Crack Sizing Research Articles

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

This paper deals with quantifying the performance of a technique for detection, location, and sizing of circumferential crack-like defects in pipelines using synthetically focused guided waves. The system employs a circumferential array of piezoelectric transducer elements. A torsional probing guided wave is excited using the array, which subsequently interacts with the reflecting features of the pipe, such as defects or weld caps. The recorded backscattered signals are synthetically focused to every point of interest in the pipe wall, to form an image of the reflecting features of the pipe. The defect image amplitude is used to estimate the defect depth, and the full width at half maximum of the defect image circumferential profile is used to estimate the circumferential extent of the defect. The imaging system is tested with data from finite element simulations and from laboratory experiments. It is found that reliable sizing of circumferential cracks in finite element simulations and experiments can be achieved if the circumferential extent of the defect is greater than 1.5 lambda(S), where lambda(S) is the shear wavelength at the frequency of inspection. This result is theoretically valid for any pipe size, any axial defect location, and any inspection frequency. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in an 8-inch pipe with a 9 dB SNR improvement.

Development of computational inversion techniques to size cracks from eddy current signals

This paper reviews the recent advance in computational inversion techniques to size defects using eddy current signals. Studies reporting the sizing of artificial slits with the aid of computational inversion techniques are briefly overviewed and then several recent studies dealing with the sizing of fatigue and stress corrosion cracks are introduced. The results of the studies confirm computational techniques for sizing slits are sufficiently mature, and even though the measured signals are polluted with high noise, the boundary profile of slits can be evaluated from the signals. Furthermore, the studies demonstrate fatigue cracks can be dealt with almost identically to artificial slits from the viewpoint of eddy current testing; fatigue cracks introduced inside weld with a rough surface are well sized using eddy current testing with the aid of computational inversion techniques. In contrast, there is a large discrepancy between artificial slits and stress corrosion cracks, and thus conventional approaches are not always applicable to the sizing of stress corrosion cracks. This paper introduces several recent activities in the author's group to overcome this problem.

Some advances in numerical analysis techniques for quantitative electromagnetic nondestructive evaluation

In this paper, some progresses in numerical techniques mainly made in our research group for the forward and inverse simulation of electromagnetic nondestructive evaluation (ENDE) signals are introduced. For the first part, efficient forward analysis schemes for the simulation of eddy current testing (ECT), remote field ECT (RFECT) and magnetic flux leakage testing (MFLT) signals are described respectively, in addition to some numerical examples. Fast and accurate ECT signal simulation is realised by introducing a database type strategy using precalculated unflawed potential field data. To meet the high accuracy requirement of the simulation of RFECT signals, a hybrid scheme using 2D and 3D geometry and a new formula for pickup signal are proposed. To improve the efficiency of MFLT signal simulation, a fast scheme is developed based on a FEM–BEM hybrid code of polarisation method. In addition, a phenomenological method is also described in the first part, which is developed for the qualitative estimation of eddy current distribution and pickup signals. The second part of this review paper is on the reconstruction of defect from the detected ENDE signals (mainly ECT signals). Reconstruction schemes based on conjugate gradient (CG) method of deterministic category and NN method, metaheuristic methods of stochastic category are developed and sizing of both artificial and natural cracks are performed by using measured signals. It is clarified through applications that a deterministic optimisation method is more efficient for treating simple cracks, while a stochastic way is prefer for defects of complicated geometry such as a stress corrosion crack and multiple cracks. In the crack modelling and parameterisation, an element of discontinuous material property is introduced to treat crack of arbitrary shape based on a given regular mesh. Several numerical models are proposed for natural cracks, which makes the reconstruction of some natural cracks become possible.

Sizing of long stress corrosion crack from 2D ECT signals by using a multisegment inverse analysis strategy

Sizing of long stress corrosion crack from 2D ECT signals by using a multisegment inverse analysis strategy

Removal of Probe Liftoff Effects on Crack Detection and Sizing in Metals by the AC Field Measurement Technique

In the alternative current field measurement (ACFM) technique, the nonzero value of liftoff distance for the magnetic sensor acts as a low-pass filter on surface crack signals, causing errors in crack detection and sizing. We present a blind deconvolution algorithm for liftoff evaluation and surface crack signal restoration. The algorithm employs the available closed-form expressions for the distribution of electromagnetic fields at the metal surface in the vicinity of a crack. To examine the accuracy of the algorithm, we use the original and the restored signals for crack sizing by a wavelet network inversion method. We present simulated and experimental results to demonstrate the role of the proposed algorithm in improving the inversion process.

