D2_Modal Analysis (Designer)

Modal Analysis

Modal Analysis 2

Step

00 Modal Analysis

Modal Analysis Summary

The presence of resonance and displacement caused by the vibration of a structure are predicted using Modal Analysis.

Through the analysis, the dynamic characteristics such as natural frequencies and mode shapes of the structure are

determined.

Important Terminology

Natural Frequency and Natural Period

• Natural Frequency: The number of times a structure oscillates (moves back and forth) in a unit time (Hz)

• Natural Period: The reciprocal of the Frequency (1/f), which represents the time required to oscillate one full cycle (sec)

• Natural Mode Shape: The shape of a vibrating structure at the state of removed external force. The lowest mode represents the most easily

deformable shape.

• Resonance occurs when the frequency of the oscillation applied to the structure is close to the natural frequency of vibration.

1st mode 2nd mode 3rd mode

• Natural mode shapes of a cantilever

A.

B.

• Although the amplitudes of A and B are different, their frequencies

and periods are the same.

• The frequency and period depend on the length.

• The number of full cycles of motion of the pendulum in one second

is referred to as natural frequency, and the time required to

complete one full cycle of motion is referred to as natural period.

• The natural frequency of a structure is directly proportional to its

stiffness and inversely proportional to its mass. If a concentrated

mass is located farther away from the boundary of the structure, a

decrease in the natural frequency will result due to the inertia effect. <Pendulums having

different amplitudes>

Modal Analysis 3

Step

00 Free Vibration

Overview of Modal Analysis

• Without any external forces acting on the structure, the structure vibrates due to its inertia force, damping and restoring forces.

Thus, the structure vibrates with a natural frequency and is not subjected to any external effects.

• Although the structure is not subjected to any load, free vibration takes place because of a non-zero initial condition.

Static

equilibrium Initial

condition

Free

vibration

Initial

displacement: A

Free vibration

Time

Period : T

Modal Analysis 4

Step

• Number of Modes:

Input the number of natural frequencies to be calculated.

• Lowest/Highest Values:

Set the range of frequencies.

The frequencies are calculated only within the range.

• Sturm Sequence Check:

Calculate the missing eigenvalues through an iterative process.

• Total Mass Calculation:

Calculate the properties corresponding to the nodal mass.

The properties include total mass, center of gravity and mass inertia moment.

• Global Mass Scaling Factor:

The parameter is used when the material density is defined by the weight instead of the mass.

The mass density is calculated by the following:

• Coupled Mass Calculation:

If this option is checked on, a distributed mass matrix is calculated instead of a general mass matrix.

It represents the values of coupled masses.

wmg )

1(

00 Overview of Modal Analysis

Modal Analysis Options

Modal 5

Step

Modal

Summary

Summary 00 Modal Analysis

- Unit: N, mm

- Geometrical Model: Modal.x_t

Material

- Cast Alloy Steel

Boundary and Load Condition

- Fixed (Holes inner part)

Results Verification

- Displacements

- Eigenvalues

- Modes Shapes

Modal 6

Step

00 Analysis Summary

Use Midas NFX (Designer) to perform the basic Modal Analysis and understand the features

- Understanding the purpose of Modal Analysis

Modal analysis is performed for the design verification of parts subjected to vibrations and periodic loads.

The analysis calculates the resonance frequency and evaluates the mode shapes of a constrained structure.

Therefore, the natural frequency determined through the analysis can be used so as not to approach the operational frequency or to coincide with the operational

frequency in certain cases (ex: ultra sonic welding)

Objectives of Tutorial

Analysis Summary

Target Model Boundary Condition (Fixed) Finite Elements Model (Tetra Mesher)

Fixed

The boundary conditions are not absolutely

required for modal analysis, but any existing

constraints must be reflected into the boundary

conditions. If the model is not fully constrained, the

Rigid Body Mode will appear (frequency near zero).

Modal 7

Step

00 Analysis Summary

Important Terminology

(Circular Frequency, ) Number of revolutions in one second

(Period, T) Time required to vibrate one full cycle (inverse of frequency)

(Frequency, f) Number of full cycles of vibration in a unit time

(Resonance) It occurs when the frequency of external forcing vibration coincides with the natural frequency of a structure or

machine.

