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Stone impact test

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STONE IMPACT TEST

Gravel, grit, and stones are frequently found lying on roads and streets. If these are thrown into the air at high speed, they can strike vehicles and damage their paintwork. If holes or cracks are formed in the paint or if it flakes off, the paint is not able to protect the vehicle body or chassis as well against corrosion.

Automobile manufacturers therefore attempt to apply coatings on the exterior of the body that are resistant to such stone impacts. The extent to which a coating is resistant to numerous small particles can be simulated in stone impact tests. In this test the extent of the damage is dependent not only on the quality of the coating, but also on the strength of the compressed air, the mass of the particles, and the duration of firing.

A standardized test procedure is required in order to ensure the results of such tests are comparable. The International Organization for Standardization (ISO) has specified a test procedure in its ISO 20567 standard. This standard consists of two parts. The multiple impact test is described in the first part (ISO 20567-1). In this test, small, sharp-​edged objects are fired in quick succession at a coated steel test panel using compressed air.

Several details of the multiple impact test of ISO standard 20567-1:

The test panels are 200 mm long, 100 mm wide and 0.07 to 1.0 mm thick. The coating or multi-​coat paint system that is to be tested is applied to the steel test panel.

The projectile material is a cast granulate with a grain size of 4 to 5 mm.

500 g of hard granulate is projected with each shot.

A coated panel is impacted with a load of granulate for a period of ten seconds using compressed air. The angle of impact is 54° and the pressure is 100 or 200 kPa. The stone impact test may be repeated one additional time.

Prior to evaluation, a 25mm wide and 120 mm to 150 mm long section of adhesive tape is applied to the test panel and abruptly removed again. This removes any loose material hanging from the panel. The adhesive tape has an adhesive force of 6 to 10 N/25 mm.

Experts assess the degree of damage visually. This is achieved by comparing the test results with images from standard ISO 20567-​1 and assigning them a rating on a scale from 0.5 to 5. If less than 0.2 % of the surface is damaged, a rating of 0.5 is awarded. This corresponds to very good resistance to repeated stone impact. If more than 81.3 % of the surface is damaged, this corresponds to a rating of 5 which is very poor resistance to impact. This assessment can also be undertaken using digital image processing.

In practice, ratings of less than or equal to 2 are accepted.

Two notes:

Car manufacturers such as Daimler, Ford, Renault, and the PSA Group have developed their own stone impact tests as has the German Association of the Automotive Industry (VDA). These are very similar to the multiple impact tests described within ISO 20567-1.

In the ISO 20567-2 standard, a procedure is described with the aim of testing how a coating resists the one-​off impact of a wedge-​shaped impactor.

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