3 Experimental Observations on the Fracture of Metals 27 3.5 Signals Recorded by the Camera At this point it is necessary to point out that the signals recorded by the camera in the optical system are not only space dependent as it occurs in static cases but are time dependent. In the process of loading a tensile specimen, one has to compare the velocity of the applied load or displacement and the process of wave propagation in the specimen. The speed of wave propagation in the specimen in the longitudinal direction depends on the particular region of the load vs. displacement plot (Fig. 3.4). In Fig. 3.8, the iso-derivative fringes of the tensile specimen studied in [4] are displayed. As the frames number change, the loci of the iso-derivatives change. The iso-derivative fringes of εv (x, P) have the same trend as the displacements V(x,P) due to the axial displacement of the specimen, and the iso-derivatives fringes of εu (x, P) are the result of the Poisson’s effect and have the same trend as U(x,P). It is possible to see, label (a) of lower row, that due to the bending effect, the iso-derivative fringes of the U(x,P) are not vertical as they ought to be in a tensile specimen but are slightly curved loci due to the bending. In Fig. 3.9, the plot load P vs. frame number of the recorded images of the iso-derivative fringes is shown, through the added labels one can see the images in Fig. 3.8 corresponding to the selected frames. Since the frame rate is given, the plot of Fig. 3.8 is also a plot of P(x,t). Fig. 3.8 Display of the finite differences iso-derivatives εv (x, P) (upper row) and εu (x, P) (lower row). Labels indicate the displayed frames corresponding to Fig. 3.9 Fig. 3.9 Plot of the relationship between acquired frames and applied load as indicated by the testing machine. Labels a, b, c . . . correlate this curve with the iso-derivatives fringe families of Fig. 3.8
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