Sharif Digital Repository

Structural system identification from vibration data is one of the most interesting research topics in the structural health monitoring area. Recently, realization and detection of the effects of damage when a structure is subjected to strong ground motion has become a great concern in earthquake and structural engineering communities. Seismic signal processing is one of the most reliable methods of detecting the structural damage during earthquakes. The structural responses during earthquakes are nonstationary with respect to both amplitude and frequency. The state-of-the-art time-frequency distributions when applied to vibration records were studied. Different methods of analysis for... 

Development of composite materials is one of the boldest technological advances of the last half of the twentieth century. This is due to their specific characteristics mostly considerably high strength-to-weight ratio, compared to metals and their highly customizing properties for different applications which make them a suitable material to be used in variety of domain from aerospace and automobiles to medical and sport equipments. Multi-stable asymmetrical laminates and non-uniform thickness laminated composite beams are being used as structural elements in a wide range of advanced engineering applications. Multi-stable asymmetrical laminates can be snapped between two or more geometries... 

The pressurized structure such as aircraft fuselage and pipeline (in the oil and water transportation) have a great importance in today’s world. There is a growing tendency in improving design factors, safety and maintenance of such valuable structures. Change of design’s philosophy during the time from infinite life to safe damage and safe life and after that damage tolerance, are a process leading to many changes in design and consequently the methods of inspection, evaluation and maintenance. In this way, using old methods like non-destructive test and evaluation (NDT/E) is encountered with different problems when damage is tiny and there is no coincidence in examination (inspection)... 

In this thesis, due to the importance of pressurized structures, method or methods that can be used to monitor the health of important structures, such as pressurized pipes (gas transmission and distribution lines or hydraulic lines), are considered. A key point in monitoring such structures is to ensure the function of pipes, elbows, Tee junctions, composite to composite and composite to metal joints. The topic that will be discussed more than anything else here is to monitor the health of the structure in the presence of cracks in some layers of composite pieces. In fact, it must be recognized before cracking occurs in all layers of composite pieces and causing overall damage to the... 

Damage detection of civil structures and infrastructures using partial modal information has been an interesting field of research. So far, three main algorithms are introduced for health monitoring of structures. The first method relies on identifying stiffness and mass properties, as physical parameters, of structures using inverse eigenvalue problem considering mass modifications in 2D and 3D shear-frame models of structures. Comparing the identified matrices with the primary values, one can detect the damage occurrence, its level and severity in the structure. Applicability of this method in health monitoring of offshore jacket platforms as one of noteworthy civil infrastructures has... 

Structural Health Monitoring (SHM) is an effective method for the early detection of structural damage, and this method can reduce the maintenance costs of structures. In this thesis, the use of artificial one-way and multi-layer artificial neural network is evaluated to determine the damage to the structure. The neural network method used in this thesis is a post-propagation method. Finally, the numerically damage determine will be evaluated using a two-dimensional frame structure and a three-dimensional frame structure by finite element method. In this method, the dynamic properties of the structure are used only in the form of the modules. For this purpose, the finite element model is... 

The process of implementing a damage detection strategy for aerospace, civil and mechanical engineering infrastructure is referred to as Structural Health Monitoring (SHM). Many portions of our technical infrastructure are approaching or exceeding their initial design life. As a result of economic issues, these civil, mechanical and aerospace structures are being used in spite of aging and the associated damage accumulation. Therefore, the ability to monitor the health of these structures is becoming increasingly important. In many cases, damage causes a structure that initially behaves in a predominantly linear manner to exhibit nonlinear response when subject to its operating environment.... 

Under conventional seismic design strategies, civil engineering structures are designed to undergo inelastic deformation to dissipate earthquake energy, leading to residual displacements and nonlinear deformations in the structure. As a result, there is an essential need for structural health monitoring methods to detect and process these nonlinear behaviors. In this dissertation, a model-free Hysteresis Loop Analysis (HLA) method is developed to identify damage and monitor the structure's performance online, under the earthquake excitations in both severe linear and nonlinear scenarios. The structural models used to validate this method includes three types of one-story, five-story, and... 

Damage identification in uniform structures using Lamb waves has been studied by the researchers; however, as most of the industrial structures are not uniform, it is necessary to study more complex structures. In this thesis, a method of identifying notch specifications as damage in a symmetric stepped beam with arbitrary number of steps using Lamb wave propagation is presented. Initially, the study will examine and compare the Lamb wave propagation in uniform, symmetric stepped and asymmetric stepped beams. In the first stage, a method for detection of damage location in symmetric stepped beam is presented. This method is based on mode conversion phenomenon of Lamb wave modes. Moreover,... 

This study investigates the effect of the variation of the properties of unreinforced brick masonry walls, such as aspect ratio, prism strength, and axial load ratio, on determining the post-earthquake damage states of the walls using fragility curves. A database comprising 245 damaged unreinforced masonry walls is collected that have been tested under the quasi-static cyclic loads up to more than three percent drift ratios. The collected walls are categorized into four damage states according to the resisting lateral force during the cyclic loading obtained from the backbone of the hysteresis curves. Then, multivariable fragility surfaces based on the drift ratio and design properties of... 

