When directly clamping fiber strands with the steel jaws, the serrations of the jaw surface will press into the comparatively soft epoxy resin and damage filaments. Independent of that, there is an abrupt switching of the fiber's strain at the transition to the clamping area due to the evenly distributed clamping pressure. To perform the load entry more carefully in the transition area especially, the clamping pressure was continually reduced in the direction of the middle of the fiber using to two measures. For one, the jaws protrude several centimeters from the specimen grip so that only indirect clamping pressure transferred via the stiffness of the jaws is applied to the fiber strand in the transitional area. For the second measure, the developers use a special compensating layer made from thermoplastic polymers with corundum filling. It is placed so that the 5mm thickness of the insert increases to 8 mm in the upper area. Due to the lower overall depth, the clamping pressure is also reduced in the transitional area.