Structural Analysis Help, Please

[gutboy17]

First off, sorry for the posting of this question. I know the problem is very basic to the structural gods, but I've been out of the structure's loop for quite sometime now. I've been assigned a project (in protest) to fix a panel that has housed my circuit boards and other electrical gizmos. The problem with the panel is after a few hours of operation cracks show up in the panel. The panel is basically a bent up aluminum sheet metal hat section that is about 12" in length, 8" in height, and 7" in depth. I also have to add some additional relays to the panel. My manager has told me that the problem is due to the hat section isn't properly supported and the aircraft vibrations and loads oscillates the panel work hardening the bend joint at the top of the hat section and thus causing cracks in the panel. She has told me to stiffen up the hat section with "gussets". My problem is that I don't know how to calculate the "gusset's" size and the fasteners required to do this (My career limits me to avionics, wires, and other electrical components), however I'm in a small company with limited resources, so I'm on my own.

My question to the group is there someone who could point me towards some references or examples on analyzing a corner gusset. Thanks for any help.

 

[JStephen]

Sounds like you really need a consultant familiar with the design in question, rather than internet help.

It might be a matter of revising materials, forming processes, etc., rather than just adding in gussets.

FYI, "structures" as used on this forum mainly means buildings and stuff, not aircraft structures.

 

[FSS]

Sounds like FEA study could be in order. Maybe getting some effects due to how the relays are being attached (localized stresses, etc.)

 

[swearingen]

Try the "Aircraft Engineering" forum on this site. I frequent that one and there are many folks there that could probably help you.

 

[drawoh]

gutboy17,

What grade of aluminium are you using?

Aircraft grade aluminium such as 7075-T6 is very difficult to bend properly, especially if the fabricator is not a proper sheet metal fabricator. Find a handbook that tells you what inside bend radii you can use. These have to be quite large for the hard aluminium grades, and the fabricators usually use a tool. The tool provides them a set of discrete bend radii. You cannot specify any number you want and expect results.

On all my aircraft sheet metal stuff, I punch a radius in each and every inside corner. When I do not see this on a sheet metal part, I assume the designer did not know what they were doing (or the fabricator did not follow my drawings). Ask around about about proper design practise for aircraft mounted equipment.

Specify something like 5052-H32, and send the job to a sheet metal shop. Chat with them about inside bend radii and available bending tools.

Other than that, you must collect information. What is your weight, how thick is your material, and how much force and vibration are you subjecting it to? The stress calculation should not be that difficult.

Is anybody doing anything to the part that you do not know about? Perhaps the bracket sits on something that twists.

I would expect gussets to work, but you may be looking at bad design practise.

JHG

 

[rb1957]

a bit more info would help ... does the panel crack in the corners ? if so, what is the bend radius of these corners ? 3t is typical practice, and maybe an electrical shop such as yours might not know.

how is the panel connected to the airplane ? if there are fasteners in the middle of the panel, these wouldn't be supporting the edges of the panel very well. ideally, i think, you'd want these fasteners to be outside the masses (such as they are) on the panel. look at the position of the masses compared on a square joining the fasteners, maybe your fasteners are grouped close together with the masses further away from this support.

how big is the panel ? ... bigger would make it more suspectable to vibration issues. breaking up the panel with little "panel breaker" stifferens" would help reduce the vibration response of the panel.

"corner gussets" implies to me that the sides of the box are not joined together (in the corners). it'd help (your manager Could know something) but i'm not sure how much.

 

[gutboy17]

Wow - This is the most help I've had on this in weeks.

First off my manager is as clueless on structures as I am (we're all wire bitting geeks). Our counter parts in structures have washed their hands of this issue since it has our electrical stuff on it and that the panel was created originally by another electrical engineer (not knowing what he was doing). Also, sorry for being in the wrong forum group.

