Degrieck, Joris

Peirs, Jan

Verleysen, Patricia

English

Ghent University Academic Bibliography

Influence of the geometry of hat-shaped specimens on the formation of adiabatic shear bands Jan Peirs, Patricia Verleysen and Joris Degrieck Department of Mechanical Construction and Production, Ghent University (UGent),  Sint-Pietersnieuwstraat 41, Ghent, Belgium E-mail: jan.peirs@UGent.be, Fax: +32 9 264 35 87 ABSTRACT The deformation behaviour of ductile metals is determined by phenomena at different physical length scales. In most ductile metals subjected to high strain-rates, the development of adiabatic shear bands (ASB) is one element of that process. In ASBs, the highly localized shear deformation occurs in a narrow band which exhibits very large strains relative to the neighbouring material. Since the deformation time is very small, the process happens adiabatically. The resulting temperature increase by plastic work causes thermal softening and further localization of plastic deformation. ASBs are observed in many applications such as machine chips, forging, ballistic impact loading and fracture. In the past, a dozen of experimental techniques have been developed to characterize the process of ASB formation. One often used technique is based on the dynamic deformation of hat-shaped specimens in a split Hopkinson pressure bar (SHPB) setup [1]. Figure 1 shows a sketch of the specimen with an indication of characteristic lengths. Due to the specific geometry of this specimen, shear strains are concentrated in a narrow region. This technique is especially popular among metallurgists because even materials that do not localize spontaneously in shear can be forced up to shearing failure. On the other hand, the determination of material properties from these experiments is not straightforward because of the complex stress state in the shearing region.   Figure 1: Hat-shaped specimen More insight in the stress distribution in the specimen is needed in order to better understand the experimental data obtained by this technique. Furthermore, as the nucleation and propagation of ASBs depends on the stress condition [2], knowledge of the stress distribution is crucial. Several researchers used different specimen dimensions in their experiments. Moreover, the outcome of these experiments is affected not only by the testing material but also by the specimen geometry. In [1] the stress distribution of one particular specimen geometry was investigated using FE simulations. The goal of this contribution is to relate the specimen dimensions with the stress distribution in the specimen. This has previously been done for other specimen geometries: e.g. truncated-conic specimens [2], dogbone-shaped tensile specimens [3] and double shear specimens [4]. In addition, the existence of an optimal specimen geometry to achieve an as pure as possible shearing stress state, is studied. The main tool in this study is the FE method. A 3D axis-symmetric model has been defined, using ABAQUS/explicit. The load is applied by a uniform velocity of the boundaries of the specimen. Strain-rate dependency of the material behaviour is modelled by the Johnson-Cook phenomenological model. Heat generated due to the plastic work is included, while heat conduction is not implemented. To overcome difficulties from extensive element distortion, ALE adaptive meshing is used. Figure 2 shows the distribution of the shear stress in the shear region of a sample.  Figure 2: Simulation of the shear stress Attention is paid on the stress tri-axiality as well as on the ratio of the hydrostatic pressure (related to r2-r1, Figure 1) to the shear stress in the shear region.  ACKNOWLEDGEMENTS The authors would like to acknowledge The Interuniversity Attraction Poles Programme (IUAP) of the Federal Science Policy of Belgium and the partners of IUAP VI (www.m3phys.be)  REFERENCES [1] Bronkhorst C.A., Cerreta E.K., Xue Q., Maudlin P.J., Mason T.A. and Gray G.T., An   experimental and numerical study of the localization behaviour of tantalum and stainless steel.  Int. J. Of Plasticity Vol 22, pp. 1304-1335, 2006. [2] Li J.R., Yu J.L. and Wei Z.G., Influence of specimen geometry on adiabatic shear instability of tungsten heavy alloys. Int. J. Impact Engineering Vol. 28, pp. 303-314, 2003. [3] Verleysen P. and Degrieck J., Experimental and numerical study of the response of steel sheet Hopkinson specimens. J. Phys. IV France Vol. 134, pp. 541-546, 2006. [4] Klepaczko J.R., Stress concentrators and rate effects in formation of adiabatic shear bands. Final technical report, European research office of the US-army, N68171-95-C-9071, 1996  

Influence of the geometry of hat-shaped specimens on the formation of adiabatic shear bands

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Influence of the geometry of hat-shaped specimens on the formation of adiabatic shear bands

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