In this study, we investigated the mechanism of the indentation size effect for Ti, whose crystal structure is hexagonal close-packed. Indentation tests were performed for two types of single-crystal Ti with different crystal orientations: (0001) and (1120). Indentation tests were also performed with different indenter directions. The hardness depends on the crystal orientation and indenter direction because Ti has large anisotropy. The shape of the impression also changes with the crystal orientation and indenter direction. The hardness increases with decreasing penetration depth for the (0001) and (1120) orientations, so the indentation size effect occurs. For the (0001) orientation, a sudden rapid increase in displacement occurs during indentation tests owing to twinning. This sudden rapid increase in displacement causes a rapid decrease in the hardness. Electron backscatter diffraction was performed around the impressions to measure the geometrically necessary (GN) dislocation density. The GN dislocation density increases with decreasing penetration depth for the (0001) and (1120) orientations. This trend corresponds to strain gradient theory. The reason for the indentation size effect in Ti is the increase in the GN dislocation density. For the (0001) orientation, the indentation size effect also can be attributed to twinning. Considering both our previous study and this study, the experimental results correspond to strain gradient theory in many cases.
The micro-pressure wave (MPW) phenomenon caused by a high-speed train entering a tunnel consists of three stages: generation of the tunnel compression wave upon train entry, propagation of the tunnel compression wave through the tunnel, and emission of the MPW outward. Model experiments using train launcher facilities (TLFs) are effective for analyzing MPWs. However, the use of those facilities to study the latter two stages, i.e., the compression wave propagation and MPW emission, is hindered by two problems: high costs required for facility operation and construction, and low data productivity. Compression wave generators can potentially provide reasonable experiments. In this study, a new, simple facility was developed for the simulation of tunnel compression waves. The proposed generator consists of small solenoid valves, a pressure chamber, compressor, power supply system, and tunnel model. The performance analyses indicate that the compression waves generated by the designed facility effectively simulate tunnel compression waves generated by train entry. The pressure rise and wavelength of the compression wave are controlled by adjusting the chamber pressure and the voltage applied to the valves. The generator is expected to provide sufficient data to develop measures against MPWs in the propagation and emission stages as it reduces the required time by over 60% compared to that required by TLFs for each generation of the compression waves.
The derailment coefficient, which is calculated based on the wheel-rail contact forces, indicates the running safety of a railway vehicle with respect to flange climb derailment. The value of the derailment coefficient changes constantly due to numerous factors associated with the vehicle and track conditions while the vehicle runs on a service line. Therefore, it is desirable to monitor the state of the wheel/rail contact in order to ensure the running safety. Recently, a new monitoring bogie, which can measure the derailment coefficient during commercial operations, has been developed and introduced into some service lines. Large-scale data have been collected by this monitoring bogie. In this paper, the temporal subtraction analysis is carried out for preparing appropriate plan for reducing the derailment coefficient based on these data. In the analysis, the vehicle running position is important for accurate calculation of the difference between two waveforms. However, the vehicle running position contains errors because of the accumulated error of integral calculation of the vehicle velocity. The present paper proposes a method which modifies the running position along track so that the two waveforms are well matched. The proposed method is based on DP matching, and the waveforms of the track irregularity of twist estimated by the monitoring bogie are used in the method. After DP matching, an example of temporal subtraction analysis of the derailment coefficient between two periods is performed. Finally, by using the long-term measurements acquired by the monitoring bogie, the monthly variation of the derailment coefficient for a certain spot on the track is shown as a practical example.