3 Experimental Observations on the Fracture of Metals 35 Fig. 3.18 Iso-derivatives plots from snapshots of the specimen of Fig. 3.17: (a), (b) t =20.09 s; (c), (d), (e) t =15.5 s Fig. 3.19 Iso-derivatives plots from snapshots of the specimen of Fig. 3.17. t =5.02 s, (a), (b), (c) t =15.5 s 3.10 Changes in Depth Produced by the Onset of the Plastic Instability In Fig. 3.12, it is shown that the narrowing of the cross section of a rectangular specimen in tension takes place also in the thickness of the specimen. Data on changes of thickness in rectangular cross-sectional specimens utilizing the moiré method and the fringe projection technique with a pitch of p=1 mm are given in [23]. The tensile specimen parameters are wo =20mm, to =0.6mm, and Lg =20 mm, the speed of pulling is vpm =8.33 μ/s. The material of the specimen is brass that has also a f.c.c. structure. This test differs from preceding examples in the sense that it is a static test; recordings are taken at increasing loadings of the tested specimen. While in the previously analyzed examples, the sensitivity vector corresponded to in-plane components of the displacement vector, in this example the sensitivity vector is in the direction perpendicular to the plane of the specimen. This means, using the literature conventional nomenclature for displacements perpendicular to a plane of reference, the displacements correspond to the W(x,P) family. The values of w(x,P) given are referred to an initial surface corresponding to the end of the elasticity range, in the beginning of the square region called in Fig. 3.20 transition region. The displayed displacements in Fig. 3.21 are total displacements with respect to the mentioned reference plane and not displacement derivatives as in the previously analyzed examples.
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