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D638-22 – Standard Test Method Technical Guide
ASTM D638-22 is the primary standard used to determine the tensile properties of unreinforced and reinforced plastics. It specifies the use of standard dumbbell-shaped test specimens under defined conditions of pretreatment, temperature, humidity, and testing machine speed. The values stated in SI units are regarded as the standard. This test method is applicable for materials with thickness up to 14 mm (0.55 in). For thicknesses exceeding this limit, specimens must be machined down.
⚠️ Important Limitation: As noted in the standard (Note 2), the constant rate of crosshead movement test does not provide a true constant strain rate on the specimen. Differences in specimen thickness can also influence results. For directly comparable data, specimens should have identical thicknesses.
📐 Specimen Geometry and Conditioning
The standard defines several types of dumbbell-shaped specimens. The choice of specimen type depends on the material’s form and rigidity. All test specimens must be conditioned in accordance with Practice D618. The standard laboratory atmosphere is maintained at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours.
| 🟦 Type | 📏 Width (mm) | 🎯 Gage Length (mm) | 📐 Overall Length (mm) |
|---|---|---|---|
| I | 13.0 | 50.0 | 165 |
| II | 6.0 | 50.0 | 183 |
| III | 19.0 | 50.0 | 246 |
| IV | 6.0 | 25.0 | 115 |
| V | 3.18 | 7.62 | 63.5 |
⚙️ Test Procedure and Speed Selection
The test is conducted by gripping the specimen ends and applying a tensile load at a constant rate of crosshead movement until the specimen fractures or reaches a specified elongation. The speed must be selected based on the specimen type and the material’s behavior.
| 🟦 Specimen Type | ⚙️ Default Speed (mm/min) | 📌 Material Guidance |
|---|---|---|
| I, II, III | 5 | Rigid and semirigid plastics |
| IV | 50 | Non-rigid plastics and elastomers |
| V | 10 | Less rigid materials in sheet form |
For materials thinner than 1.0 mm (0.04 in), such as films, ASTM D882 is the preferred test method.
📊 Key Measured Properties
The standard allows for the determination of several critical properties from the resulting stress-strain curve:
- Tensile Strength: The maximum tensile stress sustained by the specimen during the test.
- Yield Strength: The stress at which the material exhibits a specified deviation from the proportionality of stress to strain.
- Percent Elongation: The increase in gage length at the moment of yield or break, expressed as a percentage of the original gage length.
- Tensile Modulus of Elasticity: The ratio of stress to corresponding strain below the proportional limit.
💡 Special Option: ASTM D638-22 provides the specific option of determining Poisson’s Ratio at room temperature. This requires strain measurements in both the axial and transverse directions.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D638-22?
This test method covers the determination of tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature,