5
votes
Accepted
Do internal / external stresses affect modal frequency?
Consider a small piece cut out of a structure that has non-zero internal stress.
To maintain equilibrium, there must be some forces applied to the boundary of the piece. (Of course when it was part ...
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5
votes
Which mode does get excited under action of dynamic loading?
Take the Fourier Transform of the time varying driving force, this will give the frequency content of the driving force. Multiple modes of vibration can be driven at once, and will superpose with each ...
- 1,317
5
votes
Accepted
How to know which modes are excited by a given input in modal analysis?
They way to answer this question is transform the dynamics model into modal coordinates, and see what happens to the force term.
Suppose we can describe the stiffness and mass properties of the ...
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4
votes
What is the frequncy range of seismic vibrations?
The frequency and ground acceleration and other pertinent data are recorded in a seismic design espectra which is unique for each site and varies with the type of the soil on the side.
Usually ...
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3
votes
Why does the mesh size affect my modal analysis results?
I am not knowledgeable to answer this question but to lead you to read this article, which says "In order to obtain greater number of eigenmodes (and natural frequencies as well) finer mesh of ...
- 8,109
3
votes
Why does the mesh size affect my modal analysis results?
I mean the purpose of modal analysis is not to acquire nodal displacements. Displacements are random.
I disagree with both of those statements.
Part of the reason for performing a modal analysis is ...
- 526
3
votes
Accepted
What does multiplicity of a multiple natural frequency mean?
"Multiplicity" usually refers to several different modes with the same natural frequency. For example, ignoring any fault conditions and tolerances, if there are several identical planet gears, each ...
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3
votes
How to know which modes are excited by a given input in modal analysis?
Firstly, applying a constant load will not excite any modes. You need a time-varying load to induce vibration (e.g. wind, seismic, blast). Which modes are excited will depend on the frequency content ...
- 2,760
3
votes
Negative Eigenvalues in Ritz Solution of First-Order Shear Deformation Theory
The only way you can get negative eigenvalues is by including stress stiffness effects (sometimes called "geometric stiffness"), if there are compressive stresses which would cause the plate to buckle....
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2
votes
Relation between natural frequency and the thickness
We know for undamped harmonic vibration
$\omega= angular velocity \space, L=length of brick \ , m=mass \ , \\ I= \text{second moment of area of the brick section } \ , \ h= \text{the height of the ...
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2
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What is Experimental and Operational MODAL ANAYLIS?
The most basic purpose of the measurements is system identification - i.e. to determine the modal frequencies, mode shapes, and damping factors for the modes in some frequency range.
You can use ...
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2
votes
What is Experimental and Operational MODAL ANAYLIS?
Typically you are not doing modal analysis for its own sake. Typically you have a problem and you are using modal analysis as a tool to solve that problem. The goal of the modal analysis will depend ...
- 2,646
2
votes
Accepted
Analizing Vibrations Influence on Dynamic Weighing
So, look at the time history that you added in edit 3. Notice how it is never ever ever below 1. That's very telling. Let us assume that you have a MEMS accelerometer is able to read down to 0 ...
- 2,646
2
votes
Accepted
Modal Testing and Frequency Response Interpretation
You can't make much use of just one FRF, especially if none of the peak responses line up between the model and the test.
Fix the accelerometer at the position of one of your model DOFs and then ...
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2
votes
Why and how are resonance frequencies of a system dependent upon the shape, mass and the way it is constrained?
In order to answer that question you need to build up your knowledge in vibrational dynamics. This usually takes a whole semester in undergraduate physics at the latter stage of an engineering ...
- 24.3k
2
votes
Accepted
Effect of bracing on a music instrument top - is FEM modal analysis enough to predict this?
What can (and what can't) modal analysis tell me about the transfer of periodic motion from the bridge (where the strings are attached) to the soundboard?
Very often in engineering, we find ourselves ...
- 2,646
2
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Effect of bracing on a music instrument top - is FEM modal analysis enough to predict this?
Bracing in the simplest way, say a large flat sheet extended reasonably long in both directions will increase the stiffness of the sheet and hence reduce its natural frequency. This will lead to ...
- 22.1k
2
votes
Accepted
Are the modal participation factors bounded for shock response spectrum analysis?
In the section "The Multiple DOF System" the Comsol document says
It has been assumed that the mass matrix normalization of the
eigenmodes is used and that the damping matrix
can be ...
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1
vote
Why the mode frequency is approximate in modal analysis?
If a structure has 2 eigenmodes with the same eigenfrequency, then every linear combination of those modes is also an eigenmode with that same frequency.
Suppose one mode is east-west bulging and the ...
- 41
1
vote
Ansys Mechanical Modal analysis, Parametric materials, mode shape output for each parameter?
After spending a great deal of time testing, and trying out different things, this is the code that ended up working:
...
- 11
1
vote
How do I get the modes shapes (that I got from FEA of a system) in reality?
For a cantilever beam, we can either add point mass or initial constrains, encouraging the beam into the desired mode shape.
In most of the vibrations, all modes will emerge given enough time. The ...
- 22.1k
1
vote
Why and how are resonance frequencies of a system dependent upon the shape, mass and the way it is constrained?
Natural frequencies inside structures are standing waves that are reflected and propagated throughout the structure. The frequency of such wave depends on the shape of that wave and the speed at which ...
- 1,653
1
vote
Relation between natural frequency and the thickness
Natural frequencies depend upon stiffness and mass. These two properties depend on the material characteristics and geometric configurations.
In general:
Increasing stiffness, the natural frequencies ...
1
vote
Accepted
Analytical solution for wall eigenvalue analysis
Your "wall" would be referred as a "plate" in the technical literature. The eigenvalue problem for a plate is significantly more complicated than a beam, but similar ideas.
I would suggest that you ...
- 2,646
1
vote
What is Experimental and Operational MODAL ANAYLIS?
The difference between EMA (experimental modal analysis) and OMA (operational modal analysis) is that one is done under experimental conditions, which allow for you to know the excitation force, while ...
- 66
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vote
How to know which modes are excited by a given input in modal analysis?
For a static problem, you can determine the displacements of a structure using only forces and stiffness of the structure using static equations for text books:
$F_{normal} = K \Delta \\ M_{bending} =...
- 386
1
vote
FRF based material constant optimization
I have only done this sort of optimization using non-commercial software.
Rather than trying to fit "the FRF" (i.e. the complete shape of all your measured response functions) I would start by ...
- 12.5k
1
vote
How to handle modal combination in response spectrum analysis for member forces for member with more than 1 element?
You are correct that the response for each mode must be calculated before applying some modal combination methodology.
Instead of thinking about the structure as being discretized into numerous ...
- 2,760
1
vote
The distinction between lateral mass participation and vertical mass participation
From my understanding, mass participation for each mode is determined
by the general mass and the eigenvector of the mode, and eigenvector
of the mode usually consists of contribution from all the ...
- 12.5k
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