Unknown Cracks Research Articles

A Fiber-Guided Motorized Rotation Laser Scanning Thermography Technique for Impact Damage Crack Inspection in Composites

Laser Thermography manifests superior sensitivity and compatibility to detect cracks and small subsurface defects. However, the existing related systems have limitations on either inspection efficiency or unknown directional cracks due to the utilization of stationary heat sources. This article reports a Fiber-guided Motorised Rotation Laser-line Scanning Thermography (FMRLST) system aiming to rapidly inspect cracks of impact damage with unknown direction in composite laminates. An optical head with fibre delivery integrated with a rotation motor is designed and developed to generate novel scanning heating in a circumferential rotation manner. A FEM model is first proposed to simulate the principle of FMRLST testing and produce thermograms for the development of post-processing methods. A damage enhancement method based on Curvelet Transform is developed to enhance the visualization of thermal features of cracks, and purify the resulting image by suppressing the laser-line heating pattern and cancelling noise. The validation on three composite specimens with different levels of impact damage suggests the developed FMRLST system can extract unknown impact surface cracks efficiently. The remarkable sensitivity and flexibility of FMRLST to arbitrary cracks, along with the miniaturized probe-like inspection unit, present its potential in on-site thermographic inspection, and its design is promising to push the LST towards.

Tests for Structural Breaks in Time Series Analysis: A Review of Recent Development

The issue related to a structural break or change point in the econometric and statistics literature is relatively vast. In recent decades it was increasing, and it got recognized by various researchers. The debates are about a structural break or parameter instability in the econometric models. Over some time, there has been a different mechanism, and theoretical development stretching the fundamental change and strengthen the econometric literature. Estimation of structural break has undergone significant changes. Instead of exploring the presence of a known structural break, now the emphasis is on tracing multiple unknown cracks using dynamic programming. The paper an attempt has been made to review the different forms of the presence of structural break(s) over the past.

A direct sampling method for inverse scattering using far-field data

This work is concerned with a direct sampling method (DSM) for inverse acoustic scattering problems using far-field data. The method characterizes some unknown obstacles, inhomogeneous media or cracks, directly through an indicator function computed from the measured data. Using one or very few incident waves, the DSM provides quite reasonable profiles of scatterers in time-harmonic inverse acoustic scattering without a priori knowledge of either the physical properties or the number of disconnected components of the scatterer. We shall first derive the DSM using far-field data, then carry out a systematic evaluation of the performances and distinctions of the DSM using both near-field and far-field data. The numerical simulations are shown to demonstrate interesting and promising potentials of the DSM: a) ability to identify not only medium scatterers, but also obstacles, and even cracks, using measurement data from one or few incident directions, b) robustness with respect to large noise, and c) computational efficiency with only inner products involved.

Fatigue Reliability Updating Evaluation of Hydraulic Steel Gate Based on Bayes Theorem

The main objective of this paper is aim to take fatigue reliability updating evaluation of hydraulic steel gate based on the Bayes theorem. The fatigue cracks can be detected for existing structures, and fatigue reliability assessment can be updated by the nondestructive inspection. In this paper, based on the probabilistic characteristics for NDI techniques and the Bayes theorem, a fatigue reliability updating assessment model for existing steel gate components is proposed. The results showed that the fatigue reliability will increase with no cracks detected, decrease with unknown cracks detected, and change remarkably with cracks detected.

Identification of multiple cracks based on optimization methods using harmonic elastic waves

A theoretical study of the identification of multiple cracks in an elastic medium based on optimization methods using harmonic elastic waves is presented in the paper. A general interacting crack model is first used to determine the dynamic interaction between arbitrarily located and oriented cracks subjected to plane harmonic waves. The solution of this problem is then implemented into an optimization process for the identification of unknown cracks from known strain components at discrete locations. An optimization scheme based on sensitivity analysis is used to determine the length, the orientation and the position of cracks. Numerical simulation indicates that the sensitivity analysis and the optimization method used are effective in identifying multiple cracks. It is observed that convergent results can be achieved from a set of arbitrarily determined initial values of the crack parameters. Numerical examples are presented to illustrate the determination of the length, orientation, and position of different interacting cracks.

Vibration analysis and crack identification of a rotor with open cracks

A finite element model is used for flexural vibration analysis of a static (non-rotating) rotor with open cracks; the stiffness matrices of the cracked elements are obtained using transfer matrix analysis and local flexibility theorem. Through numerical simulation, the effects of the slenderness ratio and the crack depth on the mode shapes and the changes in the eigenfrequencies of the cracked rotor are investigated; the variations of the changes in eigenfrequencies with crack location are studied; and the ratios of the changes in the first two eigenfrequencies are discussed for rotors with two cracks. Then, crack identification algorithms for one unknown crack case and for two unknown cracks case are proposed, and illustrative examples are demonstrated to verify the validity of the proposed algorithms.

