points 2 and 19 fig6 a 6 i shows sections through the insert 8 taken along the section line 6 6 in fig5 wherein nine different variants are shown for the ductile insert 8 any of these variants can be chosen for the inserts 8 and 23 it is also possible to choose different variants for the inserts 8 and 23 variant of fig6 a shows a two part design of the insert 8 whose parts are in each case connected by electric conductors 26 and 27 to a detection device 25 if the insert 8 is deformed the distance between the two parts changes which results in a change of the electrical capacitance of the condenser formed by the two parts which is indicated schematically by a symbolic condenser symbol c the capacitance change can be measured by the detection device 25 which can accordingly detect deformation of the insert 8 variant of fig6 g also shows a two part design of the insert 8 whose parts are each connected by respective electric conductors 26 and 27 to a detection device 25 if the insert 8 is deformed the distance between the two parts changes so that they can come into electrical contact with one another this can be measured by the detection device 25 which can therefore detect deformation of the insert 8 obviously it is also possible to monitor a deformation of the insert 23.