dynamic response in nx nastran - structural design …€¢ random response demo dynamic response in...

5/22/2012

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IN THIS WEBINAR:

• FEMAP Dynamics Add-on Overview

• Frequency Response Demo

• Random Response Demo

Dynamic Response

in NX Nastran

Andrew Jabola

Senior Aerospace Stress Engineer

Structural Design and Analysis

andrew@structures.aero

PRESENTED BY:

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Agenda

• Introduction

• Dynamic Analysis Solutions

– Solutions

– Typical Applications

• Demo

– Frequency Response/Sine Vibration Model Setup

– Random Vibration Setup

• Conclusion

• Q/A

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Dynamic Analysis Solutions

• What is a Dynamic Analysis?– Any case where the load is time varying and inertia forces may be present

• NASTRAN Dynamics adds the following solutions to FEMAP– Transient Response

– Frequency Response

– Shock Spectrum Analysis

– Random Vibration

– DDAM Analysis

• Most Solutions offer both direct and modal solution capabilities– Direct – Applies the direct equations of motion to every grid point, providing a more accurate but more costly

answer

– Modal – Uses mode shapes to determine response and reduces overall model size

Equations of Motion – Direct Analysis

Equations of Motion – Modal Analysis

Full Mass Matrix of

System

Transforms Matrix into

Modal Coordinates

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Dynamic Analysis Solutions

• Solution Types

Type Input Output

Modal • Prescribed BC’s / No Force Input

• Natural frequencies and

corresponding mode shapes

• Stress/strain profile

Frequency

Response

• Sinusoidal excitations across a

range of frequencies

(Deterministic)

• Sinusoidal response at each

frequency

• Min/max response over frequency

range

Spectrum/Shock

• Spectrum representing the response

to a specific time history

(Deterministic)

• Maximum response if the model were

subjected to the time history

Random

• Spectrum representing probability

distribution of excitation (Non-

Deterministic)

• Response within specified range of

probabilities

Transient • Time-varying loads (Deterministic) • Time-varying response

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Typical Applications

• Typical Applications Dynamic Response– Aerospace Industry

• Heavy usage of Frequency and Random Response

– Marine• DDAM

– Vehicle Dynamics

– Civil/Structural Applications

– Heavy Equipment

– …anything where load is time varying and inertia and damping also play a factor into overall response

• Focus of webinar will be on the following use cases– Frequency Response – Sine Vibe Analysis

– Random Response – Article Verification

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Frequency Response

• Frequency Response computes the structural response to a steady-state

oscillatory excitation

– Heavily used in aerospace industry for sine vibration tests

– Provides a structural response vs. frequency for a specified range

– Used as a basis for random vibration analysis

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Frequency Response Setup

• Demonstration

– Model Overview

– Checking Modes

– Setting up Loads

– Setting up Analysis Deck

– Postprocessing Data

Model courtesy of NASA

Goddard Space Flight

Center – WFIRST

Program

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Modal Content

• Modal Mass Participation

– Ensure you’re capturing at least 80% of the mass participation or covering the

frequency range of interest

– Modal Mass Participation for first 20 modes

Mode # Frequency T1 T2 T3 R1 R2 R3

1 6.49E+01 1.00% 0.05% 2.87% 0.00% 4.10% 0.14%

2 7.99E+01 1.41% 0.24% 0.51% 0.15% 3.94% 0.01%

3 1.07E+02 0.83% 0.09% 0.04% 0.06% 0.03% 5.76%

4 1.09E+02 0.20% 4.57% 1.46% 1.99% 0.51% 0.11%

5 1.29E+02 1.41% 1.22% 0.00% 0.02% 0.18% 6.62%

6 1.94E+02 0.38% 1.32% 9.82% 0.06% 0.00% 0.00%

7 2.37E+02 0.55% 0.01% 0.22% 0.00% 0.47% 0.28%

8 2.63E+02 0.36% 0.01% 0.00% 0.00% 0.06% 0.03%

9 2.88E+02 10.29% 0.47% 0.02% 0.09% 2.06% 1.37%

10 3.24E+02 0.10% 7.98% 0.12% 2.20% 0.02% 1.28%

11 3.65E+02 0.89% 0.01% 0.33% 0.00% 0.19% 0.05%

12 3.72E+02 0.04% 0.83% 0.00% 0.18% 0.01% 0.13%

13 4.30E+02 0.47% 0.01% 0.00% 0.00% 0.04% 0.03%

14 4.59E+02 0.00% 0.00% 0.49% 0.00% 0.01% 0.00%

15 4.70E+02 0.00% 0.00% 0.01% 0.00% 0.00% 0.00%

16 4.92E+02 0.00% 0.03% 1.86% 0.01% 0.01% 0.00%

17 4.99E+02 0.00% 0.14% 0.02% 0.03% 0.00% 0.01%

18 5.05E+02 0.36% 0.06% 0.02% 0.01% 0.00% 0.00%

19 5.09E+02 0.04% 0.30% 0.00% 0.10% 0.01% 0.07%

20 5.36E+02 0.02% 0.53% 5.13% 0.06% 0.01% 0.02%

Modal Mass Participation Percentage

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Damping

• Types of Damping

• For frequency response and random response, modal

damping is typically used

• It can be specified by the following methods:

