Papers by Keyword: Shear Localization

Abstract: Numerical simulations related to polar effects in an infinite extended granular layer under shearing movement and constant vertical pressure are presented. The mechanical behavior of cohesionless granular soil is described within the framework of micro-polar (Cosserat) continuum and using an elasto-plastic constitutive relation. The influence of Cosserat rotations and couple stresses are taken into account using the mean grain size as characteristic length. Finite element method in Updated Lagrangian (UL) frame is used to consider large deformations during calculations. The numerical results demonstrate that for large shearing movement, the shear deformations within the granular layer are localized into a narrow zone. The FE-calculations indicate that the polar effects manifest by the appearance of noticeable grain rotations, high void ratios, pronounced volume changes within the localized shear zone.

Abstract: Forged and extruded TiAl products suffer from structural and chemical inhomogeneities that reduce the reliability of components. In an attempt to improve the homogeneity of the material, the feasibility of cyclic axial deformation and cyclic torsional deformation superimposed with compression, where much higher strains can be imparted into the material than during forging and extrusion, were investigated. Accordingly, during torsion superimposed with compression pronounced shear localization and cracking occurs. These difficulties can largely be overcome by cyclic axial deformation.

Abstract: The formation of shear localized chips in orthogonal machining of Ti-6Al-4V was investigated. The chips in different cutting velocities were collected and the structure of the adiabatic shear localization was examined by optical microscope and SEM after polishing and eroding. The serrated coefficient, serrated frequency and fibred coefficient were proposed in this paper to characterize the degree of serration and plastic shear flow of the chip quantitatively. Experimental results show that the width of the shear band decreased and the fibred coefficient of the shear band increased as cutting speed increased. The serrated and frequency, the serrated coefficient increased with the increase of cutting speed in the experimental range of cutting speeds.

Abstract: This paper presents a theoretical study of failure modes in sand-like granular materials under general triaxial stress conditions based on a hypoplastic model and a bifurcation analysis. The well developed constitutive model contains the void ratio as a state variable which allows the pressure and density dependent behaviour of the granular materials to be described with a single set of state-independent constitutive parameters. Based on this model a shear bifurcation condition is derived. Loading along various stress paths that can be achieved with a true triaxial test apparatus are simulated numerically. Either localized failure or uniform diffuse failure in granular samples may occur, which is determined based on whether the shear bifurcation condition is met. Effects of confining pressure and initial density are discussed.

Abstract: The effect of strain rate in the range of 10-4 to 10-1 s-1 on localization of deformation and fracture behavior of 5754 and 5182 aluminum alloys is investigated. For this study, tensile tests, interrupted tensile tests, shear band decoration, fractography and image analysis has been used. This investigation is based on experimental work and observation of the material behavior. Results show that strain rate has some effect on the mechanical properties and deformation stability of the alloys. The area of localized plastic deformation and thickness of the shear bands were found to be sensitive to the strain rate. It was also observed that localization of plastic deformation and shear band formation is an important step in the damage propagation and final fracture of the alloys. Detail of damage development, based upon micrographs of samples interrupted at different stages of straining is presented

Abstract: A number of mechanical tests and metallographic techniques have been used to investigate the mechanism of ductile fracture of AA5754 sheet. The sequence of events in the development of shear localization is clarified using in situ strain mapping on both the sample surface and through thickness direction during tensile tests. It is observed that the failure mode changes from cup-cone type to shearing with increasing Fe content in both continuous cast (CC) and direct-chill cast (DC) AA5754 sheets. However, this transition happens in CC with much lower Fe content than DC. As very little damage is found near the fracture surface, this suggests that damage may be a consequence of the shear process rather than a trigger that determines material ductility. For both CC and DC with same Fe content of 0.21%, fracture strain of CC is much lower than DC. It is postulated that this is due to the differences of particle distribution in these two materials, especially the increased fraction of stringer type structures which exist in CC material.