ASTM D638: Tensile Properties of Plastics, Explained
Representative Infinita Engineering Visual showing tensile specimen preparation, axial loading, and property evaluation for Astm D638 Tensile Properties Plastics.What Is ASTM D638?
ASTM D638 is the Standard Test Method for Tensile Properties of Plastics, covering unreinforced and reinforced plastics in standard dumbbell-shaped specimens up to 14 mm thick. Thin sheeting and film under 1.0 mm should instead use ASTM D882, which is better suited to their geometry and handling.
Specimen Types
ASTM D638 defines five specimen types (I through V), differentiated by dimensions to accommodate different material thicknesses and product forms:
- Type I: standard specimen for materials 7 mm thick or greater
- Type II: for materials under 7 mm thick — thinner gauge section, shorter gauge length than Type I
- Types III-V: additional variants for specific thickness ranges, rigid/semi-rigid sheeting, and small or brittle specimens
Specimens are machined or molded to standard dimensions with smooth, parallel gauge section surfaces — non-uniform width or thickness introduces stress concentrations that cause premature fracture outside the intended gauge length, invalidating the result.
Also Read – Advanced Materials: Types, Properties, and Applications
Test Speeds
ASTM D638 specifies five crosshead speeds (1, 2, 5, 10, and 50 mm/min), selected based on material type and the property being targeted:
- 5 mm/min: typical for rigid plastics when determining modulus and yield strength
- 50 mm/min: typical for ductile plastics when measuring elongation at break
An extensometer attached over the gauge length is required for accurate modulus and yield strength measurement, since crosshead displacement alone includes compliance from the grips and machine frame that would otherwise skew the modulus calculation.
Reported Properties
- Tensile modulus (elastic/Young’s modulus): ratio of stress to strain below the elastic limit
- Poisson’s ratio: ratio of transverse to axial strain below the elastic limit
- Tensile strength at yield: maximum stress at the transition from elastic to plastic behaviour
- Tensile strain at yield: strain at that same transition point
- Tensile strength at break: stress at ultimate failure
- Elongation at break: total percentage stretch of the gauge length at fracture
Test Setup
Testing is performed on a universal testing machine (UTM) with pneumatic or mechanical grips sized to hold the specimen securely without slipping or crushing the gauge section. Grip selection should match specimen thickness and brittleness — overly aggressive grip pressure can crack brittle plastics before the tensile load even reaches the gauge section.
Applications
- Product design: providing stress-strain data for finite element analysis (FEA) of plastic components
- Quality control: verifying incoming resin lots and molded parts meet specification
- Failure analysis: comparing failed-part properties against baseline material data
Industry Specifications Referencing ASTM D638
- ASTM D882: thin film and sheeting alternative
- ASTM D4101: master specification cross-referencing D638 for polypropylene materials
- ISO 527: international equivalent tensile testing standard for plastics
Also Read – Additive Manufacturing Testing for Aerospace: Material Properties & Standards
Conclusion
ASTM D638 is the default tensile reference for plastics precisely because its five specimen types and five speed options let it flex across an enormous range of material thicknesses and ductility levels — but matching specimen type and speed to the actual material and target property is what separates a valid result from a technically-run-but-meaningless one.
What properties are measured under ASTM D638? The test can determine tensile strength, yield strength, elongation, tensile modulus, and stress at break. These properties help describe how a plastic behaves when pulled under controlled conditions.
How is an ASTM D638 tensile test performed? A dumbbell-shaped specimen is secured in the grips of a universal testing machine. The specimen is pulled at a specified speed while force and extension are recorded until it yields or fractures.
What specimen types are used in ASTM D638? ASTM D638 defines several specimen types for different materials and thicknesses. Type I is commonly used for rigid plastics, while smaller or thicker specimens may require other geometries.
Why are dumbbell-shaped specimens used? The narrow gauge section concentrates stress away from the grips and encourages failure within the measured area. This helps reduce grip-related damage and produces more representative tensile results.
How is tensile strength calculated? Tensile strength is calculated by dividing the maximum force recorded during testing by the specimen’s original cross-sectional area. It is commonly reported in megapascals or pounds per square inch.
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