30 C. A. Sciammarella et al. Fig. 3.11 Sequence of events leading to the ductile fracture of a metallic specimen event is schematically represented by a discontinuity in the surface of the specimen (Fig. 3.11). The sequence illustrated in Fig. 3.11 is important to help in understanding the relationship between related events. For the example of Fig. 3.11, the transition from yielding to fracture can be separated into two processes: the propagation of the structural instability from the place of initiation of a plastic wave to the final location in the specimen where the actual onset of fracture takes place. The processes taking place between the initiation of the plastic wave propagation and the beginning of fracture are extremely complex, and there is a large inventory of possible outcomes. Before one can proceed in the analysis of the previously described processes leading to the fracture of a specimen, it is necessary to understand the link of captured images and physical events that are connected with these images. 3.8 Analysis of the Process of Propagation of the Plastic Instability Figure 3.12 illustrates the effect of the onset of the structural stability in a specimen of rectangular cross section. Bothwo and to are reduced and the sides of the rectangular section become curved arcs. The changes in dimensions of wo and to depend on the magnitude of the axial force P. The magnitude of these changes depend on many different variables, but this is the general trend of the signal that propagates as a wide band along the specimen as it is shown in Fig. 3.11. The recorded shape of the band depends on the space and time resolution of the optical system utilized to track the propagation of the plastic instability. To relate the dimensions of the wide band to actual measurements, data taken from [10] are plotted in Fig. 3.13. These data correspond to the maximum load of the specimen of Fig. 3.14 when the plastic wave propagation stops, and the fracture process starts. Data shown in Fig. 3.13 correspond to the specimen called No. 2 in Ref. [10], Fig. 3.14, with the following dimensions, thickness to =1mm, width wo =3.6 mm, and gage length Lg =19 mm. The specimen is made of faced centered cubic (f.c.c.) austenitic 316L stainless steel. The analyzed image is a snapshot with exposure time te =20.36 s, and the velocity of loading of the testing machine vpm =0.1 μm/s. The data were obtained analyzing the iso-derivative εv (x, P) pattern of Fig. 3.14 that corresponds to the point of maximum load in the strain vs. load plot. Figure 3.13a shows the changes of wo measured at t =20.36 s. Figure 3.13b shows the plot of the ratio rw =w/wo. The change in wo at the center of the specimen is wo = 0.29 3.6 = 0.08 or 8%. The spatial and temporal resolution satisfy the conditions required to capture the iso-derivatives that cannot be recorded in lower resolution images. Following the
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