Photothermal radiometric detection and imaging of surface cracks

This paper discusses surface crack detection by photothermal radiometric imaging (PRI). In PRI, also called dynamic infrared scanning, a surface is scanned with a spot of heat. Imperfections are detected by radiometrically sensing changes in the surface temperature of a small area in the vicinity of this spot. In the work described, cracks narrower than 25 µm (0.001 in.) in a lightly rusted steel surface have been detected. Indiscrete scanning an amplitude modulated heating beam is moved in steps, remaining at each location long enough to measure amplitude and phase of the AC temperature. Incontinuous scanning a constant intensity heating beam is moved continuously while the temperature deviations are measured. This paper presents methods of calculating amplitude and phase of surface temperature for discrete scanning and instantaneous temperature for continuous scanning across a surface crack. For a steel surface scanned by a watt-level laser beam, predicted surface temperature deviations when crossing the crack are several degrees Celsius, with expected radiometrically detected power several orders of magnitude above the detector noise. In experiments performed, both techniques easily detected narrow cracks in a smooth, clean surface. Discolorations and pits, on the other hand, generate a disturbing type of surface noise. This noise was minimized by differential detection. Based on results obtained, continuously scanned PRI with a fan-type heating beam and array detection could become a viable way of mapping surface cracks at practical scanning speeds.     

Laser Multi-mode Scanning Thermography Method for Fast Inspection of Micro-cracks in TBCs Surface

Conventional non-destructive testing methods are difficult to be applied in defect detection of thermal barrier coating (TBCs) because of some of its characteristics, such as porosity and thin thickness, etc. For detecting surface cracks in TBCs, a laser multi-modes scanning thermography (SMLT) method has been developed in this paper, combining fast scan mode using linear laser with fine scan mode using point laser on the tested specimen surface. Linear scanning has a large detection range and detection speed, and point scanning has a higher sensitivity. Through the theoretical analysis, numerical simulation and experimental verification, five unique thermal response features of the cracks stimulated by two scanning modes were discovered and summarized. These features in the thermal images include temperature sharply rising in local region, distinct increase of the area of high temperature zone, obvious ‘tailing’, ‘dislocation’ and thermal obstruction phenomenon, respectively. Therefore, with the corresponding post-processing algorithm developed here, the location and shape of surface cracks in TBCs can be efficiently detected by analyzing the information of these thermal response features. Validation tests showed that the surface cracks with the width of more than \(20\,\upmu \hbox {m}\) can be quickly detected in line-scan stage, while in point-scan stage, the \(9.5\,\upmu \hbox {m}\) wide surface cracks can be accurately detected.     

Analyses of infinite pairs of surface cracks in elastic plates

Stress intensity factors and crack opening displacements are presented for infinite pairs of surface cracks in plates subjected to remote tension by using the three dimensional weight function method developed in [7,8]. A wide range of configuration parameters is considered. The results compare very well with double edge cracks as crack aspect ratio tends to zero; with collinear cracks as it tends to infinity; with a pair of surface cracks in a wide plate when the ratio of crack length to plate width is small; and with a single surface crack in large plates when both the ratios of crack length to plate width and crack depth to plate thickness are small. Also illustrated is the significant difference between a single surface crack and the surface cracks in pairs when the ratio of crack depth to plate thickness is large.     

Integral equation approach for 3D multiple-crack problems

In this paper, a traction integral equation containing no hypersingular integrals is presented to study the interaction of multiple cracks in an infinite elastic medium. 8-node quadratic quadrilateral elements are used to discretize general crack surfaces, and special crack tip elements are employed along surface boundaries to model the variation of displacements near the crack fronts. Thus, the method possesses the merits of the traction integral equation without hypersingular integrals and those of the special crack tip elements for modeling variation of displacements near the crack tips. The stress intensity factors at the crack front are evaluated using one point formulation and the results are compared with available solutions.     

The thermal fatigue resistance of H13 steel repaired by a biomimetic laser remelting process

In this study, the biomimetic laser remelting process was adopted to repair thermal fatigue cracks on an annealed hot work die AISI H13 steel. Several treated morphologies: spot, striation and lattice were processed by a pulsed Nd:YAG laser. The ultrafine microstructure within the unit was comprised of martensite, austenite and carbides. The average microhardness of the unit is much higher than that of the original surface, even after thermal cycles. Thermal fatigue results show that both crack density and crack length are reduced due to the blocking effect of strengthening units. And the partial laser surface remelting with lattice morphology is the most effective for repairing cracks and improving the thermal fatigue resistance of all.     