Enhancements of eddy current testing techniques for quantitative nondestructive testing of key structural components of nuclear power plants

In this paper, studies on upgrade of eddy current testing (ECT) techniques for inspection of stress corrosion cracks (SCC) in key structural components of a nuclear power plant are reported. Access and scanning vehicle (robot), advanced probes for steam generator (SG) tube inspection, developments and evaluations of new ECT probes for welding joint, and ECT-based crack sizing technique are described, respectively. Based on these techniques, it is demonstrated that ECT can play as a supplement of ultrasonic testing (UT) for the quantitative inspection of welding zone. It is also proved in this work that new ECT sensors are efficient even for inspection of a stainless steel plate as thick as 15 mm.

Flaw detection of underground pipeline using electrical potential method

This paper presents a modeling approach to characterize adjacent cracks on underground gas pipe. Electrical potential method is used for capturing signals from surface cracks on the gas pipe. A three dimensional potential problem is considered for the inspection model. A defect parameter-to-output mapping is represented by means of finite element method in three dimensions. Computational methods for sizing and positioning of cracks are proposed based on our modeling approach.

Ultrasonic detectability of potentially closed cracks from cold-worked holes under loaded conditions

Many non-destructive inspections for cracks rely on detecting ultrasound scattered or reflected from the crack. Recently, several methods for second-layer ultrasonic crack detection have been developed, sometimes using automated analysis methods to interpret the complex ultrasonic responses from multiple layers. In the validation of these techniques, the assumption is often made that fatigue cracks will reflect as much ultrasound as an artificially introduced EDM notch or saw cut. However, this is not always the case for 'closed' cracks, which can have a very low ultrasonic reflectivity. Of particular concern is the case of cold-worked holes, where a residual compressive stress field exists, extending to approximately one radius from the fastener hole. In addition, if a crack has grown under fatigue loads that include high peak stresses, there can be a closed region in the plastic zone at the crack tip. To complicate matters further, interference-fit fasteners apply an additional tensile stress, which may or may not counteract the cold-worked stress field near to the fastener, depending on the amount of interference. This problem was encountered whilst validating a two-layer ultrasonic inspection technique on real cracks in a real fatigue-test wing containing cold-worked holes. Inspections were performed at four different remote load levels by stress-jacking the wing. Whilst the ultrasonic indications from some of the cracks grew with increasing remote load, others did not. Some ultrasonic indications grew both towards the fastener and away from it, suggesting that both crack-tip closure and cold-worked crack-root closure were occurring. A possible reason for some cracks not growing is that they were already being held open by interference-fit fasteners. The aim of this project was to investigate and understand the effects of cold working, interference-fit fasteners and crack-tip closure on the ultrasonic detectability and sizing of cracks. In particular, information was required about the potential improvement in defect detectability by applying a remote tensile load to the structure to overcome the residual compressive stresses. Results will be reported of an experiment designed to grow fatigue cracks to a range of lengths at cold-worked holes in a single specimen, with interference-fit fasteners installed. These cracks could then be inspected ultrasonically at various remote tensile-load levels and the results compared with EDM notches in a similar specimen. Finally, the interference-fit fasteners could be removed and the ultrasonic inspections at the various loads repeated. Initial results have confirmed that ultrasonic indication size and amplitude both increase with remote tensile load but further results will be required to determine the dependence of defect detectability on remote load.

Development of a Novel Algorithm for a Crack Detection, Localization, and Sizing in a Beam Based on Forced Response Measurements

Abstract The present work aims at the development of a method for the crack detection, localization and sizing in a beam based on the transverse force and response signals. The Timoshenko beam theory is applied for transverse vibrations of the beam model. The finite element method is used for the cracked beam forced vibration analysis. An open transverse surface crack is considered for the crack model, which contains standard five flexibility coefficients. The effect of the proportionate damping is also included. A harmonic force of known amplitude with sine-sweep frequency is used to dynamically excite the beam, up to few flexible modes, which could be provided with the help of an exciter. In practice, linear degrees of freedom (DOFs) can be measured quite accurately; however, rotational DOFs are difficult to measure accurately. All rotational DOFs, except at crack element, are eliminated by a dynamic condensation scheme; for elimination of rotational DOFs at the crack element, a new condensation scheme is implemented. The algorithm is iterative in nature and starts with a presumption that a crack is present in the beam. For an assumed crack location, flexibility coefficients are estimated with the help of forced responses. The Tikhonov regularization technique is applied in the estimation of bounded crack flexibility coefficients. These crack flexibility coefficients are used to obtain the crack size by minimizing an objective function. With the help of the estimated crack size and measured natural frequency, the crack location is updated. The procedure iterates till the crack size and location get stabilized up to the desired level of accuracy. The algorithm has a potential to detect no crack condition also. The crack flexibility and damping coefficients are estimated as a by-product. Numerical examples, with the simply supported and cantilevered beams, are given to justify the applicability and versatility of the algorithm in practice. With the numerically simulated forced responses, which have the noise contamination and the error in the natural frequency measurements, the estimated crack parameters (i.e., the crack location and size) are in good agreement.