– Critical Damping (2.5%)

– Structural Damping (5.0%)

– Q Damping (20)

• Relationship shown below

Q Damping

Critical Damping

Structural Damping

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Frequency Cards

• Responses are specified at certain frequencies based off a variety of criteria using the FREQ Cards– Frequency List Methods (explicitly define frequencies by a range)

• FREQ, FREQ1, FREQ2

– Modal Frequency Methods (list frequencies based off modes and generating points around each mode)

• FREQ3, FREQ4, FREQ5

– It is common to use the modal frequency to fill in data around modes and using the list methods to catch any missing data

– Can be combined

• Refer to NX NASTRAN QRG for more detail of FREQ –FREQ5 Cards

FREQ4 Frequency

Spread

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Random Response

• Random Response is used to look at load environments that are non-deterministic

– Uses a statistical approach to analyzing the response to a structure where the instantaneous magnitude is not known at a given time

• Applications include acoustic environments from launch, earthquake analysis, and wind pressure fluctuations

– Analysis uses frequency response as basis for determining overall statistical response by multiplying the transfer functions of the frequency response and the input PSD to develop a response spectral density

• This is in turn used to determine an RMS response, which is used to determine the statistical response of the structure

Sine Vibe Transfer

Function

Input PSDResponse Spectral

Density

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Random Response Setup

• Demonstration

– Show Input PSD

– Show Additional Analysis Setup

– Postprocess Data

Model courtesy of NASA

Goddard Space Flight

Center – WFIRST

Program

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Random Response

• Standard Input

– Power Spectral Density (PSD) function, determines the input environment for the

structure

– Specs are determined by analyzing time history responses of various environments

– Typical spec shown below (Source: GSFC GEVS-STD-7000)

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Random Response

• Random Output– Random will generate the following data

• PSDF Data– Used for looking at overall spectral densities to determine areas of high response

• CRMS Data– This is cumulative RMS value that shows how the energy adds together as you increase in frequency

• RMS Response– The RMS response of each data point of interest

» This is equal to the square root of the area under the PSDF data curve

– This RMS values is typically multiplied by a factor (typically 3) to show the highest stress within a certain probability

» 3 Sigma ensures that the highest stress due to the environment will not exceed this value 99.7% of the time

• Positive Crossings– Determines the amount of cycles per unit of time occur

» Typically used for fatigue calculations

RMS Output Value

RMS Von Mises StressAcceleration Spectral Density

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VIEW THE WEBINAR RECORDING

• If you would like to see the

demonstrations from this

presentation, you can view the

recording of this webinar at:

https://structures.aero/webinar/

dynamic-response-nx-nastran/

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Tips and Tricks

• Several Tips and Tricks

– Run Modes First!

• Understand the natural frequency of the system to get an idea of what frequencies will play a

major role in your system. This should be performed in combination with understand modal mass

participation to determine what modes will likely be significant

– Ensure that modal analysis captures full modal content

• Ensure that you are capturing at least 80% of mass participation or full range of frequencies of

interest

– Verify input by looking at response at constraint location

• This will verify that your load is correct for the model

– Turn on RESVEC

• Captures the higher modal content that is not captured in lower modes frequency content

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Conclusion

• FEMAP Dynamics Add-on adds several dynamic solutions to the FEMAP

environment:

– Transient Response

– Frequency Response

– Random Response

– Shock

– DDAM

• Bulk of analyses focus on random and frequency response, using the modal

method for solution

• Dynamic Analyses are very use for time varying loadings and any places

where inertia and damping play a major affect on the response of a structure

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Structural Design and Analysis (Structures.Aero)

Structural Analysis

• Team of 14 engineers that help our clients

design lightweight and load efficient structures.

• We service aerospace companies and other

industries that require high level analysis.

• Specialty in composites and lightweight

structures

• Tools used include hand analysis, HyperSizer,

Femap, NX Nastran, Fibersim, NX, Solid Edge,

Simcenter 3D, LS Dyna, and LMS.

Software Sales and Support

• Value added reseller providing software, training,

and support for products we use on a daily

basis.

• Support Femap, NX Nastran, Simcenter 3D,

Fibersim, Solid Edge, and HyperSizer.

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GET DYNAMIC RESPONSE

• You can acquire the Dynamic

Response add-on for Femap with

NX Nastran for less than $3K.

• Contact Marty Sivic for a quote

For questions about pricing, or to see

a demo, please contact Marty Sivic.

Marty SivicDirector of Sales

msivic@structures.aero

724-382-5290

5/22/2012

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For questions on the material covered

today, please contact Andrew Jabola.

For questions about pricing, or to see a

demo, please contact Marty Sivic.

Questions?

Marty SivicDirector of Sales

msivic@structures.aero

724-382-5290

Andrew JabolaSenior Aerospace Stress Engineer

andrew@structures.aero

703-935-2827