2015 Americas Altair Technology Conference

Fatigue Life from Sine-on-Random Excitation

2015 Americas ATC

May 7th, 2015

Jeffrey Mentley

HBM-nCode

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Agenda

1. Problem description

2. Engine speed profile

3. Extraction of sine on random loading

4. DesignLife vibration fatigue

1. Creation of FRF

2. Sine on random loading

3. Fatigue results

5. Summary

6. Questions

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• Components on or near rotating machinery can be subjected to

dynamic loads

• These loads can be random, sinusoidal, or a combination of both,

and are important in understanding the fatigue life of the

component

Problem description©

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Example test spec from MIL STD 810

Acc

eler

atio

n

Log frequency Hzf3 f4

A3 A4

W0

W1

f5

A5

Random PSD (g2/Hz)

Sinusoidal tones (g)

f2

A2

f1

A1

• Components on or near rotating machinery can be subjected to

dynamic loads

• These loads can be random, sinusoidal, or a combination of both,

and are important in understanding the fatigue life of the

component

Problem description©

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BM Engine order plot of acceleration

• The actual engine loading environment is swept sine on random

• The analysis approach being used in DesignLife is sine on random

• The RPM history is separated into blocks, each at a constant RPM

• Visually

• Time at level

• Each RPM block is represented as a sine dwell

Engine test schedule©

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Time

RP

M

• For each constant RPM block

• Calculate time spent at this RPM ( engine usage )

• Extract acceleration time history at this RPM ( measured data )

• Determine order or orders for sine tones

Engine test schedule©

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Engine Speed Profile

• For each RPMs acceleration time history

• Calculate frequency for sine tone based on order and RPM

• Remove sine tone frequency from acceleration history using band stop

frequency filter, and create a PSD for random loading

• Pass only the sine tone frequency from the acceleration history using a

band pass frequency filter

• Process statistics of sine tone history to obtain RMS of signal, and

calculate appropriate sine tone amplitude

Extracting sine on random loading©

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Random

Sine Dwells

Vibration fatigue in DesignLife Version 11.0©

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Analysis path for sine on random loading©

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OptiStruct

nCode DSP

nCode DesignLife

• Accessory bracket mounted on engine

• Modal analysis ( 1st 8 modes )

• Unit vertical acceleration sweep across frequency range ( 1 - 400 Hz )

OptiStruct analysis to obtain FRF©

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Frequency Response Functions from OptiStruct©

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Modal

Coordinates

PCH

Modal

Stresses

H3D,OP2

Modal Analysis

Frequency Response

Function

Harmonic

Analysis

Harmonic

Analysis

Complex FRF

OP2

Frequency Response

Function

Direct FRF Modal FRF

• Duty cycle used to sequence sine on random analyses

• Engine speed profile used to set each RPM duration

• Each sine on random analysis with its own PSD and sine dwell

Sine on random loading©

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Engine Speed Profile

• OptiStruct input

• Sine on random duty cycle

• SN fatigue properties

Fatigue analysis process©

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• Fatigue results for entire component

• Damage

• Life

• RMS stress

• Expected max stress

• Spectral Moments, Irregularity factor

• Results for overall engine test, and each RPM event

• Damage percentages across events

Fatigue results©

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• Approach for fatigue life using both random and sinusoidal loading

• Ability to use actual measured engine data to define loading

• Vibration duty cycle created from engine speed profile, and sine on

random input for each RPM condition

• OptiStruct used to define the frequency response function of the bracket

• DesignLife combines the FRF and vibration duty cycle, and computes the

fatigue damage

Summary©

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Questions

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measure and predict with confidence

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© 2015 HBM

Jeff Mentley

HBM-nCode

jeff.mentley@hbmncode.com