×
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

Spectrum Data
2

Spectrum Data

Spectrum Data

(OP)
I have been given a standard spectrum for a wing box at work which is grouped into 511 blocks with a max stress of 187 Mpa and a min 04 -44 mpa. Altogether it has 123million flight cycles. Is there any information available on how a standard spectrum is obtained?

As I have no understanding on this topic?

Thanks

RE: Spectrum Data

2
I assume you are looking at a fatigue spectrum for a wing box structural detail?
As far as I know there are different ways of obtaining such a spectrum.
One way goes like this:
First a number of Fatigue-flights (or missions) are defined. For example a short flight, medium flight and long flight mission.
Each of these are subdivided in a number of segments, say towing, taxiing, take-off, ascent, cruise, descent, landing, taxiing.
Again each of these segments are subdivided into a number of intervals. Each interval defines a flight or ground condition as an average load case plus a possible unit delta load case (fluctuation) plus the spectrum of fluctuation that goes with it. For example an interval of the cruise segment could be defined as a 1g cruise case at 20000 ft plus a 0g unit (10ft/s) lateral gust case plus the spectrum defining the exceedances of a certain lateral gust level (x ft/s) as function of altitude and distance traveled.  
Per interval, the load cases translate directly (linearity of the structure) into average stresses and exceedance curves of fluctuating stress levels at the considered structural detail, in your case a detail at the wing box.

Another way goes like this:
First a number of Fatigue-flights (or missions) are defined. For example a short flight, medium flight and long flight mission.
Each of these are subdivided into a number of segments, say towing, taxiing, take-off, ascent, cruise, descent, landing, taxiing.
Again each of these segments are subdivided into a number of intervals. Each interval defines a flight or ground condition as an average load case plus a Power Spectral Density function of the fluctuating load case (This is found as the (Transfer Function )^2 * Power Spectral Density function of the excitation.)
The average load cases and the PSD of the fluctuating load cases again translate directly (linearity of the structure) into average stresses and PSD of fluctuating stresses at the considered structural detail. The fluctuating stress PSD’s can be integrated to get the Variance(RMS^2) stresses or RMS stresses. From these, Stress Exceedance curves of fluctuating stress levels can be found.

Regards

Onemorechance

RE: Spectrum Data

Of course there is lots more to say on this topic. Because I was not at all complete in my previous post. I only commented on how you build up a mission spectrum for one flight.
This one flight being a long, medium or short mission.
Often a real flight is seen az a mix of these 3 missions, and the fatigue damage resulting from this one flight will determine how many flights are achievable until failure of the detail. Etc…
 
 Onemorechance

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