Long story short, I go to Texas A&M and being that I love fabbing and researching cool stuff, my professor has asked me to help out with some research. Now this is really cool because I'm not a graduate student, but will get some credits for it AND possibly get to submit my findings for publishing...which would be really really cool. Since I think metal deformation is one of the coolest things around and love bending tubing, this project is right up my alley. Now, a lot of FEA research has already been done, but nothing to back it up....which is where I come in. Since I have experience and the equipment to bend tubing, I offered up to conduct some tests and do some more elaborate research for tube bending. The problem we are tasking to solve is, if we internally pressurize the tubing, can we bend a tighter radius with minimal/less deformation/wrinkling of the inner wall. So we are going to take several samples of a standard tubing, but bend on different radius dies at different pressures. Of course being a university, we have very little funding to get a lot of work done. My the thing I need of course, are the most costly items in tube bending...the dies. Right now I have a 1.75"x 6.5" and a 1.5"x4.5" die. Since we are doing the same size tubing to keep things valid, I need about 2-3 more dies at different radii for the same sized tubing, either 1.5" or 1.75". I wouldn't mind buying them myself, but really don't have a need for 3 or 4 dies for the same diameter tubing. We are a few months away from doing this, but I'm wondering if anyone would be interested in renting/loaning some dies for the process? As I get more information, I'll post it up, but this is the beginning stage to see if we can even get the funding/operations to make it happen. I really don't think this is that difficult...get about 100' of the same tubing, 4 dies, a hydraulic pump (like an enerpac) and go to town....what do y'all think? Obviously this isn't directly along the lines of the home fabber cage builder, but could be very useful for the airline industry and who knows where else....Anyhow, what do y'all think? I am really excited to try to make this happen and be part of some cool research....

looking at the vansant site, looks like 1" tubing has 4 dies available and I could use one anyhow...so I'll probably buy one of those and try to use 1" tubing, everything is a bit cheaper then :)

I'm going to try to do different sized tubing as well, 1",1.5",1.75" pretty much anything I can do a few samples of to see if I can establish a relationship.

What came to mind on the idea of bending pressurized tubing was this:
http://www.pillsbury.com/images/products/photos/pillsburydinnerrolls.jpg

hahaha.....yeah, except hopefully this tube doesn't blow up in my face!!

What kind of pressure are you talking about here?

Seems like it would take a lot of internal pressure to keep the wall from deforming.. unless you are planning on using some type of fluid to pressurize the tube.

lets get some more details:grinpimp:

Forget about the club that belongs to your school? A couple of us have benders. I have a protools 105 hydraulic conversion with a 1.75 and 1.25 die, can't remember the radii of hand.

What kind of bender are you going to use? The SAE car has a JD2 manual bender they use. Don't remember the die size, probably pretty small. What professor is this for?

oh yeah...I forgot to check with y'all (TAMOR). This is for Dr. Wang in the ENTC department.

The tube will be pressurized with fluid, and I'm not sure of the pressures yet. It's something I may try to derive a formula based on tube size if the tube size is a factor at all.

Interesting stuff :D

I would think you'd not need very high pressures to begin with, but the pressure will surely climb *rapidly* when the tube starts to deform.

How are you planning to contain the fluid in the tube? What sort of fluid? Would tap water work?

...but the pressure will surely climb *rapidly* when the tube starts to deform...
I'm not so sure I agree with this... It's my understanding that when you bend tube, it actually strectches the tube. If so, it's quite possible that the volume of the tube will increase which in turn would cause the internal pressure to decrease.

I'm basically going to weld end caps and use an enerpac to pump it up. Not sure of the fluid yet, but I don't think it'll matter much.

