Understanding ASTM D638 for Plastic Tensile Testing

Introduction

What are the real tensile properties of the plastic parts you buy? How can engineers, quality managers, and procurement teams be sure a polymer will perform in service — and be confident the test data is comparable across suppliers and labs? ASTM D638 is the answer most industries reach for when they need reliable, auditable tensile property data for plastics.

This article gives a clear, practical guide to ASTM D638 — the standard test method that defines specimen geometry, test conditions, and reporting for tensile testing of plastics. It is written for engineers, lab managers, purchasing professionals, and potential business partners who need to understand tensile testing requirements quickly and use results for material selection, procurement, or product qualification.

Readers will learn:

Why ASTM D638 matters and how results are used in design and quality control.
Which specimen types and test speeds to choose for common plastics.
What equipment and fixturing produce repeatable, defensible data.
How ASTM D638 relates to ISO and other standards and when to combine tests for a fuller material profile.
Practical reporting tips to make test data actionable for buyers and engineers.

Standard Overview

History and scope of ASTM D638

ASTM D638, titled Standard Test Method for Tensile Properties of Plastics, was developed to provide a consistent, repeatable method for measuring tensile strength, elongation, and modulus of plastic materials. The standard covers unreinforced and reinforced plastics in the form of molded, machined, or otherwise prepared "dog-bone" specimens and gives requirements for specimen geometry, conditioning, test speeds, and reporting.

Over time D638 became the industry reference for laboratories, manufacturers, and buyers who need comparable tensile data for product development, quality control, and material specifications. The method was intentionally written to be practical for injection-molded parts and machined specimens while allowing for controlled modifications when material specifications require them.

Key points from the standard:

Specimen types: Types I–V (Type I is the preferred general-purpose specimen; Type V is for limited material availability or thin samples).
Thickness range: The standard provides guidance for specimens up to ~14 mm thick, with recommended thicknesses (commonly 3.2 mm for Type I).
Reported properties: Tensile yield, ultimate tensile strength, tensile modulus (Young’s modulus), and elongation at break — all extracted from the stress–strain curve.

Why ASTM D638 Matters for Plastics Testing?

ASTM D638 matters because it converts material behavior into quantitative, auditable data that stakeholders can use for decisions across design, procurement, and manufacturing:

Comparability: Results from ASTM D638 are widely recognized by suppliers and labs, making it easier to compare materials and verify claims during supplier qualification.
Design input: Tensile strength and modulus are fundamental inputs for component design and finite element analysis (FEA), affecting safety factors and performance predictions.
Quality control: Routine D638 testing helps detect lot-to-lot or process changes early, supporting consistent production quality.
Contract clarity: Naming ASTM D638 (including specimen Type, test speed, and extensometer method) in purchase specifications removes ambiguity and reduces inter-lab variability.

For businesses that buy or test plastics, adopting ASTM D638 as a baseline test method — and documenting exactly how it was run — reduces disputes and speeds supplier qualification.

Scope and Applicability

Materials and industries covered

ASTM D638 applies to a broad range of polymeric materials and is particularly suited to situations where a standardized, reproducible tensile property is required. Typical materials and use-cases include:

Thermoplastics: Injection-molded engineering plastics (ABS, polycarbonate, nylon, polyethylene, polypropylene) and commodity resins used in consumer products, housings, and structural components.
Thermosets & reinforced plastics: Unreinforced thermosets and some reinforced plastics when specimens can be machined or molded to the required geometry.
Filled & composite polymers: Plastics with fillers (glass beads, mineral fillers) and discontinuous reinforcements — where tensile response of the bulk material is of interest.
Thermoplastic elastomers (TPEs): When moderate elasticity coexists with plastic deformation; sometimes combined testing (e.g., D412) is desirable to capture full behavior.

