Crack Detection Capability Research Articles

A comprehensive theoretical, numerical and experimental approach for crack detection in power plant rotating machinery

In order to describe the state-of-the-art on cracked rotor related problems, the current work presents the comprehensive theoretical, numerical and experimental approach adopted by EDF for crack detection in power plant rotating machinery. The work mainly focuses on the theoretical cracked beam model developed in the past years by S. Andrieux and C. Varé and associates both numerical and experimental aspects related to the crack detection problem in either turboset or turbo pump units. The theoretical part consists of the derivation of a lumped cracked beam model from the three-dimensional formulation of the general problem of elasticity with unilateral contact conditions on the crack lips, valid for any shape and number of cracks in the beam section and extended to cracks not located in a cross-section. This leads to the assessment of the cracked beam rigidity as a function of the rotation angle, in case of pure bending load or bending plus shear load. In this way the function can be implemented in a 1D rotordynamics code. An extension of the cracked beam model taking into account the torsion behaviour is also proposed. It is based on the assumption of full adherence between crack lips, when the crack closes, and on an incremental formulation of deformation energy. An experimental validation has been carried out using different cracked samples, both in static and dynamic configurations, considering one or three elliptic cracks in the same cross-section and helix-shaped cracks. Concerning the static configuration, a good agreement between numerical and experimental results is found. It is shown to be equal to 1% maximal gap of the beam deflection. Concerning the dynamical analysis, the main well-known indicator 2× rev. bending vibration component at half critical speed is approximated at maximum by 18% near the crack position. Our experiments also allowed for the observation of the bending and torsion resonance frequency shifts determined by the extra-flexibility induced by the crack in the shaft. The validated crack model is then applied to predict the dynamical behaviour of large industrial rotating machinery and to verify the crack detection capability based on the vibratory response. With respect to 900 MW turboset units, with cracks affecting LP rotors, a map of crack detection capabilities, based on 1× rev. and 2× rev. components as a function of circumferential extension ratio and crack depth, is drawn. If the crack depth is higher than 37% of the rotor diameter, on-line measurements of 2× rev. vibratory level shift allow to detect the crack. On the opposite, 1× rev. monitoring is necessary for cracks with circumferential extension superior to 270°. It is also observed that LP rotor bending mode shift monitoring theoretically allows to detect cracks with depths equal to or greater than 20% of the rotor diameter or with circumferential extension greater than 120°. The difficulties encountered for distinguishing the LP rotor bending mode frequencies, which may also evolve in time, independently from the cracks, limit the industrial application of this latter technique. Therefore new studies will focus on the analysis of torsion dynamic behaviour and on its sensitivity to cracks. With respect to RCP units, when half of the shaft section is cracked, the 2× rev. component remains very small. Whilst the result is simply due to a small excitation, a more accurate estimation of the external forces acting on the shaft could lead to more accurate numerical predictions.

A study on the failure detection of composite materials using an acoustic emission

Fiber reinforced composite materials have been increasingly used as structural material in airplanes, in space applications, and in robot arms because of their high specific stiffness and strength. Structural design and nondestructive test techniques have evolved as increased emphasis has been placed on the durability and damage tolerance of these materials. There are several methods used to detect damaged regions of composite materials. Acoustic emission is one of these. It is a suitable technique for detection of a wide range of micro-structural failures in composite materials. In this paper, an AE signal analyzer was designed and fabricated with a resonance circuit to extract the specified frequency of an acoustic emission signal. From the tests that were completed, the disturbance noise levels, such as impact or mechanical vibration, of the fabricated AE signal analyzer had a very small value in comparison to those of the conventional AE signal analyzer. Also, the fabricated AE signal analyzer was proven to have generally the same crack detection capabilities as a conventional AE signal analyzer, under static and dynamic tensile tests of the composite materials.

