Experimental Techniques最新文献

When fastening components connected through friction contacts are subjected to tangential cyclic loads, slips may occur at the contact interfaces. Under multiple cyclic tangential load excitations, slips may be cumulative or shakedown. However, few reports have conducted experimental work on slip behaviors because local micro slips are difficult to measure. In this paper, an experimental approach to measure slips at frictional contact interfaces under cyclic loads was presented, which could directly capture slip behaviors. In this approach, local micro slips at the contact interfaces were measured by a self-powered sensor based on the principles of a triboelectric nanogenerator (TENG), and a conventional digital source meter was used to collect the voltage signals from the sensor. This approach is completely different from existing contact displacement measurement methods. The slip behaviors in a flat-on-flat contact using an established test bench were observed experimentally. The finite element model of this contact configuration was built to simulate the dynamic slips and the results were in good agreement with the experimental results.

The service environment of many core components, such as aero-engine blades, tends toward extreme. Characterization of fatigue failure behavior at high-temperatures is an important means of evaluating their structural integrity, in which digital image correlation (DIC) method shows great potential and advantages due to its full-field, noncontact, and high-temperature compatibility. For high-temperature DIC, its accuracy highly depends on the quality of high-temperature speckle, i.e., deformation carrier or sensor, fabricated on specimen surface. Herein, a parametric high-temperature speckle fabrication technique combining high-temperature dual-layer coating and femtosecond laser etching is developed. The heat-resistant black matte paint and white boron nitride paint are successively coated on the specimen surface, then femtosecond laser is used to etch the coated layer, realizing the parametric non-destructive fabrication. Based on high-throughput design and high-temperature static test, the speckle parameters and fabrication processes are optimized. The fabrication quality of optimized speckles is further verified by charactering fatigue crack propagation behavior of nickel-based superalloy GH4169 at 650℃. The speckle maintains good quality without ablative erosion, showing the advantages of parameterization, automation, high quality, and strong universality/repeatability.

This work accentuates investigation of machinability of TiB₂/Al 7075 metal matrix composite (MMC) while electrical discharge machining (EDM) with cryogenic treated Cu electrode, following Taguchi L₆₄ design of experiments (DOE). Influences of peak current (I_P), pulse on time (T_ON) and gap voltage (V_G) on material removal rate (MRR), tool wear rate (TWR) and average surface roughness (R_a) were studied. Morphology of the machined surfaces was also explored through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and elemental mapping. Results revealed increment of MRR (by about 55%) and TWR (by about 75%) on increasing I_P (from 4 A to 10 A at T_ON = 100µs at V_G = 55 V), but these responses reduced (by about 70% and 9%, respectively) on increasing V_G (from 55 V to 80 V at I_P = 4 A and T_ON = 100µs). R_a was influenced significantly by the I_P and T_ON levels. At 4 A of I_P, 200µs of T_ON and 55 V of V_G, SEM micrograph of the machined surface revealed craters, re-solidified regions, voids and cracks. On increasing the current level to 6 A keeping the other parameters constant, significant reduction of debris with larger craters were observed. On further increasing the current level to 10 A, minimal amount of debris, thicker ridges and deeper craters were witnessed from the machined surface microstructure.

Cyber-physical testing is a class of experimental methods in which a system is partitioned into physical and numerical components to study its overall behavior. Transfer systems are typically needed to capture relevant interactions by enforcing appropriate interface conditions between the physical and numerical portions. While cyber-physical testing is often performed using lumped-parameter systems, thermomechanical cyber-physical testing requires the use of thermal actuators as transfer systems to apply a thermal condition over a spatially distributed region or surface. Thus, the interactions between the numerical and physical components are of a distributed nature, i.e., a field rather than point values. In this paper, we develop and experimentally validate a state observer for temperature fields to enable thermomechanical cyber-physical testing. The state observer provides continuous estimates of temperature over a surface of the thermal actuator using only temperature measurements at discrete locations. The estimated temperature field is then leveraged to account for localized mechanical defects in the interface condition. The method consists of building a digital replica of one portion of the control plant. By imposing defects in the digital replica it is used to generate and output the response of a thermal load pattern that represents that of a physical system with mechanical defects.

