According to the strength versus load concept the strength of the tool can either be increased or the occurring stresses reduced in order to counteract fatigue failure [
2]. Measures to increase the strength include the use of different tool materials [
6], coatings [
16], tool production methods [
17], or heat treatments [
18]. Despite the various approaches, strength can only be increased up to a certain point. For example, cemented carbides, which already cost ten times more than conventional tool steel [
19], are still very sensible towards tensile stresses [
6]. Therefore, a decrease of the stresses in the die is necessary. One possibility to achieve this is to reduce the load acting on the tool. However, the load results from the forming process [
20] and is often seen as a boundary condition for tool life analysis [
3]. Because of the high effort necessary to implement process changes, an adaptation of the load through the number and layout of forming steps should only be considered in special cases [
3]. Another approach to decrease stresses is to adapt the tool layout, for example by using appropriate radii and sliding lengths [
21]. Since changes of the inner tool contour can only be made within the specified tolerances of the produced part, a change in the outer tool contour would be more universally applicable. One approach is to use split tools to reduce bending stresses [
1]. This has the disadvantage, that lubricant or workpiece material may enter the split section [
22]. To globally reduce tensile stresses, dies are usually prestressed by pressing them into reinforcement rings with an interference closure [
23]. For highly stressed parts, stripwound containers are used [
24]. With these conventional reinforcement systems, no local influence on the stress state is possible. For a combined tangential and axial prestressing, reinforcement systems with a locally adapted interference closure have been developed [
25] and successfully implemented [
26]. These systems only have a local influence on axial stresses, not different stresses resulting from non-circular cross-sections.
For a local prestressing of distinct areas in a tool, the use of so-called stress pins has been suggested [
27]. Similar to reinforcement rings, stress pins use an interference to create prestresses. The difference is, that stress pins are pressed into the die inducing a local stress from the inside. It has been shown, that stress pins could effectively reduce stresses and deflections in a hydroforming die [
27]. When implemented for the precision forging of a connection rod, the resulting stresses were increased with the use of stress pins [
27]. Although it was stated, that the effect of the pins should be examined in further detail [
28], no new investigations were carried out to date.