ASTM D638 Tensile Testing of Plastics: Overview, Specimens & Results
What Is ASTM D638?
ASTM D638 — Standard Test Method for Tensile Properties of Plastics — is the most widely used standard for measuring the tensile mechanical properties of rigid and semi-rigid thermoplastic and thermoset polymers. It defines specimen geometry, test conditions, loading rate, data acquisition, and reporting requirements for plastic tensile testing. It is the fundamental mechanical characterisation test referenced in virtually all plastic material data sheets and purchase specifications.
What Properties Does ASTM D638 Measure?
A standard dog-bone-shaped tensile specimen is pulled apart at a defined crosshead speed until fracture. The force-extension curve is recorded, and the following key properties are calculated:
- Tensile Strength at Break (or at Yield): Maximum stress sustained by the specimen at fracture or yield (MPa)
- Tensile Modulus of Elasticity (Young’s Modulus): Slope of the initial linear elastic portion of the stress-strain curve (MPa or GPa)
- Yield Strength: Stress at which the stress-strain curve first shows a horizontal tangent — indicating permanent deformation onset
- Elongation at Break: Total percentage extension of the gauge length at fracture — a measure of ductility
- Elongation at Yield: Extension at the yield point
Specimen Types and Preparation
ASTM D638 defines five specimen types (I through V) of different dimensions, accommodating different material thicknesses and product forms:
- Type I: Standard specimen for materials ≥7 mm thick
- Type II: For materials <7 mm — thinner gauge, shorter gauge length
- Type III: For very thin films
- Type IV: For elastomeric or very flexible plastics
- Type V: Miniaturised specimen for limited material availability
Specimens are machined or moulded per the standard dimensions with smooth, parallel gauge section surfaces. Specimen dimensional accuracy is critical — non-uniform width or thickness in the gauge section introduces stress concentrations that cause premature fracture outside the gauge length.
Test Conditions and Speed
ASTM D638 specifies five crosshead speeds (1, 2, 5, 10, and 50 mm/min), with the appropriate speed selected based on the material type and the property being measured:
- 5 mm/min for rigid plastics (modulus and yield strength determination)
- 50 mm/min for ductile plastics (elongation at break measurement)
An extensometer attached to the gauge length is required for accurate modulus and yield strength measurement, as crosshead displacement includes compliance from the grips and machine frame.
Industrial Applications of ASTM D638 Data
ASTM D638 tensile data is used for:
- Material selection: Comparing tensile modulus, strength, and ductility across candidate materials
- Product design: Providing stress-strain data for finite element analysis (FEA) of plastic components
- Quality control: Verifying incoming plastic resin and moulded part lots meet specifications
- Failure analysis: Comparing the failed part’s tensile properties to the specification to identify degradation
- Regulatory compliance: ASTM D638 is referenced in FDA, UL, and NSF standards for plastic materials in regulated applications
In the automotive industry, ASTM D638 data at ambient and elevated temperatures feed into bumper fascia and instrument panel design. In the electronics industry, PC and ABS enclosure tensile properties are verified per product standards. In the packaging industry, tensile data of thermoformed tray materials support design and qualification.
Conclusion
ASTM D638 is the most widely accepted standard for determining the tensile properties of plastics, including rigid and semi-rigid thermoplastics and thermosets. It provides a standardised method for measuring key mechanical properties such as tensile strength, yield strength, Young’s modulus, and elongation at break using a dog-bone shaped specimen under controlled loading conditions. This makes it one of the most important mechanical characterisation tests used in polymer material evaluation.
Because it ensures consistent specimen geometry, test speed, and reporting requirements, ASTM D638 is extensively used for material selection, product design, quality control, and failure analysis across industries such as automotive, packaging, electronics, and consumer products. The resulting stress–strain data is fundamental for design validation and engineering calculations.
Why Choose Infinita Lab for ASTM D638 Tensile Testing?
Infinita Lab provides ASTM D638 tensile testing for all plastic material types and product forms through our nationwide accredited polymer testing laboratory network. Our mechanical testing specialists provide accurate, reproducible results with full data reporting.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What properties does ASTM D638 measure? It measures tensile strength, tensile modulus, yield strength, elongation at yield, and elongation at break.
Why is the specimen dog-bone shaped? The reduced gauge section ensures fracture occurs in the center of the specimen rather than near the grips, providing more accurate results.
Which specimen types are used in ASTM D638? The standard defines Type I, II, III, IV, and V specimens depending on thickness, rigidity, and material availability.
Which machine is used for the test? A universal testing machine (UTM) equipped with suitable grips and an extensometer is used.
What is the usual test speed? Typical speeds are 5 mm/min for rigid plastics and higher speeds such as 50 mm/min for ductile materials, depending on the property being measured.