To better meet the objective requirements such as the living comfort of residents of high-rise glulam buildings, this paper uses air-borne sound insulation and impact sound insulation testing methods to carry out the measurement and analysis of sound insulation performance of the floor structures of glulam buildings. Environmental excitation and artificial excitation were used to carry out acoustic-vibration coupling experiments and analysis of the floor structure, and the living comfort of the glulam building was evaluated. The research conclusion shows that: due to the gaps in the floor structure, sound insulation valleys appear at 125 Hz and 200 Hz. The impact sound insulation performance of glulam building floors is better than the air-borne sound insulation performance. The steady-state walking excitation method can produce low-frequency vibrations of the floor structure. Under the steady-state running excitation mode, the frequency range of acoustic-vibration coupling is expanded. The correlation between floor vibration and noise is greatest under the rubber hammer continuous tapping mode among artificial excitations. This study can provide a useful reference for the optimization design and test of vibration performance of glulam building floors in the future.

An experimental apparatus for measuring the dynamic behavior of materials subjected to strain rates on the order of 10(^3) s(^{-1}) and temperatures up to 800°C with a unique triple actuation system is developed in this work. This system is based on the traditional Kolsky (or split-Hopkinson pressure) bar design, with the addition of an external furnace used to heat the specimen to the desired temperature. A synchronized triple pneumatic actuation system is used to control the motion and timing of the the sample, incident, and transmitted bars. The cold contact time (CCT), or the time during which the heated sample is in contact with the room temperature bars before compression, is measured experimentally and carefully controlled to minimize the development of a temperature gradient across the sample and avoid heating of the bars. Experiments are performed in conjunction with ultra high speed imaging and 2D digital image correlation (DIC), as well as high speed thermal imaging. To verify the viability of the proposed system, experiments were conducted on Ti-6Al-4V (wt.%) at temperatures from 25°C up to and 800°C at an average strain rate of approximately 1200 s(^{-1}).

The poly-reference least squares complex frequency-domain estimator(p_LSCF) is one of the most popular modal identification methods, which is employed to track the modal parameter of a cable-stayed bridge in this paper. In order to make p_LSCF more successfully applicable to structural modal automatic tracking, two main contributions are presented: on the one hand, p_LSCF is optimized to obtain clearer stabilization diagram, which is beneficial for physical modes extraction, and on the other hand, automatic analysis on stabilisation diagrams is perform based on density-based spatial clustering to realize continuous identification of modal parameters without the need for manual intervention. Finally, the improved p_LSCF is applied to the modal track of the Yangpu bridge located in Danzhou City, Hainan Province, China. The analysis results suggest that the improved p_LSCF offers notable benefits in mode recognition rate and accuracy compared to the original approach. This improved p_LSCF demonstrates the capability to identify closely spaced modes and delivers exceptionally precise estimations, enabling the detection of minute frequency variations. Moreover, it provides meaningful assessments of modal damping ratios.

The preparation of smart bolts is mainly achieved by pasting or embedding piezoelectric ceramic transducer (PZT) patchs on the bolt head. However, these two methods are easily affected by temperature and noise due to the presence of the adhesive layer. In this study, a novel smart bolt based on ALN piezoelectric thin film (PTF) is developed. Firstly, a probe based thin film piezoelectric coefficient testing model was established to provide a theoretical basis for measuring the internal electric field distribution of PTF sensors and the equivalent voltage applied to ALN thin films; secondly, Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), X-ray Diffraction (XRD), and Energy Dispersive Spectrometer (EDS) were used to characterize and analyze the piezoelectric layer of the PTF sensor, and the longitudinal piezoelectric coefficient was measured, providing a basis for the feasibility of the proposed new smart bolt; finally, a comparative analysis was conducted between the four performance indicators of high-temperature resistance, sensitivity, signal to noise ratio, and repeatability with the PZT adhesive smart bolt. The verification results indicate that the new smart bolt can more accurately identify the health status of the bolt.