reinforcing a cylindrical beam

[rdamus]

hello all,

i'm in the process of designing a payload section for an autonomous underwater vehicle (e.g.,

http://www.hydroidinc.com/remus600.html).

this section will be rated to 1000psi. it is Al 7075-T6.

My problem background:

the payload section must fit inline with the vehicle diameter, which is 12.75". however, the thin-walled pressure vessel that will house our electronics is only 10" OD (0.35" thick) and will not be a structural part of the assembly, i.e. it does not support a bending moment, it will only undergo compression from the hydrostatic pressure at depth. this means that when we pull the vehicle out of the water, the entire section weight will have to be transferred to a cross-member. At the moment, i have a 2" OD aluminum tube (0.2" thick) that runs the length of the assembly whose centerline is located roughly 4.5" from the longitudinal payload centerline.

My question:

I am interested in knowing whether the 2"OD tube can support the weight of the payload section. To determine this, I have used a uniform load that represents the weight of the section (~1000N/m) to calculate the bending of this tube, using the cross-sectional inertia and Young's modulus of the tube. It appears that the 2"OD tube might deflect too much, so i am interested in suggestions on how to increase the strength of this system.

Ideally, i would like to know how to model the deflection of two beams that are rigidily mounted to a common endcap in the same vertical plane, but are of different cross-sections and located an arbitrary distance from one another... very similar to a composite I-beam in my mind, but i have trouble modeling the constituitive end-condition.

thank you for any insight. below is a cross-sectional rendering

Cross section of payload:

| | | | | | | | | |Uniform Load 1000N/m
v v v v v v v v v v
---------------------\ 12.75"OD
---------------------|
10"OD electronics |
---------------------|
..........................|..............Payload Centerline
| }
---------------------| }offset is ~4.5"
2"OD tube |.............}
---------------------|
--------------------/

 

[rdamus]

Garland:

the ribs do not connect to the tube. please see:

 

[rdamus]

IRStuff:

since my section has arrays on either side, we cannot take advantage of the 12.75" hulls that you see in the REMUS brochure. this is the crux of the problem. please refer to the image i posted, or view it here:

http://www.zhrodague.net/~rdamus/hull/ribbed-mod2-front-no-array.jpg

 

[GregLocock]

So how does the load feed into the 2 inch tube?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

Only through the shear deformation of the 10" shell I would think.

I would be very reluctant to take advantage of any stiffness from the 2" tube. It would be difficult from a stiffness standpoint. I don't think you can call this a "composite beam". The FEA would give you a more realistic, less conservative answer, but, if performed properly, would be the way I would go.

 

[IRstuff]

I see, but I don't see.

The weight of your assembly appears to be resting on the vehicle body at the ends of your structure, so the vehicle walls are supporting your entire weight. Your 10-inch tube is apparently strong enough to support the weight transfer to tbe ends, which means that extending your other fins to the same diameter as the end plates would allow you support the entire weight on 7(?) cross members, rather than just two, and get rid of the 2-inch pipe.

TTFN

 

[rb1957]

sorry, i'm dumbing this down to my level ...

it sounds like you have a sensor outside of the 10" dia tube "since my section has arrays on either side, we cannot take advantage of the 12.75" hulls", so you're expecting to see water on the inside of the 12.75" outer shell ... seems a bit wierd to me, but i don't know the project as well as you do. but this means that you're going to have water pressure on all faces of the latter picture.

i assume the latter picture is structure that is well built into the structure of the sub, and that you've got loads ahappening all over the place ... ahead of this section, behind it, inertia loads, pressure loads ... i also assume that the 10" tube pretty much slides into the big hole on the latter picture (for easy removal). If this is the case, the the 10" tube is pretty much just reacting pressure and the structure of the sub is doing it's thing all around the this tube. if this is the case, then i suspect you need structure (ribs) to support (and structurally join) the fwd and rear bulkheads.

but maybe the 10" tube is built in, attached with lots of screws. in which case it can react the sub loads. i see the two tubes (10" and 2") working together, but not as a composite beam, as they don't have a shear path between them. the two beams could react external moments by having different endloads and by individually assuming a bending stress field, but i think this is alittle tricky to calc on the back of an envelope as it looks redundant. Plane sections remaining plane sounds like a sensible assumption.

but what's happening with the outside shell whilst all this is happening to the "inards" ? is the shell attached so that the interior structure is going to take all the load, and deflect without transferring load to the shell ? seems abit odd.

 

[arto]

Dude - I hope you're not using my tax dollars to get your job done by committee for free!! ;-)

Maybe hire an unemployed torpedo designer??

 

[rdamus]

rb1957:

you have hit the nail on the head. this is the key point: the the 10" tube is pretty much just reacting pressure and the structure of the sub is doing it's thing all around the this tube. the 10" housing will float in space, and all the reaction will take place along the 2" tube. this is a requirement for the design, as the radial contraction of the 10" tube must soleley be influenced by the hydrostatic pressure; this ensures that the struts (ribs) contract uniformly and the deformation at the center of the tube is roughly equivalent at either end.

i started this thread making this claim, and was interested in understanding if
1)i could do something to strengthen the system without influencing the 10" section
2) how to properly model the bending of the 2" beam - i.e., is the system a composite beam, and if not, what assumptions will guide my calcs to generate reasonable FEA analysis, for as Garland notes, "garbage in, garbage out"


arto:

dude, first off, i've never used a forum like this. i'm just curious what people have to say. the design is more ore less done. i appreciate all the commentary and activity. it definitely feels like people are interested. i hope to respond in kind in other forums on this site.

Garland and Greg and IRStuff:

thanks mucho for the insights and keeping me diligent in addressing this problem.

-rob

 

[GBor]

This has been far more interesting than I originally thought it was going to be (no offense intended). The FEA shouldn't be too difficult. Put in "good stuff" and fire away!

Garland

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

 

[rb1957]

radmus,

why do you think the outside shell is doing nothing (to react loads) ? certainly it's best placed to stiffen the structure. it sounds like you can remove the 10" tube from the model without affecting the results (since the tube is only reacting pressure) ... tho' you should check the displacements of teh surronding structure to make sure the surrounds aren't butting up against the 10" tube.

i'd start by making sure the bulkheads at both ends of the 2" tube are stiff enough to redistribute their loads into the 2" tube, you probably want some sort of stiffeners to reinforce the bulkheads. then the loading of the 2" tube is simple enough ... loads from the bulkheads and hydrostatic pressure