ISO Gravel Impact Test (ISO 20567-1): Stone-Chip Resistance Evaluation of Coatings

The ISO Gravel Impact Test, specified in ISO 20567-1, is a multi-impact test method used to evaluate the resistance of coatings to chipping caused by gravel and road debris. Widely adopted across the automotive and coatings industries, this test simulates real-world stone impact conditions to assess coating adhesion, durability, and long-term corrosion protection performance.

Technical Overview

The ISO Gravel Impact Test, formally standardized as ISO 20567-1 "Paints and varnishes — Determination of stone-chip resistance of coatings — Part 1: Multi-impact test," is a laboratory procedure designed to measure how well a coated surface withstands repeated impacts from projected gravel or grit particles. During vehicle operation, tires on road surfaces constantly launch stones, sand, and debris against body panels, rocker panels, wheel arches, and bumpers. Over time, these impacts cause coating detachment, chip formation, and eventual substrate corrosion.

In the test, a defined quantity of standardized gravel or grit is propelled by compressed air against a coated test specimen at a controlled impact angle, pressure, and temperature. The resulting damage is evaluated by removing loose coating fragments with adhesive tape and rating the extent of chipping against standardized rating scales or reference specimens. The test provides a repeatable, accelerated simulation of stone-chip exposure and is a cornerstone requirement in automotive coating qualification programs worldwide.

ISO 20567 exists in multiple parts to cover different impact scenarios:

• ISO 20567-1: Multi-impact test with gravel or grit projection, the most widely used method
• ISO 20567-2: Single-impact test with a guided free-flying impact body
• ISO 20567-3: Single-impact test with a free conical impact body

Testing Samples

Automotive Coated Metal Panels

The most common test specimens are flat steel or aluminum panels finished with a full automotive paint system, typically consisting of electrophoretic primer (e-coat), primer surfacer, basecoat, and clearcoat. Panels may be prepared in the laboratory under controlled film thickness and baking conditions, or cut directly from production body panels to reflect real manufacturing conditions.

Plastic Substrates with Coatings

Coated plastic components such as bumpers, fascias, and trim parts are frequently tested. Because polymeric substrates exhibit different impact energy absorption behavior than metal, testing plastic systems helps verify the flexibility and adhesion of the coating system under stone impact conditions.

Pre-Painted and Coil-Coated Sheets

Coil-coated metal sheets used in construction, appliance, and transportation applications may also be evaluated. Testing confirms that factory-applied coatings maintain integrity when subjected to impact from hard particles during transport, installation, and service.

Production Parts and Components

Where permitted, actual production components such as rocker panels, wheel-house liners, and underbody parts can be tested to validate geometry effects, edge coverage, and process consistency on real parts rather than flat coupons.

Testing Items

The ISO Gravel Impact Test evaluates several key performance characteristics of a coating system:

• Stone-chip resistance: the overall ability of the coating to resist chip formation under multi-impact conditions
• Adhesion loss: the degree to which impact causes intercoating or coating-to-substrate delamination
• Chip size and distribution: the number and dimensions of individual chips, rated against standardized charts
• Substrate exposure: the extent to which impact damage penetrates through all coating layers and exposes bare metal, an indicator of future corrosion risk
• Failure mode analysis: identification of the interface at which failure occurs, such as e-coat-to-substrate, primer-to-e-coat, or within a coating layer
• Temperature-dependent performance: chip resistance at ambient and sub-zero temperatures, since coatings become more brittle in cold climates

Testing Methods

Specimen Preparation and Conditioning

Test panels are prepared with the coating system of interest, and film thickness of each layer is measured and recorded. Prior to testing, specimens are typically conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a defined period. Unless otherwise agreed, the test itself is performed at 23 ± 2 °C; however, many automotive specifications require additional testing at low temperatures, commonly around -20 °C to -30 °C, to simulate winter driving conditions.

