Simplified NDE of multiple cracks by means of the potential drop technique

A method is developed for the simplified evaluation of multiple cracks, based on the d.c. potential drop technique. A subregion containing cracks is considered. The distribution of the potential drop for the cracks except for end cracks in the subregion is measured and calculated numerically by assuming a value for the depths of the end cracks and by assuming any value for the depths of other cracks. Modifying the depths of the other cracks is repeated until the difference between measured and calculated potential drops is minimized, where the depths of the end cracks are fixed. Evaluation of the centre crack in the subregion is made for different assumed depths of the end cracks. To make the evaluation independent of the depths of the end cracks, a subregion is extended to increase successively the number of cracks considered. Multiple cracks are sized well enough by this method.     

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.     

Crack interaction in NDE of multiple cracks by means of potential drop technique

A method is developed to evaluate crack interaction in NDE of multiple cracks by d.c. potential drop technique. A subregion containing m(=1,3,5,…) cracks are considered. The potential drop of the middle crack for each subregion are considered for two successive cases (m and m+2) and compared. The ratio of these two potential drops shows the effect of the first and last cracks of the subregion on the potential drop of the middle crack. Comparison of the potential drop of the middle crack with the potential drop of a single crack with the same depth and the same positions of potential probes shows the crack interaction. The method is based on the FEM. Experimental data verify the numerical results.     

Experiment and simulation study of 3D magnetic field sensing for magnetic flux leakage defect characterisation

Magnetic flux leakage (MFL) testing is widely used to detect and characterise defects in pipelines, rail tracks and other structures. The measurement of the two field components perpendicular to the test surface and parallel to the applied field in MFL systems is well established. However, it is rarely effective when the shapes of the specimens and defects with respect to the applied field are arbitrary. In order to overcome the pitfalls of traditional MFL measurement, measurement of the three-dimensional (3D) magnetic field is proposed. The study is undertaken using extensive finite element analysis (FEA) focussing on the 3D distribution of magnetic fields for defect characterisation and employing a high sensitivity 3-axis magnetic field sensor in experimental study. Several MFL tests were undertaken on steel samples, including a section of rail track. The experimental and FEA test results show that data from not only the x- and z-axes but also y-axis can give comprehensive positional information about defects in terms of shape and orientation, being especially advantageous where the defect is aligned close to parallel to the applied field. The work concludes that 3D magnetic field sensing could be used to improve the defect characterisation capabilities of existing MFL systems, especially where defects have irregular geometries.     

NDE of multiple cracks on the surface of materials by means of the potential drop technique

A method is developed to evaluate nondestructively multiple cracks on the surface of materials. The method is based on the d.c. potential drop technique. Multiple two-dimensional cracks of unknown depth are inspected, where the distance between the cracks is known. First the distribution of the potential drop between both sides of each crack is measured on the cracked surface. Next the distribution of the potential drop is calculated numerically by assuming the depth of every crack. Then by comparing the measured and calculated potential drop, a correction factor is obtained for the assumed depth of the respective cracks. Modifying the crack depth by the correction factor is repeated until the difference between the measured and calculated distributions of the potential drop is minimized. It is shown that multiple cracks are sized accurately by the present method.     

Evaluation of the shape and size of 3D cracks using microwaves

A method to evaluate the shape and size of a small 3D crack by microwaves was demonstrated. By considering the interference phenomenon occurring in the case of 3D cracks, a parameter reflecting microwave interference effect, which is a function of the position of the sensor in the direction of the crack length, was introduced into the usual dual frequency evaluation-equation. From the modified dual frequency equation, a new component named interference waveform, which is based on the amplitude of the reflection coefficient measured at two different frequencies, was obtained. On the other hand, based on the interference model, a corresponding interference waveform, which can be calculated from the assumed shape and size of a 3D crack, was also introduced. By comparing these two interference waveforms, the evaluation of the shape and size of the 3D crack were carried out. Four 3D fatigue cracks having different shape and size were treated. The shape and size of all the cracks were estimated well by using the proposed method.     

