Bridge pier is the key components for transferring loads between the bridge and foundation. Its status significantly impacts on the safety of the entire bridge. While the sudden external force (e.g. vehicle collision, explosion, earthquake etc.) could cause catastrophic properties loss and casualties. Therefore, many anti-collision implementations are used in the bridge pier. The rigid protections and soft buffer structures, which are the conventional anti-collision methods. The former cannot lower the damage to vehicle and passengers, and the latter is capable of withstanding the minor or moderate vehicle collision only. In order to overcome the shortcomings of the conventional anti-collision method, tensegrity as a prestressed tensioned structure is proposed to be integrated with the bridge pier as a shielding component. The integrated tensegrity can absorb impact energy of the vehicle-pier-collision through large deformation or localized damage to protect the core pier. Therefore, this paper proposed a detailed anti-collision design with integration of tensegrity for the bridge pier. Additionally, the assessments of its statics and dynamics are given. Furthermore, the anti-collision effect has been illustrated, numerically. The process of the vehicle-pier-collision in three different velocities were simulated by ANSYS/LS-DYNA; the energy absorption is analyzed. The relationship between deformation state and absorbed energy was also obtained. Therefore, the feasibility of the proposed design has been fully explained. It provides an option for the anti-collision design in the bridge pier.
Currently, ultrasonic testing (UT) is widely used in concrete for identifying the sub-surface or surface flaws. However, most the UT can only provide the qualitive evaluation of the flaws. Tomography technology is capable to visualize the damage and provide its positioning information in concrete by reconstructing the ultrasound. For example, the slurry leakage state in the grouting sleeve, and the location of the inclusion in the concrete pile foundation can be displayed by ultrasonic tomography. However, concrete material is highly heterogenous due to its complex components (aggregate, mortar, internal void). All those complexities can cause significant impact on the tomographic results. Especially for the aggregate, sometimes its dimension is very closed to the testing objects. It greatly affects the recognition and location of defects by ultrasonic testing. Therefore, this research proposed to reveal the influence of aggregate, defect size and the effect of type, tomographic pixels on tomographic images in concrete. An optimum transducer arrangement and tomography algorithms in terms of ray-trace method was proposed to achieve the high accurate and resolution of tomography. Finally, the comparison of tomographic images between the imaged location and the embedded location is evaluated and then tomographic states is assessed accordingly.
Half grouted sleeve connection has been widely used in the rebars connection of prefabricated concrete (PC) structure. Mostly, the implementation of grouted should be finished on site. Meanwhile, the internal defects are inevitable due to the concrete nature. Currently, there is few methods available, which can effectively and rapidly evaluate the quality of the connection. Therefore, in this paper, we propose a combination of low-frequency linear ultrasound (LUT) and nonlinear ultrasound (NLUT) to quantitatively characterize defect. The internal artificial defects are concentrated defects, and the defect content is 10%, 20%, 30% and 40% respectively. Through transmission mode was adapted for both LUT and NLUT. The UT wave propagation was distorted by different defects, which was the results of LUT. For NLUT with higher resolution, the complex distribution and different level of defect together will introduce nonlinearity. The experimental results show that the grouted defects reduce the ultrasonic energy of LUT, and increase the nonlinearity from NLUT with the increase of the defect size and randomness. The defect has a significant impact on the ultrasonic features. Therefore, Low-frequency LUT and NLUT methods are potential to realize the visualization the defects of half grouted sleeve connection.
Currently, the glass curtain wall has become very popular in contemporary architectural wall decoration method because of its aesthetics, lightweight, energy saving, and thermal insulation. However, the damage of the glass curtain wall is inevitable due to its material nature. Currently, the detection method of glass curtain wall is to use regular manual detection, It highly depend on the experiences of the inspector and are not real-time monitoring method. Therefore, it is necessary to develop a monitoring method for the evaluation of status of glass curtain wall, which can realize the real-time monitoring and high reliability. This paper proposed a combined acoustic emission and vibrational modal analysis method to achieve multi-scale damage detection for glass curtain wall: Modal analysis is used to detect structural silicone sealant failure, bolt loosening and corrosion of glass curtain wall, which refers to the first-step inspection to approximately determine the damage status. And Acoustic emission (AE) is further used to continuously monitor the glass curtain wall to provide more detailed damage evaluation. The proposed scheme is verified by COMSOL Multiphysics 5.5. The relationship between the damage degree of structural silicone sealant, bolt failure and the modal frequency of the glass panel was also obtained. And also, AE has been proved to be able to realize real-time monitoring and early warning of objects. Therefore, the feasibility of the proposed design has been fully explained. It provides an option for glass curtain wall inspection.
