Resistance to cracking

 

 

Principle
A specimen, with a drilled hole, taken from the laminate under test is rigidly clamped in a steel fixture. After imposing additional stress by heating at 50 °C for 6 h, the resistance of the specimen to cracking is assessed by visual examination.

Apparatus
1. Clamping device
2. Drilling jig, to facilitate drilling of accurate holes which are free from chipping or cracking.
3. Conditioning chamber, in accordance with EN ISO 291, with a standard atmosphere of (23 ± 2) °C and relative humidity (50 ± 5) %.
4. Electrically heated oven, provided with air circulation and capable of being maintained at (50 ± 2) °C.
5. Hand lens, with approximately x 6 magnification.
6. Lighting, of intensity 800 to 1000 lux.
7. Drilling machine, operating at less than 400 min-1.
8. Micrometer thickness gauge, as described in 5.2.1.

Test specimens
Four specimens shall be prepared, of which three shall be tested and the fourth used as a drill backing piece.
Each specimen shall be (150 ± 1) mm long, (50 ± 0,5) mm wide, and of the thickness of the sheet under test. The length of the specimen shall correspond to the cross direction of the sheet.
The specimens shall have a (10 ± 0,5) mm diameter hole drilled in their centres using a drilling jig, the four specimens being clamped together with the decorative surfaces face to face.
During the drilling operation, care shall be taken to avoid chipping, cracking or burning around the edge of the holes.
The drill shall be sharp, and the speed of the drilling machine shall not exceed 400 r/min. After the drilling has been carried out, the specimen used as a backing piece (i.e. the bottom specimen) shall be discarded.
Any specimen showing cracking, chipping or burning around the edge of the hole shall be discarded, and a replacement prepared. Replacement specimens will also be needed if any specimen movement occurs during the test.

Procedure
Measure the thickness of the laminate under test in accordance with Clause 5.
Pre-condition the specimens for 72 h in a standard atmosphere of (23 ± 2) °C and (50 ± 5) % relative humidity.
Pre-heat the clamping device (see 23.2.1) in the oven (see 23.2.4) for 2 h at (50 ± 2)  °C.
Take the specimen under test from the conditioning chamber (see 23.2.3), place it immediately in the pre-heated clamping device, and tighten the nuts firmly to prevent movement of the specimen. Make reference marks on the surface of the specimen adjacent to each clamping block to confirm absence of movement.
Place the clamping device containing the specimen in the oven at (50 ± 2) °C.
After (6 ± 0,25) h, remove the device from the oven, check the reference marks to ensure that the specimen has not moved, and immediately examine the specimen (while still hot and clamped in the device) with the naked eye and under x6 magnification for signs of cracking around the hole. The light
intensity during the examination shall be 800 to 1000 lux.
If there is any evidence of movement of the specimen in the clamps during the 6 h test period, discard the specimen without examination and repeat the whole procedure using a new specimen.
Test two further specimens using the same procedure.

Expression of results
The resistance to cracking under stress is the lowest rating of the three specimens assessed in accordance with the following rating scale.

Rating 5: No evidence of cracking.
Rating 4: Hairline cracks only visible under x 6 magnification.
Rating 3: Cracks visible with normal vision (corrected if necessary) from the edge of the hole, but not extending to either edge of the specimen.
Rating 2: A crack visible with normal vision (corrected if necessary) from the edge of the hole, extending to one edge of the specimen such that the specimen is not broken into two pieces.
Rating 1: Specimen broken into two pieces.