An alternative method for crack interaction in NDE of multiple cracks by means of potential drop technique

A procedure of calculating the crack interaction in NDE of multiple cracks by d.c. potential drop technique is newly developed. This method can be used when the distances between cracks are small or large. The equations obtained evaluate crack interaction for any case regardless of the distances between the cracks. A subregion containing m (=1,3,5,…,) cracks are considered. The potential drop of the middle crack for each subregion are considered for two successive cases (m and m+2). The ratio of these two potential drops shows the effect of the first and last cracks of the subregion on the potential drop of the middle crack. The crack interaction is shown by the ratio of the potential drop of the middle crack to the potential drop of a single crack with the same depth and the same positions of potential probes. The equations showing the crack interaction are obtained based on the numerical analysis followed by an experimental validation.     

Interfacial defects detection in plasma-sprayed ceramic coating components using two stimulated infrared thermography techniques

Two Lock-in InfraRed Thermography (LIRT) techniques were developed and applied to detect interfacial defects in a ceramic coated steel plate deposited by plasma spraying technology. Stimulations were performed either by electromagnetic waves (lamps) or by high power ultrasonic waves (sonotrode). The first one is based on remote optical heating of the area of interest; while the second technique is based on the mechanical loss angle effect, occurring locally and for particular defects such as cracks and delaminations. The potential of these two techniques was evaluated against artificial (calibrated holes) and real (disbonding) interfacial defects. The results show success and also some limitations of both LIRT techniques in function of sizes and depths of ceramic coating interfacial defects.     

基于断裂参量K因子的焊接接头等承载设计系统

以高效准确的获得含不同缺陷接头满足等承载时的形状参数为目标,结合断裂力学理论及专家系统思想,开发了基于断裂参量K因子的焊接接头等承载设计系统.结果表明,该系统不但可以高效准确的给出焊缝和母材都含缺陷以及焊缝含缺陷母材无缺陷的接头满足等承载时的形状参数,显著提高了等承载接头的设计效率及准确性,推广了等承载接头设计思想.而且能够计算含缺陷接头承受静载时的应力强度因子、J积分、裂纹尖端张开位移、临界载荷和临界裂纹尺寸以及含缺陷接头承受疲劳载荷时的许用载荷幅值和裂纹扩展速率等相关参量,为准确指导含缺陷接头形状设计及安全性评估提供了基础.     

Numerical Modeling of Ti Deformation for the Development of a Titanium and Stainless Steel Transition Joint

Finite element analysis (FEA) was used to model the joining of titanium grade 2 (Ti) to AISI 321 stainless steel (SS) transition joint of lap configuration with grooves at the interface on SS side. The hot forming of Ti for filling the grooves without defects was simulated. FEA involving large plastic flow with sticking friction condition was initially validated using compression test on cylindrical specimen at 900 °C. The barreled shape and a no-deformation zone in the sample predicted by FEA matched with those of the compression experiments. For the joining process, FEA computed the distribution of strain and hydrostatic stress in Ti and the minimum ram load required for a defect-free joint. The hot forming parameters for Ti to fill the grooves without defects and any geometrical distortion of the die were found to be 0.001 s^?1 at 900 °C. Using these conditions a defect-free Ti-SS joint was experimentally produced.     

基于三维霍尔传感器的钢轨缺陷漏磁检测系统

为检测钢轨顶面的剥离、裂纹等缺陷,设计基于三维霍尔传感器的钢轨缺陷漏磁检测系统.采用钕铁硼环形永磁体对钢轨检测区域进行垂直于钢轨方向的励磁,并将检测车的车轮轮轴同时作为励磁回路的磁扼,由ARM单片机采集三维霍尔传感器检测的缺陷漏磁场数据.在钢轨顶面人工制作一系列横向矩形凹坑缺陷样本,检测结果表明:钢轨缺陷三维漏磁检测系...     