The enclosure method for an inverse crack problem and the Mittag–Leffler function

An inverse boundary value problem for the two-dimensional Laplace equation is considered. The problem is to extract information about the location and shape of unknown curves which are a mathematical model of perfectly insulated cracks embedded in a known bounded domain, from the associated Dirichlet-to-Neumann map or its partial knowledge on the boundary of the domain. For the purpose, a new indicator function which can be calculated from the Dirichlet-to-Neumann map acting on the trace of special harmonic function with a large parameter is introduced. The harmonic function comes from the Mittag–Leffler function which is a generalization of the exponential function. A theorem concerned with the asymptotic behaviour of the indicator function with respect to such a large parameter is obtained. As a corollary, a uniqueness theorem for some partial information about unknown cracks together with a characterization of the information is established. An algorithm based on those theoretical investigations is introduced and the numerical implementation is shown. Our method is examined by some numerical experiments.

On inverse crack problems in elastostatics

AbstractIn solid mechanics, nondestructive testing has been an important technique in gathering information about unknown cracks, or defects in material. From a mathematical point of view, this is described as an inverse problem of partial differential equations, that is, the problem is to extract information about the location and shape of an unknown crack from the surface displacement field and traction on the boundary of the elastic material. By using the enclosure method introduced by Prof. Ikehata we can derive the extraction formula of an unknown linear crack from a single set of measured boundary data. Then, we need to have precise properties of a solution of the corresponding boundary value problem; for instance, an expansion formula around the crack tip. In this paper we consider the inverse problem concentrating on this point. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CRACK DETERMINATION USING LASER-MEASURED HORIZONTAL AND VERTICAL VELOCITY WAVEFORMS OF ULTRASOUND

The authors have been investigating an inverse problem of determining the position and the shape of unknown cracks in a material using velocity waveform data of the ultrasound measured with a laser interferometer. In our previous works, however, only the vertical velocities have been utilized in the crack determination, despite the fact that the horizontal and vertical velocities on the surface of the material are calculated numerically using time domain elastodynamic BIEM in 3D. We measure the particle velocities from the direction tilted from the normal direction to obtain both the horizontal and vertical velocities using the vector decomposition rules for these velocities. We can determine the unknown cracks more accurately using the horizontal and vertical velocity waveforms than using only the vertical velocity waveforms.

B112 ECT画像認識による複数き裂の可視化

This paper proposes a novel method for visualizing the multiple cracks from ECT image. Assumed two-dimensional scanned ECT signals to be a picture image, then a template matching method predicts crack shapes. Moreover, comparing ECT image of the unknown multiple cracks and that of the known crack separates into each of the cracks. Genetic algorithm performs faster search algorithm in our template matching. Shape reconstructions are achieved in the multiple cracks at intervals of 2 mm with a satisfactory degree of accuracy.

Acoustic methods for locating fire spread paths in old buildings

It has been well known for a long time that many fires in old buildings spread rapidly. The cause of this is often the penetration of smoke gas through ducts or unknown cracks or cavities in the building structure. In order to describe and analyse this problem, a number of non-destructive methods, which could be used to identify such possible smoke distribution paths in walls and floor structures, have been studied. It was found that an acoustical method showed promising results. This method was used in a couple of experiments at the Royal Palace in Stockholm and later on in laboratory experiments. The results from these experiments show that the use of standardised sound insulation measurements, together with listening tests, has the potential to locate possible fire distribution paths. Furthermore, some properties of a path, e.g. a crack length, can be deduced, a fact which can used as an additional help to locate the path.

Considerations of Elastodynamic Inverse Analysis Using Extended Kalman Filter Algorithm.

This paper is concerned with elastodynamic inverse analysis in which unknown defects, cracks and/or boundary conditions should be identified from displacement responses on the boundary of an elastic body. In the previous paper, the authors proposed the inverse analysis method for identification of unknown parameters using the extended Kalman filter algorithm and the boundary element analysis for steady-state elastodynamics. In this study, computational aspects of the inverse analysis method are investigated with respect to the influence of errors in the measured data and to arrangement of the measuring points. Furthermore, computational efficiency and accuracy are compared between the inverse analyses using the extended Kalman filter algorithm and the optimization method. Numerical simulation is carried out for some example problems to identify an unknown defect and unknown boundary values as a parameter identification problem.