38 C. A. Sciammarella et al. Fig. 3.23 (a) Loading sequence of a specimen correlated with the force-log strain diagram (b) Fig. 3.24 (a) Optical signal; (b) image of the iso-derivatives captured by a CCD camera; (c) low-resolution picture corresponding to the isoderivatives 3.12 Interpretation of the Optical Signal and Consequences of this Analysis Having reviewed literature experimental evidence on the propagation of the plastic instability, we will proceed to extract the basic information from the experimental data, first proceeding to analyze the optical signals generated by a wave front and second determining implications of these signals in the analysis of the propagation of the plastic instability taking into consideration the method used to measure velocities. First, let us consider the shape signal that is propagating along the specimen. Figure 3.24a represents a wave shape of the signal introduced by the moving end of a testing machine at the onset of plasticity. The signal is a frequency and amplitude modulated signal, the background intensity of the signal is I0 and the maximum amplitude is Imax. The pattern has amplitude and frequency modulated fringes that merge to the background intensity that is not constant but may have the components of very low frequency and amplitude static fringes. The actual shape of the signal depends on the interaction of the specimen and the machine compliance [24]. Figure 3.24b shows the iso-derivatives of a propagating wave front recorded by a CCD camera [25]. The material of the specimen is an aluminum alloy AA5083-0; the dimension of the specimen are Lg =25mm, wo =10mm, to =3mm, 0.5 ×10−2 mm/s. From Fig. 3.24b, the value for the right side of the specimen is B=4mmand for h=10 mm and applying Fig. 3.24c, B =h−wo ×tgα (3.25)
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