Fast crack profile reconstruction using pulsed eddy current signals

This paper reports fast crack profile reconstruction methods using transient slices and spectral components of pulsed eddy current (PEC) signals after a review of the state-of-the-art and current challenges. These methods provide initial approximate profiles for crack shape reconstruction, and have potential to reduce the computing time. Experimental samples, results, and reconstructions have been presented and discussed. Comparative studies of different profile reconstructions using different PEC signals such as amplitude, phase, real and imaginary values of spectral components, transient slices, have also been conducted using reconstruction Mean Square Errors (MSE). The results show that the profiles estimation using the imaginary values of spectral components is more stable and accurate than the others.     

泡沫金属直流电位法检测技术

为验证直流电位法检测泡沫金属缺陷孔洞及大小的有效性,对实物泡沫金属进行了直流电位法无损检测试验。设计制作了具有不同大小缺陷的五个试件,对各试件进行了恒流加载和表面电位分布测量,并对所测结果进行了相应的差分及滤波处理,以提高信噪比。同时,为确定有效的试验条件,运用有限元分析程序对电位检测信号进行了数值模拟。试验和数值结果表明,对泡沫金属可以利用直流电位方法检出缺陷,且缺陷大小和检测电位信号的大小有一定对应关系。这为泡沫金属缺陷的定量无损检测打下了基础。     

Modelling and experimental measurements of idealised and light-moderate RCF cracks in rails using an ACFM sensor

Alternating current field measurement (ACFM) sensors can be used to detect surface breaking defects in metal components. In rails rolling contact fatigue (RCF) cracks form due to the wheel–rail contact stresses. These cracks are surface breaking and can have complex shapes. A COMSOL model has been developed for a commercial ACFM system and RCF cracks in rails. In this paper model results are compared to experimental measurements using an ACFM pencil probe for calibration defects machined into a rail and real RCF defects (light and moderate categories; <20 mm surface length) in a rail removed from service. X-ray tomography has been used to determine the size and morphology of the real RCF cracks for input into the model. It has been shown that the model can be used to determine the change in normalised Bx signal due to the presence of calibration defects machined into a new rail. The model has also been used to compare the experimental data for the real RCF cracks to the reconstructed model cracks and a semi-elliptical approximation to their shape.     

Stochastic modelling of corrosion damage propagation in active sites from field inspection data

A stochastic model for prediction of corrosion damage evolution in active sites, applicable under professional practice conditions is presented here. The damage of a material and its evolution are determined from the damage state at a given time instant and the rate of damage occurrence. To this end, probability density function of the corrosion damage depths of the system is estimated and four models to calculate corrosion damage velocities at localized defects are shown. Their application depends on the amount of inspection reports available. This work takes into account two settings: the first considers that the system has only one inspection report and the second assumes that there are two inspection reports; this latter setting has two variations, the first, when the same defects can be identified at both inspections, and the second, when they are not identifiable. Furthermore, the work introduces a Bayesian model that allows updating corrosion damage velocity on the basis of new measurements found in successive inspection reports. The stochastic model is exemplified by inspection data from a real pipeline system. Its analysis takes into account technical specifications of the system, measured depths of corrosion defects and the number of defects. Additionally, it considers measurement errors during inspection and the variability of corrosion phenomenon under field conditions. Model robustness lies in the fact that corrosion damage estimates are based on measurements reported during inspections. It implicitly considers multiple factors, such as aggressive chemical environment, microstructure composition, operating conditions (temperature, fluid velocity, etc) intervening in the corrosion process, as well as their correlations and variability.     

The role of probe shape on the initiation of metal plasticity in nanoindentation

The dislocation nucleation stress of crystalline materials is frequently estimated from the maximum shear stress, assuming Hertzian contact up to the first “pop-in” event, which is a sudden displacement burst during load-controlled nanoindentation. However, an irregular indenter tip shape will significantly change the stress distribution, and therefore the maximum shear stress from a Hertzian estimation. Here, we assess possible errors and pitfalls of the Hertzian estimation of initial plastic yield at the nanoscale. The near-apex shapes of two Berkovich indenters, one sharp and one worn, were measured by atomic force microscopy and directly input into finite element analysis (FEA) models for “virtual” nanoindentation experiments on single-crystal tungsten. Experiments were also carried out with those indenters. Excellent agreement is found between experimental and FEA force–displacement relationships, but the discrepancies between Hertzian and FEA estimates of the shear stresses are over 25% for the sharp indenter and over 50% for the blunt indenter. This demonstrates that small irregularities in the shape of indenter tips can cause significant deviations from the Hertzian estimation of dislocation nucleation stress.     

