Composite materials for pressure applications. 1

[ella0509]

Hello all.

I've just started a new job and my first task is to investigate using composite materials in place of the current aluminium choice for some tubes with an internal pressure of 16 Bar.G @ 5degC and 0 Bar.G @ 260degC. Tubes are approximately 4" in diameter and upwards of 100" long.

I've not got much experience with these materials and wondered if any of you guys had any links or information resources to help me with my research.

Thanks in advance,
Lee

 

[MechanicalChef]

What are your basic objectives? Cost savings, weight savings, corrosive environment, etc?

Essentially, this sounds like the ideal application for a filament-wound product. Not knowing anything about your application, a wound glass or carbon tube 'should' be able to get you the pressure requirement without much hassle.

The temperature requirement pushes the general envelope on common resins systems. I can't help you there. If weight isn't a major issue (if this pipe isn't a fly-away part) Basalt fibers will tolerate 500 F without a problem, and their pricing is between E-glass and S-glass, last I was aware.

http://www.mccleananderson.com/would

be one place to start your research.

 

[Joshmo]

Lee,

yes a filament winding would be a cost effect manufactuirng process for making such tubes. But, if your design requires bending stiffness along the axis of the tube, filament winding may not get you there since zero degree plies can't be wound with this process. Additionally, material considerations should be made in regards to the working fluid and temperatures since composite properties are degraded when they are subjected to high moisture and temperature (i.e hot wet conditions). Mil handbook 17 may have some properties to help you but such properties are from flat panels, not wound tubes...

 

[ella0509]

Hi MechanicalChef and Joshmo. Thanks for your replies.

Weight saving is the primary objective. Items are to be used on vehicle systems and hence vibration etc will be an issue. The items will be held rigid in vertical position and bending stiffness would not be a requirement.

Regards,
Lee

 

[RPstress]

Depending on longitudinal stiffness required to control vibration, braiding might be very cost effective. Plenty of longitudinal fibers can be included if necessary.

http://www.braider.com

might give further pointers.

If longitudinal stiffness to handle vibration is not an issue, then, yes, a popular option would be filament winding with any of the major fibers, with a cost/weight trade off according to fiber material.

(As MechanicalChef said, E-glass, basalt and S-glass are all good options, with carbon as a distinctly higher priced option, although it would probably minimise weight. Basalt fibres have been getting more publicity recently, and may increasingly compete with glass.)

Also as MechanicalChef said, a major problem here could be the service temperature of 260 deg C, albeit lightly loaded. Getting a polymer matrix which can handle this is no great problem, but it will tend to increase costs and reduce flexibility. I think a BMI (bismaleimide) would be the best bet, but a phenolic might do it. Hexcel and Cytec will have candidates; 3M is also worth a try, as are Magnolia Plastics and YLA Inc. There are also a number of polyimide (PI) options around these days. They're still pricey, but could certainly handle the temperature. Unitech LLC and UBE both have a fair amount of publicity out there for their PI-type resins.

I'm probably a bit too aerospace biased to be any more precise.

Not sure how many expert pressure vessel composites people visit here; perhaps a more pressure vessel-related forum might give you more response.

 

[ella0509]

Thanks RPstress and others.

We've started some material suppliers off looking at potential options for the application. They are not far from a proposed material solution now, so things are going well on that front - I will be picking their brains for design tips, rules when they come back to me.

With that in mind, the things I'm starting to consider now are assembly techniques of the various composite components. I know that machining, cutting and drilling can be acheived with some materials, but that you weaken the structure if this is done incorrectly or too much. Do you or anyone else know if there are any good reference books or guidelines out there for designing and assembling composite components - its looking like a bit of a black art to me at the moment......can aluminium flange plates etc be overmoulded into the composite mix at all??

Regards,
Lee

 

[Compositepro]

Filament winding is capable of including 0 degree fibers. By taking the fibers over end domes that get cut-off after fabrication you can get low angles close to zero. By putting a ring of pins at each end of of the mandrel you can wind zero degree fibers. There is a company that sells these disposable pin rings.

http://www.advancedcomposites.com/pinring.htm

Pressure vessels are usually made by filament winding. The winding tension is probably an important part of material performance. Braiding could work but the fibers cannot be applied with tension and there are other issues that make it less commonly used for pressure vessels.
260C is at the upper limit of most BMI's and polyimides. Fibraplex has the lowest cost BMI's available at $40 to $50 per pound and they have a BMI with a Tg of 370C.

http://www.fibraplex.com/news.htm

The biggest problem with including metal inserts is the difference in coefficient of thermal expansion between materials and the very large temperature range the part will see during cure or in use. It requires very careful design and material selection. If you can manage to include all your details into a fiber preform and then resin infuse the part using RTM or VARTM you can avoid a lot of problems and cost.

 

[MechanicalChef]

While filament winding is "capable" of zero degree winds, getting a good wind w/ zeroes is problematic. Even with pin rings, pushing a thread against a flat surface using tension is hit and miss at best. For what it's worth, it might be acceptable for winding sewer pipe, but definitely not aerospace. That's why you'd ideally want some non-zero winding angle, 3-5 degrees, just to capture some of the tool's curvature. I'd probably try using pin-rings anyway to cut down on fiber slip, and to avoid an absurd amount of turnaround at the ends for such a low angle.

Regarding assemby/fastening, a natural element of filament winding is a fiber build-up at the ends (the turnaround region). While I don't have a great feel for stress and limits to drilling composite, instead of sawing off and disposing of those areas, it might be possible to utilize a portion of it as it is is a buildup. I don't know the feasibility of doing that for this application. Just a thought.