Threshold Fluence Measurement for Laser Liftoff of InP Thin Films by Selective Absorption

We explore conditions for achieving laser liftoff in epitaxially grown heterojunctions, in which single crystal thin films can be separated from their growth substrates using a selectively absorbing buried intermediate layer. Because this highly non‐linear process is subject to a variety of process instabilities, it is essential to accurately characterize the parameters resulting in liftoff. Here, we present an InP/InGaAs/InP heterojunction as a model system for such characterization. We show separation of InP thin films from single crystal InP growth substrates, wherein a ≈10 ns, Nd:YAG laser pulse selectively heats a coherently strained, buried InGaAs layer. We develop a technique to measure liftoff threshold fluences within an inhomogeneous laser spatial profile, and apply this technique to determine threshold fluences of the order 0.5 J cm^?2 for our specimens. We find that the fluence at the InGaAs layer is limited by non‐linear absorption and InP surface damage at high powers, and measure the energy transmission in an InP substrate from 0 to 8 J cm^?2. Characterization of the ejected thin films shows crack‐free, single crystal InP. Finally, we present evidence that the hot InGaAs initiates a liquid phase front that travels into the InP substrate during liftoff.

     

Eddy Current Signal Deconvolution Technique for the Improvement of Steam Generator Tubing Burst Pressure Predictions

Eddy current techniques are extremely sensitive to the presence of axial cracks in nuclear power plant steam generator tube walls, but they are equally sensitive to the presence of dents, fretting, support structures, corrosion products, and other artifacts. Eddy current signal interpretation is further complicated by cracking geometries more complex than a single axial crack. Although there has been limited success in classifying and sizing defects through artificial neural networks, the ability to predict tubing integrity has, so far, eluded modelers. In large part, this lack of success stems from an inability to distinguish crack signals from those arising from artifacts. We present here a new signal processing technique that deconvolves raw eddy current voltage signals into separate signal contributions from different sources, which allows signals associated with a dominant crack to be identified. The signal deconvolution technique, combined with artificial neural network modeling, significantly improves the prediction of tube burst pressure from bobbin-coil eddy current measurements of steam generator tubing.     

Automatic sizing functions for unstructured surface mesh generation

Accurate sizing functions are crucial for efficient generation of high‐quality meshes, but to define the sizing function is often the bottleneck in complicated mesh generation tasks because of the tedious user interaction involved. We present a novel algorithm to automatically create high‐quality sizing functions for surface mesh generation. First, the tessellation of a Computer Aided Design (CAD) model is taken as the background mesh, in which an initial sizing function is defined by considering geometrical factors and user‐specified parameters. Then, a convex nonlinear programming problem is formulated and solved efficiently to obtain a smoothed sizing function that corresponds to a mesh satisfying necessary gradient constraint conditions and containing a significantly reduced element number. Finally, this sizing function is applied in an advancing front mesher. With the aid of a walk‐through algorithm, an efficient sizing‐value query scheme is developed. Meshing experiments of some very complicated geometry models are presented to demonstrate that the proposed sizing‐function approach enables accurate and fully automatic surface mesh generation. Copyright © 2016 John Wiley & Sons, Ltd.     

Sizing of multiple cracks using magnetic flux leakage measurements

This study presents an approach to estimate the characteristics of multiple narrow-opening cracks from magnetic flux leakage (MFL) signals. The number, locations, orientations and lengths of the cracks are the objective of the inversion process. The proposed procedure provides a reliable estimation of crack parameters in two separate consecutive steps. In the first step, the Canny edge detection algorithm is used to estimate the number, locations, orientations and lengths of the cracks. Then, an inversion procedure based on space mapping is used in order to estimate the crack depths efficiently. The accuracy of the proposed algorithm is examined via simulations based on the finite element method as well as real experimental MFL data.     

Ultrasonic signal processing using Born inversion

A pulse-echo (1D) Born inversion technique, which utilizes digital signal processing on ultrasonic back-scattered signals to size defects, is briefly outlined together with its application to the measurement of spheroidal defects in metals. The theoretical derivation of the algorithm is valid for application to weak scatterers; however, it is shown experimentally that this technique can give accurate radius predictions for strong scatterers such as voids. This apparent anomaly is examined using theoretically generated exact scattering data. A methodology for testing a specific transducer's suitability for sizing a specific defect is reported and demonstrated.     