Concrete is the most widely used material in civil engineering, which has many advantages in terms of strength, accessibility, and durability. Currently, the high-strength concrete has been used in some projects. However, its failure mode is more complex than the regular concrete. Especially for the loading rate, it may significantly affect the failure mode. Therefore, it is essential to quantify the process of the failure for better application of high-strength concrete with consideration loading rate. Due to many merits of acoustic emission (AE), it has been successfully used for evaluating the progress of concrete damage. Therefore, in this study, the experiments were proposed to correlate the AE with failure mode in the high-strength concrete. As progress of the damage, the AE activities show highly related to the mechanical response. Additionally, the loading rate directly related to the failure mode is also considered. Compared with the regular concrete, the failure of the high-strength concrete shows more brittle behavior. And the phase of AE signals has been changed from obvious three stages to two stages with increase of loading rate, which can be used for further identification of failure mode.
In high-rise building structures, only using structural stiffness to resist the seismic energy is not economic and effective. Therefore, various energy dissipation devices are deployed to the structure, such as friction type energy dissipation device, Buckling Restrained Bracing (BRB) and viscous damper. Many researchers have been working on improving the performance of the dissipation devices. Though the plastic or residual deformation after earthquakes can consume the energy, the irreversible damage was introduced. In addition, its capability highly depends on the materials. Therefore, we proposes to take advantage of the mechanical bistablity to design a novel energy dissipation device, Mechanical bistability is defined as availability of two stable equilibrium configurations in the structure in response to the same loading conditions. The bistablity was realize by constructing a mechanical metamaterial: the snapping and buckling behavior were used to control the multistable response. The load-displacement curve was obtained by the analytical model. The results show that bistable stage was achieved. With bistablity and hysteretic characteristics, the proposed design can dissipate considerable energy. It provides a new strategy to develop the energy dissipation device.
Ultrasonic testing and acoustic emission are acknowledged non-destructive testing methods for pipeline inspection; however, attenuation is a significant issue in long-range structures such as pipelines. A gradient-index metamaterial lens composed of subbed unit cells is proposed to address the energy loss by amplifying the signal through wave focusing. A prototype is tested actively (ultrasonic testing) and passively (acoustic emission) in the laboratory, and significant amplification of the signal is observed. The reciprocity between the two methods is validated.
The understandings of seismic mechanisms of tensegrity are highly demanded for perfecting the design principle. Due to the characteristics of periodicity, the effect of pre-stress on the frequencies of unit cell is investigated. And the stiffness of the basic tensegrity units was derived analytically. Then the vibration analysis of the entire assembly tensegrity structure is conducted numerically. Through parametric study, the influence of the pre-stress level on strength capacity of tensegrity is investigated. The cables yielding and bars buckle are considered. The influential factors on stiffness were evaluated, which can provide a guidance to the seismic mitigation optimization.
Acoustic tomography method facilitates mapping internal defects in real-time and in-situ without destructive testing. The method requires certain number of transmitter and receiver paths to reconstruct the slowness map of scanned area depending upon the target resolution. Once the hardware component is determined, the major software output to feed into the algorithm is the time of flight. There are sophisticated signal processing methods reported in literature to determine the time of flight (TOF) with better accuracy as compared to conventional threshold-based method. The most common approaches are wavelet-based or energy-based methods, which require transforming time history signal into different domains. Domain transformation is typically applied in laboratory-scale experiments. In this paper, a new arrival time pick-up approach based on defining outliers in the derivative of transient signal in time domain is evaluated in terms of accuracy, computational effort and power as compared to threshold-based and wavelet/energy-based methods reported in literature. The waveforms from experiments is used to study the influence of materials and signal-to-noise ratio on the accuracy of detecting the fastest wave mode. In addition, waveforms are also artificially generated with fixed wave velocity using numerical models to further evaluate the performance of the different methods (outlier-based, threshold-based and energy-based). The influence of tomography quality by using these to this method performs better in accuracy and efficiency.