Analysis of fatigue crack behaviour based on dynamic response simulations and experiments for tensile-shear spot-welded joints

Fatigue crack initiation and propagation behaviours were studied based on the dynamic response simulation by the three‐dimensional finite‐element analysis (FEA) and dynamic response experiments for tensile‐shear spot‐welded joints. The entire fatigue propagation behaviour from the surface elliptical cracks at the initiation stage to the through thickness cracks at the final stage was taken into consideration during the three‐dimensional FEA dynamic response simulations. The results of the simulations and experiments found that the fatigue cracks of spot‐welded joint from initial detectable crack sizes to crack propagation behaviour could be described by three stages. Approximately one‐half of the total fatigue life was taken in stage I, which includes micro‐crack nucleation and the small crack growth process; 20% of the total fatigue life in stage II, in which the existing surface crack propagates through the thickness of sheet and 30% of the total fatigue life in stage III, during which the through thickness crack propagates along the direction of plate width to the final failure. According to the relationship between the crack length and depth and the dynamic response frequency during the simulated fatigue damage process, the definition of fatigue crack initiation and propagation stages was proposed. The analysis will provide some information for the fatigue life prediction of the spot‐welded structures.     

基于激光闪光法的立式热扩散率测量装置研究

介绍了基于激光闪光法的立式热扩散率测量装置,利用脉冲激光对样品进行均匀加热,使样品内部产生一维热流,并通过红外探测器测量样品温升信号,采用立式真空加热炉控制测量温度环境,实现室温至1600℃的热扩散率测量。用该装置测量厚度为1. 1 mm,直径为10 mm的不锈钢样品,测量结果与PTB参考数据的偏差小于1%。     

Methodology for Mapping the Residual Stress Field in Serviced Rails Using LCR Waves

Thermal barrier coatings (TBCs), as effective thermal protection separating the substrate from high-temperature combustion gases and reducing the substrate temperature, are widely used in aerospace and other fields. During the service cycle of life, surface crack defects, interface disbond defects, and coating thickness changes are the main non-destructive testing (NDT) objects of TBCs. In this paper, the main active infrared thermography NDT techniques including the optical infrared thermography testing, the ultrasonic infrared thermography testing, and the microwave thermography testing techniques are reviewed. Through the summary and highlight of the detection principle and application status of these state-of-the-art techniques, the development of the active infrared thermography DNT technique in TBCs is presented. By comparing the sensitivity, advantages, and disadvantages of the techniques in TBC NDT, can provide a significant reference for researchers to choose an appropriate method. It is noteworthy that fabrication techniques of artificial defects for calibration of the active infrared thermography NDT technique inspection of TBC systems are also reviewed. Moreover, future trends in NDT for the TBC system based on the active infrared thermography NDT technique are also discussed and analyzed.

     

Thermal striping fatigue damage in multiple edge-cracked geometries

Thermal fatigue striping damage may be caused when incompletely mixed hot and cold fluid streams pass over the surface of a component or structure containing a defect. Stress intensity factor (SIF) fluctuations are developed in response to the surface temperature fluctuations. An existing methodology for the analysis of striping damage in geometries containing a single edge‐crack geometry is extended to such an analysis of multiple edge cracks. SIFs are calculated as functions of crack depth, when an edge‐cracked plate and semi‐infinite solid, each containing multiple cracks, are subjected to thermal striping. The effect of various restraint conditions and striping frequencies on the SIF values for a stainless steel plate is examined. The degree of conservatism is shown when an assessment of thermal fatigue striping damage is based on a single, rather than multiple, crack analysis. Accurate curve fits are developed resulting in practical weight functions for an edge‐cracked plate and semi‐infinite solid.     

An elastic strip with periodic surface cracks under thermal shock

The problem of two periodic edge cracks in an elastic infinite strip located symmetrically along the free boundaries under thermal shock is investigated. It is assumed that the infinite strip is initially at constant temperature. Suddenly the surfaces containing the edge cracks are quenched by a ramp function temperature change. Very high tensile transient thermal stresses arise near the cooled surface resulting in severe damage. The degree of the severity for a subcritical crack growth mode is measured by determining the stresses intensity factors. The thermoelastic problem is treated as uncoupled quasi-static. The superposition technique is used to solve the problem. The thermal stresses obtained from the uncracked strip with opposite sign are utilized as the only external loads to formulate the perturbation problem. By expressing the displacement components in terms of finite and infinite Fourier transforms, a hypersingular integral equation is derived with the crack surface displacement as the unknown function. Numerical results for stress intensity factors are carried out and presented as a function of time, cooling rate, crack length, and periodic crack spacing.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

Liquid metal embrittlement during resistance spot welding of Zn-coated high-strength steels