A fretting fatigue crack detection feasibility study using shear wave non-destructive inspection

A study was conducted to assess the viability of using ultrasonic shear wave non-destructive inspection (NDI) methods to detect fatigue cracks nucleating in the vicinity of a contact regionin situ. Use of this method is hampered by the presence of electronic and acoustic noise in the laboratory environment and by the contact in the experimental configuration. A previously established fretting fatigue test fixture was selected, in which nominally flat pads are held in contact against a thin, flat specimen and gross sliding between the pad and specimen is eliminated. Experiments were performed with Ti-6Al-4V at 300 Hz andR=0.5 for average clamping stresses of 200 and 620 MPa, and applied fatigue stresses of 330 and 250 MPa. The shear wave response was monitored during each test, and the test was interrupted when changes in the waveform were thought to indicate a crack. Also, the effect of the contact load and the sensitivity of the technique under the contact conditions were assessed. For the lower clamping stress, a sizeable portion of life was spent nucleating cracks, and the propagation life was too short to allow interruption of the tests. At the higher clamping stress, cracks with surface lengths of ∼2.5 mm were detected on, 10 mm wide specimens in tests conducted using the shear wave NDI technique. While the presence of the contact produced changes in the ultrasonic waveform, additional changes occurred as the crack propagated that permitted crack detection. A simple waveform correlation was used post-test to quantify the waveform changes, and thereby validate the viability of this NDI method for use in contact regions. In the configuration used for this study, the shear wave NDI technique was insensitive to small cracks. Some refinements that could dramatically improve crack detection capability are discussed.

복합재료의 파손 감지를 위한 동조형 음향방출 신호분석 기법

장섬유 복합재료는 항공우주 구조물, 로봇 팔 등의 구조물에 광범위하게 적용되고 있으며, 복합재료의 비파괴 검사방법에 대한 연구는 구조의 안전성과 신뢰성을 향상시키기 위하여 매우 중요한 연구 분야로 대두되고 있다. 본 논문에서는 AE 선호의 특정주파수 성분을 추출할 수 있는 풍조회로를 가진 AE 신호 분석 장비를 설계, 제작하였다. 제작된 AE 신호 분석 장비에 충격이나 기계적 진동 등과 같은 외란을 가하였을 때 제작된 AE 신호 분석 장비의 잡음레벨은 상용 AE 신호 분석 장비의 잡음레벨보다 매우 낮은 값을 나타내었다. 또한 복합재료 시편의 정, 동적 인장시험 하에서 제작된 AE 분석 장비는 상용 장비와 통일한 수준의 복합재료 파손감지능력을 나타냄을 확인할 수 있었다. As fiber reinforced composite materials are widely used in aircraft, space structures and robot arms, the study on the non-destructive testing methods of the composite materials has become an important research area for improving their reliability and safety. In this paper, the AE signal analyzer with the resonance circuit to extract the specified frequency of the acoustic emission signal were designed and fabricated. The noise levels of the fabricated AE signal analyzer by the disturbance such as impact or mechanical vibration had a very small value comparable to those of the conventional AE signal analyzer. Also, the fabricated AE signal analyzer was proved to have about the same crack detection capabilities with the conventional AE signal analyzer under the static and dynamic tensile tests of the composite materials.

Remote structural health monitoring with serially multiplexed fiber optic acoustic emission sensors

Development and testing of a serially multiplexed fiber optic sensor system is described. The sensor differs from conventional fiber optic acoustic systems, as it is capable of sensing AE emissions at several points along the length of a single fiber. Multiplexing provides for single channel detection of cracks and their locations in large structural systems. An algorithm was developed for signal recognition and tagging of the AE waveforms for detection of crack locations. Laboratory experiments on plain concrete beams and post-tensioned FRP tendons were performed to evaluate the crack detection capability of the sensor system. The acoustic emission sensor was able to detect initiation, growth and location of the cracks in concrete as well as in the FRP tendons. The AE system is potentially suitable for applications involving health monitoring of structures following an earthquake

Embedded Fiber Optic Crack Sensor for Reinforced Concrete Structures

A distributed fiber optic sensor is developed for embedment in concrete structures. The sensor principles are simple, and, therefore, practical for crack detection and deformation measurement in civil structural applications. The distributed sensor developed in the study operates based on the intensity measurements of optical power. The sensor consists of a number of individual segments on one line, with gage lengths designed according to the structural and materials requirements. An optical time domain reflectometer is employed for interrogation of the sensor signal. This study is aimed at demonstrating the applicability of this sensor in monitoring performance of concrete structures. Crack detection methodologies were established through experiments with plain concrete beams. Structural monitoring capability of the sensor was evaluated through experiments with reinforced concrete beams. Experiments involved both monotonic and fatigue loading of the beams. The sensor was embedded in the tension zone of the beams to facilitate comparison of sensor response with visual observations. Strain gages and Bragg grating fiber optic sensors were employed for monitoring reinforcing bar strains. Experimental results are presented and discussed in terms of the crack detection capability, resolution, and practicality of the sensor in structural applications.

