×
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

FATIGUE CHARACTERIZATION

FATIGUE CHARACTERIZATION

FATIGUE CHARACTERIZATION

(OP)
I´d like to know how to characterize a material to know their fatigue properties, I was looking for information and I didn´t find anything.

RE: FATIGUE CHARACTERIZATION

Fatique calculations is highly statisticaly. The objects geometry plays just as large a role as the material itself. As far as I can remember in one formula half of the tensile strength is used, multiplied by up to six emperical and statistical factors. These are for surface ruffness, object size, relaibility, temperature, stress consentrations (geometry) and corrosion. For these factors there are tables, graphs and simple emperical relations to calculate each. Try to get hold of the book: 'Mechanical Engineering Design' by Joseph E. Shigley. It has most of the tables, graphs and relations for the factors in it. The book is also great for basic mechanical design aplications.

Some steel tables does include a unmodified fatique limit stress, noted as Se. This can be used instead of half the tensile strength.

RE: FATIGUE CHARACTERIZATION

Finding the fatigue properties of materials is quite tricky unless they are welded, because the crack initiation phase is difficult to predict.  If they are welded, then one assumes they already contain flaws and calculating fatigue performance is easier as less information is required. Characterisation is then dependant on the type of joint configuration used.

If you want to know about the basics of fatigue in welded joints you will find some useful information on my web site www.gowelding.com


RE: FATIGUE CHARACTERIZATION

I'm writing a paper on fatigue and I can give you some good titels.
Fatigue design, Eliahu zahavi

Machine design an integrated approach, Robert L. Norton

You should find something in these.

RE: FATIGUE CHARACTERIZATION

Mechanical Metallurgy by Dieter also has a good basic chapter on fatigue testing and the ins and outs.  Other than that maybe the TMS website has some books for materials engineers.  There are pifalls to be aware of regarding fatigue testing, and this is also why it is a highly researched topic.  For example, be wary of handbook values for Al-Si casting alloys.  The fatigue life is strongly dependent on porosity, which is an alloy-process variable.

RE: FATIGUE CHARACTERIZATION

I recommend that you read:
Analysis & Representation of Fatigue Data by Joseph Conway and Lars Sjodahl
and
MIL-HDBK-5, Chapter 9 (http://www.grantadesign.com/news/battelle.htm)

The Conway & Sjodahl book is out of print but definitely worth the price. MIL-5 is the material properties bible for stress analysts working on defense projects.

To create a statistically acceptable fatigue design curve, you perform the following steps (Excel functions in parentheses):
1) Linear regression of S-N data, with S as the "X" and N as the "Y" (m=Slope(Y,X) & b=Intercept(Y,X))
2) This gives you an equation that looks like Log(N)= b + m*log(S)
3) Compute the standard error of the estimate (STEXY(Y,X)
4) Your design curve is Log(N) = b + m*log(S) - k*see
5) k is a function of how "safe" you want to be. You can set k to 3, giving you a "-3sigma curve." It is better to use k as a function of the number of specimens you tested. There is more detail about that approach in the MIL-5 reference cited above.

Doug

RE: FATIGUE CHARACTERIZATION

Correction to my previous post:
Usually, X is log(N) and Y is log(S). You can try other transformations, but as I described the process, these should be used.

Doug

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