34 C. A. Sciammarella et al. 1500 1800 1200 900 300 0 600 0 10 20 30 40 50 60 Force (N) Δt = 35.18s dtot (%) Δt = 20.09s Δt = 15.75s Δt = 5.02s Fracture Fig. 3.17 Snapshots of the iso-derivatives εv of specimen No. 1 [10] at different points near the maximum load of the specimen. The strain axis corresponds to the logarithmic definition of strain [11], a lot of additional information is provided for tests of these types of specimen that is very useful for completion of the subject matter under analysis. The parameters characterizing this specimen areLo =22mm, wo =5mm, andto =0.5 mm. The material of this specimen is the same material of specimen No. 1. Different speeds of the testing machine were used for different tests of this geometry; the optical sensitivity is Sy =0.447 μm. A very interesting feature of the setup is the addition of acoustic emission (AE) equipment to capture the signals produced by the motion of dislocations and coordination with the optical signals. As it has been done with the other specimen, the iso-derivatives for the different time intervals t have been computed and are displayed in Fig. 3.18. For this specimen, it occurs Ln =1.93, that is, close to 2 as it occurs for specimen No.1. For t =35.18 s, the iso-derivative shows a uniform rate of change of 20 ×10−4, indicating that as it was determined for specimen No. 2, the wave front has passed through, and the iso-derivative pattern corresponds to the displacements “V” of Fig. 3.1. For t =20.09 (Fig. 3.18), the wave front stopped and the graphs represent the regions where the rate change of the iso-derivatives is concentrated around the middle of the neck formation. Graphs (a) and (b) give the upper edge and the middle section of the neck; while at the lower edge, the values of the iso-derivatives are smaller than those of the upper edge, the deformation at the upper edge is larger than at the lower edge. For t =5.02 s (Fig. 3.19), the maximum rate of change occurs in at the upper edge, and the slopes of the iso-derivatives of Fig. 3.17 are 27◦ and17◦, and the change of angle takes place at the middle of the section. The fracture crack reflects this change of direction. The type of fracture in the 2D plane stress condition indicates a mixed mode-1 and mode-2 fracture. This type of fracture indicates the presence of bending in the plane of the specimen increasing the strain in one edge and upper edge and reducing the stress in the lower edge.
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