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SAE J671: Specification for Vibration Damping Materials and Underbody Coatings
The SAE J671 standard provides a comprehensive classification system and performance requirements for mastic-based vibration damping materials and underbody coatings used in motor vehicles. It defines a numbering scheme that allows engineers to specify material properties based on decay rate and solids content, along with standardized test methods for key physical properties.
Classification and Numbering System
The standard uses a prefix system to denote material type: D for cut back vibration damping materials, U for cut back underbody coatings, E for water emulsions, and F for solvent-water emulsions. After the prefix, two digits indicate the type (based on decay rate at 21°C) and one or two digits indicate the class (based on solids content).
| Type | Nominal Decay Rate (dB/s) |
|---|---|
| 5 | 5 |
| 10 | 10 |
| 15 | 15 |
| 20 | 20 |
| 25 | 25 |
| 30 | 30 |
| Class | Nominal Solids Content (%) |
|---|---|
| 60 | 60 |
| 65 | 65 |
| 70 | 70 |
| 75 | 75 |
| 85 | 85 |
| 90 | 90 |
For example, a cut back underbody coating with a decay rate of 5 dB/s and solids content of 65% is designated U-565. An emulsion body damping material with medium decay rate and maximum solids content is DE-1090.
Physical Properties and Test Methods
The standard specifies minimum decay rates at three temperatures (21°C, -18°C, 38°C) to ensure consistent damping performance across operating conditions. For instance, a Type 10 material must have a minimum decay rate of 10 dB/s at 21°C, 4 dB/s at -18°C, and 4 dB/s at 38°C.
🔍 Note: Type 25 and 30 materials currently have no decay rate requirements at -18°C and 38°C, as indicated in the standard’s table.
Solids content is determined by drying a 3–5 g sample at 105°C for 3 hours. Cold adhesion is evaluated at -23°C using a slamming fixture; the material must retain specified percentages at slam angles from 50° to 70°. Underbody coatings require a minimum flash point of 38°C (Pensky-Martens closed tester). Vibration damping materials need a flash point agreed upon by the manufacturer and purchaser.
Abrasion resistance (when specified) requires at least 95% retention after shot blasting. Sagging must not exceed 6 mm on an inverted 45° panel. Sprayability criteria include a maximum flow rate of 45 s/L for underbody coatings and 57 s/L for vibration damping materials, along with stability and limits on sprayback and fogging.
⚠️ Common Mistake: Ignoring the temperature-specific decay rate requirements can lead to inadequate damping in extreme cold or heat. Always verify decay rates at all three specified temperatures.
Practical Considerations and Design Insights
Understanding the numbering system is critical for selecting the right material. The prefixes and type/class numbers allow precise communication between supplier and user. However, ensure that both vibration damping and underbody coating properties are verified if a material is used for both purposes.
🔍 Design Insight: The cold adhesion test is particularly stringent—proper surface finish (0.89–1.52 μm) and single-use panels are mandatory. Incorrect panel preparation is a common source of test failure.
Frequently Asked Questions
1. How is decay rate measured according to SAE J671?
A coated panel (500×500×6 mm, with natural frequency 145–165 Hz) is supported at nodal points, excited at fundamental frequency, and the free vibration decay is recorded in dB/s.
2. What are the cold adhesion requirements for underbody coatings?
After baking and cooling to -23°C, panels are slammed repeatedly from 10° to 90°. Minimum retention is 50% at 70° slam, 80% at 60°, and 100% at 50°.
3. What is the difference between a ‘type’ and a ‘class’?
Type refers to the decay rate category (damping performance), while class refers to the solids content range. For example, Type 10 indicates a minimum decay rate of 10 dB/s at 21°C; Class 65 indicates at least 65% solids by weight.
4. Are there any special storage requirements?
Materials must not settle or separate within 30 days at room temperature to ensure usability. Emulsion materials must also remain stable under typical working pressures.
For detailed test procedures and complete specification tables, always consult the official SAE J671 standard.
