Multi-axial non crimp fabrics for composite parts design.
This is an article about the latest ( although not new ) generation of fabrics used in the aerospace industry.
These are called non crimp fabrics and are available in uni directional or multi axial combinations.
These fabrics are not woven in a conventional way. They are made of several layers of uni-directional fibers ( glass, carbon, kevlar and other ) stitched together in several directions.
The process used is to take flat ribbons of uni directional fibers that are enzymed together and carefully stitch/ glue them in flat panels.
These panels are then laid up on top of each other in 2, 3, 4 stacks or more ( up to 8 layers for some manufacturers ) to create the final product. This enables the manufacturer to create fabrics that have stiffness in the following directions: 0, 90, +45 and -45 degrees ( as well as any other direction for custom fabrics ) on a 2D plan.
The benefits of such a technology is multiple. One will understand this much better after having seen a 3 D representation and description of a standard weaving process.
Woven fabrics.
The waving process, by design, forces the fibers to go above and under each other alternatively. Below is a pleated ribbon illustration of a standard plain weave pattern.
Once isolated, the fiber pattern looks like this:
One can clearly see that the fiber is not only going in the wanted horizontal direction but also quite dramatically in the vertical direction.
Additionally, applying pressure during layup increases the vertical deflection of the fibers. Altogether, this reduces the stiffness on the horizontal plan and creates small bending radii ( ribbon pleating here ) that also tend to introduce stress fracture application points in the laminate.
When a load is applied to a woven fabric a stress concentration occurs at every point where one fiber bundle passes over or under another. This causes unwanted stresses in the resin, which is much weaker than the fibers. Repeated loading and unloading, or cycle fatigue, will cause a breakdown of the resin leaving the fibers unsupported and free to buckle in compression loading. Fibers in stitch-bonded fabrics are straight and directly aligned with the load path.
Finally this up/ down pattern creates voids that will be filled by the resin, thus slightly increasing the weight of the laminate ( ie decreasing the fiber reinforcement rate ).
Non crimp fabrics.
Non crimp fabric on the other hand are mostly flat ( stitching introduces minimal vertical deflection locally as the yarn used is a very thin and soft polyester), thus offer a true horizontal alignment with minimum void between the fibers. The tows are spreads into flat ribbons.
A weaved fabric generally offer two reinforcement directions. That is 90 degrees and 0 degrees.
Non crimp fabrics can be assembled in any directions 0, 90 degrees and from 30 degrees to 70 degrees. Generally, it is considered that quadriaxial fabrics at 0, 90, +45 and -45 degrees offer quasi-uniform stiffness plates. Thus it is the product we use the most.
However, the best manufacturers offer the possibility to create any combination of angle and material for up to 7 layers at the customer convenience.
Overall, non crimp fabrics offer the following benefits compared to traditional weaving:
- Better mechanical properties are obtained due to higher Fibre rate compared to traditional fabrics (lower resin consumption).
- Less fiber stress concentration when load applied, thus better fatigue resistance.
- Less resin usage.
- Zero crimp provides better interply strength.
- Due to its flat structure, the stitch-bonded fabric reduces the print-through of conventional fabrics.
Modern CAD software like Dassault Catia often include composite design modules that will guide the engineer in laying up the uni-directional fabrics in the best possible directions. Finite analysis can then provide a good feedback on achieved results.
An example of this use can be found on our latest design: the SR-71 Blackbird.
Finally note that NCF are not new. The technique was studied in the 80s with the first assembly plants created in the USA and Italy in the 90s. However these were originally highly specialized products created for the space industry. This technique slowly spread and became available at a wider scale about 10 years ago. However carbon fiber quadriaxial NCF are still difficult to find on the retail market.
