Shell Thickner (Doubler) Stress Concentration Effects

[jeffhmc]

Hi Everyone,

I've posted this question in the aerospace forum, but it seems more appropriate here. I'm investigating the low damping and failure of aluminum rods in a duct. The duct is used in a heat exchanger for an aircraft with high velocity air flowing through it so it'll have negative pressure inside. The negative pressure is sucking in the sides of the duct and would cause the duct to buckle (i.e. implode), so the tie rods are placed to keep the duct from imploding. The failure occured during the vibration test, so the heat exchanger wasn't in operation. The tie rods were welded to opposing sides of the duct (a think aluminum shell). If you can imagine, it basically looks like sticking a pencil through the sides of a coffee can. Washers quite a few sizes bigger than the OD of the rods were welded to the outer face of the shell, then the rods were welded to the washer. The failure of the connection actually occured at the ends of these rods rather than on the shell (which is thought to be much weaker than the rods).

I appologize, but my experience with welding methods and their effects is limited. I was wondering if this type of weld is common and what typical problems (if any) have you seen with this type of weld? Any comments, intuitive checks, or possible resources to look up would be greatly appreicated!

Humble young engineer,
~Jeff

 

[berkshire]

jeffhmc (Structural)
I must apologize for sending you over here. It appears you are being ignored.
What is the material of the duct and the strut rods, is it a normal welding quality alloy like 6061, and was the entire assembly heat treated after welding?
Or was the weld done on the assembly and left unheat treated?
B.E.

 

[jeffhmc]

Hi Berkshire,

No need to appologize! Help of any kind is greatly appreciated. Yes, the alloy is standard 6061 for both the duct and the rods. However, neither the assembly nor the connections were heat treated after the weld, so the strength of the rod near the connection is uncertain. However, according to the materials engineer, this was not the cause of the failure of the rod at the connection.

I'm as confused as you as to why some things were or weren't done, but I'm just trying to figure out what caused the final failure. Also, I'm being asked to figure out why there was such low damping in the system (~ 0.1%). Just as a side question...have you ever seen damping this low in a system?

~Jeff

 

[berkshire]

jeffhmc (Structural)
It sounds as though you do not have any damping to speak of.
I do not know if this duct has any twists and turns in it,
but I think you would get better results with a splitter plate riveted into the duct , essentially turning the duct into a double D section back to back. Your skin friction losses would be higher, but you would not have the turbulence induced by the rods. I am presuming that you are not using any Anti Drum or similar acoustic damping material on the outside of the duct.
B.E.

 

[jeffhmc]

Hi Berkshire,

Thanks again for keeping up with this thread. I think a splitter plate is a great idea...while the current design i was investigating is too late to change, i can make the suggestion for future designs. And to answer your question...there isn't anything there to really provide damping so the low damping was expected...just didnt know how low... And no, there's no twists or turns...just a regular straight duct with slight tapering at the end.

~Jeff