×
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

Fatigue analysis maxi principal stress

Fatigue analysis maxi principal stress

Fatigue analysis maxi principal stress

(OP)
HI, I'm Alfonso.
I need explanation on a fatigue analysis using Max principal method.
I've applied 8 fatigue load condition on a fem model, with a static analysis.
I've found the critical location on the model, and allowable delta sigma from en-1993.1.9 .
Now I've 8 max pirncipal and 8 min principal stresses in this location, but I don't know how use those.

Is it correct subtract the maximum MaxPrinc and minimum MaxPrinc and compare the result with the allowable (do the same with MinPrinc)?

I've read that there are a lot of different approach.
What are the most popular methods?

thanks.

RE: Fatigue analysis maxi principal stress

The general method is to calculate the maximum principal stress range, ie. not simply the difference between the maxima but the difference between the stress components from which you calculate the principal stresses. Then it's common to use the rainflow method. I think it's all detailed in the standard you quote.

RE: Fatigue analysis maxi principal stress

Generally if one has a bunch of possible max principal stresses it
is best to create a plot of their values versus
their angle. As Corus recommends one must do this for both tensile and compression
principal stresses. The 2D plot of stress vs angle will usually show two
large critical angles. One for each principal stress (assuming planar problems).
One needs to pick load cases that give both large tensile principals and
large compressive principals.

When there are multiple channels of load inputs scan each channel
for max and min values, and at those times also record the values of the
other channels. i.e.: things may be out of phase. Then plot the principals
for each of these "simultaneous max mins". To save FEA time it is probably
best to run an FEA for a unit of load for each channel independently. Then use
superposition to recombine the loads for each set of simultaneous max mins.

After picking the critical direction from the stress vs angle
plot run all the load cases with stress output for this critical direction
and then rainflow count the result. This assumes no multiaxial stress conditions
where the principals and angles are all over the place.

RE: Fatigue analysis maxi principal stress

(OP)
Thanks all

Is it possible use von mises stress insted of max princ,if you have a multiaxial stress?

RE: Fatigue analysis maxi principal stress

you can use von Mises but fatigue is more dependent on max principal (von Mises is IMO more of static stress, combining the multi-axis stress state into a single value).

you say you have 8 stress cases. do your 8 cases happen sequentially ? are the cases assumed to cycle, +ve to -ve, when they occur ? does the stress state reduce to zero between loads ? would this be a conservative assumption ?? someone has mention "rainflow", look it up (as it applies to fatigue analysis). you need to understand how the max. principal stress can cycle at the critical location. It is not simple and difficult to write (in a blog like this).

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

RE: Fatigue analysis maxi principal stress

In elements that are severely multiaxial, as observed in a
max/min principal vs angle plot, one cannot use von Mises or just a principal.
Best method is to divide the possible angles into 10 or 20 representative
cuts and then resolve the element stress history to stresses
perpendicular (plus shear) to each cut. Then Rainflow these resulting stress
"on a cut" histories to find the critical angle. Probably also best to use
a damage parameter that includes shear. The one by Findley is probably ok, but
there are a number of others.

RE: Fatigue analysis maxi principal stress

maybe a post-FEA code (like "ncode") could help ?

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

RE: Fatigue analysis maxi principal stress

Yes ncode, or any of the other two(?) use a similar scheme.
But its expensive per seat for the commercial codes,
and the process isn't really that difficult for a few elements.
I'm tempted to write an open source version.

RE: Fatigue analysis maxi principal stress

sounds like you should draw mohr's circle for each load case.

but load sequence and cycles is vital (I mean I'd use the max principal stress for each load case as being conservative and worry about the possible load cycles before I tried to "perfect" the stress story by saying in this particular direction I have max principal stress for case 1, some normal stress and some shear stress for case 2, etc).

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

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

White Paper - How ESI is Helping Move New Medical Device Product to Market Quicker & More Cost Effic
Early Supplier Involvement has long been a strategy employed by manufacturers to produce innovative products. Now, it almost seems like a necessity. Because decisions made in the design phase can positively affect product quality and costs, this can help add value to OEM bottom lines. This white paper will discuss many facets of ESI, including why it’s so valuable today, what challenges limit the benefits of ESI, how cost is impacted, and more. Download Now
White Paper - Moving to a Driverless Future
This white paper describes what we see as the best practices to support a sustainable engineering process for autonomous vehicle design. It exposes how to use simulation and testing in common frameworks to enable design exploration, verification and validation for the development of autonomous cars at a system, software and full-vehicle level to drive a mature product development process for automated driving. Download Now
Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. 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