NCFs are a type of “engineered fabrics” to reinforce polimeric composites that are made up of multiple layers or fibers stitched together. The most used NCF’s are biaxial, triaxial and cuadriaxial fabrics, where the fiber tows are straight and with different orientations (0, 45, 90 degrees) to provide multidirectional properties.
The combination of multiple layers of fibers, stacked in just one fabric, leads to faster and cheaper preforms production processes than unilayer based processes. It also has advantages for LCM (Liquid Composite Moulding) methods as NCF present better drappability due to the good deformability of unidirectional plies versus vowen fabrics that are undulated (crimp). An additional advantage of the NCF reinforced composites is that they generally feature better mechanical properties as the tows are not crimped or woven. Furthermore the delamination resistance and impact strenght of the NCF reinforced composites is better.
One key advantage of the NCF’s is the ability to drap into relatively complex shapes without giving rise to wrinkles that normally appear with standard woven textiles and preimpregnated tapes. The excelent conformability of biaxial fabrics under “dome type” deformation conditions is caused by a slipage of the fiber tows as there is no joints that restrict this movement.
This is why the use of NCF is rapidly growing in aircraft, automotive, yatching , wind energy and complex structural components.
A lot of work is being done in NCF to push the boundaries of this materials:
Research is beeing developed to increase the limits of the deformability of NCFs through the determination of the best stitching conditions. These studies show that the deformation of the biaxial y triaxial NCF under load (bias extensión test), happens through rotation, sliding and compaction of the tows. The resistance of the NCF to biaxial deformation is dependant on the density, the tension and the position of the stitches. Higher tension on the stitches gives bigger resistance to deformation due to better sliding resistance.
The veils and binders used by the different suppliers play also an important role. A veil is a thin layer of polimeric material formed by fibers with random orientation. Binders are thermoplastic particles that act like an adhesive. Nowadays they are used to minimice fragility of the epoxi matrix on the composite.They also avoid fraying or wrong orientation and specially optimize the permeability, improving process times. Permeability of the material is a key feature. On one hand high permeability accelerates infusion processes, on the other hand too much space between the fibers can lead to a worse impregnation of the fibers worsening the fatigue behaviour. Many innovations are beeing lauched in these topics.
The manufacturing requires handling and depositing the NCF on the mouls with big precision, repeatability and productivity. Drappability modeling is also an important research area , predicting and improving the deformation of the fabrics depending on the tension applied is the basis for the good part design and process optimization.
In this line developments like Drapetest http://bit.ly/1wu2Sm4 allows to automatically characterize drapability and the formation of defects during draping and forming The tester combines the measurement of the force required for forming with an optical analysis of small-scale defects .
Video: Bombardier CSeries uses NCF