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D1621-16 – Standard Test Method Technical Guide
📐 Scope and Significance
ASTM D1621 −16 (Reapproved 2023) describes a standardized procedure for determining the compressive properties of rigid cellular materials, particularly expanded plastics. Under the jurisdiction of Committee D20.22, this test method defines the methodology for obtaining compressive stress-strain diagrams. Per Section 1.2, SI units are regarded as the standard, with parenthetical values provided for information only. This standard is approved for use by U.S. Department of Defense agencies and is technically equivalent to ISO 844.
⚠️ Safety Compliance: Section 1.3 requires users to establish appropriate safety, health, and environmental practices prior to use. Conditioning must follow Practice D618.
⚙️ Test Procedure and Compliance Correction
The test procedure involves applying a compressive load to a conditioned specimen while precisely recording load and deformation. A critical step is compliance correction. As defined in Section 3.1.1, compliance is the displacement difference between the test machine drive system and the actual specimen. Section 3.1.2 describes the compliance correction as an analytical method to eliminate this machine contribution, ensuring the stress-strain diagram reflects only the material’s intrinsic response. Force verification is performed per Practices E4, and extensometers per E83.
💡 Terminology Focus: Compressive deformation (3.1.3) is the decrease in gage length. Compressive strain (3.1.4) is this deformation divided by the original gage length.
📊 Key Measured Properties and References
The primary output is the compressive stress-strain diagram. Compressive strength is defined in Section 3.1.5 as the stress at the yield point if it occurs before 10% deformation (Fig. 1a); otherwise, it is the stress at 10% deformation (Fig. 1b).
| 📏 Property | 🟦 Definition (Section 3) | 🎯 Determining Criteria |
|---|---|---|
| Compressive Strength | Stress at yield or 10% deformation | Yield point before 10% strain, else stress at 10% deformation |
| Compressive Stress (Nominal) | Load per unit minimum area | Minimum original cross section within gage boundaries |
| Compressive Strain | Compressive deformation / Original gage length | Dimensionless ratio (change in length per unit length) |
| Compressive Yield Point | First point strain increases without stress increase | Identified on the stress-strain diagram |
| Compliance | Machine displacement minus specimen displacement | Used for analytical correction of machine artifacts |
| 📐 Supplementary Standard | ⚡ Application in D1621 |
|---|---|
| ASTM D618 | Practice for Conditioning Plastics for Testing |
| ASTM E4 | Force Calibration and Verification of Testing Machines |
| ASTM E83 | Verification and Classification of Extensometer Systems |
| ASTM E691 | Conducting Interlaboratory Studies for Precision |
| ISO 844 | Technically Equivalent International Standard (Note 1) |
❓ Frequently Asked Questions
🔍 What exactly is compressive strength per D1621?
Section 3.1.5 defines it as the stress at the yield point if one is observed before 10% deformation. If no yield point is apparent, the compressive strength is defined as the stress measured at 10% deformation.
💡 Is ASTM D1621 equivalent to an ISO standard?
Yes, Note 1 explicitly states this test method is technically equivalent to ISO 844, facilitating global harmonization in testing rigid cellular plastics.
⚡ What is the purpose of a compliance correction?
Defined in Section 3.1.1 and 3.1.2, compliance correction removes displacement contributions from the test machine from the specimen data, yielding a true stress-strain curve free from machine stiffness artifacts.
📌 Which unit system is standard for this test?
Per Section 1.2, SI units are the standard. Values provided in parentheses (inch-pound units) are for informational purposes only.