A Modified Three-rail Shear Test for Fatigue of Composite Laminates

In the standard three-rail shear test, as described in “ASTM D 4255/D 4255M The standard test method for in-plane shear properties of polymer matrix composite materials by the rail shear method” the specimen is clamped with bolts. This however, requires drilling holes through the specimen, which of course is not ideal for composite materials, because of the occurring damage around the holes. Furthermore, the rail shear test is only used for quasi-static tests, in order to determine shear stiffness and ultimate shear stress; very limited research has been done on the use of a rail shear test under fatigue loading conditions [1]. In this study, a new design of the three rail shear test is presented. To avoid the need for holes in the composite specimen, the gripping is based on friction and geometrical clamping. An exploded view of this new design is given in Fig. 1 (a), the actual design can be seen in Fig. 1 (b). The composite specimen is placed between the grip and a steel plate and five bolts are used to press a steel plate against the composite. However, the force given by the bolts does not provide sufficient normal force for friction alone to grip the specimen. Therefore, there is a notch in the steel plate with the exact width of the specimen. As a result, the specimen cannot move along the vertical axes because of the geometrical gripping (the notch) and it is not allowed to rotate in the grip, because of the friction. This results in a similar load transfer to the loaded zone between the grips, as with the standard three rail setup, without the need for drilling holes.