3 Experimental Observations on the Fracture of Metals 41 range of tensile specimens as the specimen is stretched in continuous stretching or periodic stretching in the case of fatigue. A very interesting case of wide band formation is the bands associated with the Portevin-Le Chatelier effect that manifests in the form of serrations in the stress-strain plot that appear at a critical strain and is both temperature and strain rate dependent. These serrations have variable amplitude and frequencies. To clarify ideas, it is interesting to analyze the case of the soliton wave illustrated in Fig. 3.11. As shown in this paper, and is supported in the literature, a soliton wave can only take place if the metal tested has a very homogeneous structure that satisfies the condition dσ dε = 0. In Fig. 3.11, an experimental observation of this propagation is provided within the approximation of low spatial and time resolution. Because of the required conditions in the structure of metal, the experimental observation of a soliton wave is not a very common event. However, it was the starting point for the study of the propagation of plastic waves by Hähner [28], later followed by Yoshida [6]. The analysis of Fig. 3.23 provides the most probable type of propagation that can be observed influenced by local structure changes and different plastic instabilities possible in the wave propagation. In [11], a statistical approach based in the correlation of optical signals and AE signals is utilized to get an average value of the plastic wave propagation velocity. The passage of the wave creates different isoderivatives at particular positions. Correlating these patterns with AE signals provides average speed of propagation that is utilized to relate the velocity of propagation of the signal optically detected with the speed of motion applied to the machine. 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