We have a part that we want to analyze for vibrational fatigue and (seperately) impact stresses. I have done static FEA's on the part and have tried to figure out the other things but don't know where to start. Thanks for helping.
There are two ways to due Vibrations with Finite element analysis. The First, is by doing ruining the analysis using time integration. The Second uses the mode shapes of the part to carry a modal solution in either time or frequency domain. The Pro's to use a time integration technique is that nonlinearities can be included in the analysis because the numerical integration is carried in small time intervals where the solution of the previous time interval becomes the initial condition for the new interval. A good candidate for time integration analysis is impact. Impact analysis occurs in a fairly short time with contact stress being part of the nonlinear solution. The disadvantage of time integration is that solutions may take long and frequency analysis can be done only for simple functions with fairly good size period.
Modal analysis in the other hand is more versatile. In the case of modal analysis you run an analysis that first subtract the various mode shapes and frequencies of the part or assembly. The dynamic solution is then obtained by modal superposition. Each mode at its given frequency contributes to the total solution by adding its response in either frequency or time forcing function. Advantages of the modal analysis is that is simpler to use and faster to run in both time and frequency domain. The disadvantages of modal analysis are that nonlinearities can be captured and that the analysis is an approximation due to the fact that to have a total solution the model must capture and infinite number of modes. However, there is some techniques that can tell you how accurate is you solution and if you need to capture more modes in order to get a better solution.
ProE uses modal analysis to obtained solutions to dynamic forcing functions. Here are the steps you need to do in order to run an dynamic analysis in ProE.
1) Prepare model by assigning materials properties and constraints ( and forces will be changing as a function of time or frequency).
2) Set analysis to modal and run it. You will have to specify how many modes or the frequency range that proE must search to subtract modes from the model. Remember, every part in theory has semi infinite number of modes. In FEA the maximum number of modes that can be obtained is equal to the number of degrees available. Initially you should only specify to obtain maybe 10 or 20 modes depending on the size of your model.
3) Set a dynamic analysis and check the box that tell ProE to use your previous modal analysis for the solution. In the previous analysis any forces that you set were not taking into account. However, when you set the dynamic analysis you will have to tell proe to run an base excitation or a forcing function dynamic analysis. Base analysis will be like placing your part in a shake table. You must specify also how the base excitation or forcing function changes with time or frequency. Also, make sure you specify damping for the model. A good rule of thumb is to apply damping between 2% and 5% unless you know from experience or from data collect that damping is higher or lower .
If you want to due a impact analysis in proE you will need to specify the location where the impact occurs and apply a force that will change with time in a form of an impulse. This is only an approximation since contact stress are not fully capture due to the fact that this technique approximates the impact forces by a linear impact load.
Once you run your dynamic analysis you can use those results for your fatigue analysis.
Again this is a very general description of what you need to do. Detail information of how to prepare the model and run it you may be able to find it in the online help. I hope the information I supplied will at least get you thinking what must be done. if as you prepare your model you have any specific question post them and I will or some one else in this forum will try to help you.