The interaction and the surface crack of single-crystal silicon induced by a millisecond laser

When the silicon material is irradiated by laser, it absorbs the laser energy leading to the temperature rise and the thermal stress. The damage effect includes melting, vaporation and thermal stress damage. Once the thermal stress exceeds the stress strength the crack will initiate. The silicon surface cracks induced by a millisecond laser are investigated. The experimental results show that three types of cracks are generated including cleavage crack, radial crack and circumferential crack. The cleavage crack is located within the laser spot. The radial crack and circumferential crack are located outside the laser spot. A two-dimensional spatial axisymmetric model of silicon irradiated by a 1064 nm millisecond laser is established. To assess what stresses generate and explain the generation mechanism of the different cracks, the thermal stress fields during laser irradiation and the cooling process are obtained using finite element method. The radial stress and hoop stress within the laser spot are tensile stress after the laser irradiation. The temperature in the center is the highest but the thermal stress in the center is not always highest during the laser irradiation. The cleavage cracks are induced by the tensile stress after the laser irradiation. The radial crack and the circumferential crack are generated during the laser irradiation.     

Reduction of post-weld shift in semiconductor laser packaging

A reduction of post-weld shift (PWS) in semiconductor laser packaging using a laser welding technique is presented. It is found that the PWS and its association with the power coupling loss of the laser packaging can be reduced under proper pressure constraint. Characteristics of defect mechanisms in laser welding techniques for semiconductor laser packaging are also investigated experimentally. The results in the stainless-to-Kovar joints show that the surface cracks are dependent on the Au thickness on the Kovar material. The low solubility of gold in the Kovar is identified as the defect mechanism for surface cracks. Preliminary reliability data demonstrated that these laser packages without crack defects in the welded joints are reliable.     

A comparative analysis of reticular crack on ceramic plate driven by thermal shock

Reticular crack is generally found on the surface of ceramic material that has been subjected to a thermal-shock condition. In the present study, a quantitative effect of thermal shock and quench temperature has been studied and investigated. Experimental tests were carried out to characterize the reticular crack that has been found in the Ge Kiln, which is a famous art of the ancient Chinese culture. After comparative analysis between thermal-shock cracks and the glaze crack patterns of the Ge Kiln porcelain, it is found that this study is expected to provide a powerful tool for recurrence of the long-lost firing and cooling process of the Ge Kiln porcelain.     

高强度钢手工焊接后过程的声发射实时监测研究

本针对实际工艺中使用的高强度钢，重点对手工焊接的冷却过程利用声发射技术进行实时监测。本次试验是和先前在不同型号的钢板在手工焊接过程中所产生的声发射信号进行比较的基础上进行的，主要目的是寻找高强度钢在手工焊接的冷却中所产生的活性裂纹在冷却阶段的声信号特征。     

Strength evaluation of brittle ceramics with surface defects subjected to thermal shock

The thermal shock strength of a ceramic material plate with a semi-elliptical surface crack is evaluated in this paper. The thermal stress distribution at the plate without crack is expressed by a polynomial form. The calculated thermal stresses are used to obtain the thermal stress intensity factors of semi-elliptical surface cracks using the geometric functions. Variations of thermal stress intensity factor with crack depth and time are obtained. For the strength evaluation of the material, crack growth analysis is conducted. In addition, the thermal shock resistance of the ceramic material plate is evaluated by the stress-based failure criterion and the toughness-based failure criterion. The critical temperature at which crack propagation starts is obtained. Applicability of the two failure criteria in thermal shock resistance evaluation of ceramic is identified and the importance of the semi-elliptical surface defects on the thermal shock resistance behaviour of ceramic materials is justified.     

Effect of periodic surface cracks on the interfacial fracture of thermal barrier coating system

Periodic surface cracks and interfacial debonding in thermal barrier coating (TBC) system may be induced during cooling process. The objective of this work is to investigate the effect of periodic surface cracks on the interfacial fracture of TBC system. The finite element method (FEM) incorporating cohesive zone model is used in analysis. It is found that surface crack spacing has significant effect on the initiation and propagation of short interface crack. Three different regions are identified for describing the effect of surface crack spacing. In Region I the interface crack driving force is dramatically reduced due to high surface crack density. In this case, the initiation of interfacial delamination can be delayed. Region II applies as the surface crack spacing is moderate. Analysis of this transition zone brings to the definition of normalized critical surface crack spacing. Region III arises for sufficient large surface crack spacing. In this case, the interface crack driving force reaches a steady state, where the effects of adjacent surface cracks are relatively insignificant and can be ignored. It can be concluded that an appropriately high surface crack density can enhance the durability of TBC system.     

