Resistance to impact by small-diameter ball

 

 

Principle
A specimen from the laminate under test is bonded to wood chipboard to simulate service conditions and its decorative surface is subjected to the impact of a 5 mm steel ball mounted at one end of a spring-loaded bolt. The maximum spring force for which no visible damage occurs is used as a measure of resistance to impact.

Materials
1. Fine-faced wood particleboard, complying with EN 312 (for interior fitments), (230 ± 5) mm square, with a nominal thickness of 18 to 20 mm (± 0,3 mm), a density of (680 ± 20) kg/m3, and moisture content (10 ± 3) %.
Where the specimen is bonded to chipboard, the test actually measures the impact resistance of the whole composite material, i.e. laminate, adhesive and substrate. (The correct choice of chipboard quality is very important in achieving good reproducible with this test).
2. Urea-formaldehyde adhesive, containing approximately 15 % filler, or an equivalent adhesive.
3. Contrast medium, e.g. graphite, talcum, or solution of dye in alcohol, to contrast with the color of the sheet under test.

Apparatus
1. Impact tester, consisting of an impact bolt with a 5 mm steel ball mounted at one end, which is projected once against the surface under test by the release of a compression spring. The spring compression force before release can be adjusted continuously from 0 to 90 N by means of a force setting barrel.
The Newton metre (Nm) scale also provided on the tester is only to be used for orientation, as the introduction of a non-linear scale involves relatively great inaccuracies.
The compression spring is 100 mm long when released and has a constant of (1962 ± 50) N/m. It is compressed by drawing back the impact bolt and is held in the loaded position by a retainer which engages in the bolt. It is released to deliver the impact blow by a release unit which withdraws the retainer.
2. Force-producing arrangement, (for example a scale-pan and weights), capable of being suspended from the impact bolt to exert a comprehensive force on the spring.
3. Support fixture, which clamps to the shaft of the impact tester and provides a convenient mounting of sufficient mass for the tester to be held at right angles to the surface of the specimen and to avoid recoil following the release of the impact bolt.
4. Steel plate, having dimensions of approximately 300 x 300 x 50 mm.
5. Conditioning chamber, in accordance with EN ISO 291, with a standard atmosphere of (23 ± 2) °C and relative humidity (50 ± 5) %.

Test specimens
Specimens shall be prepared by uniformly bonding a piece of the sheet under test to the wood chipboard, using the specified adhesive evenly spread at 80 to 120 g/m2.
Sufficient specimens, each (230 ± 5) mm square, shall be prepared to obtain a final result (about three is usually sufficient). The bonded specimens shall be preconditioned for at least 72 h at (23 ± 2) °C and (50 ± 5) % relative humidity before being used for the test.

Procedure
The test shall be carried out in the laboratory atmosphere.
Place the steel plate (see 20.3.4) on a convenient rigid horizontal surface and locate the specimen on it with its decorative surface uppermost. Fix the impact tester in its support fixture (see 20.3.3), load the tester, place the assembly on the specimen and release the impact bolt. Start preliminary test with a spring force of 10 N and increase by 5 N on each occasion to determine the minimum spring force at which the surface of the specimen shows damage due to impact stress.
Test further specimens for the final determination of the maximum force at which no damage occurs.
For this purpose, start with the spring force determined in the preliminary test and reduce it in suitable stages, for example 1 N, after every five strikes.
To make any damage more easily visible, the surface of the specimen shall be rubbed with a contrast medium after the test.
The distance between points of impact shall be at least 20 mm and between points of impact and the edge of the specimen at least 30 mm.
Examine the surface tested for damage at the points of impact. For the purpose of this test, damage is defined by the presence of fine hairline cracks (which are frequently concentric), continuous cracks or flaking of the decorative surface. Indentations without cracks do not count as damage.
If the test is conducted only to determine whether the impact strength of a material exceeds a limiting value, the specimen shall sustain no damage after five successive individual impact strikes with the prescribed spring force.

Expression of results
The impact resistance of the laminate under test is the maximum value of the spring force, in Newtons, for which no damage occurs in a series of five strikes.
To prove compliance with a specified limit value it is only necessary to carry out the test at the specified force.