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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 1,272
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 ...
user avatar
  • 21k
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 ...
user avatar
  • 7,491
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 ...
user avatar
  • 501
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....
user avatar
  • 12.5k
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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 2,770
3 votes

FEM modal analysis computing displacements

As modal analysis is performed in the frequency domain, your modes and amplitudes are also in the frequency domain.The modal amplitudes ($q_i$) are scaled by the same amount as the applied load. If ...
user avatar
  • 232
3 votes
Accepted

How do I conduct an experimental modal analysis with a three-axis accelerometer?

The body of your question (the title is somewhat different) asks about computing the frequency response function (FRF) between a multi-input, multi-output (MIMO) system. First off, the numerical ...
user avatar
2 votes

How do I conduct an experimental modal analysis with a three-axis accelerometer?

Disclaimer: Base on the question, I am suspecting you are looking for a theoretical explanation. Unfortunately I am not in a position offer any insight in short order. But I can offer you following ...
user avatar
2 votes

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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 2,511
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 ...
user avatar
  • 2,511
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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 12.5k
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 ...
user avatar
  • 21k
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 ...
user avatar
  • 22.6k
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: ...
user avatar
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 ...
user avatar
  • 21k
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 ...
user avatar
  • 1,603
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 ...
user avatar
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 ...
user avatar
  • 2,511
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 ...
user avatar
  • 66
1 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} =...
user avatar
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 ...
user avatar
  • 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 ...
user avatar
  • 2,770
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 ...
user avatar
  • 12.5k

Only top scored, non community-wiki answers of a minimum length are eligible