Stress concentration / grey iron / elasticity 3

[sdra2]

I have a number of questions regarding stress analysis of irregular geometry in grey iron castings.

(1) How can I evaluate the theoretical stress concentration factor (using FEA techniques in my case) for irregular geometry? I have read that for, say, a simple bar in tension with a notch, comparison of the notch root stress with the "core" figures gives the theoretical stress concentration. This makes sense as an illustrative example, but can such a method be applied to more complex shapes and, if so, how? How can a "good" figure for the nominal section stress levels be obtained? A "mid-plane" figure or something similar? Is this a dangerous general approach because it is very geometry dependent?

(2) As an alternative to (1) above, is there any value in running analyses with varying notch sizes? This will plainly give rise to a trend of stress concentration data, but without a base case to compare this against, what purpose would such data serve?

(3) Assuming that somehow I obtain a figure for the theoretical stress concentration factor (Kt), can I correct for statically loaded grey iron using Kf=1+q(Kt-1) like I might for a fatigue loading situation?

(4) If (3) above is valid, at what point does it become invalid? As Kt is based on elasticity theory, does this idea become invalid at stress levels beyond the proportional limit of the material?

Thanks in advance to anyone who may give me some useful pointers.

 

[swall]

My recollection is that since gray iron is such a brittle material, critical flaw size/stress intensity type of analysis models are not valid.

 

[NickE]

swall -- yep.

Due to the shape of the graphite in grey iron, any casting contains a large number of "notches" or stress concentrators. With each flake of graphite acting as a notch.

This is especially bad on machined surfaces, the graphite is continuous throughout the structure.

 

[metengr]

In addition to the correct responses by swall and NickE above, the actual stress concentration factor (Kt) can be calculated from the web site below based on component geometry and notch root radius;

http://www.fatiguecalculator.com/finders/findkt.htm

 

[sdra2]

Thanks to everyone who has replied to this post.

What I am basically trying to understand is: what is it that constitutes a design stress limit in a Finite Element model for a grey iron casting?

This could be an as-cast surface, or a machined surface.

If the FE model is reporting a high stress in a notch or radius, can I factor this reported stress figure down because of the "notch insensitivity" of grey iron, or is it simply the case that as there are many discontinuities within the material matrix anyway, it is not valid to use Finite Element or indeed any stress evaluation techniques to evaluate the suitability of grey iron where (macroscopic) notches are present?

Is there a sound design methodology for evaluating the suitability of grey iron castings for a given duty at all, or is it that there are so many uncertainties due to the nature of the material that only physical testing can suffice?

 

[GBor]

Great site, metengr! Here's a start for next week's "Tip Master"!

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Cockroach]

Wow MetEngr, I fully agree with GBor...and then some. Another star for your!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[dvb62]

This link should help;

http://www.machinedesign.com/ASP/strArticleID/57766/strSite/MDSite/viewSelectedArticle.asp