Also, the tube will have a pressure guage on it, but I don't think there will be a pressure increase very noteable. The volume shouldn't change much, however the tube does deform to an oval shape, which we are trying to minimuze by using the fluid internal pressure....it's the same thing you hear about people packing the tube with sand and bending, but this is a cleaner and more "scientific" method. Ideally, if all works out, we can make some sort of pump/evacuation system for the tube...I've got some wheels turning already. :)

I'm not so sure I agree with this... It's my understanding that when you bend tube, it actually strectches the tube. If so, it's quite possible that the volume of the tube will increase which in turn would cause the internal pressure to decrease.

stretches on the outside radius and compresses on the inside radius. Typically the stretching is about 60% and compression is about 40%. I don't think percentage wise the difference in volume will make much of a difference.

I've used tightly packed sand and capped ends in a pipe bender to bend 1.5" sch40 a whole 90* to stop deforming and wrinkling. It works very well. It's been done for years by shops that bend tubes around a stationary "die" by hand. Might want to look into that as well.

How is this any different than Hydroforming? My point being that the "research" in this area has already been done.

Well, I have just been brought into this project, but I don't believe much research has been done on a fluid mandrel. Hydroforming still requires a die and the pressure expands the material to the shape correct? All we are trying to do is establish a relationship for how much tighter of a radius we can bend without kinking the intrados and losing too much thickness on the extrados. Basically it would elminate having a different mandrel for each different ID of tubing used. It's just a means of support for the material, not actually "forming" it to anything, just there to help maintain shape and reduce the ovality from the bend. If there is information on this out there, please send it my way so I can compare and see what has already been done.
Greg

I see, hydro-forming doesn't bend the tube. You have to prebend it to fit into a forming die then the fluid pressure shapes the tube to the die - O.K. got it.

Fluid mandrel??

My thoughts:

You're not going to be able to control the wall thickness during a bend and the fluid pressure will probably deform the thinnest/weakest part of the tube while bending. Since no tubing is perfect and doesn't react to the bending in exactly the same way throughout the length of the bend the deformation could happen any where. Basically your going to apply a constant pressure to the entire inside surface of the tube but, what you can't control are the changing properties of the tube as you bend it. So how will you apply more pressure where it is needed and less where it isn't?

I see, hydro-forming doesn't bend the tube. You have to prebend it to fit into a forming die then the fluid pressure shapes the tube to the die - O.K. got it.

Fluid mandrel??

My thoughts:

You're not going to be able to control the wall thickness during a bend and the fluid pressure will probably deform the thinnest/weakest part of the tube while bending. Since no tubing is perfect and doesn't react to the bending in exactly the same way throughout the length of the bend the deformation could happen any where. Basically your going to apply a constant pressure to the entire inside surface of the tube but, what you can't control are the changing properties of the tube as you bend it. So how will you apply more pressure where it is needed and less where it isn't?

Typically the control on the WT during a non mandrel bend is a result of the chosen CLR and uniformity of material used.

What kind of fluid pressure do you think it will take to "deform the thinnest/weakest part of the tube while bending"?

If the material used was ASTM A513 as welded HREW 1020 mechanical tubing in a 1.5"x0.120" WT section and the extrados of the bend stretched enough to result in 0.090" WT you are still looking at needing a pressure of over 5000psi to cause any plastic deformation due to fluid pressure in this region.

Conversely I believe the attempt here is that of using a "fluid mandrel" to prevent buckling/wrinkling along the intrados.

Unfortunately while the "fluid mandrel" may be useful in controlling or reducing buckling/wrinkling along the intrados the process it does nothing to minimize wall thinning along the extrados which you are likely going to find is the limitation for any commercial applications of this process.

AddisonMckee and others have been using a "boost bend" process that involves forcing the material axially into the bend during the process to reduce wall thinning.

About to get some progress for getting this rolling. I need to know what tubes (id) you have wrinkled (if any) for 1.5" and 1.75" od. I collapsed some 1.5" x.058" a while back, so I'm going to get some more of that to see how it fairs in the experiment, but do you have any tubes that you have "wrinkled" for 1.5 and 1.75" tube?