Scattering of an induced surface electromagnetic field by fatiguecracks in ferromagnetic metals

In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented     

Improved projection algorithm to invert forward scattered light for particle sizing

An improved version of the nonlinear iterative projection algorithm is proposed and applied to the inversion of the light-scattering data for particle sizing. Compared with the original projection algorithm, the improved one is much more stable, reliable, and accurate with respect to random noise. The particle-size distributions retrieved with the improved projection algorithm are independent of the different starting solutions. The criterion for stopping iteration is fairly clearly determined and is not a user-dependent parameter of the inversion algorithm. Numerical computer simulations and experimental results for the mixture of monodispersed size distributions are presented to allow both the inversion procedure validity and the instrument performance to be evaluated.     

THE MEASUREMENT OF SURFACE BREAKING CRACKS BY THE ELECTRICAL SYSTEMS ACPD/ACFM

Abstract— The successful implementation of a fracture mechanics approach to the in-service structural integrity assessment of welded and machined components requires an accurate and reliable NDT method for detecting and sizing surface breaking cracks. This paper discusses and compares the crack depth sizing abilities of two theoretically-based NDT techniques: alternating current potential difference (ACPD) and alternating current field measurement (ACFM). Surface fatigue cracks of high and low aspect ratio are used in the investigation. It is demonstrated that ACPD is capable of accurately sizing long shallow cracks and that ACFM provides accurate crack depth predictions for small deep cracks.     

改进的遗传算法在粒径测量中的应用

在光全散射法粒径测量中,基于改进的遗传算法反演颗粒系的粒径分布.在独立模式下,粒径反演为求解约束优化问题,将改进的遗传算法与模拟退火算法相结合,克服了罚函数遗传算法反演时罚系数难以确定以致极易产生不可行收敛解的不足.在非独立模式下,采用改进的遗传算法能够在3个波长下较准确地反演粒径分布.在光全散射法中采用改进的遗传算法反演粒径分布是可行的,反演结果稳定可靠,避免了基本遗传算法容易过早收敛而使反演结果陷入局部解的缺陷.     

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.     

Optimizing the Inversion Protocol to Determine the Geometry of Vertical Cracks from Lock-in Vibrothermography

We use low intensity lock-in vibrothermography to characterize vertical cracks of any shape. The inverse problem, consisting in finding the geometry and location of the crack from vibrothermography data is ill-posed, which makes it necessary to stabilize the inversion algorithm. In previous works, we developed a stabilized inversion procedure that was able to characterize vertical cracks of rectangular shape from lock-in vibrothermography data. In this work, we extend the method to characterize vertical cracks of arbitrary shape. For this purpose, first we improve the stabilization algorithm to obtain accurate crack shape reconstructions. Then, we optimize the protocol followed to handle data before entering the algorithm. To validate the new procedure we invert both, synthetic data with added uniform noise and experimental data on samples containing calibrated artificial cracks of different shapes. The results show that very accurate reconstructions are obtained for shallow cracks. As the depth of the crack increases, the depth is precisely obtained although the shape of the crack is rounded and its area is slightly overestimated.     

ECT Inversion Using a Knowledge-Based Forward Solver

In this paper, a new approach was proposed for crack reconstruction based on ECT technique and optimization theory. A newly developed fast forward solver was applied for predicting the impedance signals during the inversion procedure. Comparing with the conventional approaches, the scanning signal due to an arbitrary shaped crack can be calculated in a significantly reduced CPU time but with higher accuracy. The conjugate gradient algorithm was employed in the optimization of the crack parameters, with the gradient calculated numerically from the electric field in the crack region. The scheme was implemented for the shape reconstruction of EDM cracks from the measured impedance data having in mind possible applications in the inspection of steam generator tubing. Several cracks embedded in conducting tube or plate were reconstructed from the observed impedance data by concerning the benchmark models proposed by T. Takagiet al. Good reconstruction results were obtained within a short CPU time. Consequently, the validity of the new approach was verified.     

Acoustic emission detection of fatigue cracks in wind turbine blades based on blind deconvolution separation

The occurrence and expansion of fatigue cracks in large wind turbine blades may lead to catastrophic blade failure. Each fatigue phase of a material has been associated with a typical set of acoustic emission (AE) signal frequency components, providing a logical base for establishing a clear connection between AE signals and the fatigue condition of a material. The relevance of efforts to relate recorded AE signals to a material's mechanical behaviour relies heavily on accurate AE signal processing. The main objective of the present study is to establish a direct correlation between the fatigue condition of a material and recorded AE signals. We introduce the blind deconvolution separation (BDS) approach because the result of AE monitoring is usually a convoluted mixture of signals from multiple sources. The method is implemented on data acquired from a fatigue test rig employing a wind turbine blade with an artificial transverse crack seeded in the surface at the base of the blade. Two different sets of fatigue loading were conducted. The convoluted signals are collected from the AE acquisition system, and the weak crack feature is extracted and analysed based on the BDS algorithm. The study reveals that the application of BDS‐based AE signal analysis is an appropriate approach for distinguishing and interpreting the different fatigue damage states of a wind turbine blade. The novel methodology proposed for fatigue crack identification will allow for improved predictive maintenance strategies for the glass‐epoxy blades of wind turbines. The experimental results clearly demonstrate that the AE signals generated by a fatigue crack on a wind turbine blade can be synchronously separated and identified. Characterizing and assessing fatigue conditions by AE monitoring based on BDS can prevent catastrophic failure and the development of secondary defects, as well as reduce unscheduled downtime and costs. The possibility of using AE monitoring to assess the fatigue condition of fibre composite blades is also considered.     

