Surface-breaking Cracks Research Articles

Assessment of Reinforced Concrete Surface Breaking Crack Using Rayleigh Wave Measurement.

An improved single sided Rayleigh wave (R-wave) measurement was suggested to characterize surface breaking crack in steel reinforced concrete structures. Numerical simulations were performed to clarify the behavior of R-waves interacting with surface breaking crack with different depths and degrees of inclinations. Through analysis of simulation results, correlations between R-wave parameters of interest and crack characteristics (depth and degree of inclination) were obtained, which were then validated by experimental measurement of concrete specimens instigated with vertical and inclined artificial cracks of different depths. Wave parameters including velocity and amplitude attenuation for each case were studied. The correlations allowed us to estimate the depth and inclination of cracks measured experimentally with acceptable discrepancies, particularly for cracks which are relatively shallow and when the crack depth is smaller than the wavelength.

X-ray asterism and the structure of cracks from indentations in silicon

The asterism observed in white radiation X-ray diffraction images (topographs) of extended cracks in silicon is investigated and found to be associated with material that is close to breakout and surrounded by extensive cracking. It is a measure of the mechanical damage occurring when the fracture planes do not follow the low-index cleavage planes associated with the crystal structure. It is not related to a propensity for some cracked wafers to shatter during subsequent high-temperature processing. There is no correlation between crack morphology and alignment of an indenter with respect to the orientation of a silicon wafer, the cracks being generated from the apices of the indenter and having threefold symmetry for Berkovich indents and fourfold symmetry for Vickers indents. X-ray diffraction imaging (XRDI) of indents does not reveal this underlying symmetry and the images exhibit a very substantial degree of variation in their extent. This arises because the XRDI contrast is sensitive to the long-range strain field around the indent and breakout reduces the extent of this long-range strain field. Breakout is also detected in the loss of symmetry in the short-range strain field imaged by scanning micro-Raman spectroscopy. Weak fourfold symmetric features at the extremes of the images, and lying along 〈110〉 directions, are discussed in the context of slip generated below the room-temperature indents. Scanning electron microscopy imaging of the region around an indent during focused ion beam milling has permitted the three-dimensional reconstruction of the crack morphology. The surface-breaking Palmqvist cracks are found to be directly connected to the median subsurface cracks, and the presence of extensive lateral cracks is a prerequisite for material breakout at indenter loads above 200 mN. The overall crack shape agrees with that predicted from simulation.

Numerical investigation of the ability of eddy current testing to size surface breaking cracks

This study attempts to estimate the limitations of eddy current testing for evaluating the depth of a crack with the aid of numerical simulations. Three-dimensional finite element simulations were conducted to obtain eddy current signals resulting from a crack in a plate made of Ni-based alloy. The simulations modelled the crack as a sufficiently long rectangular region with a constant width and depth. A uniform conductivity was given inside the modelled crack because actual cracks do not always behave as electrically insulating walls. The conductivity of the modelled crack was chosen on the basis of a recent study reporting how real cracks can be modelled from the viewpoint of electromagnetic non-destructive evaluation. This study considered three conventional eddy current probes: a pancake probe, a vertical pancake probe and a plus point probe. The results of the simulations reveal that lower frequency does not always enhance the evaluation of a deeper crack if the crack is conductive; moreover, the maximum depth of a stress corrosion crack that conventional eddy current testing can evaluate quantitatively should be limited to a few mm at most. In contrast, the simulations also demonstrate that the ability of eddy current testing to size a fatigue crack would not differ significantly from that to size an artificial slit.

Probabilistic Analysis of Stress Corrosion Crack Growth and Related Structural Reliability Considerations

The predetection evolution of stress corrosion cracking has been examined as a necessary preliminary to effective detection of such cracks. Anodic dissolution (AD) and hydrogen embrittlement (HE) have been considered to calculate the stress corrosion crack (SCC) growth in AA7050-T6 for a surface-breaking crack with blunt tip in an aqueous environment. Since these processes are not completely deterministic, several advanced statistical methods have been used to introduce probabilistic considerations. Based on the data from designed computer experiments, the computer code developed by the authors (Lee et al., 2015, “A Comprehensive Analysis of the Growth Rate of Stress Corrosion Cracks,” Proc. R. Soc. A, 471(2178), p. 20140703) to conduct deterministic stress corrosion crack growth analysis has been represented by metamodels using Gaussian process regression. Through sensitivity analysis, important variables which need to be calibrated have been identified. The dynamic Bayesian network (DBN) model and Monte Carlo simulation (MCS) have been utilized to quantify uncertainties. Statistical parameters of input variables have been obtained by a machine learning technique. The calibrated model has been validated using Bayesian hypothesis testing. Since the DBN model yields a probability of detection (POD) comparable to the probability based on binary validation data, the probabilistic model with calibrated parameters is expected to well represent the growth of a stress corrosion crack. The results also show that the reliability largely depends on the accuracy of flaw detection methods and on the critical crack length.

