## 3 Research Results and Discussion
### Macroscopic Observations  
The macroscopic observations of the shaft surface were conducted at each stage of the wear tests. The results of those observations are included in Table 5. Surfaces are distinguished by their diversity and intensity of wear and damage depending on the number of fatigue cycles. Traces of damage, mainly due to the shaft sleeve forcing in and removal process, are noted on shafts which operated for 10,000 cycles. These are insignificant surface seizures occurring over the entire axle seat circumference, especially at the end from which sleeve installation on the shaft began. It should be emphasised that the joint was \"dry\"-assembled, which could enhance the development of surface friction wear as a result of the shearing and movement of irregularities occurring at the surfaces of the elements joined together. It is the microirregularities migrating during the forcing process which were the cause of surface scratching and grooving. In addition to the traces of surface seizing, scratches of insignificant dimensions were also noted in places on the shaft surface.  
On the surfaces of shafts which operated for 100,000 fatigue cycles, micro-size plastic deformations were noted, and these are most probably the result of the local occurrence of adhesive bonds. Insignificant fretting wear traces also appear. The wear occurring on the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline Number of Cycles: 0 & Number of Cycles: 10,000 & Number of Cycles: 100,000 & Number of Cycles: 1,000,000 \\ \hline No traces of wear and plastic deformation & Insignificant surface seizure & Plastic deformation & Fretting wear traces \\ \hline \hline \end{tabular}
\end{table}
Table 5: Shaft surface condition after each fatigue cycle.  
shaft surface is due to the successive assembly and disassembly process and to the fatigue operation of the joint in rotational bending conditions.  
Moving wear products originating from the shearing of microprojections on the surface of the mating elements tightly fill the microholes and create a bigger surface area of the joint between the mating elements. That situation translates into the need to apply greater forces to remove the sleeve from the shaft and contributes to the formation of adhesive bonds, causing plastic deformation.  
Local chippings of the top layer and plastic deformations are noted on the surface of the shafts, which operated for 1,000,000 fatigue cycles. Fretting wear traces are, however, the prevailing kind of damage visible on all the surfaces under investigation. When that number of cycles was exceeded, quickly progressing fatigue wear also occurred, which resulted in shaft fracture.  
The surface of the shaft, which operated for 1,000,000 fatigue cycles, is shown in Figure 11. Visible are forcing traces in the form of scratches and skinning of the material, which do not significantly affect the life of the shaft/sleeve forced-in joint. On the shaft surface, fretting wear traces are also noted, which may cause fatigue wear development or surface pressure reduction leading to joint disconnection.  
Fretting wear traces are located on either side of the shaft axle seat and occur close to the edges of the joint with the sleeve. The movement of wear towards the deeper area of the joint is noticeable in places. What is conspicuous is that, on one side, the wear comprises the entire circumference of the shaft axle seat. The width of the wear ring in its widest place is approximately 4 mm, and the width in its narrowest place is 1 mm. On the other side of the axle seat, however, the wear is spaced randomly on the entire axle seat circumference. This situation concerns all the samples tested. The wear ring occurs each time on the opposite side in relation to the one from which forcing was performed.  
Fretting wear traces are distinguished by their grey and dark brown colour, which may prove surface oxidation.