Xiaoli Hu

Novel in situ device for investigating the tensile and fatigue behaviors of bulk materials

For investigating the static tensile and dynamic fatigue behaviors of bulk materials, a miniaturized device with separate modular tensile and fatigue actuators was developed. The fatigue actuator presented good compatibility with the tensile actuator and mainly consisted of a special flexure hinge and piezoelectric stack. In situ fatigue tests under scanning electron microscope or metallographic microscope could be carried out due to the miniaturized dimensions of the device. A displacement correction method of tensile actuator based on load sensor compliance was investigated, and the feasibility of the method was verified by the comparison tests with a commercial tensile instrument. The application of testing the storage and loss modulus as a function of frequency was explained, and the temperature rises of both the piezoelectric stack and specimen were obtained as a function of frequency. Output characteristics of the fatigue actuator were also investigated. Additionally, the discharge performance of piezoelectric stack based on various initial voltages and fatigue tests on C11000 copper was carried out. This paper shows a modularized example that combines a servo motor with a piezoelectric actuator attached to the specimen grip to realize the in situ fatigue tests.

Note: investigation on the influences of gripping methods on elastic modulus by a miniature tensile device and in situ verification.

In this paper, by gripping the specimen on various positions, including the gripping section, stress concentration transition section, and gauge length section, theoretical analysis on the influences of gripping methods on tensile elastic modulus calculation was investigated with a group of equations. Then, an image-based displacement measurement system was implemented, and the experimental verification via in situ tensile testing was carried out to verify the feasibility of the theoretical analysis by a miniature tensile device integrated with a metallographic microscope. The stress-strain curves of 2026 aluminum alloy were also obtained by gripping the specimens on various positions to illustrate the influences of gripping methods. The influence of gripping methods on elongation measurement was also investigated. This paper showed a modular calculation method of elastic modulus for the tensile testing of typical plate specimens.

Novel correction methods on a miniature tensile device based on a modular non-standard layout

A novel in situ tensile device with a large output load–volume ratio was developed for testing the mechanical properties of bulk materials. A major characteristic of the device was the modular non-standard layout, as the specimen was placed on the top plane of the device to approach the lens of an optical microscope or the electron gun of a scanning electron microscope. Accordingly, to investigate the effects of non-standard layout on tensile properties, displacement and load correction methods were given by formulas based on theoretical calculations to describe the specimens actual tensile displacement and load. Based on in situ observation, the feasibility of the correction method was verified by comparing it with the data from metallographic microscope images. The bending effects on the specimens vertical displacements and tensile load due to the installation mode were also discussed. This paper presents a modularized correction method for a horizontal-type tensile device with a non-standard layout design.

The evolution of machining-induced surface of single-crystal FCC copper via nanoindentation

The physical properties of the machining-induced new surface depend on the performance of the initial defect surface and deformed layer in the subsurface of the bulk material. In this paper, three-dimensional molecular dynamics simulations of nanoindentation are preformed on the single-point diamond turning surface of single-crystal copper comparing with that of pristine single-crystal face-centered cubic copper. The simulation results indicate that the nucleation of dislocations in the nanoindentation test on the machining-induced surface and pristine single-crystal copper is different. The dislocation embryos are gradually developed from the sites of homogeneous random nucleation around the indenter in the pristine single-crystal specimen, while the dislocation embryos derived from the vacancy-related defects are distributed in the damage layer of the subsurface beneath the machining-induced surface. The results show that the hardness of the machining-induced surface is softer than that of pristine single-crystal copper. Then, the nanocutting simulations are performed along different crystal orientations on the same crystal surface. It is shown that the crystal orientation directly influences the dislocation formation and distribution of the machining-induced surface. The crystal orientation of nanocutting is further verified to affect both residual defect generations and their propagation directions which are important in assessing the change of mechanical properties, such as hardness and Youngs modulus, after nanocutting process.

A tension stress loading unit designed for characterizing indentation response of single crystal silicon under tension stress

In this paper, a tension stress loading unit is designed to provide tension stress for brittle materials by combining the piezo actuator and the flexible hinge. The structure of the tension stress loading unit is analyzed and discussed via the theoretical method and finite element simulations. Effects of holding time, the installed specimen and hysteresis of the piezo actuator on output performances of the tension stress loading unit are studied in detail. An experiment system is established by combing the indentation testing unit and the developed tension stress loading unit to characterize indentation response of single crystal silicon under tension stress. Experiment results indicate that tension stress leads to increasing of indentation displacement for the same inden-tation load of single crystal silicon. This paper provides a new tool for studying indentation response of brittle materials under tension stress.

A three-point method for evaluating the tilt status between the indenter axis and the sample surface

Non-perpendicularity nanoindentation experiments commonly exist in previous research work and affect nanoindentation results. In this design note, a new measuring method named the three-point method is proposed to evaluate the tilt status between the indenter axis and the sample surface. The measuring principle is addressed in detail. By the proposed measuring method, the tilt status between the indenter axis and the sample surface can be evaluated by the selected three points on the sample surface. A measuring example is given to verify the feasibility of the three-point method. This method is independent of other measuring equipment and it has the potential to be used in commercial nanoindentation instruments for evaluating the tilt status between the indenter axis and the sample surface.