Focus on unidirection composites in intro texts?

[WK95]

Yes, total beginner question.

Why do many introductory or overview texts on composite materials focus primarily on unidirectional fabrics while giving minimal coverage to bidirectional? They focus a lot on undirectional composites and base a lot of the derivations and calculations on the notion of the unidirectional laminas being transversely isotropic.

If I had to guess, I'd say that unidirectional laminas are most crucial by far since they would of course be oriented in the direction of the main load. But then what is the purpose of bidirectional composites? To cover the possible small unexpected stress in unexpected directions?

In the construction manual for one general aviation aircraft (the Lancair's) I see that bidirectional fabric is used commonly at the joints.

Source: Lancair IV Construction Manual

Now, I can see that bidirectional would be used here where the loads at the bulkhead joint area is not all in a single direction hence necessitating fibers aligned in multiple directions.

Where would I use bidirectional laminas over multiple unidirectional laminas aligned in multiple directions?

In general though, it seems that unidirectional laminas might be preferred because they are easier to analyze and design with.

 

[berkshire]

WK95
The primary use for bi directional fabrics particularly for tabbing bulkheads is drapability . For this purpose the fabric is cut mostly on the bias i.e. 45*-45*, this also gives the cloth two useful fiber runs in one layup. In your illustration you would need 4 layers of unidirectional to achieve the same effect.
Trying to use unidirectional cloth on sharply contoured shapes results in bridging and unwanted strand overlapping. The bi directional cloths particularly the twill weaves have much more drapability ,and will lie into these shapes without puckers and bridges. Whilst these cloths do not always have the tensile strength of a unidirectional cloth in the major direction, used in the methods I describe they will out perform the unidirectional.
B.E.

You are judged not by what you know, but by what you can do.

 

[ESPcomposites]

Fabrication: As berkshire stated, fabrics can be desirable for reasons related to manufacturing.

Other Reasons: They may also commonly used in conjunction with unidirectional plies as a hybrid laminate. A fabric layer at the outer surface(s) of the laminate will improve abrasion resistance and also help to prevent delamination when holes are drilled. Fabrics also improve resistance to damage from impacts.

Cost: Because of the additional weaving procedure, material cost is increased. However, labor cost may be decreased because the plies are thicker (fewer plies need to be laid up).

Brian

http://www.espcomposites.com

 

[Compositepro]

Theoretically, all parts can be made from only unidirectional materials. The advantages of unitape are:
1. Maximum fiber volume can be achieved due to best fiber packing.
2. Maximum strength and modulus are obtained due to straightness of the fibers, particularly in compression.
3. You can optimize the lay-up for minimum weight because just the right amount of fiber is used in any direction.
4. Tape can be spliced to any width without any loss of properties.

The advantages of fabric are:
1. Drapeability on contours is much better because the fibers are already crimped (prebuckled) at every fiber crossing. Compressing fabric lengthwise, which is required for drape, causes the waviness of the fiber to increase. Doing this to unitape will result in wrinkles or puckers (one large out-of-plane wave in the fibers)
2. Fabric is often used as the surface ply to avoid the problem of fiber peel-out that happens with unitape when it is damaged or machined.
3. Laying-up fabric is more productive than with unitape because of better drape and thicker plies. You cannot make parts with tape plies that are too thick because the interlaminar stresses caused by cure shrinkage and structural loads between plies in different directions will cause cracking and delamination.
4.Fabric can be handled dry, whereas unitape must be in prepreg form.

 

[WK95]

Thanks for the good info guys.