Determining the propagation angle for non-vertical surface-breaking cracks and its effect on crack sizing using an ACFM sensor

Alternating current field measurement (ACFM) probes are used to detect and size cracks in a range of engineering components. Crack sizing for this, and other electromagnetic (EM) based NDT systems, relies on relating the signal obtained to the actual crack length. For cracks that do not propagate vertically, such as rolling contact fatigue cracks in rails, predicting the crack depth, which determines the rail depth to be removed by grinding, requires an assumed propagation angle into the material as no method to determine crack vertical angle from the EM signals has been reported. This paper discusses the relationship between ACFM signals and propagation angles for surface-breaking cracks using a COMSOL model. The signal accurately predicts the crack pocket length when the vertical angle is 30–90° but underestimates pocket length for shallower angles, e.g. a 50% underestimate is seen for a 3.2 mm pocket length crack propagating at a vertical angle of 10°. A new measure, the trough–peak ratio, is proposed to determine the crack vertical angle. These are verified by experimental measurements using a commercial ACFM pencil probe for cracks with a range of vertical angles between 10° and 90°.

A Simulation Study of Surface Breaking Crack in Concrete Beam Based on Time-Reversed Theory of Surface Wave

Existing non-destructive test methods are usually ineffective in detection of surface breaking cracks with large depths in concrete structure. This paper introduces a method suitable for detection of deep cracks based on surface wave time reversal theory. A numerical simulation based on the finite element model is performed to investigate surface cracks detection. A damage index was defined based on the correlation coefficient between the actuated and the reconstructed wave signals. The results demonstrated that the presence of crack had a significant influence on the propagation characteristics of surface waves along concrete. Cracks in different sizes were introduced and correlated with the damage index. Enlarging the crack depth resulted in an increase in the distortion of reconstructed signals, and a higher damage index was obtained. The results illustrated the effectiveness of the surface wave time-reversal process in identifying cracks in concrete structures.

Automated Surface Wave Measurements for Evaluating the Depth of Surface-Breaking Cracks in Concrete

The primary objective of this study is to investigate the feasibility of an innovative surface-mount sensor, made of a piezoelectric disc (PZT sensor), as a consistent source for surface wave velocity and transmission measurements in concrete structures. To this end, one concrete slab with lateral dimensions of 1500 by 1500 mm and a thickness of 200 mm was prepared in the laboratory. The concrete slab had a notch-type, surface-breaking crack at its center, with depths increasing from 0 to 100 mm at stepwise intervals of 10 mm. A PZT sensor was attached to the concrete surface and used to generate incident surface waves for surface wave measurements. Two accelerometers were used to measure the surface waves. Signals generated by the PZT sensors show a broad bandwidth with a center frequency around 40 kHz, and very good signal consistency in the frequency range from 0 to 100 kHz. Furthermore, repeatability of the surface wave velocity and transmission measurements is significantly improved compared to that obtained using manual impact sources. In addition, the PZT sensors are demonstrated to be effective for monitoring an actual surface-breaking crack in a concrete beam specimen subjected to various external loadings (compressive and flexural loading with stepwise increases). The findings in this study demonstrate that the surface mount sensor has great potential as a consistent source for surface wave velocity and transmission measurements for automated health monitoring of concrete structures.

