Impacting rods are used in various devices in practical life. One example is rock drilling, where a piston is accelerated hydraulically or pneumatically before it hits the drill rod. The hit is followed by a complicated sequence of phenomena. A stress wave, whose length is double the physical length of the piston, starts to propagate along the drill rod to finally reach the tool-process material interface. A nonlinear penetration of the bit into the rock takes place and a reflecting wave component is generated.

In the long history of rock drilling, fundamental mechanisms contributing to the effective drilling process are still unknown. To produce maximal tool penetration for each hit, rules have to be derived which specify the dimensioning of the piston and drill rod. Recent investigations, that cover the issue of optimization, are revealing that the efficiency of the drilling process strongly depends on the shape of the longitudinal wave transmitted through the drill rod to the drill bit. A well-known fact is that the cross-sectional profile of the piston is shaping the stress wave profile. Therefore, the main object of the research project is to adjust the geometrical form of the impacting piston to generate every desired stress wave form. This is an inverse problem as the cause (geometry of the piston) is concluded from the consequence (stress wave form after impact). For inverse problems, it needs to be examined if a solution exists and if the solution is unique.

A test rig is set up in order to validate the theoretical results. The stress wave from is derived from the strain gauges (SG) signal which is further processed with a signal conditioner. By recording the hit with a high-speed camera, the time when piston and rod separate may be detected. Besides, the high-speed camera is able to validate the theoretical calculations which predict that the rod moves discontinuously after the hit.