斜入射SH波厚壁管道内壁裂纹检测方法研究

厚壁管道常被用于军事装备及其他流程工业中，长期使用后内壁会产生较多微裂纹，成为影响构件安全运行的重大隐患。为此，本文针对厚壁管道内壁裂纹难以检测的问题，提出基于斜入射SH(Shear Horizontal)波的厚壁管道检测方法，对厚壁管道内壁不同深度的裂纹进行检测。本文首先对斜入射SH波的激励原理进行分析，建立声场模型优选激励频率，研究斜入射SH波与厚壁管道内壁裂纹径向深度的作用规律，并通过实验对仿真结果进行验证。研究结果表明，斜入射SH波对该型管道最佳检测频率为1MHz；随着裂纹径向深度增加，缺陷回波幅值呈现曲折型上升；斜入射SH波可有效对厚壁管道内壁轴向长8mm，径向深1mm和周向宽1mm的微裂纹进行检测，验证了斜入射SH波厚壁管道内壁裂纹检测方法的科学性和可行性。     

基于幅度调制的非线性超声相控阵成像方法*

超声相控阵是超声检测领域常用的材料缺陷定位和成像技术,可便捷快速地对裂纹、孔洞等缺陷进行成像。但是,传统超声相控阵方法对较小缺陷如闭合裂纹不太敏感。非线性超声信号因对材料性能退化以及微小缺陷敏感而广受关注。本文针对疲劳闭合裂纹检测,提出一种基于幅度调制的非线性超声相控阵成像方法,通过测量物理聚焦和虚拟聚焦两种聚焦模式下超声扩散场中的声能差,并将其作为非线性参量,实现疲劳裂纹闭合部分的定位成像和定量表征。将该法应用于7075铝合金试样疲劳裂纹的实验测量,并研究了扩散场信号延迟时间对非线性超声相控阵成像结果的影响。结果表明：相较于传统超声相控阵全聚焦法,基于幅度调制的非线性超声相控阵成像方法能够更准确地定位和成像疲劳裂纹闭合部分;延迟时间的选择对疲劳裂纹长度的表征精度影响较大,本文研究了该延迟时间的选择方法并实现了检测结果的优化。     

Detection and evaluation of the depth of surface cracks in conductive materials by using a loop antenna

A new method is developed for detection and evaluation of the depth of surface cracks in conductive materials. The main components of the measurement equipment are a conductive wire and a loop antenna, both of which are located as close as possible to the surface of the investigated material, while the antenna loop is perpendicular to the surface and parallel to the wire. Supplying of electrical potential with microwave frequency f and amplitude |_s| to the wire results in spreading of induced current close beneath the surface of the material. The induced current redistributes in the vicinity of surface cracks, and some of the current paths flow along the crack walls. This leads to generation of a time-variable magnetic field mostly within the volume of the crack, which is detected by the loop antenna. A theoretical analysis is presented of the contributions of the different types of generated magnetic fields to the potential drop with amplitude |U_s| measured by the antenna. The distribution of the ratio R(dB)=|U|/|U_s| is measured for specimens of paramagnetic and ferromagnetic steel which contain three artificial cracks with depths of 0.5 mm, 1 mm, and 2 mm, and frequency f=300 MHz of the supplied electrical potential. It turns out that artificial cracks with a depth of d 0.5 mm, located at distances from x=3 mm to x=15 mm from the wire, can be detected. The largest sensitivity of crack detection is achieved for x4 mm when the long axis of a crack is perpendicular to the wire. The crack response of the measured ratio R is proportional to the crack depth when the loop of the antenna is located above or slightly outside an artificial crack, and at least 4 mm from the wire. This relation could be used for evaluation of the depth of real surface cracks. The notable decrement of the crack response outside the crack can indicate the location of the crack tips on the surface of the specimen. It is shown that a loop antenna with a diameter of 7 mm provides the largest crack response, as well as that the developed method can be applied to both paramagnetic and ferromagnetic materials. An example of the detection of a real crack by an antenna is reported for the first time. PACS 81.70.Ex; 84.40.Ba; 84.32.Ff     

Effect of driving frequency on non-linear coupling between ultrasound transducer and target under inspection in Sonic Infrared Imaging

Sonic Infrared (IR) Imaging, also referred as vibrothermography, is a novel Nondestructive Evaluation (NDE) technology to find cracks through infrared imaging of vibration-induced crack heating. The vibration source plays an important role in the detection of cracks. In this paper, the effect of driving frequency on the ultrasound vibration to the thermal imaging is presented. The research is organized by using different frequency system and coupling materials on the same aluminum bar sample. The analysis is conducted by combination of the vibration waveforms with the IR images and signals. Correlation analysis between the acoustic energy and the thermal energy in the crack is discussed as well.     