Variation of the stress dependent magnetic flux leakage signal with defect depth and flux density

The effects of uniaxial stress on the normal (radial) component of the magnetic flux leakage (MFL) signal induced by blind-hole defects for depths of 25%, 50% and 75% of the thickness of the pipe wall were investigated with a pipe wall flux density of 1.24 T. These three defects were on the same surface as the magnetizer and sensor for the MFL signal (near side). A fourth 50% defect was on the pipe wall surface opposite the sensor (far side). Changes of as much as 47% in the MFL signal due to stresses of up to 300 MPa were observed. Increased changes in the stress dependent MFL signal were observed with increasing defect depth. Comparison of the near side and far side 50% defects indicated similar changes in the MFL_pp signal as a function of stress, although the shape of the MFL signals was qualitatively different. The stress dependent MFL signal was also investigated for the near side 50% defect for pipe wall flux densities between 0.65 T and 1.24 T. A linear increase in the effects of stress on the MFL signal with increasing flux density was observed. Results demonstrated that the variation of the MFL signal with stress is primarily a bulk stress effect, although the effect of defect-induced stress concentrations upon the various MFL signals investigated could also be observed.     

Numerical Study and Experimental Validation of Effect of Varying Fiber Crack Density on Stiffness Reduction in CFRP Composites

Representative volume element (RVE) has commonly been used to predict the stiffness of undamaged composite materials using finite element analysis (FEA). However, never has been an independently measured true microstructural damage quantity used in FEA to predict composite stiffness. Hence, in this work, measured fiber crack density in unidirectional fiber composite (generated using controlled fatigue loading) was used to predict reduction in stiffness using a RVE. It was found that the stiffness changes with change in depth of the volume element along the fiber direction and asymptotically reaches a constant value beyond a critical length called representative depth. It was argued that this representative depth should be more than the minimum of two characteristic length scales, twice of ineffective length and average length of broken fibers. Effective stiffness obtained from FEA of the optimum-sized RVE was in excellent agreement with the experimental results for given microstructural damage state.     

Electrochemical measurements of cathodic protection for reinforced concrete piles in a marine environment using embedded corrosion monitoring sensors

This study developed a sensor to monitor the corrosion of reinforced concrete structures. Concrete pile specimens with embedded sensors were used to obtain data on corrosion and cathodic protection for bridge columns in a real marine environment. Corrosion potential, cathodic protection current density, concrete resistivity, and the degree of depolarization potential were measured with the embedded sensors in concrete pile specimens. The cathodic protection (CP) state was accurately monitored by sensors installed in underwater, tidal, splash, and atmospheric zones. The protection potential measurements confirmed that the CP by Zn-mesh sacrificial anode was fairly effective in the marine pile environment. The protection current densities in the tidal, splash zones were 2–3 times higher than those in underwater and atmospheric zones. The concrete resistivity in the tidal and splash zones was decreased through the installation of both mortar-embedded Zn-mesh (sacrificial anode) and outside an FRP jacket (cover). Considering the CP, the cathodic prevention was more effective than cathodic protection.     

In-situ measurement of three-dimensional deformation behaviour of sheet and tools during stamping using borescope

An in-situ measurement approach for three-dimensional deformation of sheet and tools during stamping using borescopes was developed. The borescope, consisting of a small CCD camera with a flexible cable, was connected to personal computer and placed inside a small space between the tool cavities to measure the deforming behaviour of the sheet. The deformation behaviour of the sheet and punch, the real springback and wrinkling in shrink flanging were measured. The deformed shape of the sheet measured by the borescope was in good agreement with that measured by the laser displacement sensor. In addition, three-dimensional deforming behaviour was measured by a stereoscopic method using two borescopes. It was found that the borescopes are effective in in-situ measurement of three-dimensional deformation behaviour of the sheet and tools.