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D1938-19 – Standard Test Method Technical Guide
📐 Specimen Scope and Applicability
ASTM D1938-19 outlines the standard test method for determining the tear-propagation resistance of plastic film and thin sheeting using a single-tear (trouser tear) method. The standard specifically applies to materials with a thickness of 1 mm (0.04 in.) or less. Within this scope, “film” is arbitrarily defined as sheeting having a nominal thickness not greater than 0.25 mm (0.010 in.). A critical exclusion for this test method is any material that exhibits brittle failure during the testing process.
| 🟦 Material Constraint | 📏 Defined Limit | ⚠️ Condition |
|---|---|---|
| Maximum Sheeting Thickness | 1 mm (0.04 in.) | Method Scope |
| Maximum Film Thickness | 0.25 mm (0.010 in.) | Specific Definition |
| Brittle Failure Materials | Not Applicable | Excluded from Test |
⚙️ Test Procedure and Machine Setup
This test method employs a Constant-Rate-of-Grip Separation (CRG) testing machine, as described in Test Methods D882. The “trouser” specimen is pulled apart by the grips at a constant rate, and the force necessary to propagate the tear is calculated from the load-time chart. Specimen extension is measured directly by the separation of the grips. The testing apparatus must undergo force verification according to Practices E4, and conditioning of the specimens must follow Practice D618.
| 🛠️ Requirement | 🔬 Specification | 📄 Standard Reference |
|---|---|---|
| Testing Machine | Constant Rate of Grip Separation (CRG) | D882 |
| Force Verification | Annual/Periodic Validation | E4 |
| Specimen Conditioning | Standard Lab Atmosphere | D618 |
| Thickness Measurement | Precision Micrometer | D5947 / D6988 |
⚠️ Standard Alignment Note: While D1938-19 covers a similar concept to ISO 6383-1, they are not considered technically equivalent. The ISO 6383-1 method utilizes a larger specimen size and specifies different test speeds. Direct comparison of results between these standards is not recommended.
📊 Key Measured Properties and Reporting
The primary measured value is the force required to propagate the tear, determined from the analysis of the load-time chart. This value is often normalized by the specimen’s thickness to provide a tearing resistance value (e.g., N/mm). The values stated in SI units are regarded as the standard. The test is invalid if brittle failure occurs, as the method is designed to measure propagation resistance rather than initiation or catastrophic break strength.
💡 Best Practice for Data Quality: Ensure the tear propagates cleanly along the longitudinal axis of the specimen. Deviations such as knotty tearing (where the tear jumps ahead or changes direction) can significantly affect the recorded force and should be noted in the report.
❓ Frequently Asked Questions
🔍 What is the maximum thickness allowed for testing according to D1938?
The standard is applicable for plastic film and thin sheeting with a maximum thickness of 1 mm (0.04 in.). “Film” is specifically defined as being ≤ 0.25 mm (0.010 in.).
💡 What does the “trouser tear” method entail?
A specimen is cut into a shape resembling a pair of trousers, with a slit creating two legs. The legs are pulled apart by the testing machine to propagate the single tear along the center of the specimen.
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