×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Assessing Random Load with Yield Strength?

Assessing Random Load with Yield Strength?

Assessing Random Load with Yield Strength?

(OP)
Hi All,

I am new to space technology... I stumble upon the random vibration assessment criteria from a textbook that using yield strength when using a random load...

Why not fatigue strength as random vibration is a durability type of loading? P/s: not going into details to use Miner's rule etc.

In addition, what is the safety factor that used in space industry? My past experience in aerospace using 1.15 - 1.50 ... And the textbook (by J._Jaap_Wijker) suggesting the same... is that true?

Many thanks in advance!

RE: Assessing Random Load with Yield Strength?

What is the material?
What is keeping you from using a more conservative strength?
I seem to recall NASA using a 1.4 ultimate factor. But you should verify this for your component and customer. Surely you are working to a design spec?

RE: Assessing Random Load with Yield Strength?

Maybe there're two ways at looking at random vibration ...

can the material withstand the load as a static load, including dynamic factors ?
then can the structure withstand the necessary time interval (of random loading, to some PSD) ?

Admittedly the first seems practically useless, but I can see some point to the question.

ultimate factors ... depends on how much you tet and analyze and know the launch vibration loads. 1.15 (geeze, a fitting factor) seems low ... I've heard military will use 1.33. 1.50 is typical of commercial aviation, but may be too expensive for space

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

Random vibe isn't typically an issue with strength of material, per se, isn't it? Isn't it more of an issue of modal excitations? The modal excitations loosen screws and bolts and deteriorate electronics.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Assessing Random Load with Yield Strength?

satellites are often designed by the launch vibration ... why they test on a shaker table

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

(OP)
Thanks a lot for the overwhelming response!

The material is aluminum. The safety factor looks reasonable after checking with the launcher specification and standards.

Now I wonder should I use yield strength for random vibration if anyone has the experience to share? :)

RE: Assessing Random Load with Yield Strength?

guessing you don't want plasticity at those load levels ?

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

(OP)
@rb1957, yes, no plasticity

RE: Assessing Random Load with Yield Strength?

and hence yield strength as an allowable

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

(OP)
As I see random vibration is more like a durability than a strength check, using yield strength seems anti conservative, right?

RE: Assessing Random Load with Yield Strength?

see my first post

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

Dear Sarclee,
in random vibration you get RMS stress plot from analysis (e.g. by Patran Random utility, that postprocesses Nastran SOL111 FRF).
Maximum RMS stress value has to be multiplied by 3 to get the so called 3 sigma value.
In this way you are confident that in 99.7 % of cases that stress value shall not be exceeded.
Then your customer has surely specified a yield margin of safety defined as (material yield stress)/(SFy x 3 RMS stress)-1.
SFy is the product of includes various factors that depend from project specifications or general regulations, for example:
SF qualification_yield: 1.1
SF modelling: 1.1
Analysis SFs: 1
Overall SF Yield: 1.21

I remark it again: the above factors depend on project specifications or applicable regulations (NASA? ECSS? It depends on the project).

Regards,
Stefanix1972.

RE: Assessing Random Load with Yield Strength?

(OP)
@rb1957, I am not using static load...

Thanks stefanix1972! Just to clarify if the rms stress I obtained is at 3 sigma, I don't have to multiply by three?

MoS = Yield/ (SF*3*RMS@3sigma) - 1

Is it correct?

RE: Assessing Random Load with Yield Strength?

you're using a static allowable, so you're using a quasi-static load (like 3 sigma) to represent the loading spectrum.

there is another calc to be done ... to compare the spectrum loading with the fatigue allowable (s/N curve).

another day in paradise, or is paradise one day closer ?

RE: Assessing Random Load with Yield Strength?

Dear Sarclee,
by random analysis, if you input nominal PSD (e.g. in acceleration expressed in m/sec2) you get the RMSstress value in N/m2.

This RMSstress value has to be multiplied by three:

MoS = Yield/ (SF*3*RMSstress) - 1

In synthesis, be careful not to multiply twice by three.
You also need to pay attention to PSD curve units: if it is in g-units, RMS stress value has to be multiplied by 9.81 m/sec2 to get N/m2.

Regards,
Stefanix1972.

RE: Assessing Random Load with Yield Strength?

(OP)
@Stefanix1972

Thanks. That is what I concerned if I take 3 sigma stress, should I still take the multiplier of 3?

MoS = Yield/ (SF*3*RMSstress) - 1

I was told, not space industry, that random vibration is a durability event, either we use fatigue strength in calculating MoS or we use Miner's rule to do cumulative damage ...

But based on the comments above, it seems the space industry is using yield strength in calculating MoS for Random Vib...

RE: Assessing Random Load with Yield Strength?

Dear Sarclee,
you are right: in space industry random vibration is relevant in the 120 seconds from launch onwards, so there is no real concern for fatigue due to random vibration. There is a minor concern for fatigue for the effects of ground tests, transportation loads, launch loads.
Thermal loads can be assessed for fatigue effects if you are dealing with a spacecraft in orbit around a planet.
In aeronautics, automotive, etc, of course random fatigue has to be taken into account.
I have not a specific background on this, but I would check on Steinberg and MIL-STD-810 last versions.

Regards,
Stefanix1972.

RE: Assessing Random Load with Yield Strength?

(OP)
Thanks Stefanix1972 for clearing my doubt!

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! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close