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D5420-21 – Standard Test Method Technical Guide
📐 Scope and Significance of D5420-21
ASTM D5420-21, the Standard Test Method for Impact Resistance of Flat, Rigid Plastic Specimen by Means of a Striker Impacted by a Falling Weight (Gardner Impact), provides a standardized procedure for the relative ranking of materials based on the energy required to induce failure. The test is applicable specifically to flat, rigid plastic specimens under various specified impact conditions. SI units are regarded as the standard, with values provided in parentheses for informational purposes only.
⚠️ Important Note on Failure Interpretation: Per Section 3.2.1, failure is defined as any crack or split created by the impact that is visible to the naked eye under normal laboratory lighting conditions. Cracks usually start at the surface opposite the one that is struck, making bottom-surface inspection critical for accurate result determination.
The standard emphasizes that it does not purport to address all safety concerns associated with its use. It is the responsibility of the user to establish appropriate safety, health, and environmental practices. Practitioners are also directed to relevant material specifications for specific guidance regarding failure differences between material types.
⚙️ Test Procedure and Failure Modes
Specimens must be conditioned in accordance with Practice D618 (Conditioning Plastics for Testing) prior to evaluation. The test utilizes a striker impacted by a falling weight, differentiating it from falling dart methods such as D5628. The test is designed to determine the mean failure energy, typically employing a staircase (up-and-down) technique to converge on the precise energy level required for failure.
🟦 Specific Failure Modes per D5420
Defining what constitutes a failure is critical for consistency. The standard identifies six distinct failure criteria that must be evaluated.
| 🟦 Failure Type | 📐 Detailed Description from Standard |
|---|---|
| Complete Shatter | Complete shattering of the plaque into multiple pieces. |
| Radial Cracking | Any crack radiating out toward the edges of the plaque on either surface. |
| Puncture / Hole | A hole in the plaque, due to brittle or ductile puncture, where unobstructed light or water could pass through. |
| Brittle Splitting | Any brittle splitting of the bottom (non-struck) surface of the plaque. |
| Glassy Chip | Any glassy-type chip dislodged from or loosened from the plaque. |
💡 Operator Tip: The standard explicitly notes that failure includes any crack within or just outside the impact area of the striker. Magnification is not typically required for failure evaluation, as the standard specifies visibility under normal laboratory lighting. Consistency in lighting conditions between operators is highly recommended.
📊 Referenced Standards and Precision
The precision of D5420 is determined through interlaboratory studies conducted in accordance with Practice E691. Equivalence between testing processes can be assessed using Practice E2935. This test method is frequently referenced by material specifications such as D4066 (Nylon Injection and Extrusion Materials) and D4226 (Rigid PVC Building Products). Notably, there is no known ISO equivalent to this standard.
| 📏 Parameter | 🎯 Specification / Guidance |
|---|---|
| Apparatus | Gardner Impact system (striker + falling weight) |
| Specimen Type | Flat, rigid plastic specimens (plaques) |
| Conditioning | Per Practice D618 (Plastics) or E171 (Barrier Packaging) |
| Failure Energy | Mean Failure Energy (relative ranking) |
| Precision Standard | E691, E2935 |
| Related Terminology | D883 (Plastics), E456 (Quality & Statistics) |
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D5420?
The primary purpose is the determination of the relative ranking of materials according to the energy required to crack or break flat, rigid plastic specimens. It provides a comparative means of assessing impact resistance using the Gardner Impact configuration, rather than providing an absolute material constant.
💡 How is “failure” defined for a valid test?
Failure is defined in Section 3.2.1 as the presence of any crack or split created by the impact that can be seen by the naked eye under normal laboratory lighting conditions. The six specific criteria include complete shatter, radial cracking on either surface, puncture creating a hole, brittle splitting of the bottom surface, and glassy chips dislodged from the specimen.
⚡ How does D5420 differ from D5628 (Falling Dart)?
While both test impact resistance of rigid plastics, D5420 specifically employs a striker impacted by a falling weight (Gardner geometry). D5628 utilizes a falling dart (tup or falling mass). The impactor geometry and fixture configurations differ, which can result in varying failure modes and energy absorption values between the two methods.
📌 What conditioning standards are required before testing?
Standard conditioning must be performed in accordance with Practice D618 for general plastics testing. For flexible barrier packaging materials, Practice E171 provides the applicable conditioning guidelines. Proper conditioning ensures that temperature and humidity variables do not obscure the intrinsic impact properties of the specimen.