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• A hierarchical toughening strategy was proposed for improving the interlaminar properties of GFRP. • The highest improvement of G IC,ini and G IC,prop values reached as much as 211 % and 174 %. • The multiscale fiber bridgings and interlaminar crossings are the key to achieve high interlaminar fracture toughness. Hierarchical toughening is commonly found in natural materials such as teeth, bone and seashells, which have evolved to withstand mechanical stresses. The incorporation of this mechanism into the design of synthetic materials, such as composites, has drawn much attention in recent decades. In this work, a carbon fiber felt (CFF) made of short carbon fibers was spray-coated with carbon nanotubes (CNT) to create a hierarchical structure that can be directly interleaved into glass fiber reinforced polymer (GFRP) composites for interlaminar toughening purpose. The results showed that the highest enhancement of G IC,ini and G IC,prop values reach as much as 211 % and 174 %, respectively, which far outweighs the state of the art. The fracture surfaces as well as crack propagation behaviors were comparatively investigated to get deep insight into the toughening mechanisms. It was found that multiscale fiber bridgings and interlaminar crossings are the key to achieving high interlaminar fracture toughness of laminate composites. [ABSTRACT FROM AUTHOR]

"Interleaving" is widely used for interlaminar toughening of fiber-reinforced composites, and the structure of interleaving is one of the important factors affecting the toughening efficiency of laminates. Several experiments have demonstrated that compared to continuous and dense structures, toughening layers with structural heterogeneity can trigger multiple toughening mechanisms and have better toughening effects. On this basis, this work further investigates the application of heterogeneous toughening phases in interlaminar toughening of bidirectional GFRP. CNT was selected to construct toughening phases, which was introduced into the interlaminar of composites through efficient spraying methods. By controlling the amount of CNT, various structures of CNT toughening layers were obtained. The fracture toughness of modified laminates was tested, and their toughening mechanism was analyzed based on fracture surface observation. The results indicate that the optimal CNT usage (0.5 gsm) can increase the initial and extended values of interlayer fracture toughness by 136.0% and 82.0%, respectively. The solvent acetone sprayed with CNT can dissolve and re-precipitate a portion of the sizing agent on the surface of the fibers, which improves the bonding of the fibers to the resin. More importantly, larger discrete particles are formed between the layers, guiding the cracks to deflect in the orientation of the toughened layer. This generates additional energy dissipation and ultimately presents an optimal toughening effect. [ABSTRACT FROM AUTHOR]