SAE J400

SUMMARY OF SAE J400 (Test for Chip Resistance of Surface Coatings)

Click to view SAE J400 Test Equipment

1. Purpose
SAE J400 specifies a method to test the chip resistance of surface coatings when impacted by stones/gravel, simulating real-world conditions on automobiles.
The test is primarily used for:

• Automotive paints, organic coatings on flat metal panels.
• May be applied to anodized aluminum, plated plastics, etc., but the limitations of the method must be understood.

2. Test Principle
A device called a Gravelometer is used, consisting of:

• A vibratory hopper containing gravel.
• An air nozzle to propel gravel with compressed air.
• A panel held perpendicular to the impact direction.

Procedure:

1. Gravel falls into the compressed air stream → blasted onto the panel.
2. Apply a special tape to lift all detached coating.
3. Evaluate the degree of chipping.

3. Materials & Main Equipment

Gravel

• Naturally rounded abrasive gravel (do not use crushed stone).
• Size: passes through 5/8 inch and retained on 3/8 inch sieve.
• Gravel must be cleaned and replaced periodically as it becomes dull after use.

Tape

• 3M #898 or equivalent (strongly affects the amount of coating removed).

Temperature Conditions

• Test at room temperature or -29°C.
• Three chilling methods: A (whole machine), B (panel only), C (room temperature).

4. Panel Preparation

• Flat panels, 4" × 12".
• Three replicate panels per coating type.
• Coating thickness must be uniform. Application process strongly influences results.

5. Test Procedure

1. Adjust air pressure: 70 psi ± 3 psi.
2. Adjust gravel flow rate: hopper should empty in 7–10 seconds.
3. Propel 1 pint (0.47 L) of gravel onto the panel.
4. Remove panel, apply tape multiple times in two directions to lift detached coating.
5. Evaluate results.

6. Evaluation Method (Gravelometer Rating System)

Two methods:

Method I – Accurate Counting

• Use a 4×4 inch grid.
• Count chips in each cell → classify by size.
• Chip size categories:
  • A: <1 mm
  • B: 1–3 mm
  • C: 3–6 mm
  • D: >6 mm
• Report as:
  5B – 6A – 8C (P/T)
  (Number of type B chips is highest, then A, then C; failure in Primer/Topcoat)
• Can be shortened to:
  4 BAC (P/T)

Method II – Visual Comparison

• Compare directly with SAE standard reference panels (photos of chip levels).
• Fast, commonly used for routine inspection.

7. Reporting Results
Must record:

• Chip rating (e.g., 4 BAC).
• Test conditions different from the standard (temperature, pressure, etc.).
• Material type, thickness, surface treatment.
• Coating process and thickness of each layer.

Summary:
SAE J400 simulates gravel impact to assess the chip resistance of coatings using a Gravelometer.
Gravel is blown with compressed air onto the panel, then tape lifts the detached coating.
Results are evaluated by chip counting or visual comparison with reference images and reported in a number–letter format (10–0 & A–D).