JEC COMPOSITES MAGAZINE - Issue #49 - June 2009 - 37

Reinforcement fabric (NCF) reinforcement was constructed to provide a good combination of high stiffness along the main loading direction (0°) and shear properties, both commonly needed in the spar and root area of the blade. These hybrid NCFs were optimized in terms of their processing characteristics using Devold’s latest textile enhancing technology and then compared with a pure glass fibre solution. manufacturer’s recommended processing conditions, giving fibre volume contents of 52% +/- 2.5%. Static mechanical testing was conducted at Toho Tenax, Wuppertal. The different materials were tested in tension, compression and shear, according to ISO 527, EN 6036 and ISO 14130 respectively. All static tests were carried out at room temperature, and test results were normalized to 52% fibre volume content. Experimental Three different carbon-/glass-fibre hybrid NCFs and a reference material containing only glass fibre-based NCFs were developed (see Table 1). All three hybrids were triaxial NCFs containing Tenax STS40 24k carbon fibre in the 0° direction and standard E-glass in the +/-45° directions. Hybrid sample A was produced using a standard tricot stitch, while Hybrid B and C used Devold’s proprietary stitching technology. In addition, Hybrid C contained Devold’s advanced functional flow aid for improved fatigue and flow properties. The glass fibre reference material was a combination of a unidirectional NCF and a biaxial (+/- 45°) NCF employing the , same E-glass fibre. Both fabrics are widely used standards in the wind industry of which the UD construction contained 50 g/m2 of flow enhancing chop. The laminate construction was chosen to provide ultimate static load levels similar to those of hybrid materials. Permeability testing was conducted using Devold’s flow test bench, as shown in Figure 1. This equipment has been continuously improved over the past years and is finding increasing interest in the industry. In the VARTM-type set-up, a testing fluid is drawn through the reinforcement whilst a camera system logs the progression of the flow front. Taking the recorded pressure data into account, the system reproducibly calculates permeability constants. Permeability in all three principal material directions (x/0°, y/90°, z) can be measured, providing valuable process information for the various impregnation scenarios used by blade manufacturers. Results & discussion Infusion behaviour Processing properties are among the key requirements for blade reinforcement materials. Short impregnation times and high permeabilities (i.e. low resistance to resin flow) are required to infuse large structures effectively and in a reproducible fashion. In the past, carbon fibre was frequently considered to be less easy to impregnate than glass fibre. However, early carbon reinforcement materials were not particularly developed for the wind market, nor was there much knowledge about carbon/glass hybrids. Figure 2 shows the in-plane permeabilities, namely KDx (0°) and KDy (90°), of the investigated materials. Interestingly, the carbon/glass hybrid materials were found to mostly match or exceed the corresponding glass reference, even though no heavy chop layer was used in the hybrids. Devold’s proprietary stitch more than doubled the permeability in the critical 90° direction compared with the standard tricot stitch, drastically improving the impregnation performance. Additional enhancing by 20-30% could be achieved with Devold’s functional flow aid. Fig. 2: In-plane permeability KDx and Kdy. Fig. 1: The Devold AMT flow bench. Laminates were prepared for mechanical testing by a standard VARTM infusion process using the lay-ups shown in Table 1. Hexion’s epoxy infusion resin RIM135/RIM137 was the resin matrix. The resin was infused and cured according to the Through-the-thickness permeability KDz was measured using a thick stack of the same lay-up schedule as for the cured laminates. Again, the hybrids exceeded the transverse flow properties compared with the 100% glass solution, as shown in Figure 3. The highest permeability was achieved by Hybrid C containing the Devold advanced flow aid combined with No49 June 2009 / jec composites magazine 37

JEC COMPOSITES MAGAZINE - Issue #49 - June 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #49 - June 2009