Example:-
In CAD setup: I have a Chassis frame which has a bracket mounted on to and this bracket is holding a mass of certain number of kilograms with help of straps.. Similar to a fuel tank mounted in a truck..
FEA setup: I have created FEA model of the system with Chassis frame & brackets with 0D mass for the fuel tank. I just conducted Modal analysis in Unigraphics NX and have obtained results which has 10 modes and under each mode with certain frequency has displacement & stress values.
I see each mode exhibiting a mode shape..
My objective is to improve the bracket design in order to increase the first mode frequency of the design.
I would like to know how I need to interpret the results and looking at what parameters do I need to improve my design.
Thank you all for your insights.
Hello Krishna,
In order to improve your design you need to clearly establish your TARGETS. What are the criteria for considering your design "good" or "acceptable"? It is true that increasing the natural frequencies imply increasing the stiffness. But how stiff you want the structure to be? If you have durability issues perhaps the answer would be: I want the structure to be stiff enough so I don't bump into fatigue problems. Then you would be defining LIFE as a clear target meaning that you would know exactly when your design is good enough. As Claudio pointed out above the modal stresses alone don't tell much so you have to combine them with the generalized displacements (aka modal coordinates). Most of the finite element packages in the market allow you to use the modal superposition technique in order to solve Steady State Dynamic Analyses (aka Harmonic Analyses). In the simplest case your load would be constant amplitude but if they are not you'd have to get more familiar with frequency domain fatigue analysis (sometimes called random vibration fatigue). In case your concern is another (for instance acoustics, wear, comfort, etc.) you would have to link the stiffness to whatever other parameter of interest and clearly define a target for them so you would know when your mission is accomplished.
Hello Krishna, in addition to what Claudio and Giovanni have mentioned, if your objective is to improve the first frequency of your model, you could look at the strain energy plots for your modes. Most software allow you to request for the strain energy plots as output. Looking at these plots would give you a good idea on which area of the model you would need to stiffen to have more influence on your frequency. Stiffening areas where strain energy is less may not contribute much to the frequency increase.
Dear Krishna,
I think ANSYS or VES software can do better job for a model analysis of a chassis.
Modal analysis on car chassis structure has to be done very carefully. Modal analysis is a process to determine the dynamic characteristic of car chassis. In this regard, vibration analysis can be done using FFT especially in automobile chassis followed by some characteristics such as mode shape, natural frequencies of vehicle model. FEM can be used for model analysis considering DOFs. Amplitude vs. frequency plot using post processing option in ANSYS can be used to obtain natural frequency. Harmonic analysis result is used to verify the steady-state response of a linear structure to determine whether in which the chassis can withstand resonance, fatigue or other structural problems related to vibration during its operating life.
Ashish
- Badges
- Science topic