Detection of defect parameters using nonlinear air-coupled emission by ultrasonic guided waves at contact acoustic nonlinearities

Interaction of ultrasonic guided waves with kissing bonds (closed delaminations and incipient surface breaking cracks) gives rise to nonlinear features at the defect location. This causes higher harmonic frequency ultrasonic radiation into the ambient air, often referred to as Nonlinear Air-Coupled Emission (NACE), which may serve as a nonlinear tag to detect the defects. This paper summarizes the results of a numerical implementation and simulation study of NACE. The developed model combines a 3D time domain model for the nonlinear Lamb wave propagation in delaminated samples with a spectral solution for the nonlinear air-coupled emission. A parametric study is conducted to illustrate the potential of detecting defect location, size and shape by studying the NACE acoustic radiation patterns in different orientation planes. The simulation results prove that there is a good determination potential for the defect parameters, especially when the radiated frequency matches one of the resonance frequencies of the delaminated layer, leading to a Local Defect Resonance (LDR).

A comprehensive analysis of the growth rate of stress corrosion cracks

This paper is a synthesis of earlier results supplemented by new results to define a comprehensive analysis of the growth rate of stress corrosion cracking (SCC). Two mechanisms, anodic dissolution (AD) and hydrogen embrittlement (HE), have been considered to calculate the SCC growth rate of AA 7050-T6 for a surface-breaking crack with blunt tip in an aqueous environment. The relative contributions of each mechanism and their mutual interactions have been quantitatively assessed. Results show that AD provides critical conditions for HE, which explains in part a stepwise propagation of the crack. Finally, the total crack growth rate due to the combined effects of AD and HE has been determined, and numerical results have been compared with experimental data, and a calculation of the crack growth rate for a practical configuration has been presented.

표면파 측정을 통한 콘크리트 슬래브의 표면균열 깊이 측정

Non-contact surface wave transmission (SWT) measurements are used to evaluate the depth of a surface-breaking crack in concrete slabs. The author propose a measurement model that includes an appropriate configuration of the source and receivers, and a transmission function for the given configuration. A series of numerical simulations using a 3D finite element model is used to obtain the transmission function. Then, validity of a proposed model is verified through experimental studies. Two air-coupled sensors are used to measured surface waves across surface-breaking cracks with varying depths from 0mm to 100mm with intervals of 10mm in a concrete slab () in laboratory. As a result, the proposed method is demonstrated as to be effective for charactering the depth of a surface-breaking crack in concrete bridge deck with an average error of 10%. A discussion on practical applications of the proposed method is also included in this article.

Application of laser ultrasonic technique for non-contact detection of structural surface-breaking cracks

Abstract Based on the finite element method (FEM), the surface-breaking cracks have been investigated by using the laser-generated Rayleigh wave. The features of laser-generated Rayleigh wave interaction with cracks are analyzed in time and frequency domain. The simulation results show that the surface acoustic wave induced by the pulsed laser is sensitive to the surface-breaking cracks. As the crack depth increases, the transmission coefficients almost linearly decrease and the reflection coefficients show a dip. The corresponding experimental results have verified the feasibility of numerical calculation and reached a good agreement with simulation results. The research findings would provide a potential application for testing surface-breaking cracks of aircraft parts.

Study of Laser-Array-Source-Generated Narrow-Band Acoustic Waves Interacting with Surface-Breaking Cracks Using Finite Element Method

Based on the classical theory of laser ultrasound generation in the thermoelastic regime, the generation of ultrasound by the irradiation of a laser array source is simulated by the finite element method. The interaction of ultrasound with a surface-breaking crack is studied. By changing the space distribution of a simultaneously triggered laser array source, narrow-band acoustic waves can be obtained. The characteristics of the narrow-band acoustic waves in the frequency domain are analyzed. By comparing with the spectrum of wide-band reflected acoustic waves generated by a single laser source, the mechanism of the interaction of a surface-breaking crack on the reflected narrow-band acoustic waves is studied. According to the variation of a reflected narrow-band acoustic wave spectrum on cracks with different depths, the influence of the crack-depth variation on different components of the narrow-band acoustic waves is obtained. Results suggest that the narrow-band acoustic waves could also be sensitive to the variation of the crack depth so that it can provide an alternative option for crack-depth gaging using all-optical methods.

Ultrasound monitoring for evaluation of damage in reinforced concrete

Abstract The paper deals with automated monitoring of damage evolution in concrete elements subjected to three-point bending tests. The monitoring is based on the nonlinear interactions of traveling ultrasonic waves with micro-crack zones inside the concrete specimens and surface-breaking cracks. The developed procedure assumes semi-continuous ultrasonic testing during the element full loading cycle and generation of the power spectral density maps for the on-line assessment of the degradation process. Two damage indicators are introduced to evaluate micro- and macro-damage. The preliminary experimental results show that the proposed automated monitoring system provides an effective method for the evaluation of progressive damage in concrete.

