A new jig for mode II interlaminar fracture testing of composite materials under quasi-static and moderately high rates of loading

While it is still debated whether a pure mode II interlaminar fracture can physically exist in composites, several test methods have been proposed for its characterization. Lack of agreement between the results obtained with different test configurations has been attributed to the use of inconsistent data reduction schemes or inadequate correction factors used to correct for, e.g. the large deformations occurring with some tough modern materials. Aim of this work was to design a new jig that could provide an as pure as possible mode II crack initiation in unidirectional composites materials, that would allow a direct determination of fracture toughness, i.e. requiring almost no assumption for data reduction nor side effects correction and could be amenable to being used under impact as well as quasi-static loading conditions. The geometry of the system was designed in order to obtain great compactness, i.e. reduced masses and contained volume, making it usable with drop-weight testing machines, but at the same time enough stiffness to prevent flexural moments from closing or opening the crack faces, so granting the purity of the wanted mode, mode II, of loading. The compactness of the jig plus specimen system and the rigid confinement to which the composite specimen is subjected also grant that quite small displacements and overall deformations are reached at fracture. A static finite element analysis was conducted to optimize the jig geometry and is discussed here. Preliminary numerical and experimental results obtained with moderately high rate tests are also presented. The method employed for data reduction is based on the experimental calibration of the compliance and it is quite straightforward.