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D5628-24 – Standard Test Method Technical Guide
🔬 Specimen Geometries and Standard Configurations
ASTM D5628‑24 covers the determination of the threshold value of impact‑failure energy required to crack or break flat, rigid plastic specimens under various specified conditions of impact of a free‑falling dart (tup). The specific test geometry is a critical variable. The method includes several standard geometries, each with a unique tup diameter, support diameter, and specimen size.
| 🟦 Geometry ID | 📏 Tup Diameter | 📐 Support Hole Diameter | 🎯 Typical Specimen Dimensions |
|---|---|---|---|
| DF | 12.7 mm (0.5 in) | 76.2 mm (3.0 in) | 89 mm × 89 mm (3.5 in × 3.5 in) |
| FE | 20.0 mm (0.787 in) | 40.0 mm (1.575 in) | 60 mm × 60 mm (2.36 in × 2.36 in) |
The standard specifies that the user must select the geometry most representative of the end‑use impact conditions. Geometry FE is specifically designed to achieve technical equivalence with ISO 6603‑1 when the Bruceton Staircase method of calculation is employed.
⚙️ Test Procedure and the Bruceton Staircase Method
Specimens are conditioned strictly in accordance with Practice D618, typically at 23 ± 2 °C and 50 ± 10 % relative humidity. The test is performed by clamping the flat specimen securely in the support fixture and releasing the specified tup from a predetermined height. The Bruceton Staircase (up‑and‑down) method is the standard statistical approach used to determine the 50 % failure level. Starting from an estimated mean height, the drop height is decreased by a fixed increment after a failure and increased by the same increment after a non‑failure. This process is repeated to converge rapidly on the mean‑failure height.
💡 Key Insight on Conditioning: Conditioning parameters heavily influence the impact behavior of plastics. Proper adherence to Practice D618 is essential for obtaining repeatable and reproducible results across different laboratories.
⚠️ Critical Note on Failure Definition: According to Section 3.2.1, failure is strictly defined as the presence of any crack or split, created by the impact of the falling tup, that is visible by the naked eye under normal laboratory lighting conditions. Mere surface imprints, stress whitening, or deformation without a visible rupture are not considered failures.
📊 Key Measured Properties and Calculations
The primary result of this test method is the Mean‑Failure Energy (impact resistance), which corresponds to the energy required to produce failures in 50 % of the tested specimens. As defined in the standard, it is equal to the product of the constant drop height and the mean‑failure mass, or the product of the constant mass and the mean‑failure height.
| ⚡ Property | 📝 Definition | 🎯 Target |
|---|---|---|
| Mean‑Failure Energy | Product of drop height and mean‑failure mass (or mass and mean‑failure height) | Energy required for 50 % failures |
| Mean‑Failure Height | Height at which a standard mass will cause 50 % failures | Statistical midpoint of the test results |
The energy value is typically expressed in joules (J) or inch‑pounds (in‑lbf). The standard deviation of the impact‑failure heights can also be calculated from the staircase data, providing insight into the variability of the material’s impact resistance.
❓ Frequently Asked Questions
🔍 What constitutes a valid failure in ASTM D5628?
Per Section 3.2.1, a valid failure is the presence of any crack or split created by the falling tup that is visible to the naked eye under normal laboratory lighting. Superficial whitening or indentation that does not create a visible crack is not considered a failure, which ensures a clear and objective pass/fail criterion.
💡 How does the Bruceton Staircase method work?
This iterative up‑and‑down method is used to determine the 50 % failure level efficiently. The first specimen is tested at an estimated target height. If it fails, the drop height is decreased by a fixed increment for the next specimen. If it passes, the height is increased. Results are statistically analyzed to calculate the mean‑failure energy and standard deviation.
⚡ What is the relationship between D5628 and ISO 6603‑1?
ASTM D5628‑24 and ISO 6603‑1 are technically equivalent only when Geometry FE (20 mm tup, 40 mm support) and the Bruceton Staircase calculation method are used. This allows direct correlation of results between the two international standards, facilitating global material specifications.
📌 Why is conditioning critical to this test?
Polymer impact behavior is highly sensitive to temperature and moisture content. Conditioning per Practice D618 ensures the measured impact resistance is intrinsic to the material and not skewed by uncontrolled environmental variables, which is critical for accurate material selection and quality control.