125 W

Figure 7.1. Standard specimens a. Bend specimen; b. Compact tension specimen

The specimens have to be provided with a fatigue crack. In order to ensure that cracking occurs at the right place, the specimens contain a starter notch. In thick members fatigue cracks usually start at a corner (figure 7.2a). Such cracking behaviour results in an irreproducible, curved crack-front, not suitable for a standard test. It can be avoided by providing the specimens with a chevron notch (figure 7.2b). This notch forces initiation of the crack in the centre, which enhances the probability of a relatively straight crack front, and it has the additional advantage that the fatigue crack starts almost immediately upon cycling.

After the test, the size and shape of the fatigue crack can be determined easily. The fatigue area and the final fracture area have a different topography and therefore a different reflectivity to incident light, which w

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