(Mode Shape)

Mode shapes represent the shape components of free vibration of a structure without being subjected to any

external load. The most easily deformable shape component is the first mode. The higher the mode, the lesser

the contribution toward the deformation of the structure due to the corresponding mode.

Free Vibration

Time

Period: T

θ

ω t

A

m

k

Tf n

2

1

2

1

Modal 8

Step

Procedure

Model & LBC > Geometry > Import

5

3

01 Click [ ] (New).

Click [Geometry] - [Import].

Model: Select Modal.x_t.

Check [Search Contact Faces].

Click [Open].

Note: Tutorial models are included

in the folder, Manuals / Tutorials /

Files, in the installed program

folder.

1

2

3

4

Click [New], then all the menus will be

activated.

If the option [Search Contact Faces]

is checked on when importing the

CAD model, all the faces in contact

will be automatically defined as

welded contacts.

5

4

Check File type and Length Unit

1

2

Modal 9

Step

Procedure

Model & LBC > Geometry > Material 02 Click [Geometry] - [Material].

In the Material Library, select the

Group Steel.

Select Cast Alloy Steel.

Click [OK].

1

2

4

2 3 1

3

4

Modal 10

Step

Procedure

Model & LBC > Geometry > Material (Material-Cast Alloy Steel) 03 After selecting the model in the work

window, right-click Geometry.

Select [Material] > [Cast Alloy Steel]

1

2 1

2

Right-click each part under Geometry

to assign them with different materials.

To assign the same material on all the

parts, simply right-click Geometry and

select the material.

Modal 11

Step

Procedure

Model & LBC > Boundary > Support 04 Click [Boundary] - [Support].

Name: Enter Fix.

Target: Select 4 Planes.

(Refer To picture).

Condition: Select Fixed.

Click [OK].

1

2

3

4

3

3

5

2

4

5

1

Modal 12

Step

Procedure

Model & LBC > Mesh > Auto Mesh 05 Click [Mesh] - [Auto Mesh].

Select the entire model as the target.

Select High Speed Tetra Mesher.

Click [Option] button.

Verify that High-Order Element is

checked on.

Click [OK].

1

2

3

2

3

6

Click [ ] (Select All) Icon to select

the total model displayed on the work

window.

4

4

5

6

5

1

2

Modal 13

Step

Procedure

Analysis & Results > Analysis Case > General 06 Click [Analysis Case] - [General].

Name: Enter “Modal”.

Analysis type: Select [Modal].

Click [OK].

1

2

3

2

3

1

Modal 14

Step

Procedure

Analysis & Results > Analysis > Perform 07 Click [Analysis] - [Perform].

Click [OK].

Save As:

Enter “Modal”.

Click [Save(S)].

1

2

3

4

Once midas NFX is executed, the

solver becomes engaged. Click “Stop

Execution!” to interrupt the calculation.

1

2

3

4

Modal 15

Step

Procedure

Analysis & Results Works Tree > Modal > Modal Analysis > MODE 1 08 Click [ ] (Isometric2) Icon.

Select Deform > Deform+Undeform

(Transparent).

In the Analysis & Results Works

Tree, double-click

MODE 1, TOTAL DISPLACEMENT.

1

2

3

1

2

Natural Frequency

of Mode 1

The total displacements from the

results of modal analysis do not

represent “real” values. Therefore,

ignore the total displacements, and

check only the natural frequencies

and the modal shapes.

3

Modal 16

Step

Procedure

09 In the Analysis & Results Works

Tree, double-click

MODE 3, TOTAL DISPLACEMENT.

1

Analysis & Results Works Tree > Modal > Modal Analysis > MODE 3

Natural Frequency

of Mode 3

1

Modal 17

Step

Modal

Summary

Summary 00 Modal Analysis

- Unit: N, mm

- Geometrical Model:

Hanger.x_t

Boundary Condition

- Pinned

Results Verification

- Mode Analysis Results

- Mode Shapes

- Natural frequencies

Modal 18

Step

00 Analysis Summary

Understanding the basics of Modal Analysis

- Define the basically required conditions for performing modal analysis (Material-Mass density, Boundary Condition)

- Define the number of modes in Analysis Case – General - Analysis Control option.

