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Frequently Asked Questions (FAQ)
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Since surfaces only have the directions x- and y- in the plane, it is necessary to define which should be the hoop stress and the axial stress. In the following example, sigma_x should be the axial stress and sigma_y the hoop stress.
The example consists of an inclined circular container (Figure 01). After modeling, the program tries to align the local axis systems on the global axis system (Figure 02). In the present case, however, the x-axis should run along the container for all surfaces. This orientation can be achieved as follows.
First, the z-axis of all surfaces must point inwards or outwards. In the example, the outward direction has been selected. If this is not the case for a surface, you can right-click the surface and use the function "Reverse Local Axis System" to move the z-axis to the other surface side. Then, select all surfaces and select the Axes tab in the surface dialog box. Figure 03 shows the dialog box. In this case, one of the axially extending boundary lines has been selected for the orientation. Figure 04 shows the now aligned local axis systems. All x-axes are axial and all y-axes are circumferential.
Figure 05 shows the results of the membrane stresses axial (sigma-x, m) and over the circumference (sigma-y, m).
AnswerBoth RFEM and RSTAB are ideally suited for use in mechanical engineering and are used by numerous users in daily practice. Due to the modular structure, you only need the add-on modules required for your applications.
Main Programs RFEM or RSTABThe basic programs define the structures and materials as well as the actions.RSTAB supports you best when it comes to truss or frame-type structures
In addition, RFEM provides the best support if you want to additionally calculate structures with surface or solid elements.
Dynamic AnalyzesIf seismic calculations or vibration analyzes are necessary, the RF-/DYNAM Pro add-on modules provide suitable tools for the determination of natural frequencies and shapes, the analysis of forced vibrations, the generation of equivalent loads, or for the nonlinear time history analysis.If you have any further questions on the Dlubal programs, please do not hesitate to contact the sales department.
Calculation of stresses and comparison with the limit stresses
Modeling of Pipelines (only in RFEM)
- RF-PIPING Design
Design of Pipelines (only in RFEM)
AnswerIn RF-MOVE Surfaces, you can also select a circle as a set of lines. A simple example is demonstrated in the video.
AnswerWith the time history monitor, you can view all results over a period of time. In this case, it is also possible to select several parts of the structure and then export the results directly to Excel.
AnswerWith the FE mesh refinement, it is also possible to create an aligned FE mesh in the program. Thus, the automatic FE mesh generator can be controlled to a certain extent. However, it is not possible to use it for setting of a specified mesh geometry.
AnswerThe most common causes are listed below:
1. SingularitiesSingularities appear in a limited range due to the concentration of the result values that depend on the stress. They are conditioned by the FEM methodology: In theory, the stiffness and/or the stress in an infinite size concentrate on an infinitesimal small area. Therefore, the singularities occur especially at point supports, load application locations, reentrant corners, or in the area of stiffness peaks.If the result value of the stress peak is greater and the area of this stress peak is smaller in the case of a finer FE mesh, the singularity is very likely to occur.The recommendations for dealing with the singularity locations are included in the following technical articles of our Knowledge Base, for example:
2. Unrealistic Support DefinitionRigid supports (infinitely stiff supports) are rather unrealistic in many cases. Therefore, it is recommended to display the supports as elastic supports. In this case, the stiffness of the adjacent structural components should be estimated realistically.For checking purposes, the deformation diagram is suitable, possibly with a stronger precamber, as well as the result display of the support reactions or contact stresses. For a better overview, the simplest possible loads should be used for the check.
3. Incorrectly Defined Directions / NonlinearitiesA mistake in the direction definition, for example, of loads, member hinges, or line and surface releases, is often the cause of the unrealistic behavior. When using the local or rotated coordinate systems as a reference systems, you must pay attention to the correct definition. For example, nonlinearities defined in the opposite direction are typical for the supports that fail due to tension or compression.Incorrectly defined loads can be easily identified by displaying the loading. The loads applied for the calculation can be easily displayed in the Results navigator by using the "Load Distribution" option.Furthermore, modeling inaccuracies can also lead to the incorrect definition of directions. By importing a DXF file, you can introduce inaccuracies into the model, for example, nodes that are not on top of each other or the lines skewed in the wrong direction.The "Regenerate Model" feature is very helpful for treating the minor inaccuracies.Incorrectly defined releases and hinges can usually be identified by means of the deformation image and the diagrams of internal forces. Again, it is recommended to work with simple loads for checking purposes.
4. Model does not correspond to realityIt may often happen that not all external or internal influences from a structure to be modeled have been considered sufficiently and accurately in the model. Supports or supporting structural components may not have been modeled or they are in a wrong place. The realistic estimation of the stiffness of the adjacent structural components is also important. Provided that it has been over- or underestimated, the load transfer in the model is sometimes changed significantly.However, it is possible to simple check the deformation, possibly by using a stronger precamber.The following questions may help you to find the solution if the real structure is known: Is the magnitude of the deformations close to reality? Is the deformation diagram qualitatively consistent with my expectations?A good example is presented in the following Knowledge Base article:
AnswerYes, it is possible. The FAQ video shows step by step how to create a model of such a component with quadrangle surfaces.
The "Without membrane tension" stiffness type describes the plate and plane stiffness of surfaces.The plate stiffness including the shear stiffness perpendicular to the surface plane is defined linearly elastic and the plate stiffness in the surface plane is defined nonlinearly elastic with the material model according to "Drucker-Prager" depending on the defined thickness and the assigned material.In order to ensure that the surface behavioe is "Without tension" in the pane direction, the nonlinear material model reacts with yielding tensile stress fy,t going to almost zero in the elements subjected to tension in combination with a relatively small strain hardening modulus Ep. On the other hand, in the case of compressive forces, the elements remain virtually unaffected linearly elastic due to the relatively high yielded compressive stress fy,c and react with an unrestricted compression transmission.Since the "Without membrane tension" stiffness type basically modifies the pane-related degrees of freedom of the surfaces, it is only applied to the surfaces of the relevant model types 3D, 2D - XZ (uX/uZ/φY) and 2D - XY (uX/uY/φZ).To better describe the partial nonlinearity of this surface stiffness type, it was renamed from "Without tension" to "Without membrane tension" as of the Version RFEM 5.06.1103 on 4.02.2016.The Knowledge Base article "Modeling Approaches for Shear/Hole Bearing Connections by Means of FEa" shows possible application.
AnswerIn RFEM, it is possible to create a blind hole in a solid by means of a solid intersection.The conversion requires two valid solids. One solid represents the structural component to be drilled, and the other solid represents a simplified drill. The simplified drill solid should consist of a cylindrical shank with an applied conical tip. The diameter of the shank is to be modeled with the blind hole diameter and the length of the shank is greater than the blind hole depth.For the intersection, the drill solid referring to the solid to be drilled should be placed in the position of the planned blind hole. It is important that the drill solid protrudes slightly above the base volume. Then, mark both solids and in the shortcut menu, select the "New Compound Solid" option. By specifying which Solid B (a drill solid) should be subtracted from which Solid A (a solid to be drilled) and activating the "Solid B as hole" option, the program deactivates both initial solids and creates a new compound solid. This solid represents the solid with a blind hole and depends intelligently on the geometry specifications of both initial solids.
AnswerBasically, elements in RFEM are rigidly connected with each other. To solve this problem, RFEM provides two elements. The line hinges and the line releases. The difference is explained in the link below.
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Wind Simulation & Wind Load Generation
With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.
The generated wind loads acting on these objects can be imported to RFEM or RSTAB.
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