Multiaxial non-crimp fabrics have been known on the market for a long time. Multiaxial non-crimp fabrics are understood to be structures made from a plurality of superimposed fiber layers, wherein the fiber layers comprise sheets of reinforcing yarns arranged parallel to each other. The superimposed fiber layers can be connected and secured to each other via a plurality of sewing or knitting threads arranged parallel to each other and running parallel to each other and forming stitches, such that the multiaxial non-crimp fabric is stabilized in this way. The sewing or knitting threads thereby form the zero-degree direction of the multiaxial non-crimp fabric.
The fiber layers are superimposed such that the reinforcing fibers of the layers are oriented parallel to each other or alternately crosswise. The angles are virtually infinitely adjustable. Usually, however, for multiaxial non-crimp fabrics angles of 0°, 90°, plus or minus 25°, plus or minus 30°, plus or minus 45°, or plus or minus 60° are set and the structure is selected such that a symmetrical structure with respect to the zero-degree direction results. Multiaxial non-crimp fabrics of this type can be produced e.g. by means of standard warp knitting looms or stitch bonding machines.
Fiber composite components produced using multiaxial non-crimp fabrics are suited in a superb way to directly counteract the forces introduced from the directions of stress of the component and thus ensure high tenacities. The adaptation in the multiaxial non-crimp fabrics, with respect to the fiber densities and fiber angles, to the load directions present in the component enables low specific weights.
Multiaxial non-crimp fabrics can be used due to their structure especially for the manufacturing of complex structures. The multiaxial non-crimp fabrics are thereby laid without matrix material in a mold and e.g. for shaping, they are adapted to the contours using increased temperatures. After cooling, a stable, so-called preform is obtained, into which the matrix material required for producing the composite component can subsequently be introduced via infusion or injection, or also by the application of vacuum. Known methods in this case are the so-called liquid molding (LM) method, or methods related thereto such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RH), liquid resin infusion (LRI), or resin infusion flexible tooling (RIFT).
It is important on the one hand for the preform that the fibers within the layers as well as the individual fiber layers are secured against each other to a sufficient extent. On the other hand, with respect of the required three-dimensional shaping, a good drapability of the multiaxial non-crimp fabrics is required. Finally, it is also important that the multiaxial non-crimp fabric shaped into the preform can be easily penetrated by the matrix resin which is introduced via the above listed methods.
Multiaxial non-crimp fabrics and the manufacture thereof are described for example in DE 102 52 671 C1, DE 199 13 647 B4, DE 20 2004 007 601 U1, EP 0 361 796 A1, or U.S. Pat. No. 6,890,476 B3. According to DE 10 2005 033 107 B3, initially individual mats made from unidirectionally arranged fibers or fiber bundles are produced, in which said fibers or fiber bundles are caught in stitches by binding threads and all binding threads envelop and secure only one fiber or only one fiber bundle. In a second step, a plurality of layers of mats produced in this way are superimposed at different angles to each other and connected to each other.
EP 1 352 118 A1 discloses multiaxial non-crimp fabrics, for which the layers of the reinforcing fibers are held together by means of fusible sewing yarns. The use of fusible yarns allows, according to one of the embodiments of EP 1 352 118 A1, a shift of the layers against one another during the shaping of the multiaxial non-crimp fabrics above the melting temperature of the sewing threads and a stabilization of the form during subsequent cooling below the melting temperature, such that the sewing stitches function as an in situ binding means. The tension in the sewing yarns leads, according to the statements of EP 1 352 118 A1, initially to the formation of channel zones in the composite, resulting in a better infiltration of matrix resin. Heating the composite structure above the melting temperature of the sewing yarns results then in a reduction of tension for the sewing yarns and as a result thereof in a reduction of the waviness of the reinforcing fibers. The proportion of sewing threads in the non-crimp fabric should, according to EP 1 352 118 A1, preferably lie in the range of 0.5-10 wt. %,
Often, sewing threads made from thermoplastic polymers such as polyamide or polyester are used, as is disclosed in EP 1 057 605 B1 for example. According to information from U.S. Pat. No. 6,890,476 B1, the threads used there have a linear density of approximately 70 dtex. WO 98/10128 discloses multiaxial non-crimp fabrics made from several superimposed layers, deposited at an angle, of reinforcing fibers, said layers being sewn or knitted to each other via sewing threads. WO 98/10128 discloses multiaxial non-crimp fabrics in which the stitch chains of the sewing threads have a gauge of 5 rows per 25.4 mm width (=1 inch) for example and a stitch width generally in the range from approximately 3.2 to approximately 6.4 mm (⅛-¼ inch). The sewing threads used therein have a linear density of at least approximately 80 dtex. In U.S. Pat. No. 4,857,379 B1 as well, yarns made for example from polyester were used for connecting the reinforcing yarns by means of e.g. knitting or weaving processes, wherein the yarns used there have a linear density of 50 to 3300 dtex.
DE 198 02 135 relates to multiaxial non-crimp fabrics for e.g. ballistic applications, for which superimposed layers of warp and weft threads arranged parallel to each other respectively are connected to each other by binding threads. For the multiaxial non-crimp fabrics shown in DE 198 02 135, the threads parallel to each other have a distance from each other, and the loops formed by the binding threads wind around the warp or weft threads respectively. For the binding threads used, linear densities in the range between 140 and 930 dtex are indicated. For the multiaxial non-crimp fabrics disclosed in WO 2005/028724 as well, several layers of reinforcing yarns with high linear density and arranged unidirectionally or parallel to each other are connected by binding threads that interweave between said reinforcing yarns and loop around the individual reinforcing yarns. The reinforcing yarns are separated from each other within the layers. As binding threads, yarns, for example, made from polyvinyl alcohol with a linear density of 75 denier or elastomer yarns based on polyurethane with a linear density of 1120 denier are used.
Also, randomly-laid fiber mats or non-wovens, or staple fiber fabrics or mats, are to some extent laid between the layers made from reinforcing fibers in order to improve e.g. the impregnatability of the fabrics or to improve e.g. the impact strength. Multiaxial non-crimp fabrics having such mat-like intermediate layers are disclosed for example in DE 35 35 272 C2, EP 0 323 571 A1, or US 2008/0289743 A1.
The results show that today's multiaxial non-crimp fabrics can absolutely have a good drapability and that their impregnatability with matrix resin can be satisfactory. A good level of characteristic values can be achieved for components that are produced using multiaxial non-crimp fabrics, with respect to flexural strength or tensile strength. However, these components often show an unsatisfactory level of characteristic values with regard to compression stressees and impact stresses.
The disadvantages of the unsatisfactory mechanical tenacities under compression loading and impact loading have been sufficiently serious thus far that, in spite of the above-mentioned better suitability of the materials especially for complex components, the somewhat longer established, so-called prepreg technology is employed, and thus a greater expenditure of time and higher production expenditures are accepted.