Using aligned nanofibres for identifying the toughening micromechanisms in nanofibre interleaved laminates Lode Daelemans a , Sam van der Heijden a , Ives De Baere b , Hubert Rahier c , Wim Van Paepegem b, ** , Karen De Clerck a, * a Department of Textiles, Ghent University, Technologiepark-Zwijnaarde 907, B-9052 Zwijnaarde, Belgium b Department of Materials Science and Engineering, Ghent University, Technologiepark-Zwijnaarde 903, B-9052 Zwijnaarde, Belgium c Department Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium article info Article history: Received 12 October 2015 Received in revised form 17 November 2015 Accepted 19 November 2015 Available online 2 December 2015 Keywords: Nano particles Delamination Fibre bridging Damage tolerance Electro-spinning abstract The susceptibility to delamination is one of the main concerns in many advanced laminated composite applications. Laminates interleaved with electrospun nanofibrous veils provide a potential solution in order to increase the material's resistance to interlaminar fracture. Previous studies have shown that nanofibres are able to bridge microcracks in the laminates resulting in an increased interlaminar fracture toughness (IFT). However, the exact micromechanisms resulting in these nanofibre bridging zones are still unclear. In this article, aligned nanofibrous structures are used to identify and study the different micromechanisms which take place during Mode II crack propagation. Three nanofibrous veil mor- phologies with a distinct orientation of the nanofibres are used: (1) a random deposition of nanofibres, (2) nanofibres oriented parallel to the crack growth direction, and (3) nanofibres oriented perpendicular to the crack growth direction. A thorough analysis of the fracture surface of tested specimens and crack path behaviour is performed in order to determine the micromechanisms associated with the devel- opment of nanofibre bridging zones. A strong effect of the nanofibre orientation distribution on the Mode II IFT and the underlying toughening mechanisms was observed: different micromechanisms were observed depending on the nanofibre orientation. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Delaminations between reinforcing plies remain one of the most important types of damage encountered in composite lami- nates during service. Hence, the susceptibility to delamination of composite laminates is a critical design factor in many applications [1e3]. It is often expressed in terms of interlaminar fracture toughness (IFT) and a lot of research is dedicated to developing methods that increase the IFT of composite laminates. A recently proposed method of increasing the IFT is the inter- leaving of laminates with electrospun nanofibrous veils [4e21]. The nanoscale diameter of electrospun nanofibres offers the possibility of obtaining very thin veils with a very high surface area to volume ratio and superior mechanical performance compared to the bulk polymer. Although the use of electrospun nanofibrous veils offers many benefits compared to traditional toughening methods, the research on composites enhanced with electrospun thermoplastic nanofibrous veils is still limited. Several studies indicate that electrospun polyamide nanofibrous veils have a lot of potential to be used as an interlaminar toughener [4e6,9,14,17,19]. In a previous article by the authors [4], the development of nanofibre bridging zones was suggested to be the main mechanism resulting in an increased toughness of PA nanofibre interleaved laminates. How- ever, a detailed analysis of the fracture process and the nanofibre bridging development in nanofibre interleaved laminates has yet to be performed. Hence, it is necessary to investigate the fracture behaviour of the nanofibre interleaved composites in detail in order to determine the critical factors for toughening. In this article, the authors aim to provide thorough insight into the micromechanisms that govern the interlaminar fracture of nanofibre interleaved composite laminates. The focus will be on Mode II IFT as delamination growth during service often occurs under Mode II dominated loadings. Mode II IFT can also be related * Corresponding author. ** Corresponding author. E-mail addresses: Karen.DeClerck@UGent.be (K. De Clerck), Wim.VanPaepegem@ UGent.be (W. Van Paepegem). Contents lists available at ScienceDirect Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech http://dx.doi.org/10.1016/j.compscitech.2015.11.021 0266-3538/© 2015 Elsevier Ltd. All rights reserved. Composites Science and Technology 124 (2016) 17e26