Introduction
All fatigue is dynamically induced. That is, there must be some level of dynamic loading in order for
fatigue damage to occur. It is probably a true statement to say that nothing in real life is actually static,
or not moving at all. Even slight changes in temperature will cause stress fluctuations in an otherwise
apparently static structure. Some dynamic loading is hardly detectable, changes very slowly, and is quite
repeatable while other types are quite noticeable and very random in nature such as engine noise from an
automobile.
The pseudo-static approach for calculating a stress time response, where unit stresses are associated with
load time histories, is valid if the frequency of the input loading is below the lowest natural frequency of
the structure. However, for cases where the dynamic response of the structure comes into play, the usage
of transient response or random response is appropriate to compute fatigue life.
Objective
? Perform analysis using transient results
? Perform analysis using the modal superposition method
? Random Vibration Fatigue analysis
? Run comparative studies
Analysis Using Transient Results
Up to this point we have strictly used linear elastic FE results from static load cases where we have
associated the time variation of the loading to externally defined time histories. This is the most common
usage of MSC Fatigue and perfectly valid for most components and structures which are fairly stiff in
nature. Thus the name quasi-static. The assumption is made that dynamic effects are third or fourth order
contributions to fatigue life and therefore ignored.
There are times, however, where the dynamics of the structure can significantly affect the fatigue life of
the product, especially when the mass of the structure is large and the operating loads approach or even
pass through the natural frequencies of the structure such as the dynamics of an entire vehicle body as
shown by the bus to the right.
In these cases it is generally better to use a dynamic FE analysis to capture all the important dynamic
effects. All time variations of the loading are defined directly in the FE model and a direct or modal
dynamic transient analysis is performed. There is no need for any externally defined and associated time
histories as with the pseudo-static method. The drawback however, is that you cannot separate the loads.
They must all be defined in the same FE analysis. Investigation of the influence each load may have on
fatigue life requires a new FE analysis to be run each time.
To illustrate the use of transient results in MSC Fatigue, follow this mini-exercise:
Transient Keyhole Job
The geometry is the same keyhole model. Open a new database called keyhole and import the MSC
Nastran Output2 file call, key_tran.op2. In addition to this transient analysis, we are also going to
compare the answers to an equivalent pseudo static analysis, so also read in the Output2 file,
key_stat.op2. Remember to read the model and results for the first file and only the results for the second
file in the order specified here.
In this version of the keyhole model, the static load case results were determined using a 30 Newton
loading at the same point of application as the original keyhole problem, the results from which, when
scaled by the load time history should give roughly equivalent stress time histories for all nodes as does
the modal transient analysis. This of course does not take into account any dynamic effects that the mass
distribution may have on the dynamic behavior and resulting stress results. However, with this simple
model and a very evenly distributed mass, there should not be a large difference.
Access the main MSC Fatigue form and read in the saved job called transient using the file transient.fin.
You will also need static.fin, so copy this file while you are at it. Systematically open the Solution
Params..., the Material Info..., and the Loading Info... forms and follow the explanations of each to
understand the setup. Note that we are running a Crack Initiation analysis.
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