Webs and mode of failure 1

[koopas]

Hello all and happy new year:

I understand that a beam web is to take mostly shear loads while the tensile loads are carried by the caps. Here are my questions:

1. When cutting out a section of web and adding a doubler, why calculate the load transfer capability based on Ftu and not Fsu? Using Fsu would half the number of fasteners. Would the web ever see a load above Fsu?

2. May be a stupid question but...I've been wondering why webs as we know it (thin sheets) are so efficient in taking shear loads. Is a beam that's made up of a rectangular cross section (i.e. no cross sectional area chanage between the caps and the web) grossly inefficient?

3. What is the failure mode of a web in aircraft applications? I don't think the web will fail in shear but rather buckle when you transform the shear loading into the principal tensile stresses. Is that correct?

4. Can one idealize the fuselage skin between two stringers as being the web of a beam? (the caps are the stringers in this case).

5. Last, besides in-plane tensile loading due the hoop stress in the fuselage skin, I understand that the skin sees in-plane shear stress (as in a typical beam application). Does the fuselage skin see much out-of-plane bending? What other loads does the skin encounter?

Thanks,
Alex

 

[edbgtr]

Hi Alex

Happy New Year to you too.

It's been 7 days since you posted this question, and I suspect it's not because no one can answer it, but because I think you need to read the classic text books on the subject first before coming to this forum for an explanation.

Please beg, borrow or steal the following books on the subject, study the theory of shear flow and the inter-related shear and bending stress distributions of I-beams, and then come back to this forum with your specific questions relating to failure modes of shear panel members in semi-monocoque construction. The books are Peery, Bruhn & Supplement, Prof Niu's 2 books on metal airframe construction, Megson, Rivello, and any others you can find. Also visit the NACA website for the major research and development work done in the 1940's and 50's regarding this subject. Know the history and you'll know where it came from and where you can go with it.

Ed.

 

[koopas]

Ed,

Thanks for your reply.

I understand the basic theory of shear stress in beams and shear flow, as I studied it in school. In fact, I reviewed the topics lately from mechanics of materials books before posting on here. I also read Niu's Airframe and Stress Analysis volume, which was helpful, but left me with some unclear points.

I can see that my questions may infer that I never picked up a book on the subject but they really attempt to confirm knowledge that I've read about, thus allowing me to proceed with peace of mind with sound fundamentals.

Frankly, I will say that my area of concentration in engineering school was not structures, and neither do I have a Master's degree. Essentially, I am familiar with the P/A, MC/I, VQ/IT, etc. equations and that's about it. I guess that's the education you get with just a bachelor's in aerospace engr (I wouldn't call myself a bad student either

Likewise, my level of experience in the "real world" is minimal so I haven't been exposed to a whole lot. Indeed, sometimes I feel that my job as an airline liaison engineer is over my head considering some of the decisions I have to make. That bothers me, and I study quite a bit on my own time to remedy that. Add the fact that the mentorship is poor where I work (everyone is too busy and overworked...no time for coaching), the work conditions ruthless, but yet the degree of responsibility high coupled with a low degree of authority. So I either read books (with some varied success of understanding), ask Boeing (they're getting sick of me in service engineering), or come here. Whenever my company can afford to send me to training, I bombard the instructor with questions and I swear he's added me to their "black list".

I can picture what goes on in a "beam" type part that in tension or compression (1D), and substantiate repairs accordingly. However, when I get to 2D panels, I struggle because my exposure to them has been limited. Besides a bit of in-plane analysis using Mohr Circle, nothing academically. Thus, I am trying to learn more. That's why I come to this forum for some "virtual" training, for which I feel very grateful for. People have been very helpful to me and I can honestly say that I am a better engineer today than I was a year ago when I started fresh out of school with my shining degree...but the learning curve remains steep and the mechanics brilliant in asking questions I can't answer

I think I'll stop because I am just rambling now...

Have a good weekend, and thanks for reading.

Alex

 

[jetmaker]

Alex,

You need to understand what type of shear beam you are working with... diagonal tension, IDT, or shear resistant.

I assume from some of your comments that you are refering to a diagonal tnsion shear beam. So here are some comments.

1)You look at tension because the strength of th doubler must be able to transfer the load across the diagonal tension of the web. In one direction, the web buckles while in the other it is in pure tension. Therefore, a doubler must be capable of carrying the tension load.

2) Shear web beams (i.e. thin web) are very efficient because they are designed only to carry a tension load across the diagonal. These types of webs could be easily replaced with a diagonal cable and still perform the basic structural function as a thin web.

3) you need to review the 3 basic types of shear beams as each has different failure modes. For a shear web beam, you are correct in that in one direction, the web buckles. However, this does not constitute failure as the web is still transfering loads through tension along the principal axis.

4) can not answer this one as I am not sure what the exact answer is. In a fuselage skin, you have shear, hoop, and longitudinal loads. In a diagonal tension shear beam, you assume only shear loads in the web.

5) Again, not sure on the answer for this one. Maybe Ed can help us out.

regards,
jetmaker

 

[edbgtr]

Hi Jetmaker
The fuselage skin/stringer/longeron shell forms a beam that bends and twists from the various loads applied to it.
In torsion it is a tube. In bending it is a multi-boomed (end-load member) box beam connected by skin shear members. The shear loads are carried by the skin.
The out of plane bending results from the non-uniform pressure differential across the skin due to changing AoA to the oncoming airstream combined with the pressurization of the shell. Hanging antennae, blisters etc. directly onto the skin exacerbates this condition.
Regards,
Ed

 

[SuperStress]

Alex,

To answer your question about the fundamantal reason for the I-beam shape, think about the moments.

For the case of a simple cantilevered beam, moment builds in the beam proportional to the distance from the point of load application.

The farther apart your beam caps are, the lower the streses in the caps. Added bonus! The ability of the beam to resist these moments increases by the square of the distance between the caps (your Ad^2 term).

So now that your caps are sufficiently far apart to have a reasonably efficient beam, take a look at the required thickness for your web. Ignoring any contribution from the caps, for a rectangular web cross section, shear stress peaks at the neutral axis and equals 1.5V/A.

Since you have such a large height, the required thickness is small. The problem you face now is web stability.

Web stiffener spacing is defined by the buckling criteria.

And yes, intermediate diagonal tension (or pure diagonal tension) beams are more efficient than a shear resistant beam.

Hope that helps.

SuperStress