1. The results of stereofractographic measurements of the height of the straight zone are in satisfactory agreement with the results of determination of the cracking resistance by the standard methods: there is a linear relationship between both groups of data for the examined loading rates and temperatures up to the temperature of start of subcritical growth rate. This confirms that fracture toughness can be evaluated independently by stereofractographic measurement of the height of the stretch zone. 2. In all the test conditions examined, the direction of the initial stage of crack propagation greatly differs from the main direction of crack growth.