Most of structural destructions take place because of components fracture. This event occurred by cracks that their development is considered as a serious threat for the treatment of structure. Therefore the exhibition and distinction methods of crack were the subject of wide verifications during last decade. Considering the local hardness differences arising from cracking and its effect on the treatment of structure, this article studies the distinction of internal crack in cantilever beam. Specimens were placed on harn10nic analyses and the base of recorded reaction was analyzed by Wavelet transform. Also this text offers the sensitiveness of the above method due to the depth. angle and situation of crack, and possibility of its identification, in various conditions.

Structural and crack parameters in a continuous mass model are identified using Observer Kalman filter Identification (OKID) and Eigen Realization Algorithm (ERA). Markov parameters are extracted from the input and out responses from which the state space model of the structural system is determined using Hankel matrix and singular value decomposition by Eigen Realization algorithm. The structural parameters are identified from the state space model. This method is applied to a lumped mass system and a cantilever which are excited with a harmonic excitation at its free end and the acceleration responses at all nodes are measured. The stiffness and damping parameters are identified from the extracted matrices using Newton-Raphson method on the structure. Later, cracks are introduced in the cantilever and all structural parameters are assumed as known priori, the unknown crack parameters such as normalized crack depth and its location are identified using OKID/ERA. The parameters extracted by using this algorithm are compared with other structural identification methods available in the literature. The main advantage of this algorithm is good accuracy of identified structural parameters.

This article identifies crack in beams under moving forces (a moving load, a moving mass, a moving oscillator and a four degree of freedom moving system) using the Hilbert Huang transform. Timely identification and repair of damage in structures (such as cracks in bridge decks), especially in those structures that are always under dynamic loads, is very important. In recent years, new methods have been proposed based on the recorded dynamic responses of the structure. most of these methods are based on developed model updating methods which are usually very costly computationally. To overcome this problem, this study presents a method based on the Hilbert-Huang transform. In this method, the bridge is modeled in the form of an Euler-Bernoulli beam and the vehicle is also modeled in different conditions as a moving load, a moving mass, a moving oscillator and a moving system. The results show that the proposed method is able to detect cracks in bridge decks with an acceptable accuracy in all cases.

Early crack detection in structures prevents the occurrence of damage. Therefore, a challenge exists in the literature to provide efficient methods in the field of structural health monitoring. Many of the researches that have been done on the crack identification in structures are based on the models that ignore crack closure effects that make a significant error in the crack identification. Since it is more difficult to identify breathing crack than other damages, the purpose of this research is providing an efficient algorithm to identify breathing crack in beam type structures which are important elements in various types of structures. Because it's an important and significant issue to present a perfect model that is able to accurately explain behavior of damaged structure, in this paper a cracked beam has been modeled by ''breathing crack'' model which explains beam responses more accurately than other common models. In order to calculate natural frequencies of the beam accurately, in this research the fatigue crack model is used, which considers crack as breathing one with opening and closing behaviour. Then the problem of identifying crack parameters (location and depth of the crack) is defined as an optimization problem with the aim of minimizing the differences between natural frequencies calculated by the model and measured natural frequencies. In order to choose an appropriate algorithm to identify breathing crack, algorithms among various meta-heuristic algorithms are selected, which are able to identify the crack using only two natural frequencies. This ability is more valuable when it is desired not to damage structure during health monitoring procedure based on using frequency response of structure. Regarding the surveys conducted, the optimization problem is solved using cat swarm optimization (CSO) algorithm. Moreover, for validation the results obtained for different crack parameters have been compared with those of experimental tests. The results indicate that the proposed method has good accuracy.

Because of importance of damage identification by vibration parameters, there are several methods, among which, index method, because of its abilities, have important role in identifying damage in civil structures exact property of index method was unknown before. In this research, by using suitable model of beam with transverse crack on it we have shown that index method can be used on a simply supported beam to identify transverse crack on it, so we could study properties and strength of index method exactly. Through these studies, the important results have been driven, such as high indexes are made on nodes of beam and these indexes are moving toward crack by increasing the depth of crack. In experimental section of this project, the index methods have been used on simply supported aliminum beam with transverse notch on it by using modal analysis. Then experimental and theory results have been used for comparison purposes.

Concrete is the most important material in geotechnical projects and constructions which that due to the cyclical loading and fatigues were always under failures. The fatigue is natural events were occurred in under cyclical loaded materials which causes very complex failures in under low stress and in elastic behavior. Although the concrete behavior and changes under axial loading are studied, but failure mechanism of fatigue and crack propagation on concrete (as the main factor to failure) is on obscurity. The investigation of crack propagation process from the beginning to failure under cyclic axial loading can be identified the fatigue mechanism. This study tired to monitoring and modeling the crack propagation mechanism under the cyclic uniaxial compressive strength (UCS ) in axial loading of concrete specimens which help to understands of crack propagation and developed path in the body. For this purpose, the concrete specimens are tested by UCS, take photos repetitively (in 5 loading-unloading cycles) and used the image-processing techniques (IPT) for crack detection, propagation and simulation of the failure in concrete. The simulation results shown the IPT is good performance to crack propagation detections and propagation process is perfectly simulated by IPT.

Examining the mechanical properties of polymer materials has particular importance due to the increasing usage of these materials. In this paper, the crack development process was identified using the experimental data obtained from the tensile tests of polyvinyl chloride polymer (PVC) to reduce the experimental test number. Some pre-cracks were created on the experimental samples by slotting and broaching methods and the experimental results of force versus displacement were recorded for each sample. In the next step, some mathematical pre-processing operations including equating the sampling intervals using interpolation and integration using trapezoidal methods were performed on the experimental data. In the third stage, by using the pre-processed data and the Least Squares (LS) identification method, the mathematical model of the system, the frequency domain discrete transfer function of the crack development process, was obtained. Finally, after evaluating and verifying the obtained mathematical model with other experimental results, it was proposed to predict the crack development process. The obtained results showed that by identifying the crack development process for limited samples, it is possible to avoid the number of experimental tests that are time-consuming and costly. In addition, the second-order two-stage least squares method with five unknown parameters was promisingly able to estimate crack development dynamics with acceptable accuracy, particularly in the first half of the development process.