![]() ![]() The transitions are performed along the line five inches away from the fixed edge. Two different transition methods will be used for this purpose-one with CTRIA3s (Figure 9-10) and one with RSPLINEs (Figure 9-11). Since you are not interested in the stresses outside of this region, you can represent the rest of the region with a coarser mesh. Hypothetically consider that you are interested in the stresses along the line one inch from the fixed edge. See The RBE3 Element, 327 for a description of the RBE3 element. The load can then be applied to grid point 500 and distributed to the rest of the edge grid points. This grid point is connected to all the edge grid points. For convenience, a RBE3 element is created along the free edge by defining a dependent point (grid point 500) at the center. Five different loadings-as shown in Table 9-2 are applied to this structure along the free edge (right end). For clarity, only grid points of interest are labeled. This problem involves a plate fixed at the left end with a dimension of 14 in by 9 in, and modeled with 1 in by 1 in CQUAD4 elements. The spline element assumes a linear interpolation for displacement and torsion along the axis of the spline, a quadratic interpolation for rotations normal to the axis of the spline, and a cubic interpolation for displacements normal to the axis of the spline.Ĭonsider an example problem as shown in Figure 9-9. ![]() Displacements for intermediate grid points are interpolated from the equations of an elastic beam passing through these grid points. In MSC Nastran, the RSPLINE element is an elastic interpolation element that connects N end points. The midside nodes, if used, should be located as close to the center of the edge as possible.Īs an alternative mesh transition method, a spline element can be used. For the CHEXA element, you should use it with eight nodes (no midside nodes), or use it with 20 nodes (all midside nodes). A CQUAD8 element with all midside nodes deleted is excessively stiff and therefore is inferior to a CQUAD4. ![]() When using a higher-order element, such as a CQUAD8 or a CHEXA (nine to 20 nodes) for any purpose, the general recommendation is to include all midside nodes. This method is not recommended because it severely distorts the stress distribution in the elements adjacent to the change in mesh size. It is also very tempting to selectively delete midside nodes of higher-order elements (e.g., CQUAD8) and use them as transition elements as shown in Figure 9-8(b). One common method of performing a transition is to use an intermediate belt of triangular elements as shown in Figure 9-8(a). The transition from a coarse mesh to a fine mesh, or vice versa, may not always be an easy task. Transition from a Coarse Mesh to a Fine Mesh However, if this localized stress variation occurs away from areas of interest, the increase (or decrease) in stress caused by the transition should cause no concern. In this case, the local stress rises (or decreases) due to the effect of the transition in other words, the results may be conservative (or unconservative) in an area near a transition. However, a problem arises when the transition occurs in an area of interest. Normally, these stress anomalies are localized and dissipate quickly as you move away from the transition.
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