Composite material

[ben1793]

So I have been reading up on composite materials and have been geting a bit confused, from what I have read, composite materials consist of a matrix and reinforcement, the reinforcement can be fibers, I read that the way the fibres are placed will effect the machanical properties of the material does anyone know how this will be effected?

Edited March 9, 2012 by ben1793

[DrRocket]

So I have been reading up on composite materials and have been geting a bit confused, from what I have read, composite materials consist of a matrix and reinforcement, the reinforcement can be fibers, I read that the way the fibres are placed will effect the machanical properties of the material does anyone know how this will be effected?

 

Fibers basically carry axial stress well and shear not so well.

 

Think of a Chinese handcuff. When you pull on it the fibers try to align themselves with the direction of the applied force. That also leads to a (macroscopic) Poisson ratio that can exceed 1/2, something that is impossible in a homogeneous material.

 

Not only does fiber direction affect properties, but also the quality of the manufacture, including bonding or lack thereof, can dramatically affect the overall mechanical capability of the structure. Surface treatment of te fiber and selection of the bonding resin are very important.

 

The study of the mechanics of composite materials is a major sub-area of mechanical engineering and there are several universities with strong faculty in that area. Virginia Tech is one of them. http://www.coursehero.com/file/5192787/CompositesatVirginiaTech/

Edited March 9, 2012 by DrRocket

[ben1793]

Thanks for the reply, the example you gave made it much more easier to visualise.

I looked further into composite materials and started reading about laminates, would the lay direction of laminates effect the material?

[InigoMontoya]

would the lay direction of laminates effect the material?

Absolutely.

 

Think of plywood. It's stronger in one direction than another, yes? It's not a perfect analogy to composite materials, but it's very close.

[ben1793]

Thanks for the help, I looked into plywood and I read that the layers where all plywood (the same material) I thought that the whole point of laminates was to combine the properties of 2 or more materials combined to combine both properties for example like the windshield of a vehicle, I read that it was a combination of glass and hard plastic. Unless the information I read was incorrect?

Edited March 10, 2012 by ben1793

[zapatos]

Thanks for the help, I looked into plywood and I read that the layers where all plywood (the same material) I thought that the whole point of laminates was to combine the properties of 2 or more materials combined to combine both properties for example like the windshield of a vehicle, I read that it was a combination of glass and hard plastic. Unless the information I read was incorrect?

Also I was wondering how would the amount of matrix material and reinforcement material effect the properties of the material? I mean lfor example having more reinforcement, would this make this composite material stronger?

Plywood gets its desirable properties by aligning the wood grain of alternate plys at right angles to each other. AFAIK plywood does not use any other material (except glue of course).

[ben1793]

Just like plywood are all laminates layered up at a right angles?

Edited March 10, 2012 by ben1793

[zapatos]

So if plywood is layered up at right angles doesnt that mean each 90 degree angle wouldnt that mean that with 2 sheets of plywood layed in right angles the plywood would be strong from all directions? I ask because Inigo says it is only strong in one direction.

Just like plywood are all laminates layered up at a right angles?

One sheet of plywood is made up of multiple plys, or veneers, layered at right angles, usually an odd number of plys. So yes, a sheet of plywood is strong in all directions.

 

Sorry, but I can't help you with all types of laminates. I know more about wood than most other materials.

[ben1793]

Ok, thanks I understand it better now, but back to ordionary composition consisting of matrix and renforcements, how would the ratio of matrix/reinforcement effect the strength of the composite material?

[InigoMontoya]

One sheet of plywood is made up of multiple plys, or veneers, layered at right angles, usually an odd number of plys. So yes, a sheet of plywood is strong in all directions.

True, that's the point of plywood, but at least in my experience, it resists bending the best (ie, is strongest) in the axis perpendicular to the outer layer's grain. This being due to the strength contribution of any given layer being a function of the cube of the distance of that layer from the center of the sheet. In other words, strong in all directions does not equate to equally strong in all directions.

 

True, one could counter this via proper selection of thickness for each layer, but my personal experience (read: observations that plywood is NOT equally strong in all directions) implies that the manufacturers do not do this (I'd guess for cost reasons).

 

 

 

Ok, thanks I understand it better now, but back to ordionary composition consisting of matrix and renforcements, how would the ratio of matrix/reinforcement effect the strength of the composite material?

If you don't put enough matrix in the mix to fill all intersticial voids, your resistance to torsion or bending of any kind is complete and utter crap as the matrix cannot effectively transfer shear loads.

 

If you put too much matrix in the mix, your tensile strength will be reduced.

Edited March 10, 2012 by InigoMontoya

[ben1793]

Thank you so much, made it very clear and easy for me to understand.

 

 

[zapatos]

True, that's the point of plywood, but at least in my experience, it resists bending the best (ie, is strongest) in the axis perpendicular to the outer layer's grain. This being due to the strength contribution of any given layer being a function of the cube of the distance of that layer from the center of the sheet. In other words, strong in all directions does not equate to equally strong in all directions.

 

True, one could counter this via proper selection of thickness for each layer, but my personal experience (read: observations that plywood is NOT equally strong in all directions) implies that the manufacturers do not do this (I'd guess for cost reasons).

Yes, thanks for the clarification. My comments didn't really stay in line with the question in the OP.

[DrRocket]

Just like plywood are all laminates layered up at a right angles?

 

No.

 

The lamina that make up a laminate are designed to meet the specific application at hand.