Method and Theory for a Multi-Gauge Distributed Fiber Optic Crack Sensor

A distributed sensor is developed for detection of cracks and measurement of crack widths in composite materials. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. Return loss measurements at the interfaces between adjoining segments provide the basis for crack detection and measurement. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The quasi-distributed sensors were embedded in reinforced concrete beams in order to evaluate their crack detection capability. Experiments involved both monotonic as well as fatigue loading. The sensors were embedded in the tension zone of the concrete beams in order to facilitate comparison of sensor response with visual crack observations. Experimental results in terms of crack detection capability and resolution are presented and discussed.

Method and Theory for a Multi-Gauge Distributed Fiber Optic Crack Sensor

A distributed sensor is developed for detection of cracks and measurement of crack widths in composite materials. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. Return loss measurements at the interfaces between adjoining segments provide the basis for crack detection and measurement. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The quasi-distributed sensors were embedded in reinforced concrete beams in order to evaluate their crack detection capability. Experiments involved both monotonic as well as fatigue loading. The sensors were embedded in the tension zone of the concrete beams in order to facilitate comparison of sensor response with visual crack observations. Experimental results in terms of crack detection capability and resolution are presented and discussed.

Method and Theory for a Multi-Gauge Distributed Fiber Optic Crack Sensor

A distributed sensor is developed for detection of cracks and measurement of crack widths in composite materials. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. Return loss measurements at the interfaces between adjoining segments provide the basis for crack detection and measurement. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The quasi-distributed sensors were embedded in reinforced concrete beams in order to evaluate their crack detection capability. Experiments involved both monotonic as well as fatigue loading. The sensors were embedded in the tension zone of the concrete beams in order to facilitate comparison of sensor response with visual crack observations. Experimental results in terms of crack detection capability and resolution are presented and discussed.

Crack detection characterization of strain sensing grids

This paper presents a new probabilistic framework for evaluating measurements to detect anomalies such as cracks. The new framework accounts for the role of errors and the role of policy parameters such as the maximum allowable rate of false alarms and the required minimum probability of detecting critical anomalies. The framework is applied to the detection of cracks using strain sensing grids. This problem is studied and illustrated by examples from the viewpoint of both analysis and design. In analysis, the crack detection capability of a strain sensing grid is probabilistically characterized. In design, the specification of grid spacing and pattern is determined at various operating conditions that include the role of errors, minimum crack size that must be detected and other parameters. The computational techniques used to study strain sensing grids is discussed in detail. The main results indicate that under plausible operating conditions, a relatively large number of strain sensing stations (more than one station per three times the square of the minimum crack size to be detected) is needed when the coefficient of variation of the possible errors exceeds 1%. The implications on the required technology for strain sensing grids to detect cracks is briefly discussed.

Bayesian Reliability Analysis for Non-Periodic Inspection with Estimation of Uncertain Parameters

The present study concentrates on the development of the Bayesian reliability analysis for estimating appropriate values of several uncertain parameters in probability distributions and generating appropriate non-periodic structural inspection schedules using small sample data gathered during in-service inspections. A pressurized fuselage structure of transport-type aircraft with multiple fatigue-critical elements is accepted as a structural model in this analysis. This element is a multiple component of two-bay fail-safe structure, designed by the damage tolerance principle, which consists of three frames and a skin panel subjected to cyclic stress due to differential pressure. Fatigue cracks are initiated and propagated from a hole for a rivet to connect the skin panel to the center frame. Probabilistic factors considered in this model are fatigue crack initiation and propagation, failure rates before and after crack initiation, yield stress, fracture toughness, cyclic stress range and crack detection capability of detailed visual inspection. Uncertain parameters estimated from inspection data such as the number of detected cracks, crack sizes and whether or not failures were detected are a scale parameter in a probability density function of fatigue crack initiation and a parameter in a simplified fatigue crack propagation equation. Both parameters can not be a priori estimated because of the paucity of pertinent data. Monte Carlo simulation techniques are utilized in order to generate a failure process in the structural element and to evaluate the validity of the proposed Bayesian reliability analysis.