Fracture Toughness Characterization of High-Pressure Pipe Girth Welds Using Single-Edge Notched Tension [SE(T)] Specimens

Abstract The safety and reliability of large-diameter pipelines for the transport of fluid hydrocarbons is being improved by the development of high-strength steels, advanced weld technologies, and strain-based design (SBD) methodologies. In SBD, a limit is imposed on the applied strains rather than the applied stresses. For high-pressure pipelines, SBD requires an assured strength overmatch for the weld metal as compared to the base material, in order to avoid strain localization in the weldment during service. Achieving the required level of strength overmatch, as well as acceptable ductility and low-temperature fracture toughness, is a challenge as the pipe strength increases. Published studies show that low constraint geometries such as single-edge tension [SE(T)] or shallow-notched single-edge bend [SE(B)] specimens represent a better match to the constraint conditions of surface-breaking circumferential cracks in large-diameter pipelines during service (Shen, G., Bouchard, R., Gianetto, J. A., and Tyson, W. R., “Fracture Toughness Evaluation of High Strength Steel Pipe,” Proceedings of PVP2008, ASME Pressure Vessel and Piping Division Conference, Chicago, IL, July 27–31, ASME, New York, 2008). However, the SE(T) geometry is not included in any of the most widely used elastic-plastic fracture mechanics (EPFM) test standards. A procedure has been developed for performing and analyzing SE(T) toughness tests using a single-specimen technique that includes formulas for calculating the J-integral and crack-tip opening displacement, as well as for estimating crack size using rotation-corrected elastic unloading compliance. Here, crack-resistance curves and critical toughness values obtained from shallow-crack SE(T) specimens (a0/W ≈ 0.25) are compared to shallow-crack (a0/W ≈ 0.25) SE(B) specimens. We believe that the SE(T) methodology is mature enough to be considered for inclusion in future revisions of EPFM standards such as ASTM E1820 and ISO 12135, although additional work is needed to establish validity limits for SE(T) specimens.

Measurement of crack-induced non-propagating Lamb wave modes under varying crack widths

This study aims to measure non-propagating Lamb wave modes (NPLs) for detection of a surface-breaking fatigue crack. Crack-induced NPLs are the attractive feature to identify and localize an invisible crack, because they are not spatially propagated but significantly contribute to local Lamb mode amplitudes at a crack interface. By extracting only the crack-induced NPLs from Lamb wavefields measured at multi-spatial points, the crack is able to be identified and localized without relying on baseline data obtained from the pristine condition of a target structure. First, the existence of the crack-induced NPLs are analytically proved using a modal expansion method. Then, a NPL filter based on a frequency–wavenumber analysis is proposed so that only NPLs are isolated from Lamb wavefields. Subsequently, the theoretical proof and the proposed NPL filter are numerically and experimentally validated through fatigue crack detection tests. In particular, the quantities of NPL generation at the real fatigue crack interface are investigated under changing driving frequencies as well as crack widths. The numerical and experimental results confirm that NPLs remarkably appear at open crack interfaces, while they are rarely induced at closed crack interfaces.

Condition Assessment of Cementitious Materials Using Surface Waves in Ultrasonic Frequency Range

Surface waves propagating in a medium provide information about the mechanical properties and condition of the material. Variations in the material condition can be inferred from changes in the surface wave characteristics. Multichannel analysis of surface waves (MASW) is a well-established surface wave method used for determination of the shear-wave profile of the soil layers near the surface. The MASW test configuration is also applicable to assess the condition of construction materials using appropriate frequency range. Previous studies on the detection of surface-breaking cracks in concrete elements, using the dispersion and attenuation of ultrasonic waves, were successful; however, a complete damage assessment of the whole element was not in the scope of these studies. In this study, different wave characteristics, such as Rayleigh wave velocity, wave attenuation, and phase velocity dispersion, are investigated to evaluate their sensitivity to the damage in a medium. The condition of a test specimen, which is a half-space medium made of cement and sand, is evaluated using ultrasonic transducers for different damage cases. The recorded signals are processed using the Fourier and wavelet transforms to determine the surface wave characteristics. A new dispersion index (DI) is introduced, which represents the global correlation between the dispersion of phase velocity and damage level. All features are found to be capable of reflecting the damage in the test medium with different levels of sensitivity. Among the investigated parameters, the proposed dispersion index shows high sensitivity and linear correlation with the damage.

