Effect of roughness on imaging and sizing rough crack-like defects using ultrasonic arrays

All naturally occurring crack-like defects in solid structures are rough to some degree. Based on simulated array data for incident and scattered longitudinal waves from various rough cracks and the total focusing method (TFM) imaging algorithm, the effect of roughness on defect imaging and characterization is examined. The array data are simulated by using a previously validated model based on the Kirchhoff approximation to predict the rough crack scattering matrix. The scattering matrix describes the scattered field for all possible incident and scattering directions. The performance of TFM imaging for detecting rough crack-like defects is investigated by considering the averaged scattering coefficient over specified angular coverage ranges and the maximum scattered amplitude in the final image. It is shown that roughness can be either beneficial or detrimental to the detectability of a crack-like defect, depending on the defect characteristics, such as length, roughness, correlation length, orientation angle, and array inspection configuration. It is also shown that roughness can cause underestimation of the crack length if using an image-based approach for sizing.     

Longitudinal wave scattering from rough crack-like defects

The roughness of crack-like defects affects ultrasonic wave scattering and this, in turn, affects defect detection and characterization. The first part of this paper is concerned with the efficient numerical modeling of scattering from rough cracks, i.e., a finite element local scattering (FELS) model. The scattered field is presented in the form of a scattering matrix, which describes the far-field scattering coefficient for all possible combinations of incident and scattering directions. The scattering matrices for many different realizations of rough cracks are simulated using both a FELS model and a model based on the Kirchhoff approximation. It is shown that the difference between scattering matrices extracted from the Kirchhoff model and the FELS model is less than 8%, for rough cracks with a standard deviation less than 0.3 wavelengths and a correlation length longer than 0.5 wavelengths, at incident and scattering angles ranging from -80° to 80° relative to the normal direction of the mean surface. Because the Kirchhoff model is significantly more efficient than the FELS model, it is used for subsequent simulations in which many realizations of rough cracks are studied to gain insight into the statistical nature of the scattering process. In line with previous work, a distinction is made between the coherent and diffuse contributions to the overall scattered field, in which the former represents the ensemble average over multiple surface realizations. The coherent and diffuse contributions of scattered field from various types of rough cracks are simulated. It is shown that surface roughness directly affects the coherent contribution to scattering behavior, whereas the diffuse contribution is affected by both surface roughness and correlation length, especially for rougher cracks.     

Fatigue behavior prediction of welded joints by using an integrated fracture mechanics approach

Current fracture mechanics methods for fatigue assessment of welded joints are based on long crack behavior. The present work introduces a method to predict the fatigue strength of welded joints by means of an integrated fracture mechanics approach (IFMA) that takes into account the fatigue behavior of short cracks. This methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, function of crack length. Firstly, the proposed fracture mechanic method is introduced and compared with the traditional fracture mechanic approach, used mainly for fitness for purpose assessment of welded joints with cracks or other crack-like defects. Then, the method is used for several theoretical and parametric applications to show its ability to predict the influence of different mechanical, geometrical and microstructural parameters in the definition of the fatigue resistance of welded joints. The influence of plate thickness, initial crack length and reinforcement angle on fatigue strength of butt-welded joints has been analysed and results show good agreement with experimental trends. Finally, the method is applied to predict and analyze the fatigue behavior of butt welded and non-load-carrying transverse fillet welded joints, and estimated and experimental results are analysed and compared.     

Sizing Cracks in Power Plant Components Using Array Based Ultrasonic Techniques

New techniques developed using the array transducer, to image and size the bottom-surface and near-through-wall crack-like defects in thick carbon steel components are discussed. Three studies are reported here including (a) Optimal beam steering angle for focused and unfocused inspection using phased array method for bottom-surface crack sizing. (b) A front wall correction algorithm for sizing of near-through-wall crack-like defects. (c) A small aperture technique for sizing of near-through-wall crack-like defects. A Finite Difference Time Domain (FDTD) based simulation was used to study and verify the experimental observations. The application of time domain scheme relative arrival time technique (RATT), to measure the size of the near-through wall crack-like defects for the leak before break (LBB) criterion, was also investigated and found to be insufficient. A conventional SAFT algorithm was used for improving the sizing using the small aperture technique.     