Design of interdigital transducers for crack detection in plates

Interdigital transducers (IDT) for non-destructive evaluation (NDE) of cracks in plates are designed based on an analytical model established previously. Key considerations include mode selectivity, excitation strength, collimation of wave and cost. The advantage of mode selectivity of IDT over PZT patch is presented both analytically and experimentally. Effects of parameters, namely finger spacing, width, length, number of fingers, and the size of IDT, on the excitation strength and mode selectivity are considered. This led to the design of a mobile double-sided IDT as an efficient device where excitation strength is strong and focused. The device was fabricated in-house using commercially available piezoelectric ceramics and used to develop a procedure for accurate identification of the direction and extent of cracks in plates. Three aluminum plates, one with a linear deep crack, another with a piecewise linear shallower crack and the third with a curved crack, were used to illustrate the accuracy and efficiency of both the proposed device and procedure for effective NDE.     

An infrared thermal image processing framework based on superpixel algorithm to detect cracks on metal surface

Infrared thermography has been used increasingly as an effective non-destructive technique to detect cracks on metal surface. Due to many factors, infrared thermal image has low definition compared to visible image. The contrasts between cracks and sound areas in different thermal image frames of a specimen vary greatly with the recorded time. An accurate detection can only be obtained by glancing over the whole thermal video, which is a laborious work. Moreover, experience of the operator has a great important influence on the accuracy of detection result. In this paper, an infrared thermal image processing framework based on superpixel algorithm is proposed to accomplish crack detection automatically. Two popular superpixel algorithms are compared and one of them is selected to generate superpixels in this application. Combined features of superpixels were selected from both the raw gray level image and the high-pass filtered image. Fuzzy c-means clustering is used to cluster superpixels in order to segment infrared thermal image. Experimental results show that the proposed framework can recognize cracks on metal surface through infrared thermal image automatically.     

闭合裂纹非共线混频超声检测试验研究

针对结构中微裂纹检测难题,发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上,进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析,结果表明,可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量,实现有无闭合裂纹的判识;通过移动激励探头的位置,控制两列入射声波在试件中的交汇位置,实现试件中不同深度位置的混频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布,实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。      

A model for crack formation during active solid pyrolysis of a char-forming solid: crack patterns; surface area generation; volatile mass efflux

A model is developed for the formation and propagation of cracks in a material sample that is heated at its top surface, pyrolyses, and then thermally degrades to form char. In this work the sample is heated uniformly over its entire top surface by a hypothetical flame (a heat source). The pyrolysis mechanism is described by a one-step overall reaction that is dependent nonlinearly on the temperature (Arrhenius form). Stresses develop in response to the thermal degradation of the material by means of a shrinkage strain caused by local mass loss during pyrolysis. When the principal stress exceeds a prescribed threshold value, the material forms a local crack. Cracks are found to generally originate at the surface in response to heating, but occasionally they form in the bulk, away from ever-changing material boundaries. The resulting cracks evolve and form patterns whose characteristics are described. Quantities examined in detail are: the crack spacing in the pyrolysis zone; the crack length evolution; the formation and nature of crack loops which are defined as individual cracks that have joined to form loops that are disconnected from the remaining material; the formation of enhanced pyrolysis area; and the impact of all of the former quantities on mass flux. It is determined that the mass flux from the sample can be greatly enhanced over its nominal (non-cracking) counterpart. The mass efflux profile qualitatively resembles those observed in Cone Calorimeter tests.     

Conditioning-induced elastic nonlinearity in hysteretic media

The definition and measurement of the nonlinear elastic properties of a sample is of great importance for a large number of applications, including characterization of material performances and damage detection. However, such measurements are often influenced by spurious effects due to a combination of nonlinearity and nonequilibrium phenomena. We will present experimental data to show how nonlinearity due to small cracks in concrete samples increases as a consequence of conditioning, i.e., after having perturbed them with a constant amplitude excitation. In addition, our experimental data highlight “memory effects,” i.e., they show that when the excitation is removed, the elastic modulus does not return instantaneously to the initial value.     

Oscillating cracks in glassy films on silicon substrates

Cracks that propagate with near-perfect sinusoidal form are reported in amorphous silicon-rich silica films deposited onto (001) silicon substrates by plasma-enhanced chemical vapour deposition and subjected to thermal annealing. The cracks are shown to result from high tensile stresses that develop in the film during thermal annealing at temperatures in the range up to 700°C, a process shown to be correlated with the loss of hydrogen from the films. Two distinct modes of crack propagation are reported: straight cracks that propagate along directions parallel to [100] cube-edge directions in the substrate, and oscillating cracks that propagate with sinusoidal form parallel to [110] diagonal directions. Sections through the cracks show that the oscillating cracks have a complex three-dimensional structure that extends through the glassy film and into the underlying silicon substrate. This involves a correlated oscillation between the crystallographic orientation of the crack in the surface plane and that of the crack extension into the substrate. Whereas a complete theoretical treatment of this behaviour would be extremely complicated, a simple theory is developed to demonstrate that an oscillating crack has a minimum energy per unit length for a particular wavelength and amplitude that depends upon the physical parameters of both film and substrate. The energy at this minimum is shown to be lower than that of a straight crack for certain parameter ranges so that the oscillating geometry is preferred.     

