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D2990-17 – Standard Test Method Technical Guide
The ASTM D2990-17 standard provides standardized methods for evaluating the time-dependent deformation (creep) and failure under sustained load (creep-rupture) of plastics. These tests are critical for engineering design where materials are subjected to long-term static loads under specified environmental conditions.
🧪 Scope and Applicability
These test methods cover the determination of tensile and compressive creep and creep-rupture of plastics under specified environmental conditions (see Section 3.2). Flexural creep is measured using three-point loading per Test Methods D790, though four-point loading using the equipment and principles from D6272 is permitted as an option. For creep-rupture measurements, tension is the preferred stress mode because some ductile plastics do not rupture in flexure or compression. The data obtained is deemed relevant and appropriate for use in engineering design, with SI units regarded as the standard.
💡 Technical Note: While ASTM D2990 and ISO 899 (Parts 1 and 2) address the same subject matter, their technical content differs significantly, and results cannot be directly compared. A critical distinction is that compressive creep is addressed in D2990 but is entirely absent from ISO 899.
| 🟦 Stress Mode | 📏 Measured Property | 📐 Loading Configuration |
|---|---|---|
| Tension | Creep & Creep-Rupture | Uniaxial (per D638) |
| Compression | Creep | Uniaxial (per D695) |
| Flexure | Creep | 3-Point (Primary, per D790) / 4-Point (Optional, per D6272) |
📐 Specimen Geometry and Reference Standards
Specimen geometries are derived from well-established ASTM standards rather than being uniquely defined within D2990-17 itself. Tensile creep specimens must conform to Test Method D638, while compressive specimens follow Test Method D695. Flexural specimens utilize the standard dimensions and loading noses from Test Methods D790. Before testing, all specimens must be conditioned according to Practice D618, and their physical dimensions must be verified using Test Methods D5947 to ensure accurate stress calculations.
| 🎯 Test Type | ⚡ Source Standard (Geometry) | 📏 Key Fixture Requirement |
|---|---|---|
| Tensile Creep/Rupture | D638 | Grips allowing axial alignment |
| Compressive Creep | D695 | Self-aligning compression platens |
| Flexural Creep | D790 / D6272 (Option) | 3-point or 4-point bending fixture |
⚠️ Critical Note: The standard emphasizes that creep behavior in plastics is highly sensitive to environmental conditions. Strict control of temperature and humidity per the test plan is mandatory, and a specific safety warning is provided in Section 6.8.2 regarding potential hazards.
📊 Key Terminology and Measured Properties
Accurate interpretation of results requires an understanding of the standard’s specific definitions. Creep strain (Section 3.2.2) is defined as the total strain at any given time produced by the applied stress. This includes the sum of elastic and non-elastic strain, as plastics exhibit a wide spectrum of retardation times, making it impractical to separate the elastic component for engineering formulas. The creep modulus (Section 3.2.1) is calculated as the ratio of the initial applied stress to the creep strain at a specific time, typically expressed in GPa or psi.
❓ Frequently Asked Questions
🔍 Why is tension the preferred stress mode for creep-rupture testing according to D2990?
As stated in Section 1.3, tension is preferred because many ductile plastics will not undergo rupture in flexure or compression, making uniaxial tension the most reliable and definitive mode for generating rupture data.
💡 How does ASTM D2990-17 relate to the ISO 899 standard?
Per Note 1, the standards address the same subject matter but contain technical differences preventing direct comparison of results. D2990 is unique in that it includes a comprehensive method for compressive creep, a stress mode entirely omitted from the ISO 899 series.
⚡ Why does D2990 define “creep strain” as the total strain rather than strictly viscoelastic strain?
Based on Section 3.2.2.1, plastics have a broad spectrum of retardation times. Because the instantaneous elastic portion cannot be reliably separated from the time-dependent non-elastic portion in standard engineering practice, “creep strain” in this standard refers to the sum of both components.
📌 Is four-point loading permitted for flexural creep testing in this standard?
Yes. Section 1.2 explicitly states that four-point loading, using the equipment and principles described in Test Method D6272, is permitted as an option alongside the primary three-point loading method from D790.