Surface Acoustic Wave-based Characterization of Randomly Distributed Surface Cracks

In this paper we present an approach to characterize surface breaking cracks by analayzing propagation of surface acoustic waves. We generate surface acoustic waves with plane wave fronts using a line-focused pulsed-laser to study scattering of SAWs on surface microcracks. A homogenized behavior of the randomly scattered and interferometrically detected field, i.e. the coherent waveform, is gained by spatially averaging the measurements. The data is studied in the time and frequency domain. The experimental results show the attenuation of the coherent SAWs due to scattering and the strongly distorted shape indicates the presence of dispersion. These parameters can be used to characterize the density and depth distribution of the cracks.

A Fully Non-contact, Air-Coupled Ultrasonic Measurement of Surface Breaking Cracks in Concrete

Previous researchers have shown that the surface wave transmission coefficient across a surface breaking crack in concrete can be used to estimate the depth of the crack. This study is the first to investigate a fully non-contact ultrasonic technique using air-coupled transducers which can reliably measure the surface wave transmission coefficient across a surface breaking crack in concrete. Using this setup, the transmission coefficient of Rayleigh surface waves is measured for three notches with different depths (0.5, 1, and 2 cm). Then, the relationship between the transmission coefficient and the normalized notch depth is experimentally reconstructed and compared with analytical and numerical results. It is demonstrated that the proposed fully non-contact technique is efficient and promising as a field application in civil infrastructure. The advantages of the proposed non-contact technique compared to existing methods are also discussed.

Accurate depth measurement of small surface-breaking cracks using an ultrasonic array post-processing technique

In this paper, the half-skip configuration of the Total Focusing Method (TFM) is used to image and size surface-breaking cracks. The TFM is an ultrasonic array post-processing technique which is used to synthetically focus at every image point in a target region. This paper considers the case of inspecting for cracks which have initiated from the far surface of a parallel-sided sample using an array on the near surface. Typically, only direct ray paths between the array and image points are included in the TFM algorithm and therefore the image obtained for this case consists only of root and tip indications; no specular reflection from the crack faces is captured. The tip indication often has such a poor signal-to-noise ratio that reliable crack depth measurement is challenging. With the Half-Skip TFM, instead of using directly-scattered signals, the image is formed using ultrasonic ray paths corresponding to the ultrasound that has reflected off the back surface and has then undergone specular reflection from the crack face back to the array. The technique is applied to experimental and simulated array data and is shown to measure the depth of small cracks (depth <1mm) with greater reliability than methods which rely on tip diffraction.

Quantitative simulation of ultrasonic time of flight diffraction technique in 2D geometries using Huygens–Fresnel diffraction model: Theory and experimental comparison

This article presents an analytical approach for simulation of ultrasonic diffracted wave signals from cracks in two-dimensional geometries based on a novel Huygens–Fresnel Diffraction Model (HFDM). The model employs the frequency domain far-field displacement expressions derived by Miller and Pursey in 2D for a line source located on the free surface boundary of a semi-infinite elastic medium. At each frequency in the bandwidth of a pulsed excitation, the complex diffracted field is obtained by summation of displacements due to the unblocked virtual sources located in the section containing a vertical crack. The time-domain diffracted wave signal amplitudes in a general isotropic solid are obtained by standard Fast Fourier Transform (FFT) procedures. The wedge based finite aperture transducer refracted beam profiles were modelled by treating the finite dimension transducer as an array of line sources. The proposed model is able to evaluate back-wall signal amplitude and lateral wave signal amplitude, quantitatively. The model predicted range-dependent diffracted amplitudes from the edge of a bottom surface-breaking crack in the isotropic steel specimen were compared with Geometrical Theory of Diffraction (GTD) results. The good agreement confirms the validity of the HFDM method. The simulated ultrasonic time-of-flight diffraction (TOFD) A-scan signals for surface-breaking crack lengths 2mm and 4mm in a 10mm thick aluminium specimen were compared quantitatively with the experimental results. Finally, important applications of HFDM method to the ultrasonic quantitative non-destructive evaluation are discussed.