On the Edge: Integrated Finite Element Analysis with Femap Express14 Nov, 2005 By: Russell Brook Cadalyst
Integrated, process-driven finite element analysis for Solid Edge reduces physical prototypes and the cost of manufacturing.
Market pressures to reduce design cycle time, increase speed to market and improve quality are driving the growth in the use of digital simulation throughout product lifecycles. Femap Express, found in Solid Edge, allows designers to perform first-pass analysis and validate that parts are fit for purpose while keeping costs low. By moving analysis to an earlier stage in the design cycle, Solid Edge users can ensure their designs are strong enough and function as intended to avoid costly recalls.
Today's analysts and engineers no longer have to build separate, meshed geometric models based on their CAD files in order to run a FEA (finite element analysis). Femap Express is an FEA application closely integrated with Solid Edge, allowing FEA technologies to work together within a common user interface. Femap enables engineers to solve real problems early in the design, leading to higher quality products at lower cost.
Analysis settings and results are stored with the component, so if the part changes, the results are modified without needing to repeat the process of applying loads and constraints.
A step by step process is provided through the Solid Edge SmartStep ribbon bar, users are first prompted to select the body (figure 1). Most models have only one; however, Solid Edge is capable of creating multiple bodies in one file. You can use simplified parts instead of the as-designed parts. This capability is particularly useful because much of the detail is insignificant, has no real influence on the results and unnecessarily complicates the analysis.
Figure 1. Examples of Solid Edge's toolbars for analysis.
Types of Analysis
Static Analysis (Stress): Static analysis is used to calculate the stresses in a component to help predict strength and durability for given loading conditions. For example, when a wire is pulled tight, it stretches (undergoes strain). The amount it stretches is proportional to the load divided by the cross-sectional area of the wire: n = F/A. Failures occur when the load exceeds a critical value for the material; the tensile strength multiplied by the cross-sectional area of the wire, Fc = nt A.
Modal Analysis: Nearly all objects, when hit, struck, plucked, strummed or somehow disturbed, vibrate. If you drop a yardstick or pencil on the floor or pluck a guitar string, it begins to vibrate. Objects tend to vibrate at a particular frequency. This frequency or frequencies is known as the natural frequency of the object. The shape of the part when vibrating at one of these frequencies is the mode shape. Modal analysis is used to determine the natural frequency and shape of the part at this frequency. If the operating environment contains a loading condition at a frequency close to a natural frequency, the part can vibrate with an increasing amplitude and fail.
Sheet Metal Analysis: For thin bodies, a different type of meshing approach is required. For sheet metal parts, you can extract and mesh a mid-surface using 2D plate elements instead of 3D solid elements. These structures are represented much more efficiently using 2D elements without compromising accuracy, with minimal solution time and use of computer resources. You can use sheet metal mid-surface for both static and modal analysis.
Choosing the Material
Accurate material data is critical for analysis and has a huge influence on the results. Choosing the correct material is also important for designing parts that are suitable for their application. Material properties available in Solid Edge are accessed directly from the part, if the material properties were set. The material step allows materials to be changed if the designer wants to test alternative materials (figure 2). If the material property is not detected in the part, the program prompts you to choose a material.
Figure 2. The Materials table.
Placing a Load
Loads (pressure or force) are placed on edges or evenly across a face. You can place multiple loads at the same time. Because the program can analyze parts in context of an assembly, you can use other components within the assembly to set the force vectors as well as edges of the part (figure 3). Units such as N, Nm and lbf are set in File Properties.
Figure 3. Defining the loads.
For a load to have an effect on a part, it needs to be held in place or constrained. The part is then constrained along faces or edges by simply choosing where to hold the part. Once the program gathers all the analysis criteria, users are notified that they can perform an analysis. Loads are applied to the model, and then underlying Femap technology automatically creates the mesh. NX Nastran is a standard feature of Femap Express and is used to solve the analysis.
The system collects all the information and input that it needs to perform the analysis. Loads, constraints and material properties are applied to the finite element model, and the process invites the user to analyze the part. The model is automatically meshed and the analysis performed (figure 4).
Figure 4. The Femap Express dialog box displays the progress while it is analyzing.
The mesh contains the data on material and structural properties that define how the part will react to certain load conditions. An optimized 3D mesh is automatically applied directly to the Solid Edge model, based on model proportions and level of detail to determine the mesh density. The same model, when simplified, will have different mesh size than the as designed. You can refine the mesh for greater accuracy. Femap technology is used to mesh a model.
During the analysis, the software calculates von Mises stresses for the component. You can choose how the results display depending on what criteria you need. You can turn deflection and finite element contours on or off in the display. The program can animate the results with any combination of these settings. Animation speed is adjustable, and you can save the animation as a separate AVI file. The model window displays the FEA criteria and a gradient scale to help read the results.
Stress: Results display the mesh and colored stress contours that show areas of stress build up, and a scale that shows the values using units that have been set for the stress analysis, such as kPA, PSI and BAR, plus many others (figure 5).
Figure 5. An example of the results of a stress test.
Displacement -- Stress: Results display mesh and colored displacement contours that show where the model deflects, and a scale that shows the values using units that have been set for the stress analysis, such as mm, cm and inches (figure 6).
Figure 6. An example of a displacement stress test results.
Factor of Safety: Results display mesh and colored factor of safety contours indicating areas where the model exceeds the determined FOS (factor of safety), and a scale that shows by how much a component is within or outside its FOS (figure 7).
Figure 7. Factor of safety results.
Natural Frequency: Results display the four major natural frequencies of the part and the shape of the part at those frequencies (figure 8).
Figure 8. Results for a natural frequency analysis.
Mid-Surface: The sheet metal mid-surface is a method of analyzing parts using 2D plate elements to make the analysis more efficient. Femap Express can display mid-surface results using both static and modal analysis (figure 9).
Figure 9. A sheet metal mid-surface analysis.
Results are quick and easy to save using the Femap Express report generation tool. A detailed HTML report contains in-depth analysis information on part and material properties, constraint information and modal and/or static results. Stress contour images graphically show where stress is concentrated; maximum deformation occurs or where parts exceed a factor of safety.
The Save Animation tool allows users to save individual pictures in BMP, JPG or TIF format. A second option saves the currently displayed results animation as an AVI file.
See you On the Edge next month.
About the Author: Russell Brook
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