Anyway, for info on the panel. The panel is .04" thick 2024-T3 AL. Bend radius' min. .12"R, relief's min. .16"R. Again, Panel is basically a hat section 12" in length, 7" in depth", and 8" in height. The electrical components are mounted on the top of the hat section and weigh about 2 lbs, weight is pretty much distributed evely on top. The panel straddles other electrical components (so it can't be lowered in height). The panel is supported at the base of each side panel by six fastener sets. The fasteners are standard aircraft screws, washers, and locknuts. The fasteners are spaced evenly at the base of the side panels (they are about 3/8" from the bottom edge of the side panels). The structure that this is mounted to is a avionics rack that is constructed of some healthy aluminum extrusion pieces that are riveted together.

I've been looking through some structures books (Michael Niu, Bruhn, etc.), but I'm not finding an example that would relate to my problem. I appreciate all the help so far, so any hints on where to look to resolve this problem would be a big help.

P.S. - As for the FEA, I've tried FEA in SolidWorks and I got yelled at by a structural engineer for using the Cosmos Express software. I understand that it's accuracy is limited (you get what you pay for!), but it is the only FEA software that I could figure out on my own.

 

[rb1957]

i wouldn't use FE on this.

you say the panel is a hat section ... i think this means a large channel section, with a large flat panel with a stiffnening flange along the top and a moounting flange along the base. if this is so, then the panel would be somewhat flexible and i guess that the small masses on the panel are getting driven by the airplane vibrations.

can you fill in the sides (to make a 5 sided box) ? use the same AlAl, same process to make formed flanges to rivet to the top and bottom flanges. that would stiffen up the box alot and be easy to make.

good luck

 

[gutboy17]

rb1957-

We can't add the panels to the 4th and 5th sides (front and back) due to the front needs to be open for access to the components mounted under the panel's hat section. The back has to be left open also so as not to interfer with the various harnesses. This is why we were thinking of adding small triangular gussets to the four corners to tie the top surface panel to the two side panels to stiffen up the structure. I just need direction on where to find the equations to analysis the rivets used and the gussets strengths.

Thanks.

 

[swearingen]

I had a similar problem with vibration cracking of a channel shaped support. The solution will be tough to spell out, but here it goes:

Imagine your channel shape as a C. What I did is close the C (moved the tips closer together) only at the ends. In other words, bolt the center of the 12" long tips at their standard width (7" or 8" - I can't tell which from your description). Then, as you approach each end, bend the side walls in slightly and bolt them down. You can also bend them out (make the C tips move farther apart) if you don't have room to move them together. What this does is effectively kink the walls of your channel providing what is essentially a stiffening element in the center of the channel. It allows the tips of the channel (where it's connected) to take far more moment than it does now, thereby relieving much of the moment stresses at the bend. This may not be practical in your application, but it worked for me. It was some instrumentation mounted on some vibrating machinery, specifically a coffee bean vibrator.

 

[rb1957]

whilst there is some analysis help, somewhere, i'd suggest a more experimental approach. since these things crack quickly it shouldn't take long to show that you've improved the situation. part of the reason for this approach is that i don't think you have the numbers to do the analysis (the vibration spectrum). how exactly is the panel oriented in the plane, once it's mounted in the rack ? it sounds like the cracking is being driven by out-of-plane bending of the panel. it be nice to know if this was the up/down, the port/stbd, or the fore/aft direction.

the masses involved are small; how far offset form the panel surface are they ? this would affect the bending at the base of the panel.

i'd suggest formed triangluar gussets, same gauge as the panel, formed flanges to pick up the panel and the other flange, 45deg angle (so its as long as the width of the panel flange) ... 7" or 8" ?.

can you add stiffeners (formed angles) to the face of the panel ? This would help enormously to increase the bending stiffness of the panel, and might be worth rearranging things on the panel to make space for them). Maybe the back face, outside the box ?

as for filling in the sides, they'd still be effective if they had to have cut-outs to access things inside the box. the point would be to have a constant flange as far away for the panel as possible to make an effective bending section. this can be achieved with cut-outs in the wbes.

 

[drawoh]

gutboy17,

Aluminium 2024-T3 is a hard, aircraft grade aluminium. It is difficult to bend properly. I have seen even 6061-T6 crack upon bending. The regular sheet metal shops do not want to work with this material.

I have seen electronics boxes made of this stuff. It looks like they were fabricated in hangars by aircraft maintenance people. They did an impressive job of bending, and a rotten job of drilling holes neatly.