Robust damage assessment of multiple cracks based on the frequency response function and the Constitutive Relation Error updating method

The aim of this paper is to present a damage assessment technique for the non-destructive detection and sizing of multiple open cracks in beams. The constitutive relation error updating method is used for the identification of the location and the size of multi-cracks in a simply supported beam.The present identification method is illustrated through numerical examples including double and triple cracks. Moreover, the efficiency and robustness of the proposed method is demonstrated through various numerical simulations in regard to the non-dimensional crack depth and the crack location.It is demonstrated that the constitutive relation error updating method can detect the number of cracks on the beam and can estimate both the crack positions and sizes with satisfactory precision, even if 10% or 20% noise levels has been added to the simulations, and only few degrees of freedom are used for the identification procedure.

Detection of Transverse Cracks in a Composite Beam Using Combined Features of Lamb Wave and Vibration Techniques in Ann Environment

Abstract The detection, location and sizing of transverse cracks in a composite beam, by combining damage features of Lamb wave and vibration based techniques in artificial neural network (ANN) environment, using numerical finite element model, is discussed. Four damage features, time of flight (TOF) and amplitude ratio, which are Lamb wave based features and first and second natural frequencies, which are vibration based features were used as input to ANN. The output of ANN was crack location and depth. It was demonstrated that through the simultaneous employment of features from the two modalities in an ANN environment, the sizing could be done more effectively.

Inverse method for detection and sizing of cracks in thin sections using a hybrid genetic algorithm based signal parametrisation

A hybrid-GA method, based on signal parameterization, has been reported here for the improved detection and sizing of surface cracks of small sizes/depths in thin sections. The method relies on parameterizing the composite reference from the defect into its individual components i.e., the crack tip diffracted echo and the corner trap echo and subsequently use the relative arrival time technique (RATT). The phased array ultrasonic technique was employed in the investigation. Both experimental and simulated signals were used in the study. It is shown through both simulations and experiments that the hybrid-GA is successful in parameterizing both non-overlapping and overlapping echoes encountered in thin sections. It is additionally shown that the hybrid-GA improves the signal to noise ratio and correct for under-sampling of data.

Multiple cracks effects and identification

An important rotor fault, which can lead to catastrophic failure if undetected, is fatigue crack in the shaft. The cracked rotor problem received the first attention in 1970, since when the interest among the researchers started. Vibration behavior of cracked structures, in particular cracked rotors, has received considerable attention in the last two decades. The problem of damage and crack detection in structural components has acquired important role in recent years. However, the studies are mainly dealt with single crack. If the structure is cracked in at least two positions, the problem of crack sizing and location becomes decidedly more complex. Relatively few authors have addressed the multi-crack assessment for structures. The objective of this present study is to summarize the different studies on double/multi-cracks and to note the influences, identification methods in vibration structures such as beams, rotors, pipes, etc. And thus this paper brings out the state of the research on multiple cracks effects and their identification.

Using AC field measurement data at an arbitrary liftoff distance to size long surface-breaking cracks in ferrous metals

We propose a model-based inversion method to size long surface-breaking cracks in ferrous metals using alternative current field measurement (ACFM) data at an arbitrary liftoff distance. This method employs conjugate gradients optimization to invert measured surface ACFM signal to crack depth. To alleviate the adverse effect of sensor liftoff uncertainty on crack sizing, we propose a blind de-convolution algorithm for reconstructing respective surface ACFM crack signal. In this algorithm, the partially known filter function associated with the sensor liftoff is estimated from which the surface crack signal can be restored. The validity of the proposed inversion method is demonstrated by comparing the actual and predicted depths of several simulated and machine-made long cracks in mild steel test blocks.

Circumferential higher order guided wave modes for the detection and sizing of cracks and pinholes in pipe support regions

The non-dispersive propagation of ultrasonic guided wave higher order modes cluster (HOMC) traveling along the circumferential direction in a hollow cylinder and its interaction with defects in pipe support regions is reported. These circumferential guided waves were generated in mild steel (MS) pipe specimens containing artificially created axial notches (simulating axial cracks) and pinholes (simulating pinhole-like defects) of different sizes in order to simulate conditions such as cracking and corrosion under pipe supports. The characteristics of these guided waves were also studied as a function of parameters related to how they were generated; namely, using: (a) 2.25 MHz linear phased array transducer, (b) 2.25 MHz conventional circular transducer and (c) 1 MHz conventional circular transducer. These higher frequency modes were explored for their ability to detect and size defects. Because of access limitations to the pipe support region in actual field testing, the transducer was always placed at a fixed circumferential position and moved axially along the length of the pipe. The defect position along the circumference was ascertained from the time of flight while the defect size was estimated using the amplitude data. The signals obtained for all three transducer configurations are compared for their ability to locate, detect and size the above-mentioned defects. It was shown that at these relatively higher frequencies, the guided wave modes exhibit small dispersion and have the ability to provide improved imaging of small size defects throughout the cross-section of the pipe.

