dies, extensometry, and reporting, since permitted options change between versions.

Dumbbell Specimens and Thickness Limits

ASTM D638 covers rigid and semi-rigid plastic specimens up to a defined thickness; thinner sheeting and film are tested to ASTM D882 instead. The method defines five specimen types so geometry can be matched to the material and its thickness, as described below.

Table 1. Specimen types defined in ASTM D638.
Type I	The default for rigid and semi-rigid plastics at common thicknesses
Type II	Used when a Type I specimen does not break within the narrow section
Type III	For thicker specimens within the method’s range
Type IV	For comparing materials of differing rigidity, such as a rigid plastic against a more flexible one
Type V	For small or thin specimens where material is limited

Conditioning and Test Speed

Plastic properties shift with preparation, temperature, humidity, and loading rate, so ASTM D638 fixes the conditioning and the test speed (commonly 5, 50, or 500 mm/min, selected by specimen type and material). These conditions must match the cited specification whenever results support acceptance or supplier comparison.

Equipment for ASTM D638

The test runs on a universal testing machine configured for plastics, with force and strain measured in a controlled, repeatable way. A typical setup includes a UTM with an appropriate load-cell capacity, grips suited to plastic specimens, an extensometer (contact or non-contact) for the strain range, and tools to measure specimen width and thickness for the stress calculations. To configure a plastics tensile system around your specimen sizes and throughput, request pricing for a matched setup.

Machining Plastic Specimens

Specimen quality is decisive for plastics. Edges must be smooth and notch-free, and machining must avoid the heat and internal stress that distort results. CNC preparation produces clean, repeatable dumbbells across a batch and holds the tight width and radius tolerances the method depends on.