The multi-impact stone chip test, also known as repeated gravel impact testing, evaluates the ability of paints and coatings to withstand damage from gravel and road debris striking the surface many times in rapid succession. Conducted to ISO 20567-1, ASTM D3170, and related automotive industry standards, it is one of the most important qualification tests for vehicle coating systems. This article explains the test principle, applicable standards, sample requirements, test procedure, evaluation methods, and typical applications.
Technical Overview
The multi-impact stone chip test is an accelerated laboratory procedure that simulates the repeated impact of stones, gravel, and road debris on coated surfaces. In this test, a defined quantity of standard gravel is propelled by compressed air against a coated test panel mounted at a controlled angle, distance, and temperature. Unlike single-impact methods, which strike the specimen with one guided impact body, the multi-impact method delivers hundreds of small impacts over a short blast duration, closely reproducing the cumulative chipping conditions experienced by vehicle body panels, rocker areas, and underbody components in real service.
The test is widely used in the automotive industry to assess the chip resistance of complete coating systems, typically consisting of electrocoat (e-coat), primer or stone-chip primer, basecoat, and clearcoat. It evaluates both the cohesive strength of individual coating layers and the adhesive strength of the interfaces between layers and the substrate. Testing is frequently performed at low temperature, because coatings become more brittle in cold climates and stone chipping most commonly causes severe damage during winter driving. Accredited third-party laboratories routinely perform this test for paint suppliers, tier-one component manufacturers, and vehicle OEMs.
Testing Samples
Standard Flat Test Panels
Flat metal panels are the most common test specimens. They are usually prepared from cold-rolled steel, galvanized steel, or aluminum sheet, coated with the full production coating system applied under specified film thicknesses and curing conditions. Typical panel dimensions are in the range of 100 mm × 200 mm, although exact sizes depend on the selected standard and customer specification. A minimum of three replicate panels per condition is generally recommended to ensure statistically meaningful results.
Plastic Substrates and Components
Painted plastic substrates such as polypropylene (PP), ABS, and polycarbonate blends are also tested, particularly for bumper fascias, side claddings, and exterior trim parts. Because plastic parts behave differently from metal under impact, substrate type and pretreatment must be clearly documented in the test report.
Actual Vehicle and Machine Parts
Real components such as hoods, fenders, door sills, wheel arch liners, and chassis parts can be tested when the equipment accommodates their geometry. Testing actual parts provides the most realistic assessment but requires special fixtures to position the surface at the specified impact angle and distance.
Testing Items
The multi-impact stone chip test can be used to determine the following characteristics:
- Overall chipping resistance rating of the complete coating system
- Adhesion loss and delamination between coating layers (for example, e-coat to substrate, primer to e-coat, or basecoat to primer)
- Cohesive failure within individual coating layers
- Degree of substrate exposure and the size distribution of chips
- Effect of test temperature on coating brittleness (typically comparing room temperature and low-temperature conditions)
- Corrosion propagation from chip points, when combined with subsequent neutral salt spray exposure
Testing Methods
Applicable Standards
The most frequently applied standards include:
- ISO 20567-1 – Paints and varnishes – Determination of stone-chip resistance – Part 1: Multi-impact testing
- ASTM D3170 – Standard Test Method for Chipping Resistance of Coatings
- VDA 621-427 – German automotive industry method for multi-impact gravel testing
- GB/T 31588.1 – Chinese national standard adopting the ISO 20567-1 multi-impact method
Related single-impact procedures, such as ISO 20567-2 and SAE J400, may be specified by some OEMs and are often performed alongside the multi-impact test for a complete chip resistance profile.
Test Apparatus
The test is performed with a gravel impact (stone chipping) tester, which consists of a compressed air supply with pressure regulation, a blast gun or nozzle, a gravel feeding system or hopper, a specimen holder with adjustable angle, and a protective blast chamber. Many systems are equipped with a temperature conditioning device so that specimens can be tested immediately after being cooled to the specified low temperature. Standard road gravel of controlled size grading is used as the impact medium.