Defect Detection of Industrial Radiography Images of Ammonia Pipes by a Sparse Coding Model

Pipeline transportation systems for liquid anhydrous ammonia require periodic inspections for pipe defects. Defects such as crack-like flaws, including those due to stress corrosion and fatigue crack growth and fatigue of welded joints, can be tested by in situ radiography testing (RT). It is required that the detection technique should reveal the defect region without any significant distortion. Imaging noise such as that due to radiation scattering reduces contrast over the whole or part of the image, causing a reduction in image quality. Various image processing methods can be utilized to improve the outcome of RT of pipes. The sparse coding model is a powerful contrast improvement algorithm, but it tends to eliminate imaging details, passing them off as noise. We present a novel implementation of the sparse coding model based on a probabilistic interpretation to improve defect detection in ammonia pipelines radiography images. The performance of this procedure was evaluated in radiographic images of defect and corrosion regions for different pipe types. For ammonia pipes, the technique was found to offer a significant improvement in defect detection while preserving imaging details.     

Specific features of the space distribution of the signal of an eddy-current converter caused by cracks of different lengths

We calculate and analyze the space distribution of the signal of an eddy current converter for crack-like defects of different lengths. We have established the differences in eddy current signals between short and long cracks. We also present recommendations for the optimal selection of the parameters of an eddy current converter depending on the crack length. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 4, pp. 121–124, July–August, 2007.     

Further development of flaw plane identification with application to crack sizing

The flaw plane identification technique (FPIT) has been further extended to the pitch-catch and tandem scan modes with application to the sizing of penny-like smooth cracks. The influence of statistical noise and converted waves on the crack surface is discussed in detail, and methods to remove these effects are described. Sizing accuracy in the three scan modes (pulse-echo, pitch-catch and tandem) is analysed and compared in the longitudinal and transverse directions. This technique proves to be effective for sizing void- and crack-like artificial defects and is easy to carry out automatically.     

兰姆波在裂纹处的模态转换及散射特性研究

该文采用散射矩阵表征兰姆波在缺陷处的模态转换和散射特性, 研究了S₀模态兰姆波与亚波长级槽形裂纹的交互作用. 由板中导波模态位移场的特性, 提出了只存在零阶模态时的模态分离方法. 求解了S₀模态在不同尺寸裂纹处的多模态散射矩阵. 结果表明, S₀模态入射到裂纹时, 散射波动场周向分布不均, 与入射角度相关;散射特征的强弱由裂纹长度决定, 长度越短, 散射特征越明显;各散射模态的能量分配由裂纹的深度决定, 随着裂纹深度增加, 散射的S₀和SH₀模态能量增加, 而A₀模态先增加后急剧减小.     

基于SINTAP-FAD方法的含裂纹缺陷钢结构构件安全性评定研究

在施工和运营过程中,钢结构构件上出现的裂纹缺陷可能会扩展而导致其发生断裂破坏,因此对含裂纹缺陷钢结构构件的安全性进行评估具有重要意义。首先,引入断裂力学K_IC断裂判据和失效评定图（FAD）概念,并介绍了结构整体性评估程序（SINTAP）的原理;给出了裂纹缺陷评定所需材料参数的确定方法,以及含裂纹缺陷构件的断裂力学参数和塑性屈服荷载的计算方法;然后,提出了基于SINTAP-FAD的含裂纹缺陷钢结构构件的安全性评估流程。最后,以华南和华东两个电厂锅炉钢结构大板梁的焊接缺陷为例,进行了基于K_IC断裂判据和SINTAP-FAD的裂纹缺陷安全性评定研究。结果表明,基于SINTAP-FAD的方法为含裂纹缺陷工作的钢结构构件的安全性评估提供了一种行之有效的方法,考虑多裂纹耦合作用的安全性评定方法有待进一步研究。     

