×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

PSD(G^2/Hz) to time history for fatigue analysis
6

PSD(G^2/Hz) to time history for fatigue analysis

PSD(G^2/Hz) to time history for fatigue analysis

(OP)
I have a question concerning converting PSD random vibration levels into a representative block of time history.  I am able to write a software code that converts G's vs. Hz in the frequency domain back into a G's vs. time in the time domain by including a random phase number.  I have a specification for a military avionics unit which has a bracket that is of concern.  We are convinced that the most appropriate way to perform a fatigue analysis on the bracket is by the strain life method, because we believe it is possible even w/ 3 sigma clipping that we may be experiencing periodic stresses that exceed yield for a small percentage of the time.  The bracket material is aluminum 6061-T6 and we have the ability to produce an FEA model in Ansys.  The following is the process I am considering using to generate an estimate of fatigue life

CAD model--->FEA model                       |
PSD spec--->Conversion to time history (?)   |

------>
Input time history into FEA model as an inertial loading event and track the strain history in the area of interest (Multi-linear material model used).

-----> Rain flow counting  --->  Strain life method coupled with Miner's cummulative damage-----> Result


The difficulty I am having is being able to convert the PSD in (G**2/Hz) into a (G/Hz) profile so I can perform an inverse FFT to calculate a time history block.  Does anyone know of how to go about tackling this problem?


Thanks


  




RE: PSD(G^2/Hz) to time history for fatigue analysis

Integrate the G^2/Hz curve with respect to Hz (G^2/Hz * Hz = G^2).  Take the square root of the result to obtain G's.

RE: PSD(G^2/Hz) to time history for fatigue analysis

2
I have posted a Matlab program to synthesize a time history to satisfy a PSD at:

http://www.vibrationdata.com/matlab.htm

The program is called: psd_syn.m

The program can be downloaded via:

Username: vibration
Password: quake

Tom Irvine

RE: PSD(G^2/Hz) to time history for fatigue analysis

ThetaJC-
Note that when you go back to time history from the PSD profile you inevitably lose all history of acceleration sign changes (changes in direction).  You will end up with only positive values.  Now you may be able to INFER sign changes by cusps in the time plot...

Tunalover

RE: PSD(G^2/Hz) to time history for fatigue analysis

2
btrueblood appears to have answered your question for you. However, you must be careful when using this sort of random-phase multisine for fatigue calcs.

The problem is with the crest factor of your reconstituded time signal. Crest factor is defined as the ratio of the maximum absolute value in the time history divided by the rms value of the time history. The crest factor using a random-phase multi sine will probably be 3 or more because at some time points you will find that (purely by chance) you get a very high amplitude.

The time data which was originally used to generate the PSD probably had a much lower crest factor (assuming it came from real measurements on a real structure). In this case your fatigue predictions will possibly be over-consevative (no bad thing, perhaps!). Ideally, if you had knowledge of the crest factor of the original measurement, you should ensure that the crest factor of your reconstituded signal is approximately the same. This crest factor optimisation is relatively easy to achieve (especially as you seem to have a good understanding of FFTs).

M

--
Dr Michael F Platten

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources