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Volume 26, Number 2, August 2020

This paper deals with an application of adaptive blind source separation (BSS) method, equivariant adaptive separation via independence (EASI), and Teager Energy Operator (TEO) for online identification of structural modal parameters. The aim of adaptive BSS methods is recovering a set of independent sources from their unknown linear mixtures in each step when a new sample is received. In the proposed approach, firstly, the EASI method is used to decompose structural responses into independent sources at each instance. Secondly, the TEO based demodulation method with discrete energy separation algorithm (DESA-1) is applied to each independent source, and the instantaneous frequencies and damping ratios are extracted. The DESA-1 method can provide the fast time response and has high resolution so it is suitable for online problems. This paper also compares the performance of DESA-1 algorithm with Hilbert transform (HT) method. Compared to HT method, the DESA-1 method requires smaller amounts of samples to estimate and has a smaller computational complexity and faster adaption due to instantaneous characteristic. Furthermore, due to high resolution of the DESA-1 algorithm, it is very sensitive to noise and outliers. The effectiveness of the proposed approach has been validated using synthetic examples and a benchmark structure.

Key Words
adaptive blind source separation; discrete energy separation algorithm; equivariant adaptive separation via independence algorithm; online structural identification; Teager-Energy Operator

Department of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran.

This numerical study demonstrates the porosity conditions and the intensity of the interactions with the aggressive agents. It is established that the density as well as the elastic modulus are correlated to ultrasonic velocity The following investigation assessed the effects of cement grade and porosity on tensile strength, flexural and compressive of Ultra High Performance Concrete (UHPC) as a numerical model in PLAXIS 2d Software. Initially, the existing strength-porosity equations were investigated. Furthermore, comparisons of the proposed equations with the existing models suggested the high accuracy of the proposed equations in predicting, cement grade concrete strength. The outcome obtained showed a ductile failure when un-corroded reinforced concrete demonstrates several bending-induced cracks transfer to the steel reinforcement. Moreover, the outcome also showed a brittle failure when wider but fewer transverse cracks occurred under bending loads. Sustained loading as well as initial pre-cracked condition during the corrosion development have shown to have significant impact on the corrosion behavior of concrete properties. Moreover, greater porosity was generally associated with lower compressive, flexural, and tensile strength. Higher cement grade, on the other hand, resulted in lower reduction in concrete strength. This finding highlighted the critical role of cement strength grade in determining the mechanical properties of concrete.

Key Words
strength-porosity relationship; cement grade; mechanical properties of concrete; PLAXIS 2d; UHPC

(1) Rayed Alyousef, Hisham Alabduljabbar, Abdeliazim Mustafa Mohamed:
Department of Civil Engineering, College of Engineering, Prince Sattam bin Abdulaziz University, Al-kharj 11942, Saudi Arabia;
(2) Abdulaziz Alaskar:
Department of Civil Engineering, College of Engineering, King Saud University, Riyadh 11362, Saudi Arabia;
(3) Kittisak Jermsittiparsert:
Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Vietnam;
(4) Kittisak Jermsittiparsert:
Faculty of Social Sciences and Humanities, Ton Duc Thang University, Ho Chi Minh City, Vietnam;
(5) Lanh Si Ho:
Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam.

The guided wave technique is commonly used in structural health monitoring as the guided waves can propagate far in the structures without much energy loss. The guided waves are conventionally generated by the surface-mounted piezoelectric wafer active sensor (PWAS). However, there is still lack of understanding of the wave propagation in layered structures, especially in structures made of anisotropic materials such as carbon fiber reinforced polymer (CFRP) composites. In this paper, the Rayleigh-Lamb wave strain tuning curves in a PWAS-mounted unidirectional CFRP plate are analytically derived using the normal mode expansion (NME) method. The excitation frequency spectrum is then multiplied by the tuning curves to calculate the frequency response spectrum. The corresponding time domain responses are obtained through the inverse Fourier transform. The theoretical calculations are validated through finite element analysis and an experimental study. The PWAS responses under the free, debonded and bonded CFRP conditions are investigated and compared. The results demonstrate that the amplitude and travelling time of wave packet can be used to evaluate the CFRP bonding conditions. The method can work on a baseline-free manner.

Key Words
guided waves; anisotropic; layered plate; piezoelectric wafer active sensor; normal mode expansion; debonding

(1) Lingfang Li, Yong Xia:
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong;
(2) Hanfei Mei, Victor Giurgiutiu:
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA;
(3) Mohammad Faisal Haider:
Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305, USA;
(4) Dimitris Rizos:
Department of Civil Engineering, University of South Carolina, Columbia, SC 29208, USA.

Conventional Monte Carlo simulation-based methods for seismic risk assessment of water networks often require excessive computational time costs due to the hydraulic analysis. In this study, an Artificial Neural Network-based surrogate model was proposed to efficiently evaluate the flow-based system reliability of water distribution networks. The surrogate model was constructed with appropriate training parameters through trial-and-error procedures. Furthermore, a deep neural network with hidden layers and neurons was composed for the high-dimensional network. For network training, the input of the neural network was defined as the damage states of the k-dimensional network facilities, and the output was defined as the network system performance. To generate training data, random sampling was performed between earthquake magnitudes of 5.0 and 7.5, and hydraulic analyses were conducted to evaluate network performance. For a hydraulic simulation, EPANET-based MATLAB code was developed, and a pressure-driven analysis approach was adopted to represent an unsteady-state network. To demonstrate the constructed surrogate model, the actual water distribution network of A-city, South Korea, was adopted, and the network map was reconstructed from the geographic information system data. The surrogate model was able to predict network performance within a 3% relative error at trained epicenters in drastically reduced time. In addition, the accuracy of the surrogate model was estimated to within 3% relative error (5% for network performance lower than 0.2) at different epicenters to verify the robustness of the epicenter location. Therefore, it is concluded that ANN-based surrogate model can be utilized as an alternative model for efficient seismic risk assessment to within 5% of relative error.

Key Words
Aartificial Neural Networks; surrogate model; accelerated Monte Carlo simulation; seismic risk assessment; flow-based system reliability

(1) Sungsik Yoon, Hyung-Jo Jung:
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea;
(2) Young-Joo Lee:
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea.

Nonlinear vibration of sandwich plate with functionally graded material (FGM) core and carbon nano tubes reinforced (CNTs) nano-composite layers by considering temperature-dependent material properties are studied in this paper. Base on Classical plate theory (CPT), the governing partial differential equations of motion for sandwich plate are derived using Hamilton principle. The Galerkin procedure and multiple scales perturbation method are used to find relation between nonlinear frequency and amplitude of vibration response. The dynamic responses of the sandwich plate are also investigated in both time and frequency domains. Then, the effects of nonlinearity, excitation, power law index of FG core, volume fraction of carbon nanotube, the function of material variations of FG core, temperature changes, scale transformation parameter and damping factor on the frequency responses are investigated.

Key Words
nonlinear vibration; sandwich plate; FG core and FG-CNTs face sheets; nano-composite; multiple scales perturbation method

Department of Solid Mechanics, Faculty of Mechanical Engineering, University of Kashan, Kashan 87317-53153, Iran.

Among anti-seismic technologies, base isolation is a very effective means of mitigating damage to structural and nonstructural components, such as equipment. However, most seismic isolation systems are designed for mitigating only horizontal seismic responses because the realization of a vertical isolation system (VIS) is difficult. The difficulty is primarily due to conflicting isolation stiffness demands in the static and dynamic states for a VIS, which requires sufficient rigidity to support the self-weight of the isolated object in the static state, but sufficient flexibility to lengthen the isolation period and uncouple the ground motion in the dynamic state. To overcome this problem, a semi-active VIS, called the piezoelectric inertia-type vertical isolation system (PIVIS), is proposed in this study. PIVIS is composed of a piezoelectric friction damper (PFD) and a leverage mechanism with a counterweight. The counterweight provides an uplifting force in the static state and an extra inertial force in the dynamic state; therefore, the effective vertical stiffness of PIVIS is higher in the static state and lower in the dynamic state. The PFD provides a controllable friction force for PIVIS to further prevent its excessive displacement. For experimental verification, a shaking table test was conducted on a prototype PIVIS controlled by a simple controller. The experimental results well agree with the theoretical results. To further investigate the isolation performance of PIVIS, the seismic responses of PIVIS were simulated numerically by considering 14 vertical ground motions with different characteristics. The responses of PIVIS were compared with those of a traditional VIS and a passive system (PIVIS without control). The numerical results demonstrate that compared with the traditional and passive systems, PIVIS can effectively suppress isolation displacement in all kinds of earthquake with various peak ground accelerations and frequency content while maintaining its isolation efficiency. The proposed system is particularly effective for nearfault earthquakes with long-period components, for which it prevents resonant-like motion.

Key Words
vertical isolation; leverage mechanism; piezoelectric actuator; semi-active friction damper; inertia type; nearfault earthquake; anti-resonance

(1) Lyan-Ywan Lu, Kun-An Hsiao:
Department of Civil Engineering, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan;
(2) Ging-Long Lin:
Department of Construction Engineering, National Kaohsiung University of Science and Technology, 1 University Road, Kaohsiung 824, Taiwan;
(3) Yi-Siang Chen:
Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Sec. 4, Taipei 106, Taiwan.

In this manuscript, static and dynamic behaviors of geometrically imperfect carbon nanotubes (CNTs) subject to different types of end conditions are investigated. The Doublet Mechanics (DM) theory, which is length scale dependent theory, is used in the analysis. The Euler-Bernoulli kinematic and nonlinear mid-plane stretching effect are considered through analysis. The governing equation of imperfect CNTs is a sixth order nonlinear integro-partial-differential equation. The buckling problem is discretized via the differential-integral-quadrature method (DIQM) and then it is solved using Newton's method. The equation of linear vibration problem is discretized using DIQM and then solved as a linear eigenvalue problem to get natural frequencies and corresponding mode shapes. The DIQM results are compared with analytical ones available in the literature and excellent agreement is obtained. The numerical results are depicted to illustrate the influence of length scale parameter, imperfection amplitude and shear foundation constant on critical buckling load, post-buckling configuration and linear vibration behavior. The current model is effective in designing of NEMS, nano-sensor and nano-actuator manufactured by CNTs.

Key Words
imperfect CNTs; doublet mechanics theory; differential-integral-quadrature method (DIQM); buckling; vibration

(1) Mohamed A. Eltaher:
Mechanical Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah, Saudi Arabia;
(2) Mohamed A. Eltaher:
Mechanical Design & Production Department, Faculty of Engineering, Zagazig University, P.O. Box 44519, Zagazig, Egypt;
(3) Nazira Mohamed, Salwa A. Mohamed:
Department of Engineering Mathematics, Faculty of Engineering, Zagazig University, P.O. Box 44519, Zagazig, Egypt.

Despite proven effectiveness and accuracy in laboratories, the existing damage assessment based on guided ultrasonic waves (GUWs) or acoustic emission (AE) confronts challenges when extended to real-world structural health monitoring (SHM) for railway tracks. Central to the concerns are the extremely complex signal appearance due to highly dispersive and multimodal wave features, restriction on transducer installations, and severe contaminations of ambient noise. It remains a critical yet unsolved problem along with recent attempts to implement SHM in bourgeoning high-speed railway (HSR). By leveraging authors' continued endeavours, an SHM framework, based on actively generated diffuse ultrasonic waves (DUWs) and a benchmark-free condition contrast algorithm, has been developed and deployed via an all-in-one SHM system. Miniaturized lead zirconate titanate (PZT) wafers are utilized to generate and acquire DUWs in long-range railway tracks. Fatigue cracks in the tracks show unique contact behaviours under different conditions of external loads and further disturb DUW propagation. By contrast DUW propagation traits, fatigue cracks in railway tracks can be characterised quantitatively and the holistic health status of the tracks can be evaluated in a real-time manner. Compared with GUW- or AE-based methods, the DUW-driven inspection philosophy exhibits immunity to ambient noise and measurement uncertainty, less dependence on baseline signals, use of significantly reduced number of transducers, and high robustness in atrocious engineering conditions. Conformance tests are performed on HSR tracks, in which the evolution of fatigue damage is monitored continuously and quantitatively, demonstrating effectiveness, adaptability, reliability and robustness of DUW-driven SHM towards HSR applications.

