A device for utilising the DCB test geometry at intermediate opening velocities

Abstract
Interlaminar mode-I fracture characterisations of polymer matrix composites are generally realised statically using the Double Cantilever Beam (DCB) test geometry. For several applications, it is of interest to use this geometry at intermediate opening velocities (0.1 to 10 m/s ) in order to measure evolutions of the fracture toughness with the loading rate. We show theoretically that a careful choice of the opening conditions is required for controlling the loading at the crack tip. In particular a symmetrical displacement imposed to the two arms of the DCB limits the vibrations of the sample. An experimetital device is realised for reproducing experimentally these conditions on a glass/polyester composite. Up to the initiation of the crack propagation, the force and displacement signals are free of oscillations and the sample is mechanically equilibrated. It is then possible to interpret easily the data and measure the initiation value of G_IC with the same methods that are used statically. The situation is different for the propagating value of G_IC, since we observe that the sample is destabilised during the propagation of the crack.