The layout of structural bolts used in infrastructures are determined based on the mechanical and design requirements. Minimum spacing is set to facilitate construction and control stress concentration, and maximum spacing is set to prevent the intrusion of water between plates and provide sufficiently equal force distribution to each bolt. Ultrasonic testing is one of the most widely used and rapid in-situ inspection methods to evaluate the status of bolted connections; however, scattering from hole boundaries may mask reflections from the cracked and corroded surfaces. In current practice, the ultrasonic-based inspectability of bolted connections for the presence of crack and corrosion as well as pretension loss is not a design criterion. In this paper, the inspectability is added as a design variable within the boundaries of design limits that control spacing as well as distribution of bolts at the connection. The ideal spatial distribution of different bolt groups is proposed to detect the critical crack length and area loss hidden between plates. The proposed bolt distribution is numerically tested to show the minimized influence of hole scatters to the ultrasonic inspectability of defects. The regression model is built to predict crack position and size.
Welding is a key manufacturing process for many industries and may introduce defects into the welded parts causing significant negative impacts, potentially ruining high-cost pieces. Therefore, a real-time process monitoring method is important to implement for avoiding producing a low-quality weld. Due to high surface temperature and possible contamination of surface by contact transducers, the welding process should be monitored via non-contact transducers. In this paper, airborne acoustic emission (AE) transducers tuned at 60 kHz and non-contact ultrasonic testing (UT) transducers tuned at 500 kHz are implemented for real time weld monitoring. AE is a passive nondestructive evaluation method that listens for the process noise, and provides information about the uniformity of manufacturing process. UT provides more quantitative information about weld defects. One of the most common weld defects as burn-through is investigated. The influences of weld defects on AE signatures (time-driven data) and UT signals (received signal energy, change in peak frequency) are presented. The level of burn-through damage is defined by using single method or combine AE/UT methods.
The spline component of gearbox structure is a non-redundant element that requires early detection of flaws for preventing catastrophic failures. The acoustic emission (AE) method is a direct way of detecting active flaws; however, the method suffers from the influence of background noise and location/sensor based pattern recognition method. It is important to identify the source mechanism and adapt it to different test conditions and sensors. In this paper, the fatigue crack growth of a notched and flattened gearbox spline component is monitored using the AE method in a laboratory environment. The test sample has the major details of the spline component on a flattened geometry. The AE data is continuously collected together with strain gauges strategically positions on the structure. The fatigue test characteristics are 4 Hz frequency and 0.1 as the ratio of minimum to maximum loading in tensile regime. It is observed that there are significant amount of continuous emissions released from the notch tip due to the formation of plastic deformation and slow crack growth. The frequency spectra of continuous emissions and burst emissions are compared to understand the difference of sudden crack growth and gradual crack growth. The predicted crack growth rate is compared with the AE data using the cumulative AE events at the notch tip. The source mechanism of sudden crack growth is obtained solving the inverse mathematical problem from output signal to input signal. The spline component of gearbox structure is a non-redundant element that requires early detection of flaws for preventing catastrophic failures. In this paper, the fatigue crack growth of a notched and flattened gearbox spline component is monitored using the AE method The AE data is continuously collected together with strain gauges. There are significant amount of continuous emissions released from the notch tip due to the formation of plastic deformation and slow crack growth. The source mechanism of sudden crack growth is obtained solving the inverse mathematical problem from output signal to input signal.
Based on the micromechanical approaches, high frequency elastic waves are generated in carbon fiber/polymer composites due to three main damage sources as fiber breakage, matrix crack or delamination. The occurrence of damage mode depends on the ratios of energy release rates. Simultaneous generation of damage modes such as multiple fiber breakage called as fiber fragmentation influences the characteristics of propagating elastic waves which are used for detecting, locating and understanding damage modes for acoustic emission method. Understanding the wave characteristics via experimental methods is difficult as the control of damage mode sequence is a challenge. In this paper, wave propagations due to single or multiple fiber breakages positioned at various locations along the laminate and through thickness in different composite lay-ups are studied using dynamic finite element models. Fiber breakage is defined as a point load with the rise time of 1 μsec. The load amplitude is identified using stress-strain curve of carbon fiber. The amplitude of matrix cracking is defined as crack opening displacement as a boundary load to the finite element model. The positions and amplitudes of damage modes are varied to understand the characteristics of waveform signatures. Each lamina is defined as a different plate and orthotropic material properties are entered as input to the model. The paper shows that multi-mode simultaneous damage in composites causes complex waveform generation, which makes pattern recognition based on amplitude and frequency real time a challenging task.
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