Industries that routinely rely on ASTM D638 data include:

Automotive: Interior and exterior components, under-the-hood plastics, and small structural parts where tensile strength and elongation inform part design.
Consumer electronics & appliances: Housings, connectors, and snap-fit parts where material toughness and yield are critical.
Medical devices: Non-implantable polymer components and prototypes where traceable testing and documentation support regulatory submissions and supplier audits.
Packaging & consumer goods: Rigid packaging, closures, and functional components requiring predictable mechanical performance.
Aerospace (non-structural): Interior fixtures and polymer components where baseline tensile properties are required for qualification but structural composite standards (e.g., D3039) govern primary load-bearing elements.

Note: ASTM D638 is not a one-size-fits-all method. It targets specimens that can be produced as dog-bone shapes with specific dimensions and thickness ranges. Very thin films, fibers, or highly anisotropic laminates generally need specialized standards.

When to use ASTM D638 vs. alternative tests

Choosing the correct tensile or mechanical test depends on the material form, the information required, and contractual or regulatory obligations. Use the following guidance to decide when ASTM D638 is the right choice and when alternatives are better:

Use ASTM D638 when:
- You can produce Type I–V dog-bone specimens (molded or machined) with the dimensions specified in the standard.
- The goal is to obtain baseline tensile properties (tensile strength, yield, elongation at break, and tensile modulus) for material specification, comparison, or QC acceptance criteria.
- Procurement documents or supplier contracts explicitly call for ASTM D638 results for material acceptance.
Choose alternatives when:
- Thin films or sheets: ASTM D882 (Tensile Properties of Thin Plastic Sheeting) or relevant ISO film standards are more appropriate for films and foils.
- Fibers, threads, or yarns: Use fiber-specific standards (e.g., ASTM D3822) that account for cross-sectional area and clamping methods.
- Elastomers & highly elastic materials: ASTM D412 is tailored to rubber-like materials and provides more accurate results for high-elongation materials.
- Reinforced composites: For continuous-fiber laminates, use composite tensile standards such as ASTM D3039/D638 (only if specimen geometry and laminate type permit) or specialized composite methods.
- International projects: If the contract requires ISO reporting, ISO 527-2 may be preferred — be aware that numerical results may differ slightly due to geometry and procedural differences.
Practical decision tips:
- If your material and production method allow standard dog-bone specimens, start with ASTM D638 as the baseline and document specimen orientation, molding conditions, and conditioning environment.
- When working across suppliers or geographies, clarify whether ASTM D638 or ISO 527 is contractually required — asking upfront avoids re-testing and disputes.
- When in doubt, request a small pilot test: have samples tested under ASTM D638 and the alternative standard to compare results and evaluate which method better reflects your in-service performance needs.

For turnkey testing services, specimen preparation, or equipment that supports both ASTM D638 and alternative methods, contact WTS. WTS can advise on specimen molding, orientation effects, and which standards will provide the most relevant, defensible data for procurement and design.

Apparatus and Equipment

Tensile testing machines and grips

Reliable equipment is critical to get repeatable ASTM D638 results. The right setup reduces test-to-test variability and ensures that measured differences reflect material behavior — not machine artifacts. Below is a practical guide to selecting, configuring, and maintaining equipment for ASTM D638 tensile testing.

Core equipment list

Universal Testing Machine (UTM): A frame with suitable load capacity, precise crosshead control, and low machine compliance. Choose a model sized to your expected test loads and with a reputation for stable, linear force measurement.
Load cells: High-quality load cells matched to expected maximum load. Avoid massively oversizing the load cell (which reduces resolution) or undersizing it (which risks overload). Ensure traceable calibration certificates are available.
Grips & fixturing: A selection of grips compatible with ASTM D638 specimens — flat serrated grips, pneumatic wedge grips, and optional cushioning/face plates for thin specimens.
Extensometers & strain measurement: Contact clip-on extensometers for moderate elongation and high accuracy; non-contact video or laser extensometers for delicate specimens or large strains.
Environmental chamber / conditioning oven: For temperature- or humidity-controlled tests when specified by the standard or project requirements.
Data acquisition & software: Software that records force and displacement at high resolution, generates stress–strain curves, and calculates tensile strength, modulus, yield, and elongation values.
Specimen preparation tools: Punches, machining fixtures, and micrometers for measuring thickness and gauge length accurately.
Calibration & traceability tools: Calibration weights, certified gauge blocks, and access to an accredited calibration lab for periodic verification.

