重型板式给料机论文(10)

洛阳理工学院毕业设计论文 an element or chip-separation criterion

The application of non-linear finite element techniques to the simulation of metal forming processes resulted also in finite element modelling of metal cutting processes. More sophisticated models have been developed, mainly aiming at the determination of the residual stress and strain, the temperature distribution and the prediction of the cutting forces. The two methods employed were the Eulerian-based approach, where cutting is simulated from the steady state, avoiding, therefore, the need for a chip-separation criterion, but requiring that the shape of the chip must be known in advance; and the Lagrangian approach, where cutting can be simulated from the incipient to the steady state, allowing for the prediction of the chip geometry and the residual stresses in the workpiece. Note, however, that a chip-separation criterion must be provided, to enable the separation of the chip from the workpiece.

Chip-separation criteria, based on a geometrical consideration, have been suggested in, but see also the criteria based on the critical value of the strain energy density in and the work by Ceretti et al. who developed a cutting model by deleting elements having reached a critical value of accumulated damage.

In almost all of the finite element models developed so far, non-commercial FE codes have been employed. The application of commercial FE codes is, therefore, desirable for the industrial use of the FE method for the simulation of metal cutting processes, by enabling tool makers to optimize cutting tool design, and users to evaluate the effect of cutting process parameters on the quality of machined parts prior to expensive and time-consuming experimental testing.

In the present work, the commercial implicit finite element code MARC was employed to construct a coupled thermo-mechanical finite element model of plane-strain orthogonal metal cutting with continuous chip formation The material is modelled as elastic-plastic, while its flow stress is taken as a function of strain, strain-rate, and temperature, so as to represent the real behaviour in cutting, where strain and strain-rate values, as well as the temperature rise, are large. The entire cutting process is simulated, i.e. from the initial to the steady state, and a geometrical chip-separation criterion, based on a critical distance value at the tool tip regime of the workpiece, is implemented into the MARC code by employing the rezoning feature.

2. Finite element model

The finite element model used for the plane-strain orthogonal metal cutting simulation is based on the updated Lagrangian formulation as provided by the MARC code. The plane-strain assumption constitutes a reasonable approximation, since in real metal cutting processes the width of cut is at least five times greater than the depth of cut, therefore, the chip is produced under nearly plane strain conditions.

The dimensions and the initial finite element mesh of both the workpiece and the tool are shown in Fig.1(a).The upper part of the mesh, which constitutes the workpiece, is finer, to enable the stress, strain, strain-rate and temperature in the chip and the tool tip regime to be accurately predicted. For the dimensions of the rest a coarser mesh is sufficient, as the boundaries of the mesh do not influence the predictions. The vertical displacement of the nodes, Y, at the lower boundary of the workpiece, and the horizontal displacement of the nodes, X, at the left boundary, are zero. The workpiece consists of 1340 four-node isoparametric plane-strain quadrilateral elements and 1428 nodes. This kind of lower-order elements has been proven to be more accurate in analyzing large-strain plasticity problems, compared to higher-order elements with eight nodes.

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洛阳理工学院毕业设计论文 However , as in this element bilinear interpolation functions are used, the strains tend to be constant throughout the element, resulting in poor representation of the shear behaviour， which is the dominant mode of deformation in cutting. In order to improve the shear characteristics, alternative interpolation functions are used, by employing the assumed strain formulation provided by the MARC code.

Fig.1. Chip formation in simulated orthogonal metal cutting: (a) initial undeformed state; (b) shape of the deformed chip after a tool path of 0.48 mm; (c) shape of the deformed chip after a tool path of 1.44 mm; (d) shape of the deformed chip after a tool path of 2.58 mm.

The tool consists of 425 four-node isoparametric quadrilateral planar heat-transfer elements, as nodes. The lower half of its mesh, expected to be in contact with the chip, is modelled with a finer mesh, in order to be able to predict the temperature field developed in the tool.

2.1. Chip-separation criterion

To investigate the chip formation and its separation from the workpiece, a chip-or element-separation criterion has been implemented into the MARC finite element code. Cutting is supposed to take place at the line representing the undeformed chip thickness, therefore, separation is assumed to occur only at the nodes lying along this line. The criterion for the separation of nodes in front of the tool tip is based on a geometrical consideration. When the tool tip approaches a node within a small critical distance, that node separates from

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Fig.2. Schematic diagram of the Geometrical separation criterion: (a) before element separation, D > Dc; (b) after element separation, D≤ Dc. 洛阳理工学院毕业设计论文 the workpiece and becomes part of the chip (see Fig.2).

When the distance D, between the tool tip and the node B, becomes equal or less than the pre-defined critical value Dc, a rezoning step is conducted. The connectivity of the element E2 changes and a new node BH replaces the node B in that e …… 此处隐藏：4520字，全部文档内容请下载后查看。喜欢就下载吧 ……