The Splitting of Wood-Base Building Boards Due to Edge Screwing

A method for evaluating the edge splitting tendency of building boards based on the method for testing internal bond, i.e. Z-strength, is described. The edge splitting tendency here is defined as the relative decrease in internal bond when a screw is threaded into the cut edge of a sample after a given predrilling, thereby initiating splitting or delamination in the sample. The reproducibility of the method for various boards is given. The screw recommended for the test is a wood screw with an outer diameter of 3.45 mm, but the effect of some other types of screws, as well as the screw hold in the edge and face, is reported. The edge splitting tendency increases with density for fiber building boards. Heat treatment increases their edge splitting tendency. At high relative humidities, i.e. high board moisture contents, the edge splitting tendency decreases. If the screw, is threaded into the sample at a high relative humidity and the internal bond is evaluated after reconditioning to a low relative humidity, the positive effect of the higher moisture content during threading is maintained. This method might also be suitable for the evaluation of other forms of damage in which crack propagation initiates edge splitting and delamination.

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