1. What Tensile Testing Measures
Tensile testing measures how a material carries load while a testing machine pulls the specimen in tension. It can reveal tensile strength, modulus, strain, elongation, and the failure point. These properties help engineers assess whether a material can resist service loads, compare materials, verify specifications, and evaluate changes in formulation, processing, reinforcement, or manufacturing quality.
ASTM D638 measures tensile properties of plastics using standardized specimen geometries, commonly dumbbell-shaped specimens. It applies to many unreinforced and reinforced plastics when the specimen and material fit the method. ASTM D3039 focuses specifically on polymer matrix composite materials. It typically uses straight-sided coupon specimens and requires close control of laminate construction, fiber orientation, specimen alignment, gripping, conditioning, and strain measurement because these factors influence composite response.
The key difference lies in how each standard treats material structure. A relatively homogeneous plastic specimen distributes load differently from a laminate, where fiber direction, matrix, stacking sequence, void content, and fiber orientation control stiffness and failure. D3039 therefore produces data suited to composite design, qualification, research, and structural analysis. D638 remains the more appropriate route for conventional tensile characterization of plastics. Select the method that matches the material architecture and the specification governing the component or material system.
| D638 characterizes plastic tensile behavior through a standard plastic specimen. D3039 characterizes the directional tensile response of a composite laminate and treats fiber orientation, layup, tabs, and strain measurement as critical test variables. |
2. Full Comparison
Do not choose the method from resin type alone. Choose it from the material architecture. High-modulus fiber-reinforced polymer matrix composites require a composite-specific tensile method.
| PARAMETER | ASTM D638 | ASTM D3039 |
|---|---|---|
| Material focus | General plastics, including many reinforced plastics | High-modulus fiber-reinforced polymer matrix composites |
| Specimen form | Shaped plastic tensile specimen | Flat laminate coupon, often tabbed |
| Directional effects | Usually less dominant for isotropic plastics | Central to test design and reporting |
| Strain measurement | Extensometer or specified method | Strain gages or extensometry often used for modulus and Poisson ratio |
| Primary outputs | Tensile strength, modulus, elongation, yield behavior | Ultimate tensile strength and strain, chord modulus, Poisson ratio, transition strain as required |
| Best fit | Resin and plastic product qualification | Composite laminate design, QA, and structural property data |
3. Materials Covered
The key distinction is reinforcement architecture. D3039 addresses laminates where high-modulus fibers drive directional behavior.
ASTM D638-Rigid Plastic
- ASTM D638 Materials
- Unreinforced thermoplastics and thermosets
- Filled and short-fiber plastic compounds where D638 remains applicable
- Molded or machined plastic specimens used for material qualification
ASTM D3039
- ASTM D3039 Materials
- Carbon fiber reinforced polymer laminates
- Glass fiber reinforced polymer laminates
- Balanced and symmetric polymer matrix composite laminates tested in a defined material direction
4. How Each Test Is Run
Both methods use tensile loading, but composite testing requires tighter control of alignment, tabs, layup orientation, and strain measurement.
ASTM D638-Rigid Plastic
- Prepare the required D638 specimen and measure the cross section.
- Align the specimen in tensile grips and load it at the specified rate.
- Calculate tensile properties from force and extension or strain data.
ASTM D3039
- Prepare a flat composite coupon in the required laminate direction and add tabs when the test configuration requires them.
- Align the coupon carefully to minimize bending and load it in tension.
- Measure load and strain to determine the required strength, strain, modulus, and Poisson response.
5. Testing Equipment
| EQUIPMENT | ASTM D638 | ASTM D3039 |
|---|---|---|
| Universal testing machine | Required | Required |
| Grips | Plastic tensile grips | Composite grips with suitable pressure and tab support |
| Strain measurement | Extensometer as required | Extensometer or strain gages commonly used |
| Specimen preparation | Molded or machined dog-bone specimen | Precision-cut laminate coupon with controlled fiber orientation |
6. Applications
ASTM D638-Rigid Plastic
- Plastic resin qualification
- Molded component material verification
- QC for engineering thermoplastics and filled compounds
ASTM D3039
- Aerospace and automotive composite laminate qualification
- Composite R&D and laminate property development
- Structural design data for carbon or glass fiber reinforced polymer systems
7. Strengths and Limitations
| STANDARD | STRENGTH | LIMITATION |
|---|---|---|
| ASTM D638 | Uses standardized plastic tensile specimens and supports broad plastic material comparison. | It does not capture the full specimen and reporting requirements that high-modulus composite laminates need. |
| ASTM D3039 | Controls the variables that strongly affect laminate tensile response, including orientation, stacking sequence, alignment, and strain measurement. | Composite coupon preparation, tabbing, alignment, and valid failure mode requirements increase test complexity. |
8. Which Standard Should You Run?
| IF THIS DESCRIBES YOU | RUN THIS | WHY |
|---|---|---|
| You have an unreinforced plastic or a conventional plastic compound | ASTM D638 | D638 provides the standard tensile framework for general plastics. |
| You have a carbon, glass, or other high-modulus fiber polymer matrix laminate | ASTM D3039/D3039M | D3039 addresses the directional behavior and specimen controls required for composite laminates. |
| Your material contains reinforcement but you are unsure which method applies | Review reinforcement type, fiber modulus, orientation, and product specification first | The reinforcement architecture determines whether a general plastic or composite-specific tensile method gives valid data. |
Still unsure? Infinita Lab can review your material, specimen form, governing specification, and reporting goal before you submit samples for ASTM D638 or ASTM D3039 testing. Talk to our team.
9. Frequently Asked Questions
Does ASTM D638 cover reinforced plastics?
Yes, D638 can cover reinforced plastics within its scope. For resin-matrix composites reinforced with oriented high-modulus fibers, ASTM directs testing to D3039/D3039M.
Why does ASTM D3039 often use tabs?
Tabs can protect the coupon from grip damage and help transfer load into the laminate. The need and design depend on the material and test configuration.
Can I compare D638 and D3039 tensile strength values?
Only with strong context. Different specimen geometries, fiber orientations, gripping systems, and failure modes can change the measured values.
Which method should I use for chopped-fiber molded plastic?
Check the material architecture and governing specification. Many short-fiber molded compounds use D638, while high-modulus oriented composite laminates generally use D3039/D3039M.
