1. Introduction High performance structural composites are year by year replacing metals in load-critical structures of aircrafts and other industrial applications. This ten- dency is clearly driven by the need for structural ma- terials with superior mechanical properties, low spe- cific weight and corrosion resistance. On the other hand, during their service life, multi-layered com- posites are prone to delaminations, as a result of out of plane loads. The presence of delaminations dra- matically decreases their residual compression prop- erties and also affects other in-plane properties of the composite. Additionally, polymer matrix composites are sensitive to microcracking formation within the polymer phase due to service-induced loading or even during the manufacturing. Under fatigue load- ing conditions that a real-world structure experiences on its service life, the initially minor microcracks are linked together to create significantly larger cracks. Cracks and delaminations work together to result in remarkable suppression of the load-bearing capacity of the composite part, even with minimal visual in- dication of damage being present. The interlaminar fracture toughness is the key-char- acteristic in initiation and propagation of delamina- tions in fibre reinforced polymer (FRP) composites. A constant concern of various researchers has been the enhancement of the fracture behavior of high performance structural composites, especially car- bon fibre reinforced polymers (CFRP). In this direc- 914 Mode II fracture toughening and healing of composites using supramolecular polymer interlayers V. Kostopoulos 1* , A. Kotrotsos 1 , A. Baltopoulos 1 , S. Tsantzalis 1 , P. Tsokanas 1 , T. Loutas 1 , A. W. Bosman 2 1 Applied Mechanics Laboratory, Department of Mechanical Engineering and Aeronautics, University of Patras, 26504 Rio-Patras, Greece 2 SupraPolix BV, Horsten 1, 5612 AX Eindhoven, The Netherlands Received 18 May 2016; accepted in revised form 25 June 2016 Abstract. This study focuses on the transfer of the healing functionality of supramolecular polymers (SP) to fibre reinforced composites through interleaving. SPs exhibiting self-healing based on hydrogen bonds were formed into films and were successfully incorporated into carbon fibre composites. The effect of the SP interleaves on in-plane fracture toughness and the subsequent healing capability of the hybrid composites were investigated under mode II fracture loading. The fracture toughness showed considerable increase since the maximum load (P max ) of the hybrid composite approximately doubled, and consequently the mode II interlaminar fracture toughness energy (G IIC ) exhibited an increase reaching nearly 100% compared to the reference composite. The healing component was activated using external heat. P max and G IIC recovery after activation were measured, exhibiting a healing efficiency after the first healing cycle close to 85% for Pmax and 100% for G IIC , even- tually dropping to 80% for P max while G IIC was retained around 100% even after the fourth healing cycle. Acoustic Emission activity during the tests was monitored and was found to be strongly reduced due to the presence of the SP. Keywords: smart polymers, CFRPs, supramolecular polymers, fracture toughness, self-healing eXPRESS Polymer Letters Vol.10, No.11 (2016) 914–926 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2016.85 * Corresponding author, e-mail: kostopoulos@mech.upatras.gr © BME-PT