Liquid Zn-assisted embrittlement during resistance spot welding of Zn-coated high strength steels induces risks of surface cracking. This paper presents a state-of-the-art review on the surface crack evolution due to liquid metal embrittlement during spot welding. Several aspects of cracking have been discussed such as macroscopic and microscopic features of liquid Zn-assisted cracks, prevalent crack locations in spot welds, sensitivity to welding conditions, and susceptibility of different types of steels and Zn-based coatings. Subsequently, the influence of governing factors like temperature, stress, microstructure, and nature of coating are presented by correlating thermo-mechanical studies with actual spot welding investigations on Zn-coated steels. Finally, the possible mechanisms of embrittlement are discussed and suitable methods to suppress surface crack formation during spot welding are suggested.

This is the winning review of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

A simplified approach for ductile failure analysis of semi-elliptical surface cracks

This paper presents a new simplified approach for ductile failure analysis of semi-elliptical surface cracks under uniform tension by representing the load as a multiplication of two separate functions; a crack geometry function and a material deformation function. Experimental data of specimens with wide range of crack depth, crack width, and thickness are used in the implementation of this approach. The semi-elliptical surface crack is treated as an equivalent single edge crack. An appropriate form for such a treatment is carefully selected and studied. Both the geometry and deformation functions for the studied data are developed. For the first time, an _pl value is developed for such a complex geometry. The existence of load separation, an equivalent one-dimensional crack length, and an _pl value may allow the elastic-plastic fracture parameter J to be evaluated using a single specimen for this geometry. Both the geometry function _pl and are also developed using Mattheck et al. limit load formula which is based on the Dugdale model. They agree well with those developed from the experimental data within the tested range. Finally, a key curve is developed and used in the prediction of the load-displacement records.     

新的估算表面裂纹应力强度因子经验公式

该文给出了新的估算拉伸和纯弯曲载荷下表面裂纹应力强度因子的经验公式。根据疲劳裂纹扩展的数值模拟结果确定强度因子分布函数;利用按已知应力强度因子分布函数求裂纹形状及相应应力强度因子的方法计算给定尺寸的表面裂纹的应力强度因子;通过对数值结果的曲线回归得到估算表面裂纹应力强度因子经验公式。利用该公式对有限厚度和宽度平板内表面裂纹的应力强度因子进行了估算,并与已知的半椭圆形表面裂纹的应力强度因子解进行了比较。该文结果为估算表面裂纹应力强度因子提供了一种新的途径。     

Coupled behaviour of interacting dielectric cracks in piezoelectric materials

Existing studies indicate that the commonly used electrically impermeable and permeable crack models may be inadequate in evaluating the fracture behaviour of piezoelectric materials in some cases. In this paper, a dielectric crack model based on the real electric boundary condition is used to study the electromechanical behaviour of interacting cracks arbitrarily oriented in an infinite piezoelectric medium. The electric boundary condition along the crack surfaces is governed by the opening displacement of the cracks. The formulation of this nonlinear problem is based on modelling the cracks using distributed dislocations and solving the resulting nonlinear singular integral equations using Chebyshev polynomials. Numerical simulation is conducted to show the effect of crack orientation, crack interaction and electric boundary condition upon the fracture behaviour of cracked piezoelectric media.     

Detectability of Subsurface Defects with Different Width-to-Depth Ratios in Concrete Structures Using Pulsed Thermography

The active thermography technique is one of the most effective nondestructive tests for evaluating subsurface delaminations in concrete structures. The limitation of this method, which has been studied for some time, is that the width of the smallest detectable defect should be at least two times larger than its depth. However, controversy on this matter remains for concrete material with largely uncertain homogeneity, although the development of the infrared (IR) detector technology improved the above-mentioned limitation. In this study, the pulsed thermography (PT) technique is therefore conducted in the laboratory to investigate the detectability of delaminations with the width-to-depth ratio (w2d) ranging from 1.0 to 7.9 by using a long IR wavelength detector with a focal plane array of 640 \(\times \) 480 pixels. The study focuses on the w2d ratio lower than 2.0. A concrete specimen was made with 12 embedded simulated delaminations having different sizes and depths. The results showed that a combination of PT and pulsed phase thermography can be used to detect delaminations with a w2d ratio equal or greater than 1.25. In addition, the absolute contrast above the delamination increases with the higher w2d ratio, indicating that even for a relatively deep delamination, it is still detectable if a delamination is provided by appropriate heat energy and its size is sufficiently large. Finally, the study also indicates that as the amount of heating energy provided is increased, the greater accuracy in predicting the depth can be obtained.     

