Rate-dependent fracture behavior of composite-metal adhesively bonded structure under symmetric separation loading

The failure behaviors of composite-metal adhesively bonded structures under various loading conditions have garnered significant interest in the field of structural mechanics. This research aims to explore the effects of displacement rates on the fracture behavior, particularly the fracture toughness, of such joints under symmetric separation loading, and to uncover the underlying mechanisms. Experimental tests were performed using a Double Cantilever Beam (DCB) with matched bending stiffness, and fracture toughness was evaluated for both quasi-static and dynamic fracture tests. An electromagnetic split Hopkinson bar system was employed to dynamically load the DCB specimens over a wide range of rates, showing a clear change in fracture toughness with increasing displacement rates. In the case of quasi-static loading, fracture toughness remains nearly constant. However, with dynamic loading, it rises significantly as the displacement rate increases, highlighting a strong displacement rate effect. Especially at a displacement rate of 22 m/s, the fracture toughness reached 4370 J/m², which is 264 % greater than that observed under quasi-static conditions. Furthermore, scanning electron microscopy observations reveal that the adhesive layer exhibits pronounced ductile fracture characteristics across all displacement rates tested. Compared to quasi-static loading, dynamic loading results in shorter crack propagation distances within the adhesive layer, more significant damage, and areas of greater strength are more likely to fail.