Coupled horizontal and vertical bending vibrations of a stationary shaft with two cracks

This paper investigates the coupled bending vibrations of a stationary shaft with two cracks. It is known from the literature that, when a crack exists in a shaft, the bending, torsional, and longitudinal vibrations are coupled. This study focuses on the horizontal and vertical planes of a cracked shaft, whose bending vibrations are caused by a vertical excitation, in the clamped end of the model. When the crack orientations are not symmetrical to the vertical plane, a response in the horizontal plane is observed due to the presence of the cracks. The crack orientation is defined by the rotational angle of the crack, a parameter which affects the horizontal response. When more cracks appear in a shaft, then the coupling becomes stronger or weaker depending on the relative crack orientations. It is shown that a double peak appears in the vibration spectrum of a cracked or multi-cracked shaft.Modeling the crack in the traditional manner, as a spring, yields analytical results for the horizontal response as a function of the rotational angle and the depths of the two cracks. A 2×2 compliance matrix, containing two non-diagonal terms (those responsible for the coupling) serves to model the crack. Using the Euler–Bernoulli beam theory, the equations for the natural frequencies and the coupled response of the shaft are defined. The experimental coupled response and eigenfrequency measurements for the corresponding planes are presented. The double peak was also experimentally observed.     

Interaction of fatigue crack with vibration and ultrasound in metallic rod and its use for quantitative characterization

According to the principle of synchronous demodulation,the modulation information in the output signal of vibro-acoustic modulation test was investigated and the nonlinear interaction of sound and vibration with the crack interface in a metallic rod was analyzed.A swept signal excitation was used to obtain a proper high frequency parameter in the test of aluminum rod samples with different sizes of cracks and the modulation information in the output signal was analyzed.It was found that the ultrasound wave in the structure can be separated into two parts.One is the wave which passes through the crack interface modulated by the low frequency vibration,and other wave has no any changes.The nonlinear modulation model was modified and a quantitative method for crack detection was proposed.It is demonstrated by comparing with the ultrasonic C-scanning that this method can be used to estimate the fatigue crack size in the metallic rod.     

Porcelain Insulator Crack Location and Surface States Pattern Recognition Based on Hyperspectral Technology

A porcelain insulator is an important part to ensure that the insulation requirements of power equipment can be met. Under the influence of their structure, porcelain insulators are prone to mechanical damage and cracks, which will reduce their insulation performance. After a long-term operation, crack expansion will eventually lead to breakdown and safety hazards. Therefore, it is of great significance to detect insulator cracks to ensure the safe and reliable operation of a power grid. However, most traditional methods of insulator crack detection involve offline detection or contact measurement, which is not conducive to the online monitoring of equipment. Hyperspectral imaging technology is a noncontact detection technology containing three-dimensional (3D) spatial spectral information, whereby the data provide more information and the measuring method has a higher safety than electric detection methods. Therefore, a model of positioning and state classification of porcelain insulators based on hyperspectral technology is proposed. In this model, image data were used to extract edges to locate cracks, and spectral information was used to classify the surface states of porcelain insulators with EfficientNet. Lastly, crack extraction was realized, and the recognition accuracy of cracks and normal states was 96.9%. Through an analysis of the results, it is proven that the crack detection method of a porcelain insulator based on hyperspectral technology is an effective non-contact online monitoring approach, which has broad application prospects in the era of the Internet of Things with the rapid development of electric power.     

CRACK DETECTION IN BEAM-TYPE STRUCTURES USING FREQUENCY DATA

A practical method to non-destructively locate and estimate size of a crack by using changes in natural frequencies of a structure is presented. First, a crack detection algorithm to locate and size cracks in beam-type structures using a few natural frequencies is outlined. A crack location model and a crack size model are formulated by relating fractional changes in modal energy to changes in natural frequencies due to damage such as cracks or other geometrical changes. Next, the feasibility and practicality of the crack detection scheme are evaluated for several damage scenarios by locating and sizing cracks in test beams for which a few natural frequencies are available. By applying the approach to the test beams, it is observed that crack can be confidently located with a relatively small localization error. It is also observed that crack size can be estimated with a relatively small size error.