SAMPE Journal - March/April 2011 - 45

that these crossovers act as built-in delamination arresting mechanisms and may account for the reduced delamination size of AP-PLY laminates. Thermoplastic Figure 18 and Figure 19 show the section cuts for the TP specimens impacted at 25 J. As BVID is considerably higher for the TP specimens overall less damage is visible, as well as less difference for both laminate configurations. It is clear however that less and smaller delaminations are present in the AP-PLY configuration as compared to the baseline. Also slightly more fibre breakage in the Baseline seems contrary to the deeper indentation. Whether fibre breakage or delaminations are favourable depends on the application. In general, delaminations are more of a problem in compression whereas fibre breakage causes more concern in tension. As these specimens were not tailored to any specific application but do show interestingly different behaviour, there seems to be enough room for optimizing a lay-up with all known characteristics. Conclusions A new fibre architecture is presented that shows improved damage tolerance for all the patterns tested compared to its unidirectional counterparts, for two material systems. This can be explained by the multiple load path provided by the interweaving, halving the number of ply interfaces and the creation of physical barriers for a delamination to grow. The influence of impact location is an important topic for future research. Although the worst case of the new pattern has the same residual strength as its unidirectional counterpart, a solution for the heterogeneity of the pattern could make it even more damage tolerant. A smaller bandwidth or a different stacking sequence could be the solution for this. Only quasi-isotropic lay-ups were compared in this research. More often other lay-ups are used, to meet specific loading conditions. Other lay-ups should be tested, and perhaps a combination with the earlier described dispersed stacking sequences could be beneficial. In this research only 45° and 90° interface angles were tested, where in theory all angles are possible. It could be the case that for very small angles matrix rich areas are created as the fibres do not fit up properly, as well as for more than two interwoven layers. Further research will be focused on determining the best pattern and the influence of AP-PLY patterns on properties such as open hole compression and bolt-bearing Possible applications include damage tolerance dominated structures, (fuselage or wing), but the applications are not limited to that. The ability to replace woven fabrics, which are often used for improved bearing properties or (on the surface) for improving hole quality during drilling, makes this concept usable to a large variety of applications including sandwich with thin facesheets.

References 1. N.K. Naik, Y.C. Sekher, S. Meduri, Damage in Woven-fabric Composites Subjected to Low-velocity Impact, Composites Science and Technology, 60, pp. 731-744, 2000. 2. Z. Gürdal, “Design Tailoring of Laminated Composite Structures,” 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference, 18-21 April, Austin, Texas, AIAA, pp. 2005-2164, 2005. 3. C. Lopes et al., Stacking Sequence Dispersion and TowPlacement for Improved Damage Tolerance, 49th AIAA/ASME/ ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Schaumburg Illinois, 2008. 4. Z. Gürdal, B. Tatting, C. Wu, Tow-Placement Technology and Fabrication Issues for Laminated Composite Structures, 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference, 18-21 April, Austin, Texas, AIAA, pp. 2005-2164, 2005. 5. American Society for the Testing of Materials, ASTM D7136. 6. American Society for the Testing of Materials, ASTM D7137. 7. G.A.O. Davies, R. Olsson, Impact on Composite Structures, The Aeronautical Journal, pp. 541-563, November 2004.

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SAMPE Journal, Volume 47, No. 2, March/April 2011 45

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SAMPE Journal - March/April 2011

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