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This work presents the development of a meso-scale 3-axis milling machine with a nanometer resolution. Regarding the Z-axis design of the meso-scale 3-axis milling machine, it mainly includes a pagoda structure, the high speed air bearing spindle, two HR8 ultrasonic motors, a laser diffraction grating interferometer system (LDGI), and a coaxial counter-balance system for the spindle. The optimal geometrical dimensions of the pagoda structure have been determined by ANSYS software. According to the simulation results, the maximum static deformation along the z-axis of the pagoda was about 5.05 nm. The designed meso-scale 3-axis milling machine is equipped with an X-Y coplanar positioning stage with nanometer resolution, including the low-cost components and two ultrasonic motors, in order to reduce the complexity and error resulted from the combination of a long-stroke stage and short-travel stage. The coplanar stage developed by Nation Taiwan University was integrated with two laser diffraction grating interferometer system as displacement feedback sensors, so that a two-axis closed-loop control was possible. The positioning accuracy of the coplanar stage was about 50nm after error compensation using the gate mode, base on the test results. A circular positioning test with the radius of 7 mm using the developed stage was tested, and the overall roundness error was about 0.094 mm based on the preliminary test results. A preliminary cutting tests for the oxygen-free copper using a polycrystalline diamond tool have been investigated with respect to different depth of cut and cutting speed on the developed meso-scale machine tool.

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The objective of this study is to measure the crack propagation speed in three types of self-compacting concrete reinforced with steel fibers loaded under four different loading rates. Central-notched prismatic beams with two types of fibers (13 mm and 30 mm in length), three fiber volume ratios, 0.51%, 0.77% and 1.23%, were fabricated. Four strain gages were glued on one side of the specimen notch to measure the crack propagation velocity, a fifth one at the notch tip to estimate the strain rates upon the initiation of a cohesive crack and the stress-free crack. A servo-hydraulic testing machine and a drop-weight impact device were employed to conduct three-point bending tests at four loading-point displacement rates, the former to perform tests at 2.2 μm/s, 22 mm/s and the latter for those at 1.77 m/s, 2.66 m/s, respectively. With lower fiber contents, smooth mode-I cracks were formed, the crack speed reached the order of 1 mm/s and 20 m/s. However, crack velocities up to 1417 m/s were obtained for the concrete with high content of fibers under impact loading. This value is fairly close to the theoretically predicted terminal crack velocity of 1600–1700 m/s. Numerical simulations based on cohesive theories of fracture and preliminary results based on the technique of Digital Image Correlation are also presented to complement those obtained from the strain gages. In addition, the toughness indices are calculated under all four loading rates. Strain hardening (softening) behavior accounting from the initiation of the first crack is observed for all three types of concrete at low (high) loading rates. Significant enhancement in the energy absorption capacity is observed with increased fiber content.