Effect of Stress on Ultrasonic Response in Detection and Sizing of Cracks

Abstract In different NDT techniques huge developments have been achieved during the last few years with regard to crack detection and sizing. In a large range of materials, cracks are one of the most dangerous defect types. A crack is a planar reflector, which is sometimes extremely difficult to detect and to be sized. A crack can be totally open, partly closed or even totally closed because of compressive stresses. The effects of stresses cause problems, for instance, in in-service inspections of nuclear power plants in detection and sizing of closed cracks. This phenomenon causes similar effects in all kinds of plants and components. In this study some experimental inspections have been carried out as well as some FEM calculations of stress field around the crack and compared to corresponding measurements in literature. Materials used for this study are austenitic and ferritic piping steel. The dynamic load applied to the cracks was in form of different thermal cycles. The maximum temperature variations were from 20°C to 600°C depending on each dynamic loading cycle. Different types of ultrasonic methods were used in the measurements. The effect of closure on the response of normal ultrasonic practical probes was recorded. Dynamic loading during ultrasonic measurement gives clear evidence on the effect of the crack closure as well as on the amplitude variation limits in ultrasonic testing.

Sizing of surface-breaking cracks in complex geometry components by ultrasonic Time-of-Flight Diffraction (TOFD) technique

Sizing a surface-breaking crack in a complex geometry component, for example a steam turbine's solid rotor shaft, is a challenging inspection task. This paper deals with experimental work for the evaluation of the Time-of-Flight Diffraction (TOFD) technique for sizing of surface-breaking cracks in such a complex geometry component. The experimental results show that with the help of the mathematical model explained here, TOFD can size such cracks with reasonable accuracy.

An automated time–frequency approach for ultrasonic monitoring of fastener hole cracks

Angle beam ultrasonic inspection methods are well established for detection and sizing of fatigue cracks originating from fastener holes, and have recently been applied to in situ monitoring of such cracks using permanently attached transducers. Prior work utilizing a dual angle beam method has demonstrated the efficacy of an energy ratio algorithm for detection and sizing of cracks in open holes. This energy ratio is a measure of how much a crack opens under load, and is the ratio of transmitted energy for the specimen under tensile loading to the energy under no load, normalized by the ratio computed for the undamaged hole. Considered here is an extension of this method which utilizes multiple time and frequency windows of the received signals for computing the energy ratio with the goal of achieving the earliest possible detection of cracking. These windows allow different spatial regions to be selectively interrogated, and windows corresponding to the actual crack geometry should be more effective at detecting the crack. An automated algorithm is developed and implemented to permit multiple time–frequency windows to be effectively analyzed without requiring manual interpretation. Results are shown which indicate that cracking in open holes can be detected significantly earlier in the fatiguing process compared to the previously implemented full-window energy ratio method. Integral to the method is the assumption that there is a significant change in crack closure as a function of load.

Ultrasonic sizing of short surface cracks

This paper presents a method for ultrasonic sizing of surface cracks based on time domain and frequency domain Rayleigh wave near-field analysis. The procedure allows for the entire range of ratio of crack depth to Rayleigh wavelength a/ λ to be covered with one single measurement. In the time domain the time-of-flight method was extended to cracks smaller than the wavelength by correlation of the time delay of the transmitted Rayleigh wave with the crack depth. In the frequency domain, the inverse scattering problem was solved by comparison of the measured scattering coefficients and central frequencies of the reflected and transmitted Rayleigh waves with theoretical curves. The sizing procedure was demonstrated experimentally with narrow slots and real fatigue cracks. The out-of-plane displacement component was measured pointwise in the scattered near field by means of laser interferometry. The determination of the scattering parameters in the near field was enabled by a procedure that allows for the Rayleigh wave to be separated from the other modes scattered at the defect. The experimental results showed good accuracy and repeatability down to the smallest available ratio of crack depth to Rayleigh wavelength a/ λ = 0.15.

Intelligent system supporting defect identification and sizing using eddy current testing signals in nuclear power plants

Two premier issues in eddy current testing, namely the automatic identification of cracks from their noisy backgrounds and the sizing of cracks with complex morphologies, are dealt with in this paper. A signal processing method based on statistical pattern recognition is established to realise automatic crack identification, and a method for estimating crack depth is proposed, using a new concept, the depth sizing index, which is analytically constructed from raw measurement signals based on system optimisation theory. In addition, a surface crack shape reconstruction algorithm, using a deconvolution filter, is proposed to evaluate the crack length.