- In order to avoid resonance of a structure to which a vibrating part is attached, modal analysis is performed to check the natural frequencies and mode shapes.

Objectives of Tutorial

Analysis Summary

Target Model Boundary Condition (Pinned) Finite Elements Model (Tetra Mesher)

Modal 19

Step

Procedure

Model & LBC > Geometry > Import 01 Click [ ] (New)

Click [Geometry] - [Import].

Model: Select Hanger.x_t.

Click [Open].

Note: Tutorial models are included in

the folder, Manuals / Tutorials / Files,

in the installed program folder.

1

2

3

4

Click [New], then all the menus will be

activated.

4

3

Check File type and Length Unit

1

2

Modal 20

Step

Procedure

Model & LBC > Geometry > Simplify

1

02 Click [Geometry] – [Simplify]

Target: Select the total Model.

Fillet (Radius): Enter “1”.

Click [Find].

Click [Select All].

Click [Remove].

Click [Close].

1

2

3

4

5

6

Delete holes and fillets unnecessary

for analysis.

Upon entering values and clicking

[Find], all the fillets/holes to be

deleted will be displayed by colors.

7

2

3

4

5 6 7

Modal 21

Step

Procedure

Model & LBC > Geometry > Material 03 Click [Geometry] - [Material].

ID: “2”, Name: Enter “Steel”.

Elastic Modulus: Enter “2.1e5”.

Poisson's Ration: Enter “0.3”.

Mass Density: Enter “7.9e-6”.

Click [OK].

1

2

3

“Mass Density” must be specified to

perform Modal Analysis.

4

2 2

3

4

1

Modal 22

Step

Procedure

Model & LBC > Geometry > Material (Material Assignment) 04 After selecting the model in the work

window, right-click the mouse.

Select [Material] > [Steel].

1

1

2

2

Modal 23

Step

Procedure

Model & LBC > Boundary > Support 05 Click [Boundary] - [Support].

Name: Enter “Support”.

Target: Select the 8 surfaces.

(Refer to Picture)

Condition: Select [Pinned].

Click [OK].

1

2

3

4

1

5

2

3

4

5

3

Select the 8 cylindrical surfaces of the

two holes.

Modal 24

Step

Procedure

Model & LBC > Mesh > Auto Mesh

3

06 Click [Mesh] - [Auto Mesh].

Target: Select the total Model. (Refer

to Picture)

Click [OK].

1

2

3

2

1

Modal 25

Step

Procedure

Analysis and Results > Analysis Case > General 07 Click [Analysis Case] – [General].

Name: Enter “Hanger”.

Analysis Type: Select [Modal].

1

2

2

1

Modal 26

Step

Procedure

Analysis & Results > Analysis > Modeling 08 Click [ ] (Analysis Control) Icon.

In the [Analysis Control] window,

open the tab [Eigenvalue] and in

Eigenvectors > Number of Modes:

Enter “20”.

Click [OK].

Click [OK].

1

2

3

4

2 1

3

4

Number of modes: Entering “20” will

automatically calculate the modes

from 1 to 20.

Modal 27

Step

Procedure

Analysis & Results > Analysis > Perform 09 Click [Analysis] - [Perform].

Click [OK].

Save As: Enter “Hanger”.

Click [Save(S)].

1

2

3

4

2

3

4

1

Once midas NFX ix executed, the

solver becomes engaged. Click

“Stop Execution!” to interrupt the

calculation.

Modal 28

Step

Procedure

Analysis & Results Works Tree > Hanger > Modal Analysis > MODE 1 10 Click [ ] (Isometric1) Icon.

Select Deform> Deform+Undeform

(Transparent).

In the Analysis & Results Works

Tree, double-click

MODE 1, TOTAL DISPLACEMENTS.

1

2

1

2

3 3

Natural Frequency

of Mode 1

The total displacements from the

results of modal analysis do not

represent “real” values. Therefore,

ignore the total displacements,

and check only the natural

frequencies and the modal

shapes.

Modal 29

Step

Procedure

Analysis & Results Works Tree > Hanger > Modal Analysis > MODE 3 11 In the Analysis & Results Works

Tree, double-click

MODE 3, TOTAL DISPLACEMENTS.

1

Natural Frequency

of Mode 3

1