Automatic Crack Detection and Characterization During Ultrasonic Inspection

The creation of a non-destructive technique that enables the automatic detection of defects is desirable, and TOFD (Time-Of-Flight Diffraction) technique is gaining rapid prominence due to its high accuracy in detecting, positioning and sizing flaws in steel structures. In this type of imaging, cracks are characterized using sets of hyperbolas, where summit positions correspond to crack tip positions. However, ultrasonic diffracted signals are often too low and difficult to distinguish from noise, and when large structures are inspected, the quantity of data can be extremely large, with the area of interest being very small in comparison to the image size. This paper describes a method that avoids the image formation, replacing it with a sparse matrix (as there is no reason to store and operate on an excessive number of zeros), and automates crack detection by analyzing the curve formed by the sparse matrix elements. The sparse matrix is formed using Split-Spectrum Processing, which enhances the signal-to-noise ratio. The Randomized Hough transform is then applied on the sparse matrix elements to detect the hyperbolas that characterize the crack defects.     

Modified Kitagawa–Takahashi diagram accounting for finite notch depths

The Kitagawa–Takahashi diagram in its commonly used form allows to predict, for cracks of given length and stress range, the allowable stress range for infinite life. However, caution is advised if a crack emanates not directly from the plane surface but from a sharp, crack-like notch instead. In this contribution, it is shown that taking the crack length equal to the total flaw depth (sum of notch depth and crack length) gives non-conservative results. Based on a simple mechanical model, a 3-dimensional Kitagawa–Takahashi diagram considering the build-up of crack growth resistance as well as the influence of the notch depth is developed. Comparison of model predictions and experimental results shows good agreement.     

Endurance diagram

For the design of components that do not contain any defects and that are subjected to both alternating and mean stresses, a Soderberg diagram is normally used in a ‘safe life’ approach. However, structures may contain defects or cracks where a Soderberg diagram will not be sufficient. In this paper the development of an endurance diagram is presented for components that may contain crack-like defects left in the structure during the fabrication process. This diagram is based on the threshold stress intensity factor in fatigue crack growth, ΔK_th, and the critical stress intensity factor, K_c.     

Simulation of fatigue crack growth in components with random defects

The paper presents a probabilistic method for the simulation of fatigue crack growth from crack-like defects in the combined operating and residual stress fields of an arbitrary component. The component geometry and stress distribution are taken from a standard finite element stress analysis. Number, size and location of crack-like defects are ‘drawn’ from probability distributions. The presented fatigue assessment methodology has been implemented in a newly developed finite-element post-processor, P • FAT, and is useful for the reliability assessment of fatigue critical components. General features of the finite element post-processor have been presented. Important features, such as (i) the determination of the life-controlling defect, (ii) growth of short and long cracks, (iii) fatigue strength and fatigue life distribution and (iv) probability of component fatigue failure, have been treated and discussed. Short and long crack growth measurements have been presented and used for verification of the crack growth model presented.     

Matrix fatigue cracking in fiber composites

A model is developed for fatigue growth of matrix cracks in metals reinforced with aligned continuous elastic fibers. The mechanics of elastic cracks bridged by frictionally constrained fibers is used to develop the model, which provides estimates of the tip value of the stress intensity factor amplitude, ΔK_TIP. It is found that when the applied load amplitude is held fixed during fatigue crack growth, ΔK_TIP, and thus the rate of growth approach an asymptotic value independent of crack length. The residual strength after fatigue crack growth is also discussed. In some cases, the residual strength is unaffected by prior fatigue growth. But, in another regime, the matrix crack length allows fibers to begin breaking before the matrix crack grows. The strength is then inversely proportional to the square root of fatigue crack length.     

Characterization of Surface-Breaking Cracks Using One Tangential Component of Magnetic Leakage Field Measurements

We propose a procedure for full characterization of rectangular surface-breaking cracks based on measurements of only one tangential component of the magnetic field with the magnetic flux leakage (MFL) technique. The parameters of interest include orientation, length, and depth of the cracks. We assume that the length and the depth of the investigated cracks are much larger than the crack width, so that the variation of the MFL response with respect to the width is negligible. Our procedure employs fast direct methods that provide reliable estimation of the crack parameters in three separate consecutive steps. We propose denoising and correction techniques as well. We confirmed the accuracy of the methods by simulations based on the finite-element method (FEM) as well as by experimental MFL observations. A procedure is proposed for full characterization of rectangular surface breaking cracks based on measurements of only one tangential component of the magnetic field with the magnetic flux leakage (MFL) technique. The parameters of interest include orientation, length and depth of the cracks. We assume that the length and the depth of the investigated cracks are much larger than the crack width such that the variation of the MFL response with respect to the width is negligible. The proposed procedure employs fast direct methods which provide reliable estimation of the crack parameters in three separate consecutive steps. De-noising and correction techniques are proposed as well. The accuracy of the proposed estimation methods is examined via simulations based on the finite element method (FEM) as well as experimental MFL data.     