Distinctive features of the effect of laser treatment on the corrosion-fatigue fracture of structural steel

The dependence of the susceptibility of surface-hardened 40KhMFA steel (_u=950 MPa, _0.2=780 MPa) to crack initiation and propagation on the mode of laser treatment is studied by the methods of fracture mechanics. The influence of laser treatment on the initiation of a fatigue crack is ambiguous: in air, it results in a substantial increase in the fatigue threshold, while in a 3% NaCl solution, the fatigue threshold decreases. The growth rate of short fatigue cracks decreases in both air and corrosive media. The positive effect of laser treatment on the kinetics of crack growth and the threshold SIF becomes more pronounced as the yield limit of the surface layer _0.2 increases. The depth of the influence of laser treatment on crack growth is not restricted to the hardened surface layer due to the redistribution of residual stresses. Unlike the base material, short cracks in the surface layer hardened by a laser are characterized by a high degree of crack closure, which proves to be the main factor that provokes an increase in the fatigue crack-growth resistance.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 6, pp. 40–52, November – December, 1994.     

Influence of stress intensity and crack speed on fracture surface topography: mirror to mist to macroscopic bifurcation

The development of roughness on the fracture surfaces of a brittle, glassy, epoxy resin from the mirror-to-mist transition to macroscopic bifurcation has been investigated using optical microscopy, scanning electron microscopy (SEM) and contact and non-contact laser profilometry. Most of the observations were made on specimens fractured in edge-notched tension. In a series of tests the initial crack length was varied to obtain fracture surfaces formed by accelerating and decelerating cracks without macroscopic bifurcation (specimen A) and by cracks which accelerated continuously to macroscopic bifurcation (specimen B). Some observations were made on specimens tested in compact tension to study changes in fracture surface topography associated with crack arrest in stick-slip fracture. There was a close correlation between the topographical detail revealed by the different techniques. In specimen A the roughness increased progressively from the mirror-to-mist transition and reached a maximum before decreasing as the crack decelerated. The topographical features revealed by optical microscopy and SEM were the same for accelerating and decelerating cracks at the same roughness value. In specimen B the roughness increased continuously to macroscopic bifurcation. There was a close similarity between the topographical features at all levels of roughness. A simple model for the basic step involved in roughness formation is presented which involves an element of the crack tip tilting out of the plane of the main crack before stopping (micro-bifurcation). The scale of micro-bifurcation ranged from 3 m in the early stages of mist, when the crack velocity was close to 10% of the shear wave velocity, to the full width of the specimen (6 mm) at macroscopic bifurcation. The micro-bifurcation process develops from crack surface undulations and does not involve micro-crack nucleating ahead of the main crack. It is concluded that the relationships between crack velocity and dynamic stress intensity, and the value of the limiting crack velocity, must be interpreted in terms of micro-mechanical processes at the crack tip which are strongly dependent on specific material characteristics.Emeritus Goldsmiths' Professor Metallurgy, University of Cambridge, U.K.     

微波无损检测热障涂层下金属表面裂缝的参数优化

基于微波在介电材料中的传播理论及其对金属界面特性的敏感性, 利用CST-微波工作室(computer simulation technology-microwave studio)对微波检测热障涂层下金属的裂缝进行了仿真计算, 研究了热障涂层的厚度、 裂缝方向对检测结果的影响, 仿真计算了热障涂层厚度为400 μm、 裂缝长边方向平行于矩形波导探头长边的不同宽度的裂缝。结果表明: 热障涂层厚度不同时, 微波检测金属表面裂缝的敏感工作频率不同; 裂缝方向与波导口长边的夹角为50°～55°时检测敏感度低。裂缝宽度小于8 μm时用本研究中的频率范围无法检测, 裂缝宽度在10～30 μm时检测效果不明显, 裂缝宽度在30 μm～1 mm范围内, 裂缝越宽微波的反射系数相位差越大。因此, 在合适的工作频率下能利用微波无损检测技术对热障涂层下金属表面的裂缝进行无损检测。     

A new boundary integral equation method for cracked 2-D anisotropic bodies

By using integration by parts to the traditional boundary integral formulation, a traction boundary integral equation for cracked 2-D anisotropic bodies is derived. The new traction integral equation involves only singularity of order 1/r and no hypersingular term appears. The dislocation densities on the crack surface are introduced and the relations between stress intensity factors and dislocation densities near the crack tip are induced to calculate the stress intensity factors. The boundary element method based on the new equation is established and the singular interpolation functions are introduced to model the singularity of the dislocation density (in the order of ) for crack tip elements. The proposed method can be directly used for the 2-D anisotropic body containing cracks of arbitrary geometric shapes. Several numerical examples demonstrate the validity and accuracy of BEM based on the new boundary integral equation.