JEC COMPOSITES MAGAZINE - Issue #48 - May 2009 - 73

secondary resin component into the textile. Another potential benefit of the soluble-veil approach is the added functionality. This may include performance benefits such as added surface or fire smoke and toxicity (FST) protection. In the future design of integrated structures, this discrete interlaminar toughening element could be selectively placed where it is needed within a component. A major advantage of the soluble-veil approach to the PRIFORM technology platform lies in the fact that it no longer requires an intermediate dwell period within the cure cycle. The standard soluble fibre requires a dwell period for complete dissolution and diffusion (see Figure 1). This cycle reduction was possible thanks to the characteristics of the veil filaments. No premature dissolution was observed during the resin incorporation stage. As soon as curing had started, the dissolution mechanism was initiated and it was completed during ramp-up. There have been recent competitive approaches through the use of alternative polymer types or intermediate forms. These are often found to provide limited performance and do not typically offer the balance of toughness enhancement together with FST performance and environmental resistance. Figure 4 shows that the soluble veil NCF provides a comparable cross-section to that of standard NCF . Further development of this material has led to evaluating the impact-to-drape of the veil inclusion. By managing the stitching patterns used, it was observed that the veil presence had little effect on the drapability of the NCF . Fig. 4: Cross-section comparison of standard NCF (left) to soluble veil NCF (right). Fig. 2: Controlled dissolution of the soluble veil (post-injection). Unidirectional grades In addition to the veil multiaxial concept, a stitched unidirectional grade was also assessed. This product involved sandwiching carbon tows between glass filaments and a soluble veil, and holding them in place with polyester stitching. This configuration is illustrated in Figure 5. Veil interleaved stitched non-crimp fabrics The aerospace industry has been using stitched non-crimp fabrics for a long time [5]. Although the Airbus A380 aircraft has given NCFs their first commercial aircraft primary structural application [6], it can be argued that they have not been fully exploited in demanding structural applications. This may be due to their typical low performance with standard LRI systems. Here, poor damage tolerance and notch sensitivity is a design limitation and the presence of polyester stitching can initiate inservice matrix micro-cracking. The incorporation of non-woven intermediates in stitched multiaxial textiles is not a new concept, though the integration of an epoxy-soluble medium into these textiles is, especially when it offers primary structural prepreg performance. The typical inclusion of non-soluble intermediate layers offers a trade-off in thermal performance against limited toughness gains. The configuration of a veil-modified NCF is shown in Figure 3. Fig. 5: Stitched unidirectional veil textile construction and cross-section. With glass yarns present in the textile construction, additional non-contributing material is added to the composite. Evaluating the cross-sections in this configuration has shown that the crimp levels – from both stitching and the supporting glass filaments – will reduce the plane-laminate performance. The extent of this crimping is illustrated in Figure 5. The reduced mechanical performance in dry textile configurations is well known and understood in the composites industry. Little progress has been made in closing the gap on axial-performance levels between textile process technologies and those of prepreg tapes. Property reductions of 15-20% are commonly expected for 0° axial strength. Fig. 3: Construction example of veil interleafed NCF. No48 May 2009 / jec composites magazine 73

JEC COMPOSITES MAGAZINE - Issue #48 - May 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #48 - May 2009