Effect of colloidal silica on the strength and energy absorption of glass fiber/epoxy interphases

Prior research has demonstrated that fiber-sizings can be designed to yield composite materials that simultaneously possess high energy absorption and structural properties. The improved mechanical properties resulted from control of the fiber surface chemistry and nano-scale topological features within the fiber–matrix interphase. The present study further explains the role of sizing chemistry and surface roughness on composite material performance. Model and commercial glass fiber epoxy specimens were fabricated using these fiber sizing systems resulting in interphase regions with varied surface topology and chemical functionality. Micromechanical measurements were performed using the microdroplet adhesion test method to quantify the fiber–matrix interfacial properties. Improvement in energy absorption and interfacial shear strength due to the presence of the nano-scale silica were quantified. Inspection of the failure modes revealed that the existence of colloidal silica promotes crack propagation along a more tortuous path within the interphase that results in progressive failure and contributes to increased energy dissipation.     

Controlling microarray spot morphology with polymer liftoff arrays

Biological arrays are hindered by the lack of uniformity in the deposition of biomaterials. Efforts aimed at improving this deposition have focused on altering the composition of the solution or the tool used to deposit the material. However, little attention has been paid to controlling material deposition by constraining the physical and chemical topography of the surface. Here we present the use of a hybrid hydrophilic/hydrophobic micropatterned surface to direct the deposition of spotted DNA on microarrays. These polymer "liftoff" arrays combine the hydrophobic surface properties of di-p-xylylene (Parylene) with photolithographically etched hydrophilic openings within the polymer. We show that the flow pattern of solutes on these substrates favors the concentration of dissolved material into the mesoscopic openings underlying the printed spot, resulting in significantly improved uniformity of deposition. Moreover, the micropatterned surface allows for increased replication of spotted materials. Finally, these polymer liftoff arrays display reduced array-to-array variation, improving the reproducibility of data acquisition. We envision that these novel substrates can be generalized to produce more uniform arrays of other patterned biomaterials.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

自然环境下路面裂缝的识别

论文提出了一种基于分数阶微分和图像形态学的路面裂缝检测算法。分数阶微分能有效增强信号中、高频部分,非线性保留信号的低频部分,通过构建分数阶微分掩模算子,增强裂缝信息特别是平滑区域中弱信号信息。利用图像形态学算子提取裂缝,通过组合中值滤波去除孤立噪声点。实验结果表明,该算法比传统算法能更有效地检测出细小裂缝信息,是一种具有较强鲁棒性且高效实用算法。     

Inverse method of identification for three-dimensional subsurface cracks in a half-space

A procedure is presented which is well suited for three-dimensional subsurface crack identification in a half-space through the inversion of measured surface displacements. The investigation began with the linear, forward problem of generating contour maps of surface deformation produced by a fracture of known geometry and loading which is embedded in a finite medium. The fundamental solutions for tensile and shear multipoles in a half-space provided an efficient mathematical representation of the three-dimensional fracture. The inverse problem of crack identification centers on the development of a hybrid of the Marquardt–Levenberg algorithm. Initial guesses for the constrained set of search variables were determined heuristically from the correspondences between crack geometry and loading and the resulting uplift at the free surface. Physical measurements of surface deformation were taken for a cube of transparent acrylic polyester in which a fracture was hydraulically pressurized. Displacements induced at the surface of the specimen, which were measured by laser interferometry, had a strong correlation with predictions of the computational model (coupled with a finite element discretization). Numerical tests demonstrate the robustness of the inverse methodology even in the presence of the random and systematic errors corresponding to the experimental interferometric measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of stress intensity factors for surface cracks using fatigue crack growth data

The fatigue growth of semi-elliptical surface cracks in a structural steel under constant amplitude tensile cycling loading is investigated. The AC potential technique is used for sizing and monitoring the profile development of the cracks. The data are used to determine the stress intensity factors along the entire crack front at different stages of crack growth where the effects of front face, finite thickness and finite width are continuously changing. Various numerical solutions are compared with the experimental results     

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.