Impact Exposure Procedure

The specimen is mounted in a gravel projection apparatus at the specified impact angle, most commonly 54° to the horizontal for automotive fascia simulation, although other angles such as 90° may be specified by agreement. A defined mass of standardized grit or gravel, typically 500 g per specimen, is loaded into the projection chamber. Compressed air at a controlled pressure, generally 2.0 bar (0.2 MPa), propels the grit against the coated surface. Key controlled parameters include:

• Air pressure and projection velocity
• Grit type, size distribution, hardness, and quantity
• Impact angle and distance between nozzle and specimen
• Specimen temperature during impact
• Number of passes or exposure cycles

Post-Treatment and Evaluation

After impact exposure, loose coating fragments are removed by applying and removing pressure-sensitive adhesive tape over the impacted area. The damage pattern is then assessed using one or more of the following approaches:

• Visual comparison with standardized chipping reference charts or photographs
• Rating of chip size classes and chip counts per defined area
• Measurement of the total affected or delaminated area, often expressed as a percentage
• Rating classification agreed between customer and supplier, such as characteristic values from 0 (no damage) to 5 (severe damage), with lower values indicating better resistance

For corrosion-sensitive applications, specimens may additionally be subjected to salt spray exposure after the gravel impact test to evaluate whether chip sites initiate red rust, combining mechanical damage assessment with corrosion propagation evaluation.

Applications

The ISO Gravel Impact Test is applied extensively across multiple industries and product development stages:

• Automotive OEM approval: qualification of paint systems, materials, and suppliers against vehicle manufacturer specifications
• Paint and coating development: formulation optimization of primers, basecoats, and clearcoats for improved chip resistance
• Production quality control: routine monitoring of coating application parameters such as film build, flash-off, and cure conditions in paint shops
• Pretreatment and e-coat validation: verification that phosphate or zirconium conversion coatings and cathodic e-coat layers provide robust adhesion under impact
• Commercial vehicles, buses, and railway rolling stock: evaluation of exterior coatings exposed to severe ballast and roadway debris
• Aftermarket refinish coatings: performance comparison of repair paint systems with original equipment finishes
• Protective films and armor layers: assessment of stone-chip protection films applied to vulnerable vehicle zones

FAQ

What is the difference between ISO 20567-1 and ASTM D3170?

Both standards evaluate chip resistance by gravel projection, but they differ in apparatus geometry, grit specification, impact angle, and evaluation methodology. ISO 20567-1 defines a multi-impact test with tightly controlled projection conditions and flexible evaluation by agreement, while ASTM D3170 (the U.S. chip resistance test) uses its own gravel specification and damage rating system. SAE J400 is another related North American method. Results from different standards are not directly interchangeable without correlation testing.

Why is low-temperature gravel impact testing performed?

Coatings, especially clearcoats and primers, become harder and more brittle as temperature decreases. A paint system that performs well at room temperature may crack and delaminate when struck by stones in freezing conditions. Low-temperature testing therefore reflects the worst-case service environment for vehicles operated in cold climates.

What do the rating results mean?

Ratings describe the severity of chipping, considering chip count, chip size, and the depth of damage. A low characteristic value or small affected area indicates strong stone-chip resistance and good intercoating adhesion, whereas a high value signals extensive delamination and potential corrosion risk. Acceptance limits are typically defined in the customer's technical specification.

Which factors most influence test results?

Coating film thickness, cure quality, interlayer adhesion, substrate pretreatment, grit hardness and sharpness, air pressure, impact angle, and specimen temperature all significantly affect outcome. Strict control of these parameters is essential for reproducible results, which is why calibrated equipment and standardized grit are required.

Can the test predict real-world field performance?

The test provides a controlled, accelerated approximation of stone impact exposure and is highly effective for comparative evaluation and specification compliance. Because actual road exposure involves variables such as debris velocity, particle geometry, humidity, and thermal cycling, laboratory results are best interpreted alongside vehicle proving-ground and field corrosion testing for complete performance validation.