The relationship between microscopic compression failure lines and crack-nucleation is reported. Static and reversed cyclic bending load was applied to the specimens made from wood of Agathis species (Agathis sp.). The maximum subjected load was +/-450 approximately 650 kgf/cm2. The observation of the microscopic compression failure line and cracks nucleated at the root of the specimen was made every half cycle of load.
The microscopic compression creases nucleate at the tip of the notch on the one side face of the specimen in the compression cycle. The length of the microscopic compression creases shortened in the tension cycle. This is because the microscopic compression creases are stretched parallel to the cell axis. In the next compression cycle, the microscopic compression creases appear again. After repeating these processes several times and when the shape of the microscopic compression creases has not yet recovered to its original shape a large part of the microscopic compression creases change into cracks. In other word, the cracks nucleate. Mostly, the tip of the microscopic compression creases preceeds the tip of the cracks. The microscopic compression creases and the cracks propagate by steps, and after repeating these processes several times failure of the specimen occurs. The cracks nucleate in the compression cycle and sometimes in the tension cycle too. The cracks follow microscopic compression creases. The microscopic compression creases and the cracks run at nearly right angles to the cell axis.