ASTM D638 Tensile Testing of Plastics
ASTM D638 helps to determine plastic material tensile properties. A Universal Testing Machine measures the force required to break a specimen and the extent to which the specimen stretches or elongates to that breaking point. Poisson's Ratio is useful data for plastics engineering design purposes. This test technique is required by some material specifications, but there are some procedural changes that take precedence when conforming to the specification. So, before applying this test method, it is advised to consult that material specification.
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ASTM D638 Tensile Testing of Plastics
- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
ASTM D638 Tensile Testing of Plastics Overview
ASTM D638 is the primary standard for measuring the tensile properties of plastics. The test pulls a dumbbell-shaped specimen to failure in uniaxial tension and records stress and strain continuously throughout. From the resulting stress-strain curve, the standard defines how to measure tensile strength, elongation at yield, elongation at break, tensile modulus (Young’s modulus), and yield strength – the complete set of tensile properties needed to characterize and compare plastic materials for engineering applications.
ASTM D638 defines five specimen types (Type I through Type V) of different sizes to accommodate different thickness ranges and material amounts. Type I and Type II are the most common and are used for materials that do not neck significantly before failure. The test speed varies by material type and the property being measured – lower speeds (1 mm/min or 5 mm/min) are used for modulus determination to minimize rate effects, while higher speeds (50 mm/min or 500 mm/min) are used for ductile materials where elongation at break is the primary interest. An extensometer must be used for modulus measurement; crosshead displacement is not acceptable due to grip compliance errors.
D638 is referenced by virtually every plastic material specification and is used throughout the plastic product supply chain from resin development and compounding through molded part qualification and production quality control. Tensile strength and modulus are the two properties most commonly specified on plastic material data sheets, and ASTM D638 is the method by which those values are determined and reported.
ASTM D638 Tensile Testing of Plastics Scope, Applications, and Benefits
Scope
ASTM D638 applies to rigid and semi-rigid thermoplastic and thermoset materials that can be prepared as flat dumbbell specimens. It is not applicable to highly flexible elastomers (use ASTM D412 for those). The standard covers a wide range of material forms including injection-molded specimens, compression-molded specimens, and specimens machined from sheet, rod, or plate stock. Key evaluation areas include:
- Tensile strength at yield and at break
- Elongation at yield and elongation at break (%)
- Tensile modulus (Young’s modulus) from the initial linear region of the stress-strain curve
- Effect of reinforcement, fillers, and additives on tensile properties
- Effect of processing conditions (mold temperature, injection speed, annealing) on properties
- Lot-to-lot material consistency in production quality control
Applications
- Plastic resin and compound qualification against material specifications
- Comparison of competing plastic grades for a given application
- Structural plastic component design input data generation
- Filled and reinforced composite plastic characterization
- Environmental aging and chemical resistance effect on mechanical properties
- Production incoming material inspection and certification verification
Benefits
- Comprehensive characterization of tensile behavior in a single test
- Universally accepted and referenced in plastic material data sheets globally
- Multiple specimen types accommodate different material forms and thickness ranges
- Extensometer requirement ensures accurate modulus data free of machine compliance error
- Sensitive to formulation, processing, and aging changes that affect functional performance
- Results directly usable in finite element analysis and structural design calculations
ASTM D638 Tensile Testing of Plastics Process
Specimen Preparation
Dumbbell specimens are injection molded, compression molded, or machined from stock material in the specified type and dimensions
1Conditioning
Specimens are conditioned at 23 degrees Celsius and 50% relative humidity for a minimum of 40 hours
2Modulus Measurement
For modulus determination, an extensometer is attached to the gauge section and specimens are loaded at 1 or 5 mm/min.
3Tensile Testing to Failure
Specimens are loaded at the test speed appropriate for the material until fracture.
4ASTM D638 Tensile Testing of Plastics Technical Specifications
| Parameter | Details |
|---|---|
| Standard | ASTM D638 |
| Specimen Types | Type I (preferred), Type II, Type III, Type IV, Type V |
| Type I Gauge Length | 50 mm |
| Test Speed Range | 1 mm/min to 500 mm/min depending on material and property |
| Modulus Measurement | Requires extensometer; crosshead displacement not acceptable |
| Applicable Materials | Rigid and semi-rigid thermoplastics and thermosets |
Instrumentation Used for ASTM D638 Tensile Testing of Plastics
- Universal tensile testing machine with appropriate load cell
- Contacting or video extensometer for modulus measurement
- Dumbbell specimen dies and fixtures for specimen preparation
- Micrometer for specimen thickness and width measurement
- Environmental conditioning chamber
- Data acquisition and stress-strain curve analysis software
ASTM D638 Tensile Testing of Plastics Results and Deliverables
- Stress-strain curve for each specimen
- Tensile modulus (GPa or MPa)
- Yield strength and elongation at yield where applicable
- Tensile strength at break and elongation at break
- Statistical summary across replicate specimens
- Quality assurance documentation
Frequently Asked Questions
ASTM D638 measures key tensile properties such as tensile strength, tensile modulus, elongation at break, and yield strength, providing a comprehensive assessment of a plastic's behavior under tensile loading.
The standard is suitable for many rigid and semi-rigid thermoplastics and thermosets, provided specimens can be prepared in the required geometry. Material thickness and processing method may influence specimen selection.
The standard specifies multiple specimen geometries to accommodate different material thicknesses and forms. Using the appropriate specimen ensures consistent stress distribution and reliable, comparable test results.
Tensile properties can vary significantly with crosshead speed, temperature, and humidity. ASTM D638 specifies controlled test conditions to improve repeatability and enable meaningful comparisons between materials.
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