The purpose of Stone Impact Test
“In the automotive Industry, multi-layer paint coatings are applied to car bodies for protection. Grit, road-metal and other materials can damage these coatings in such a way that individual layers come off or the whole coating delaminates from the substrate”
According to ASTM:
“The test method (Chipping resistance of Coatings) covers the determination of a resistance of coatings to chipping damage by stones
or other flying objects…”
In other words…
All car users do expect a certain resistance under normal usage against chipping of the coating or material used for car manufacturing.
These quality expectations for car users need to be tested, verified and monitored:
– During the material or coating design
– During production of equipment at defined intervals
– To compare different materials or coating formulations
– To evaluate new materials
– With a procedure which exclude user dependency (“human error”) as far as possible
– With a procedure which is quick, safe for the operator, reliable and repeatable
– With a procedure which generates numerical results
The difference between “VDA” and “SAE” standard
The majority of different test procedures of Stone Impact Tests can be sorted into two main groups:
• Testing according “SAE”• Testing according “VDA” (Association of German Car Builder)
– The base of the SAE test method has been described in the standard “Test for Chip Resistance on Surface Coating” SAE J-400.
ASTM D3170/D3170M -14 is describing a very similar method.
– The details for the VDA test method are described in the ISO 20567-1
– Several global OEMs in the car industry develop individual requirements for Stone Impact Testing which are based on either of the above procedures and standards.
The main difference between the two testing procedures are:
– Type, size and shape of material to be shot onto the coating
– Angle of impact
– Air pressure to be used
– Distance of coated surface to end of acceleration tube
– Acceleration tube diameter
Key parameter of testing according VDA / ISO 20567-1
– Defined shot material : “Chilled-iron grit” according ISO 11124-2 but with a size between 3.5 – 5 mm (GH Diamant 06302)
– Amount of shot material: 500 g
– Angle of impact from 54°
– Air pressure 2 bar
– Test time: 10 s
– Distance between specimen and end of acceleration tube: 290 mm
– Acceleration tube diameter: 30 mm
Key parameter of testing according SAE SAE-J 400 / ASTM D 3170
– Defined shot material : “Water Eroded Stones“ = Gravel Shot acc. to SAE J400/ASTM D 3170
– Amount of shot material: 550 ml (1pt) / Approx 250 to 3000 stones
– Adjustable angle of impact from 15° up to 90°, in 15° steps
– Air pressure 4,8 bar
– Test time: 7-10 s
– Distance between specimen and end of acceleration tube: 349 mm
– Acceleration tube diameter: 53 mm
The application of stone impact test in automobile coating industry
• Most car manufacturers have focused their requirements for the stone hammer blow testing on one of the established methods (either SAE or VDA).
• Some car manufactures use follow the stone hammer blow testing according the established standards with no modifications.
• Some care manufacturers have developed individual variants of the SAE or VDA methods like:
– PSA: D24 1312– Daimler: DBL 5416– Ford: FLTM BI 157-06– Renault: D24 1702