The standard "Volvo STD 1024,7132 Test method - Resistance to stone impact," developed by the Volvo Car Company, describes a laboratory procedure for testing and evaluating the resistance of surface coating systems (including the substrate) to fragmentation by gravel impact (shock waves from large stones). Abrasive cutting effects, such as blasting with sharp projectiles, stones, or steel grit, are not covered by this standard. Alternative testing methods must be used.
This test method is designed to reproduce the impact of gravel impacting the exposed machined surfaces of a vehicle. The specific purpose of the test is to evaluate surface finishes on flat test panels. If the results are documented and interpreted with due consideration of deviations from the original test procedure, it is possible to extend such tests to finished parts.
This standard does not apply to Volvo Cars. The "Volvo VCS 1024,71329 Stone chip resistance" standard applies to Volvo Cars. This standard is compatible to some extent with the "SAE J400 Chip resistance test of surface coatings" standard.
This test involves launching controlled airflow through a controlled air jet of gravel material onto a test panel. The gravelometer is designed to contain gravel, a test panel holder, and a gravel ejection mechanism.
The ejection mechanism, located in the center front of the test panel, consists of an air nozzle at the base of an inverted T-tube. The T-tube's handle points upward and is connected to a rail or inclined plane (depending on the gravelometer model) into which gravel is poured.
Gravel falling into the air blast is thrown towards the test panel, which is usually placed perpendicular to the direction of the impacting gravel.
All tests should be performed on panels under controlled temperature conditions, typically at room temperature (23 degrees plus/minus 5 degrees) or cold temperature (minus 20 degrees plus/minus 3 degrees).
After a gravel impact, a piece of tape should be applied to the impacted surface, and then the tape should be peeled back to remove any remaining flakes of paint. Stone chip damage is determined by measuring the number and size of all stone chips. Stone chip damage is defined as a defect that penetrates at least one of the surface treatment layers, such as a clear coat.
Various types of equipment are available, but the gravelometer used in the tests must meet the design specifications of the SAE J400 standard. Two examples of gravelometers designed to this design specification are shown in the figures provided in the standard.
Gravel should be water-eroded silt gravel, not crushed limestone or rock. Before use, new gravel should be sifted through a wire mesh screen. Gravel should pass through a 15-16 mm sieve but should be retained on a 9-10 mm sieve. Gravel should also be washed before its first use.
The gravelometer also has a sieve at its base to separate broken rock fragments and dust smaller than 9-10 mm. The gravel retained in this sieve can be reused. However, even this reusable gravel tends to dull or crumble after repeated impacts, so periodic gravel replacement is necessary after reasonable service. If used daily, the gravel should be replaced annually.
Masking tape (natural-colored crepe paper) should be used in a 100 mm width. Other tapes with equivalent adhesion properties can also be used. The tape should preferably be slightly transparent to facilitate assessment of good bubble-free contact with the substrate.
If the tape to be used has exceeded the stated shelf life (typically 12 months), the peel force must be verified and confirmed.
When tested against stainless steel in accordance with ASTM D3330/D3330M standard method A, the mean value of the peel force shall not be less than 16 N/24 mm, with the following exceptions and remarks:
For comparative testing of different surface treatment systems, it is important that the tape adhesion is uniform on each surface. If the tape adhesion deviates from the norm, the peel force should be checked.
Gravelometer tests are generally performed at room temperature (23°C plus/minus 5°C) or, generally, at cold conditions (-20°C plus/minus 3°C) (unless otherwise agreed by the contracting parties). If tests are to be performed under cold conditions, they should be performed in a cold room where both the gravelometer and test panels can be conditioned to the specified temperature. A note indicating that tests were performed at -20°C means that the surface temperature of the test panel maintained this temperature throughout the entire test cycle. This cannot be achieved by simply cooling the test panel before mounting it on the gravelometer for testing.
When assessing stone chip damage, a magnifying glass (approximately 7x) and a graduated scale should be used. The scale resolution should be 0,1 mm. In cases of poor contrast, oblique light is recommended to detect any damage. For darker colors, a white scale is recommended to achieve good contrast.
Unless otherwise agreed, test panels must be flat and fit into the panel holder of the gravel meter. The test panel material, sheet thickness, and pretreatment (e.g., phosphating or anodizing) must be the same for all tests in a series and must be as representative of the actual part as possible. This includes the thickness of the coating layers. Any deviation from this could lead to misleading test results. The determination of these parameters is to be agreed upon between the contracting parties.
If there is agreement to use test panels/test pieces cut from finished parts, finding the right size and flatness of surface may be difficult. In this case, agreement on how the test will be performed must also be reached.
Different performance examples:
Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the "Volvo STD 1024, 7132 Test method - Stone impact resistance" standard.
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