Key Words
diffuse ultrasonic wave; structural health monitoring; high-speed railway; fatigue crack

(1) Kai Wang:
Interdisciplinary Division of Aeronautical and Aviation Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR;
(2) Wuxiong Cao, Zhongqing Su:
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR;
(3) Zhongqing Su:
The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, P.R. China;
(4) Zhongqing Su:
National Rail Transit Electrification and Automation Engineering Technology Research Center (Hong Kong Branch), The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR;
(5) Pengxiang Wang, Xiongjie Zhang, Lijun Chen:
Southwest Jiaotong University Railway Development Co., Ltd., Chengdu, 610073, P.R. China;
(6) Ruiqi Guan, Ye Lu:
Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia.

Due to the undeniable importance of approximating the concrete compressive strength (CSC) in civil engineering, this paper focuses on presenting four novel optimizations of multi-layer perceptron (MLP) neural network, namely artificial bee colony (ABC-MLP), grasshopper optimization algorithm (GOA-MLP), shuffled frog leaping algorithm (SFLA-MLP), and salp swarm algorithm (SSA-MLP) for predicting this crucial parameter. The used dataset consists of 103 rows of information concerning seven influential parameters (cement, slag, water, fly ash, superplasticizer, fine aggregate, and coarse aggregate). In this work, the bestfitted complexity of each ensemble is determined by a population-based sensitivity analysis. The GOA distinguished its self by the least complexity (population size = 50) and emerged as the second time-effective optimizer. Referring to the prediction results, all tested algorithms are able to construct reliable networks. However, the SSA (Correlation = 0.9652 and Error = 1.3939) and GOA (Correlation = 0.9629 and Error = 1.3922) performed more accurately than ABC (Correlation = 0.7060 and Error = 4.0161) and SFLA (Correlation = 0.8890 and Error = 2.5480). Therefore, the SSA-MLP and GOA-MLP can be promising alternatives to laboratorial and traditional CSC evaluative methods.

Key Words
concrete compressive strength; neural computing; metaheuristic optimization algorithms

(1) Xinyan Ma:
China Airport Planning & Design Institute Co., Ltd., Beijing 100101, China;
(2) Loke Kok Foong:
Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Vietnam;
(3) Loke Kok Foong:
Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam;
(4) Armin Morasaei:
of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran;
(5) Aria Ghabussi:
Department of Civil Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran;
(6) Zongjie Lyu:
Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam;
(7) Zongjie Lyu:
Faculty of Civil Engineering, Duy Tan University, Da Nang 550000, Vietnam.

In this paper, a new higher order shear deformation model is developed for static analysis of functionally graded beams with considering porosities that may possibly occur inside the functionally graded materials (FGMs) during their fabrication. The model account for higher-order variation of transverse shear strain through the depth of the beam and satisfies the zero traction boundary conditions on the surfaces of the beam without using shear correction factors. The present work aims to study the effect of the distribution forms of porosity on the bending of simply supported FG beam. Based on the present higher-order shear deformation model, the equations of motion are derived by the principle of virtual works. Navier type solution method was used to obtain displacement and stresses, and the numerical results are compared with those available in the literature. A comprehensive parametric study is carried out to assess the effects of volume fraction index, porosity fraction index, and geometry on the bending of imperfect FG beams. It can be concluded that the proposed model is simple and precise for the resolution of the behavior of flexural FGM beams while taking into account the shape of distribution of the porosity.

Key Words
functionally graded materials; bending; volume fraction of porosity; Navier's solution; shear deformation theory

(1) Department of Mechanical Engineering, University of Tiaret, BP 78 Zaaroura, 14000 Tiaret, Algeria;
(2) Laboratory of Geomatics and Sustainable Development, University of Tiaret, Algeria.

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