Fig.  7: Load/displacement curve for a specimen without patch
The course of the load/displacement curve is described by drops in the force signal that increase over time. The non-smooth load/displacement behaviour in the plastic region leads to the conclusion that this phenomenon can be related to the so-called “Portevin-Le Chatelier” (PLC) effect. The temperature “flashes” are slip bands running through the material. For more information on the PLC effect see for instance [16]. Number and size of the drops in the force signal correspond to the frequency and intensity of the detected slip bands running through the specimen. Monitoring the region around the hole, where crack onset is expected, the increase in number and intensity can be seen. Slip bands running through the hole pile up and local temperature differences increase over testing time, as shown in Fig. 8.