Typical Test Procedure
While exact parameters depend on the selected standard and customer specification, a typical multi-impact test proceeds as follows:
- Prepare and mark coated specimens, and record film thicknesses of each layer.
- Condition the specimens to the specified test temperature, commonly −20 °C for low-temperature evaluation, until the entire panel reaches thermal equilibrium.
- Mount the specimen in the holder at the specified angle and distance from the nozzle.
- Charge the blast system with the specified mass of gravel, commonly about 500 g for ISO 20567-1 conditions and about 300 g for ASTM D3170 conditions.
- Discharge the gravel at the specified air pressure, typically around 200 kPa (2 bar) for ISO-type tests and approximately 480 kPa (70 psi) for ASTM D3170, over a short blast duration of roughly ten seconds.
- Remove loosely adhering debris, then apply and strip a pressure-sensitive adhesive tape across the impacted area to detach any coating that has lost adhesion.
Evaluation of Results
The damaged area is assessed by counting chips within defined size classes, by determining the percentage of affected area, or by comparison with standard reference photographs, and the result is commonly expressed as a numerical chipping resistance rating. Equally important is the identification of the failure interface: failure within a single layer indicates cohesive weakness, whereas separation between layers or exposure of bare metal indicates adhesive failure that directly compromises corrosion protection. When required, chipped specimens can be subjected to neutral salt spray exposure in accordance with ISO 9227 or ASTM B117, after which rust creep from the chip points is measured to evaluate long-term corrosion performance.
Applications
The multi-impact stone chip test is applied across many industries and product development stages:
- Automotive OEM paint system approval and material specification compliance
- Quality control and incoming inspection of coating materials at vehicle plants
- Development and benchmarking of stone-chip primers, e-coats, basecoats, and clearcoats
- Qualification of painted plastic exterior parts by tier-one suppliers
- Commercial vehicles, buses, and rail rolling stock exposed to ballast and road debris
- Agricultural and construction machinery operating in abrasive environments
- Aftermarket refinish coating validation and warranty failure analysis
FAQ
What is the difference between multi-impact and single-impact stone chip testing?
Multi-impact testing blasts the specimen with a large number of small gravel particles to simulate cumulative road chipping, whereas single-impact methods such as ISO 20567-2 and SAE J400 launch a single defined impact body at the surface. Multi-impact testing reproduces the statistical, repetitive nature of real stone chip damage and is the more common specification for automotive coating approval.
Why is the test often performed at low temperature?
Coating materials become harder and more brittle at low temperature, so chips more easily penetrate to the substrate. Because severe stone chipping occurs mainly during cold-season driving, testing at around −20 °C gives a more conservative and realistic assessment of field performance than testing at room temperature alone.
How are test results rated?
Results are typically expressed as a chipping resistance rating derived from the number and size of chips or the percentage of damaged area, often on a numerical scale referenced to standard photographs. The report should also state the failure location, such as cohesive failure within the primer or adhesive failure exposing the e-coat or substrate, since this determines the corrosion risk.
What samples and how many are required?
Flat coated panels of approximately 100 mm × 200 mm with the complete production coating system are standard. At least three replicate specimens per test condition are recommended, and panels must be coated, cured, and aged according to the specified procedure before testing.
Can the test be combined with corrosion testing?
Yes. After the gravel impact and tape removal, specimens are frequently placed in a neutral salt spray chamber according to ISO 9227 or ASTM B117. Measuring rust creep from the chip points after a defined exposure period shows how well the coating system protects the steel once chipping has occurred.
How should a testing laboratory be selected for this test?
Buyers should choose accredited third-party laboratories that operate gravel impact equipment compliant with ISO 20567-1, ASTM D3170, or the relevant OEM specification, maintain controlled gravel grading and calibrated air pressure, and can document test temperature, impact angle, and full evaluation results in the test report.