In this thesis, supervised and unsupervised deep learning-based frameworks were proposed for distributed damage detection and quantification in concrete using sinusoidal ultrasonic response signals. Before the main study on ultrasonic-based concrete damage assessment, a preliminary study was performed on deep learning-based concrete compressive strength prediction. In this study, convolutional neural networks were utilized to predict the compressive strength of concrete through its mix proportions. The Genetic algorithm was employed to find the optimum number of filters in each convolutional layer of the convolutional neural networks. The proposed framework demonstrated high accuracy in... 

In this dissertation, the location and severity of the damage are obtained using a new robust Power Spectral Density (PSD) sensitivity-based model updating method. PSD of structural response is highly sensitive metric to parameter change and provides vast information about the structure. The proposed damage detection method uses the sensitivities of measured responses in frequency domain, for FE model updating in an efficient way, by developing a quasi-linear sensitivity equation of structural response. The system of equations are solved using Least Square method. Only stiffness parameters were updated through model updating and change in mass parameters were neglected. In this study, proper... 

The main goal of this research is detection, localization, and determining the severity of structural damages. The importance of this objective is to prevent abrupt destruction and severe human and financial losses. Damage detection is divided into local and global detection, and global detection contains static and vibration-based methods. In this research, the vibration-based method is used. Due to the complexity of this method, accurate and powerful tools are required for detecting effective features in damages; Deep Learning (DL), which is part of a broader family of machine learning methods based on artificial neural networks (ANN), could be used. Accordingly, in this project, the... 

Civil structures are on the verge of changing which leads energy dissipation capacity to decline. Structural Health Monitoring (SHM) as a process in order to implement a damage detection strategy and assess the condition of structure plays a key role in structural reliability. Earthquake is a recognized factor in variation of structures condition, inasmuch as inelastic behavior of a building subjected to design level earthquakes is plausible. In this study Hilbert Hunag Transformation (HHT) is superseded by Ensemble Empirical Mode decomposition (EEMD) and Hilbert Transform (HT) together. Albeit this method is closely resemble HHT, EEMD brings more appropriate Intrinsic Mode Functions (IMFs).... 

The purpose of the study: The purpose of this study is to identify the crack position in homogeneous plates through health monitoring methods based on vibration. Identifying structural damage is one of the issues that attention can prevent the sudden collapse of structures and causing heavy loss of life and property. For this reason, extensive studies have been conducted in this field in recent decades. In this study, plates are examined as one of the most used structural members, and their damage is identified through a proposed method based on wavelet transform. Research method: In this study, through the wavelet transform method, the dynamic responses of a homogeneous plate excited by a... 

Structural health monitoring (SHM) has been motivated in aircraft and civil industries over the two last decades. Ultrasonic guided wave propagation technique is one of the best damage detection methods in airplane thin wall structures because of their large distance traveling and low attenuation.
In fact by efficient design of sensors pattern one can effectively pick up the changes that happen in propagated signals due to presence of any kind of structural damage. This could be effectively achieved by numerical simulation techniques like finite element modeling. To obtain a damage detection algorithm based on instantaneous baseline data, here, finite element technique is employed for... 

Principal component analysis (PCA) is a conventional tool for dynamic system analysis. In this paper, a new damage diagnosis method is presented based on sensitivities of principal components obtained from PCA of Frequency response functions (FRFs). Damage identification, determination of its location and estimation of the damage severity are conducted by an innovative well established sensitivity relation of PCA results. Then, sensitivity matrix is obtained by using measured frequency of damaged structure, and comparison of PCA of the intact structure with PCA from FRFs which are measured at sensors locations. Damage location and its severity are calculated by model updating procedure. This... 

Blind source separation methods such as independent component analysis (ICA) and second order blind identification (SOBI) have shown considerable potential in the area of ambient vibration system identification. The objective of these methods is to separate the modal responses, or sources, from the measured output responses, without the knowledge of excitation. Several frequency domain and time domain methods have been proposed and successfully implemented in the literature. Whereas frequency-domain methods pose several challenges typical of dealing with signals in the frequency-domain, popular time domain methods such as NExT/ERA and SSI pose limitations in dealing with noise, low sensor... 

In this dissertation a new approach is introduced for damage detection in offshore jacket platforms. The procedure uses the measured ambient vibration responses and the corresponding readable natural frequencies and mode shapes of the structural systems. Since offshore platforms are composed of heavy topsides supported by jacket structures, participation of the first mode is dominant in each direction in the response of the structure under field excitations. Moreover, ambient vibrations such as wave loads and boat impacts only excite the first modes of the structure. Therefore, it is difficult to find higher modes and the pertinent frequencies by use of accelerometers data. The introduced... 

This study presents the best layout with optimal number of sensors for an arbitrary monitoring area based on reference free damage detection and localization. In recent years, damage detection approaches with the concept of instantaneous baseline measurements in Structural Health Monitoring (SHM) with lamb wave propagation are being developed. An array of spars transducers is obligatory in SHM with lamb wave propagation, so that designing the best one is a concern for many researchers. To obtain the best sensors layout, damage localization process should be done for all definable sensors layouts in many different crack positions. Obviously, doing this seems not practical as wave propagation...