Short ED, again

[koopas]

Hi all,

I am still a relatively new and inexperienced airline structural engineer. I have a question about fastener short edge distance relative to a part's edge, again (I posted a similar message a few months ago, apologies if sections are redundant)

I'd like to know if the following methodology is correct when analyzing short ED conditions.

If the short ED is parallel to the load path (part's edge is perpendicular to load path), I'd use the empirical formula for shearout:

Pall = Fsu_material * t * (2ED + 0.766D) or
Pall = Fsu_material * 2t * (ED + 0.383D)

Then, I ensure that the Pall you just calculated (using the actual, shorter ED) is greater than the Pall from the SRM/MILHDBK 5 (based on 2D). If it is, the short ED condition is OK.

For short ED's that are normal to the load path (part's edge is parallel to the load path), I would do a net tension check using Pall = Ftu*A_net. Then, I would ensure that Pall is higher than the MIL-HDBK joint allowable Pall.

Please correct me if I am wrong. Do you agree so far?



The last time I posted this question, Philcondit responded with:

&quot;One thing to remember about the shear-out formula: It may be unconservative for e/D < 1.5. This can be appreciated if you envision the &quot;ligament&quot; as a beam, under a distributed load from the fastener. For e/D > 1.5, this beam is a shear critical beam, while a smaller e/D becomes bending critical (a picture here would be helpful, I know). So, how do you analyze a shy e/D less than 1.5 you ask? Treat it as an idealized lug and analyze according to Bruhn.

For e/D perpendicular to the load path, look at Petersons handbook for stress concentrations. They have a case for a hole near an edge. If you take the Kt's for various e/D's and normalize to a 2D configuration, you have a relative stress factor. To take it one step further, if you knew the fatigue life of the original part (maybe DT inspection, Service Bulletins, or fleet history), you could take the inverse of the relative stress to the fourth power (typical for aluminum)and get a relative life ratio to multiply your unmodified life by. Fun Stuff!&quot;

You lost me when you said &quot;ligament&quot; and &quot;Treat it as an idealized lug and analyze according to Bruhn.&quot; Could you digress a little? Oh, I don't own Bruhn.

Likewise ,for short ED normal to the load path, I don't know how to do the analysis with Peterson's Kt's. I have never dealt with Kt's. Is that related to crack formation, growth, and propagation? I can see that for this type of short ED, the part is susceptible of developing a crack from the hole all the way to the edge due to the tensile load and reduced net area. For this type of loading, you actually recommend performing some sort of fatigue analysis instead of a simple net area check?

(should I get Bruhn and Peterson's books?)

Thanks for your wisdom!
Alex

 

[koopas]

Sparweb,

The shear-out equation can be found in Douglas and Boeing structural training manuals.

Yes, the 0.383 coefficient comes from the 50-deg. angle with the line of the rivet row.

Pso=Fsu * t * 2(e - S)

where e = edge distance, S = (1/2)(D)*sin(50) = 0.383*D

Hope that helps,
Alex

P.S. Did you get my latest email? Thanks for your reply to my bending moment & shim post. I'll try to read the Bruhn ref. tomorrow.

 

[Sparweb]

Looked it up first thing this morning when I arrived at the office, and lo-and-behold, it's all in Chapter D1 of Bruhn.

BTW, if you've got both the Douglas and Boeing training manuals handy, what makes you think I can help you?

STF