Have you inspected the parts and verified that the bends are .12"? Bending sheet metal is like welding. You need a skilled worker doing it.

You should be using a softer, more ductile aluminium grade. I do not think there are crash-safety issues here.

1. I do not think that 2024-T3 and 7075-T6 require all that much more energy to fracture completely than does 5052-H32.

2. If the 5052-H32 does not crack, it is a heck of a lot stronger.

How about making your mount as a squished hexagon, with four sides instead of two?

Other than that, I suggest thicker material. Most of my electronics mounts are .081" thick aluminium. Please remember that your factor of safety is actually your (and my) factor of ignorance. If they want absolutely low weight, they should be prepared to pay for a qualified structural designer.

I like how your structural engineers do not have time to help you, but they do have time to yell at you for using SolidWorks Cosmos Express. Maybe you should chat with a carefully selected manager about office politics.

JHG

 

[Ron]

Are your cracks from the vibration (fatigue), overload (which will accelerate fatigue failure), or fabrication?

The solution will depend on which of these prevails. If the parts are small (which from your description they seem to be), then you might want to consider fabrication in a T-0 condition, then heat or aging treatment to gain strength for the completed part. The treatment will depend on the alloy. You might also want to consider a hard anodizing to improve the surface characteristics. Lastly, what about isolating the vibrations with a damper?

 

[JStephen]

I assume this is on some sort of aircraft?

That being the case, I would expect there would be more to the design than just tinkering until it didn't crack in a few hours. Does it have to be designed for any particular lifetime, or number of cycles of vibration? Or for any particular accelerations? Are there any codes or industry rules that govern the design? Does anyone have to certify that the design is good?

Or is aircraft engineering just a whole lot simpler than I imagined?

 

[rb1957]

simpler and more difficult at the same time !

the authorities don't specify a fatigue life, that's left to the manufacturer. the manfacturer (OEM or STC) is expected to appreciate the potential fatigue loads on the strcuture and to specify either a safe-life or (much more commonly today) an inspection program. For something like this, i'd expect an infinite life (ie no mandatory retirement life, and no structural inspections).

For this panel, it looks like the fatigue (vibration?) loads weren't anticipated so now they're trying to fix the design. it does seem odd that their structural people aren't getting involved (and are leaving it for the wires guys).

 

[Ron]

rb1957...also, you might want to consider using an extruded section such as a rectangular tube and adapt it to your application. Then you wouldn't have the bend radius issues.

 

[drawoh]

gutboy17,

If the fabricator of your bracket is not the one causing the crack, your first analysis should be for the natural frequency of your bracket. It sounds like the aircraft is vibrating at that frequency.

That analysis is beyond the capability of SolidWorks Cosmos Express, which does not even give you displacement values.

The seventh edition of Roark's Formulas for Stress and Strain has some tables on vibration. The Mechanical Design Handbook, Harold A. Rothbart, McGraw Hill, has tables for vibration modes for all sorts of stuff.

Failing that, I think you are going to have to reason with your structural design people.

JHG

 

[pba]

I have no experience of aircraft engineering so this may be totally wrong - Anyone please shout me down if this is silly.

Aluminium and its alloys is prone to fatigue failure and this is made worse by stress concentrations such as those caused by bedning the aluminium.

Steel is not prone to fatigue failure (at reasonable stresses). It is also stronger than aluminium. Its main disadvantage is that it is heavier.

This support is small so i'd guess that the weight is not a problem so why not try one made out ofthe same thickness of sheet steel?

 

[Kwan]

In my opinion, a hat channel that is 7"X8" with only .040 thickness is a little weak. A gusset sounds like a cost effective fix.

The structure should have been substantiated to requirements such as crash loads (g level). I would start by digesting this stress report.

Look at the weight that was used. Ensure that the weight used is correct (don't forget weight of wires, they can be heavier that the components).

You should find that the side load is reacted by bending across the .040 thickness. Now, free body the gusset, and write your new margins in the stress report (this is where you need someone looking over your shoulder). Be very careful with the gussets. If you don't place them in a location with proper support, you may drive your cracks to another location.

Sounds easy? It takes quite some time (with good mentors) to develop strain checking skills.