Grips — types and selection tips

Grips are a common source of test variability. Choose grips that hold the ASTM D638 dog-bone specimens without causing premature failure at the grip area.

Flat serrated grips: Good for standard rigid plastics; provide positive bite without crushing thin specimens.
Pneumatic wedge grips: Offer consistent clamping pressure and reduce slippage on ductile plastics. Pressure control avoids over-clamping delicate specimens.
Self-tightening or wedge-action grips: Suitable for higher loads and can reduce grip-related failures when specimen shoulders are consistent.
Specialized jaw faces: Use smooth or cushioned faces for thin or filled specimens to avoid stress concentrations; replace worn jaw faces regularly.

Extensometers & strain measurement

Strain measurement method directly affects modulus and elongation results. ASTM D638 requires reporting the strain measurement technique used.

Clip-on (contact) extensometers: High accuracy for modulus calculations; ideal for low-to-moderate strains when gauge length is accessible.
Non-contact video extensometers: Best for high-elongation materials or when extensometer attachment could alter specimen behavior.
Machine crosshead displacement: Not recommended as the sole strain measurement for precise modulus values because it includes machine compliance and grip slip.

Environmental control and specimen conditioning

ASTM D638 results are sensitive to temperature and humidity. Where conditioning or controlled test temperature is required, an environmental chamber is essential.

Common conditioning practices: standardized pre-conditioning (e.g., 23°C / 50% RH) or project-specified conditions.
For high-temperature testing, ensure the extensometer and grips are rated for the test temperature or use remote, non-contact strain measurement.

Data acquisition, software, and reporting

Modern test software should:

Record synchronized force and displacement at appropriate sample rates.
Compute stress–strain curves and extract ASTM D638 properties automatically (tensile strength, yield, modulus, elongation at break).
Allow customization of reports, inclusion of specimen photos, and insertion of test parameters (specimen Type, gauge length, extensometer type, test speed, temperature).

Calibration, verification, and maintenance

Routine calibration and preventive maintenance keep test results credible and audit-ready:

Calibration frequency: Follow national or industry guidance — typically annual calibration by an accredited lab, with in-house verification between calibrations.
Verification checks: Run quick verification tests with calibrated weights or certified reference samples before critical test runs.
Maintenance: Inspect grips and jaw faces for wear, check load-cell connectors, and keep the crosshead lead screws and guides lubricated per manufacturer guidance.
Document everything: Maintain calibration certificates, maintenance logs, and software version records as part of quality management documentation.

Procurement checklist (what to request from suppliers)

When buying a tensile tester or accessories, include the following requirements in your RFQ to get comparable quotations:

Machine frame model and maximum load capacity
Available load cell capacities and traceable calibration certificates
Crosshead speed range and resolution
Compatible grips and jaw faces for ASTM D638 specimens
Extensometer options (clip-on and non-contact)
Environmental chamber compatibility (if required)
Data acquisition sample rate and reporting capabilities
Warranty, on-site installation, and training options
Local service & calibration support or recommended accredited labs

Safety and operator training

Provide operator training that covers:

Specimen gripping techniques to prevent slippage and premature failure
Extensometer mounting and safe removal procedures
Interpreting stress–strain curves and recognizing anomalous runs
Emergency stop procedures and safe maintenance practices

WTS equipment & services

WTS offers universal testing machines, extensometers, grips, and turnkey testing solutions configured for ASTM D638 workflows. For labs and manufacturers seeking robust, audit-ready tensile testing, WTS provides:

Testing frames sized for polymer testing with multiple load-cell options.
Compatibility with clip-on and non-contact extensometers.
Calibration and on-site installation services, plus operator training.
Customized solutions for labs with specific temperature or throughput requirements.

Variations & Related Standards

ASTM D638 variants and related test methods (ISO equivalents)

While ASTM D638 is widely used for tensile testing of plastics, a number of related standards exist to address different specimen forms, international practices, and material families. Knowing the differences helps avoid misinterpretation of reported data and ensures contractual clarity.

