USE OF DIFFERENT METHODS OF FATIGUE CRACK CLOSURE MEASUREMENT AND THEIR COMPARISON

The paper contains results of a set of experimental works performed on two types of steels with different mechanical properties, particularly strength and yield stress, with the aim to contribute to the explanation of doubts about possibilities of measurement of fatigue crack closure conditions. Measurement on CT specimens was performed using: (i) videoextensometer on edges of crack mouth - initiation notch, (ii) mechanical semiconductor extensometer near the crack mouth, (iii) mechanical extensometer on opposite side to crack mouth, i.e. in the compression area, (iv) strain gauges at the immediate vicinity of crack tip and (v) strain gauges placed on the side area near opposite edge. Some differences of measurement results are compared and discussed from the viewpoint of pin hole clearance and other aspects like near crack tip plastic zone. It was shown that insufficient clearance fit strongly affects results particularly in the case of higher strength steel.