Structural damage identification using transfer matrix with lumped crack properties

A novel damage detection scheme is developed for detecting multiple cracks in beams, based on a transfer matrix (TM) approach. Lumped Crack TM of a beam element with multiple cracks is derived based on lumped crack properties. A cracked beam element is assumed as two intact beam elements connected with a hinge or torsional spring. The crack is modelled as an element of zero length and mass, but with elastic properties. Lumped crack approach is simpler for multiple cracks than the possible alternative methods. The state vector at a node includes displacements, forces and moments at that node; when it is multiplied with TM the state vector at the adjacent node can be obtained. The crack identification strategy used here, involves measuring the initial state vector at a node, in the zone of interest. The displacements at the adjacent nodes are measured and these are predicted using TM. Using an optimization algorithm the unknown crack parameters in the TM are solved by minimizing the deviation between measured and predicted displacements. The method is verified using several numerical models as well as experiments with cracked beams. The TM is shown to be suitable for local identification and also fast and accurate compare to other methods.     

Statistical analysis and reliability prediction with short fatigue crack data

This paper proposes a probability model to describe the growth of short fatigue cracks. The model defines the length of each crack in a specimen as a random quantity, which is a function of randomly varying local properties of the material microstructure. Once the model has been described, the paper addresses two questions: first, statistical inference, i.e. the fitting of the model parameters to data on crack lengths; and secondly, predicting the future behaviour of observed cracks or cracks in a new specimen. By defining failure of a specimen to be the time at which the largest crack exceeds a certain length, the solution to the prediction problem can be used to calculate a probability that the specimen has failed at any future time. The probability model for crack lengths is called a population model, and the statistical inference uses the ideas of Bayesian statistics. Both these concepts are described. With a population model, the solution to statistical inference and prediction requires quite complicated Monte Carlo simulation techniques, which are also described.     

基于部分散射系数矩阵的缺陷识别方法研究

为了在相控阵超声成像检测时实现缺陷的定性分析,采用部分散射系数矩阵表征传感器采集到的部分散射场信息,并利用部分散射系数矩阵图进行缺陷识别。以相控阵超声全阵列采集获取到的全矩阵数据为基础,结合声波传播过程的衰减性,采用逆向补偿的方法将全矩阵数据补偿到以缺陷为中心的同一圆周上,以此获取缺陷的部分散射信息,并采用部分散射系数矩阵来表征。通过分析缺陷的部分散射系数矩阵图特征,对缺陷进行定性分析和判别。采用此方法对两种典型的缺陷进行仿真实验,结果显示,部分散射系数矩阵图不仅对圆孔和裂纹两类不同缺陷表现出差异性,对同类型不同尺寸和不同角度的裂纹也有明显的不同之处,表明利用该方法可实现缺陷的定性识别。     

Statistical comparison of two fracture criteria

Two fracture criteria are compared within the stochastic model of brittle fracture of a component with the random ensemble of crack-like defects.

The component comprises a random array of internal defects, represented by equivalent cracks. Under assumption of stochastic independence of defects and Poisson distribution of defects population the probability of fracture is determined. This probability is dependent on local fracture criterion applied to an arbitrary crack. The maximum normal stress and energy release rate criteria were applied to check as to whether they may yield significantly different prediction of above probability.

Computer calculations show that with the increase of defects density the difference in predictions of above two criteria became unsignificant and for both limiting fracture-stress curves converges to maximum stress theory. For low densities energy release rate criterion yields more conservative predictions in almost all range of λ. The limiting fracture-stress curves deviate in this case from both the maximum stress and von Mises theories.