ISO 527-2 — International counterpart: ISO 527-2 is the most commonly referenced international equivalent for tensile testing of plastics. It serves the same purpose as ASTM D638 (tensile strength, elongation, modulus) but uses different specimen geometries and procedural details. When working with global suppliers or customers, confirm which standard is required and, if necessary, run both methods on representative samples to compare results.
ASTM D412 — Elastomers & rubbers: Designed for vulcanized rubber and other highly elastic materials. Use D412 instead of D638 for materials that show very high elongation and elastic recovery.
ASTM D882 — Thin films & sheets: Intended for thin plastic films and sheeting where dog-bone specimens are impractical. Results reflect film behavior and are not directly comparable to D638 values.
ASTM D3039 / D3518 — Continuous-fiber composites: For unidirectional laminates and composite panels where tensile loading interacts with fiber orientation and layup; these methods use different specimen geometries and gripping methods.
Other ISO test methods — ISO has complementary documents for films, fibres, and composites; always confirm the required ISO part (for example, ISO 527-1 gives general principles while ISO 527-2 gives specimen and test details).

When to combine tests (e.g., D412, D790)

Combining ASTM D638 with complementary test methods gives a more complete mechanical profile of a material. Below are common combinations and their rationale:

D638 + D790 (Tensile + Flexural): Use when both tensile and bending behaviors affect part performance — for example, housings or covers that experience bending loads and tensile stresses from assembly features.
D638 + D412 (Tensile + Elastomer testing): Useful for thermoplastic elastomers (TPEs) and hybrid materials that show both rubber-like and plastic-like behaviors. D412 captures high-elongation elastic response while D638 provides baseline tensile modulus and yield data.
D638 + ISO 527: Run both if contracts span regions with different test preferences, or when you need to compare legacy data generated to an ISO method with new ASTM data. Document differences and, if required, include conversion notes or parallel testing results.
D638 + ASTM D3039 (for short-fiber-filled polymers vs. continuous-fiber composites): When material families differ in reinforcement type, combine tests to isolate matrix behavior (D638) and laminate/tape performance (D3039).

Best practices when combining tests:

Source all specimens from the same production batch and, where relevant, the same molded orientation to reduce variability between different test methods.
Record and publish all test parameters so that comparisons are transparent (standard version, specimen Type, gauge length, test speed, temperature/humidity, and strain measurement method).
If comparisons between standards are required for contract acceptance, include a short pilot study in your procurement or qualification plan to quantify typical differences.

FAQs

1. What is ASTM D638 used for?

ASTM D638 is used to determine the tensile properties of plastics, including tensile strength, elongation, and modulus of elasticity. It provides standardized procedures for ensuring accurate and comparable results.

2. Can ASTM D638 be used for rubber or elastomers?

No, ASTM D638 is primarily intended for rigid and semi-rigid plastics. For rubber or highly elastic materials, ASTM D412 is the more appropriate standard.

3. What type of specimen is used in ASTM D638?

The test uses “dog-bone” shaped specimens with specific dimensions depending on the material thickness and form. These shapes help ensure that failure occurs in the gauge section and not near the grips.

4. What is the difference between ASTM D638 and ISO 527?

Both standards are used for tensile testing of plastics, but they have differences in specimen dimensions, test speeds, and other procedural details. ASTM D638 is more common in North America, while ISO 527 is widely used internationally.

5. How accurate is ASTM D638 testing?

When performed correctly with calibrated equipment and following the standard’s guidelines, ASTM D638 provides highly accurate and repeatable results.

Conclusion

ASTM D638 remains one of the most important and widely used standards in the plastics industry. From material selection to product development, it plays a critical role in ensuring performance, safety, and compliance. By understanding its scope, methodology, and applications, engineers, manufacturers, and quality control teams can make more informed decisions and produce better products.

At WTS Machinery, we understand the importance of precision and compliance in plastics testing. Our advanced tensile testing solutions, combined with expert support, help businesses achieve reliable results that meet ASTM D638 requirements. Whether you need equipment, consultation, or testing services, WTS is your trusted partner in quality assurance.