stiffness

[klaas2]

sorry for my english

but if you have a complicted construction like several tubing ,and for get about the weight

i only look at stiffness and compare alu with composite´s

and due to bonding and hole´s can you not general say that they come out with the same stiffness

again i know there is a differance here i only look at the total stiffness

i am not a fea engineer but come about the same stiffnees

am i on the wrong track?????

 

[GBor]

"Composites" means a lot of things. We would need to know what materials make up the composite and to which aluminum you are comparing it. If you are referring to tubing, we need to know inner or outer diameter and wall thickness. For the composite tube or shaft, we would need to know fiber directions and fabrication method.

I can use a carbon fiber in an epoxy matrix, lay the fiber along the axis of the shaft, and produce a beam that is MUCH stiffer than any aluminum. I can use those same materials, but lay the fiber at 45 degrees off-axis and get a shaft that is weaker in bending, but perhaps stonger in torsion. There are many options.

Also, by "Total Stiffness", are you referring to simply the "EI" of the beam?

I appreciate your attempt at English and it appears to be much better than any foreign language I may speak, but if I am understanding you correctly, I would say that you are on the wrong track.

Garland

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

 

[Spirit]

klaas2,

the first thing I would say is that one wouldn't use the tube layout when considering building a structure (my guess is that your problem is a car chassis ... ?) in composite rather than aluminum. This to say that it is not easy to answer your question. It would be useful also to know what kind of stiffness is prioritary for you: torsional? bending? lateral?
One of the advantage of composites is the high stiffness/weight ratio, but if weight is not a problem I would rather go on more conventional materials, like steel.

Regards,

SD

'Ability is 10% inspiration and 90% perspiration.'

 

[klaas2]

ok i take a example let say a bike you all know that and how the constructions

so construction is molded tube
and composite i mean carbon
as the young modulus of carbon let say 225 and of alu 70

so there huge differance

only becuause of the bonding of carbon fiber tubing i come to the same rough stiffness as when i do it out of alu in let say ones piece
assume we have max dia 24 mm as stiffness comes in dia
how bigger the more siffness but the bonding of tubing brings back the huge advantage of carbon fibers in this construction
here to say that the tubing max one direction only can be 24 mm other side max 80 mm so more not round but a ellipse shape


or............am i wrong

 

[rb1957]

i think it would be easier to modify the fiber lay-out in the composite tube to get the same stiffness as an Al tube, like GBor described.

i'd start with the tube profile i want (maybe you want an elliptical tube for aerodynamic reasons, maybe you want a round tube to simplify manufacture). I'd build up the composite fibers (direction, number of plies) to equal (or exceed) the stiffness of the Al tube.

good luck