ASTM D638 Explained: Everything You Need to Know About Plastic Tensile Testing
How to follow ASTM D638 for plastics tensile testing step by step
ASTM D638 is the internationally recognized standard method for determining the tensile properties of plastics. Whether you're testing materials for product development, regulatory compliance, or quality control, understanding and correctly applying this method is essential for accurate, consistent results.
This comprehensive guide walks you through the full D638 testing process, including specimen setup, testing equipment, extensometer options, material compatibility, and the data you can expect to collect. It also includes integrated examples of how CubeTen automates and improves each step.
Official Standard: Click here
What Does ASTM D638 Measure?
ASTM D638 is used to evaluate the tensile behavior of plastics under uniaxial stress. The method measures:
- Tensile strength (at yield and break)
Tensile strength is the maximum force a material can withstand while being stretched. At yield, it’s the point where plastic deformation begins. At break, it’s the point where the specimen physically fractures. - Strain and elongation at break
Strain is the percentage change in length during testing. Elongation at break shows how much the material stretches before failure. It’s a key indicator of ductility and flexibility. - Modulus of elasticity (Young’s modulus)
This measures a material’s stiffness — how much it resists deformation under load. It’s calculated from the slope of the linear portion of the stress-strain curve. Higher modulus = stiffer material. - Yield strain
Yield strain is the amount of strain a material experiences at the yield point. It marks the transition from elastic (reversible) to plastic (permanent) deformation. Beyond this, the material won’t return to its original shape. - Stress-strain behavior
This curve shows how a material responds to tension from start to failure. It visually represents strength, elasticity, yield, and fracture behavior. It’s the most comprehensive snapshot of material performance under load.
This data helps engineers, product designers, and quality assurance teams ensure that materials meet performance expectations under load.
When Should You Use ASTM D638 or ISO 527-2?
ASTM D638 is applicable when testing:
- Rigid and semi-rigid thermoplastics
- Thermosets
- Reinforced plastics
- Injection-moulded or machined plastic parts
It is the go-to tensile test method for most quality control and product development processes involving plastics, particularly in North America.For international or globally aligned applications, ISO 527-2 is a closely related standard. It outlines a similar methodology for determining the tensile properties of plastics and is commonly referenced in Europe and other regions.
How to efficiently Perform a Tensile Strength Test on Plastics (ASTM D638/ISO 527-2)
Performing a tensile strength test according to ASTM D638 (or ISO 527-2) involves a clear, repeatable sequence, especially when using an automated system like CubeTen. Below is the typical workflow:
The specimen is then secured with a consistent gripping force. No operator intervention needed.
This workflow removes operator variability and dramatically reduces time per test, while ensuring full compliance with ASTM and ISO testing standards.
A system configured for ASTM D638 must include:
- A tensile frame with controlled crosshead speed
- A calibrated load cell appropriate to expected failure loads
- Proper grips to hold the specimen without slippage or damage
- A strain measurement system (extensometer)
- Accurate thickness and width measurement tools
It is the go-to tensile test method for most quality control and product development processes involving plastics, particularly in North America.For international or globally aligned applications, ISO 527-2 is a closely related standard. It outlines a similar methodology for determining the tensile properties of plastics and is commonly referenced in Europe and other regions.
CubeTen supports three extensometer technologies — all optimized to meet the accuracy and compliance requirements of ASTM D638 and the equivalent international standard, ISO 527.
| Type | Description | Use Case |
|---|---|---|
| Crosshead Displacement | Built-in, calibrated displacement system for basic strain measurements. | Low/High-elongation materials or quick tests. |
| Automated Physical Extensometer | Tracks specimen elongation during testing using mechanical gauge arms. | Rigid plastics where high repeatability and accuracy are needed. |
| Video Extensometer | Non-contact, high-resolution strain tracking across the gauge length. | Delicate samples or high-elongation materials. |
ASTM D638 Specimen Types
ASTM D638 defines five specimen geometries, with Type I and Type IV being the most commonly used.
| Header | Thickness | Width | Length | Use Case |
|---|---|---|---|---|
| Type I | 3.2 mm | 13 mm | 165 mm | Standard for rigid plastics |
| Type IV | 2.0 mm | 6 mm | 115 mm | Thin plastics and films |
CubeTen is optimized to handle both types through automated gripping, alignment, and extensometer placement.
Specimen Reference: https://www.astm.org/d0638-14.html
Specimen Measurement & Alignment
Why it’s important
For a tensile test to be accurate and repeatable, each specimen must be measured and aligned correctly. Even minor errors can lead to:
- Uneven load distribution
- Premature specimen failure
- Incorrect strain and modulus data
How CubeTen ensures precision
- CubeTen automatically measures the width and thickness of each sample using calibrated sensors. No manual tools needed.
- Measurements happen instantly during the loading stage, saving time and improving consistency.
- A built-in smart vision system detects the edges of each specimen, including those with different colours or surface finishes.
- The system auto-aligns each sample in the grips with precision, using self-centering mechanisms and alignment verification.
Data Review, Integration, and Consistency
All test results can be reviewed, compared, and exported via the LabsCubed Portal, which offers:
- Interactive stress-strain visualization
- Batch testing reports and comparisons
- Integration with LIMS, ERP, or external systems via API
- Direct data export (CSV, PDF)
With CubeTen, results are consistent and repeatable, even across shifts or operators, a critical factor for high-throughput labs or multi-site operations. In traditional vertical testing machines, manual handling introduces variability at nearly every step:
- Slight differences in how operators position specimens in the grips can lead to misalignment
- Inconsistent gauge length marking and measurement affect strain accuracy
- Variations in grip pressure or clamping technique can cause premature slippage or breakage
Each of these issues contributes to test-to-test variation, even with calibrated equipment. Beyond data variability, these repetitive setup steps consume valuable operator time, time that could be redirected toward analysis or higher-value tasks.
CubeTen automates alignment, gripping, and measurement, eliminating these sources of human error and delivering precise, standardized inputs across every test. This not only boosts consistency but also maximizes lab efficiency through automation.
Looking to automate ASTM D638 tensile testing?
Download the CubeTen brochure to discover how automation can help your lab save time on repetitive tasks, reduce data variations, and let your team focus on the work that really matters.
