ASTM D638 Tensile Testing Methods for Plastics

What Is ASTM D638?

ASTM D638 is the most widely used standardized test method for determining the tensile properties of rigid and semi-rigid plastic materials. Developed and maintained by ASTM International Committee D20, it defines the procedures, specimen geometry, equipment requirements, conditioning protocols, and calculation methods for characterizing how a plastic behaves under uniaxial tensile force until failure.

The tensile data generated by ASTM D638 — tensile strength, tensile modulus, elongation at Yield, and Poisson’s Ratio — form a robust mechanical property dataset that engineers use for plastic material selection, structural design calculations, quality control, and regulatory compliance across virtually every industry that uses polymer components.

Why Tensile Testing Is Fundamental for Plastics

Every plastic component in service experiences some form of tensile stress — whether from direct pulling forces, bending-induced surface tension, thermal expansion differential, or assembly preloads. Without quantitative tensile property data, reliable product design is impossible. ASTM D638 provides the standardized measurement framework that ensures data generated in one laboratory is comparable to data generated in another — enabling global material specification, supplier qualification, and regulatory submission with confidence.

Scope of ASTM D638

ASTM D638 applies to:

• Unreinforced and short-fiber reinforced thermoplastics and thermosets
• Specimen thicknesses from 1.0 mm to 14 mm (0.04 to 0.55 in.)
• Molded, extruded, machined, or cut-from-sheet specimens in standard dumbbell (dog-bone) geometry

For thin films and sheeting with a thickness below 1.0 mm, ASTM D882 is the appropriate standard. For fiber-reinforced polymer matrix composites with high-modulus fibers, ASTM D3039 applies. For vulcanized rubbers and elastomers, ASTM D412 is used.

Specimen Types in ASTM D638

ASTM D638 defines five specimen geometries (Type I through Type V) differing in overall dimensions and gauge length, each suited to different material thicknesses and stiffness ranges:

Type I: The preferred and most commonly used specimen for rigid and semi-rigid plastics. Overall length 165 mm, narrow section width 13 mm, gauge length 50 mm.

Type II: A shorter alternative to Type I for materials that break outside the gauge section with Type I.

Type III: Required for materials with a thickness between 7 mm and 14 mm.

Type IV: Used for non-rigid (flexible) plastics up to 4 mm thick.

Type V: A miniaturized specimen used when material quantity is limited — with a 25 mm gauge length.

Specimens are either injection-molded directly to shape or machined from flat sheet using a precision die cutter to ensure smooth, defect-free edges. Edge quality is critical — notches, tool marks, or burrs on the specimen edges will cause premature failure and artificially low tensile values.

ASTM D638 Test Procedure

Conditioning

Specimens must be conditioned at 23°C ± 2°C and 50% ± 5% relative humidity for a minimum of 40 hours before testing per ASTM D618 Method A. This ensures moisture equilibrium and eliminates variability in testing due to ambient humidity fluctuations — particularly important for hygroscopic materials such as nylon, which absorbs moisture and significantly changes its mechanical properties.

Equipment Setup

Testing is performed on a Universal Testing Machine (UTM) with:

• Pneumatic or mechanical grips that hold the specimen without slippage or crushing
• A load cell with sufficient capacity for the expected failure force (typically 1–10 kN for most rigid plastics)
• An extensometer — clip-on, optical, or video-based — for accurate strain measurement in the gauge section

Crosshead-derived displacement is not acceptable as a substitute for an extensometer when reporting tensile modulus — the standard requires Class B-2 or better extensometers per ASTM E83 for modulus determination.

Test Speed

The crosshead speed is selected based on material type per ASTM D638 Table 1 — typically 5 mm/min for rigid plastics and up to 500 mm/min for flexible grades. Testing speed affects tensile properties in polymers, so the correct speed must be selected and reported with results.

Data Collection and Calculations

The UTM continuously records force and crosshead displacement (or extensometer strain) throughout the test. From the raw data, the following properties are calculated:

• Tensile Strength at Yield (MPa): Maximum stress before the material begins to deform plastically
• Tensile Strength at Break (MPa): Stress at the point of specimen fracture
• Elongation at Yield (%): StrYieldt the yield point
• Elongation at Break (%): Total strain accumulated at fracture — a measure of ductility
• Tensile Modulus / Young’s Modulus (MPa or GPa): Slope of the linear stress-strain region — stiffness of the material
• Poisson’s Ratio (optional): the Ratio of transverse-to-axial strain in the elastic region

How ASTM D638 Differs from ISO 527

ASTM D638 and ISO 527-2 cover the same fundamental measurement but are not directly equivalent:

• Specimen geometry differs — ISO uses different overall dimensions and a 50 mm or 75 mm gauge length
• Modulus calculation method differs — ISO 527 calculates modulus between 0.05% and 0.25% strain; ASTM D638 uses the linear portion of the curve.
• Test speed selection differs between the two standards

Results from the two standards are often close but should not be directly compared without noting which standard was followed. North American manufacturers typically specify ASTM D638; European and Asian suppliers often specify ISO 527-2.

Industry Applications

Automotive: Tensile testing qualifies structural plastic underhood components, bumper fascias, fuel system housings, and interior trim materials against OEM performance specifications.

Aerospace: Lightweight plastic and composite housings, bracket components, and non-structural parts require ASTM D638 tensile data for weight-optimized design and material approval.

Electronics: Connector housings, enclosures, and insulation materials are qualified by tensile testing to verify structural integrity under assembly and service loads.

Medical Devices: Polymer components for implants, diagnostic equipment, and drug-delivery devices require tensile data for design validation and FDA 510(k) submissions.

Packaging: Tensile testing of packaging polymers confirms sufficient strength and ductility to withstand filling, sealing, handling, and distribution forces

Conclusion

ASTM D638 tensile testing — measuring tensile strength, modulus, elongation, and yield behavior across rigid and semi-rigid plastics using standardized dumbbell specimens, controlled conditioning, and extensometer-verified strain measurement — provides the foundational mechanical property dataset that engineers rely on for plastic material selection, structural design, supplier qualification, and regulatory compliance across automotive, aerospace, electronics, medical, and packaging applications. Selecting the right specimen type, test speed, and conditioning protocol for the specific material and end-use requirements determines whether ASTM D638 data accurately represent in-service mechanical behavior — making rigorous test execution as critical to reliable material characterization as any design or formulation decision.

Why Choose Infinita Lab for ASTM D638 Testing?

Infinita Lab addresses the most frustrating pain points in the ASTM D638 testing process — complexity, coordination, and confidentiality. Our platform is built for secure, simplified support, allowing engineering and R&D teams to focus on what matters most: innovation. From specimen preparation and conditioning through testing and final report delivery, we orchestrate every detail — fast, seamlessly, and behind the scenes.

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. Request a Quote.

Frequently Asked Questions