×
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

Design with Yield or Ultimate stress?

Design with Yield or Ultimate stress?

Design with Yield or Ultimate stress?

(OP)
Hello Guys,

i my attempts to get practical experience with FEM, i reverse engineered an excisting part and used actual (fairly accurate) worst case loading.

After refinement the stress in the tooth fillets are considerabel above yield. The stress is actualy around 390MPa for ductile iron, which has a yield of 320MPa.

Basically this would mean the part would yield in  the filled in service, but i'm reluctant to accept this, since the actual part has been in service for years without any failure in the fillet with the "too high" stresses.

Simply put, i'm confused how to deal with this..


Now my question: do you guys "accept" parts for prototype testing  even if the FEM is above yield (but below ultimate)?


p.s. again, the stress is real and not a hot spot. The high stress is the location one would expect it to be in the actual part. Furthermore i double checked the loading with actual test report data.

RE: Design with Yield or Ultimate stress?

Is this loading static or cyclic?

RE: Design with Yield or Ultimate stress?

(OP)
TGS4,

yes it it cyclic.

The stress (P1) in the fillet fluctuates per revolution of the part.

The lowest value of P1 is near zero, so the fluctuation is almost equal to the max. value of P1, the 390MPa.
 

RE: Design with Yield or Ultimate stress?

A stress at a fillet radius is a localised (peak) stress where stresses above yield would be acceptable for static cases. As the stress is cyclic then failure would probably be by fatigue. You'd need to check the stress against the SN curve for that material to see how many cycles it could be designed for.  

Tata  

RE: Design with Yield or Ultimate stress?

(OP)
Since it was a truck wheel end, i was kind of surprised that it seems to be designed for fatigue.

Is it common to design for fatigue and not for yield for such critical components?

It seems to me the risks are a lot higher this way(what will happen in service?,loads not underestimated?,subpar material casted?). Furthermore extensive fatigue testing should also be done, which is all but cheap.

Just curious.


Regards,




 

RE: Design with Yield or Ultimate stress?

Even if the stress was just under yield it may fail from fatigue damage, depending on the number of cycles. There is a general rule of thumb though that if the stress range is below half UTS then it has 'infinite' life. For a wheel I would have thought such a limit would apply. Best to check on the actual value for that material.  

Tata  

RE: Design with Yield or Ultimate stress?

(OP)
I looked it up,

Yield strenght: 320MPa
Bending fatigue limit(non-symmetric): 380MPa

Since the stress cycles from near zero to Max. P1(320MPa), it seems to be designed for fatigue after all.

So, I was wrong in my initial assumption.

Thank you all!

 

RE: Design with Yield or Ultimate stress?

It might not matter in your particular case, but in general you should Walkerize the component stresses from FEA to determine mean stress and stress amplitude for life calculations.
Components are often designed to meet a fatigue life, rather than a simple yield criteria, especially when considering concentrated stresses, so your situation is not uncommon.

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