Numerical study on thermal stress cutting of silicon wafers using two-line laser beams

We propose a method of cleaving silicon wafers using two-line laser beams. The base principle is separating the silicon wafer using crack propagation caused by laser-induced thermal stress. Specifically, this method uses two-line laser beams parallel to the cutting line such that the movements of the laser beam along the cutting line can be omitted, which is necessary when using a point beam. To demonstrate the proposed method, 3D numerical analysis of a heat transfer and thermo-elasticity model was performed. Crack propagation was evaluated by comparing the stress intensity factor (SIF) at the crack tip with the fracture toughness of silicon, where crack propagation is assumed begin when the SIF exceeds the fracture toughness. The influences of laser power, line beam width, and distance between two laser beams were also investigated. The simulation results showed that the proposed method is appropriate for cleaving silicon wafers without any thermal damage.

     

Dynamic mixed mode crack propagation behavior of structural bonded joints

The stress field around the dynamically propagating interface crack tip under a remote mixed mode loading condition has been studied with the aid of dynamic photoelastic method. The variation of stress field around the dynamic interface crack tip is photographed by using the Cranz-Shardin type camera having 10⁶ fps rate. The dynamically propagating crack velocities and the shapes of isochromatic fringe loops are characterized for varying mixed load conditions in double cantilever beam (DCB) specimens. The dynamic interface crack tip complex stress intensity factors,K ₁ andK ₂, determined by a hybrid-experimental method are found to increase as the load mixture ratio of y/x (vertical/horizontal) values. Furthermore, it is found that the dynamically propagating interface crack velocities are highly dependent upon the varying mixed mode loading conditions and that the velocities are significantly small compared to those under the mode I impact loading conditions obtained by Shukla (Singh & Shukla, 1996a, b) and Rosakis (Rosakis et al., 1998) in the USA.     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

Deformations of a simplified flip chip structure under thermal testing inspected using a real-time Moiré technique

Deformations of a Si-epoxy-FR4 (simplified flip chip) structure under thermal testing were inspected with a real-time Moiré technique. Specimens without cracks and specimens with a crack at the silicon-epoxy interface were prepared. The measurement results showed that the maximum deformation appeared at the edge. When the specimen was cooled to 20 °C, there was residual plastic deformation in the specimen. The creep effect was more dominant in the FR4-epoxy interface. Upon cooling to 20 °C, the specimen experienced partial strain recovery. To characterize the behavior of the interfacial crack, stress intensity factors K_I and K_II, and the strain energy release rate G in the vicinity of the crack tip were calculated using the measured deformations to conduct a quantitative study. It was observed that a sharp strain gradient occurred at the crack tip. K_I and K_II were dependent on temperature, and G was dominated by K_I for the interfacial crack in the specimen.     

Moving finite element analyses for fast crack propagation in sheet metal

The conventional fracture mechanics parameters K_IC and/or J_IC are used as fracture toughness criteria necessary for the start of crack propagation under plane strain conditions. These criteria are defined only for small-scale yielding or infinitesimal deformation, though actual fractures involve large plastic deformation. Hence, measurement of fracture resistance during crack propagation is difficult with the conventional parameters.Estimating the mechanical conditions around the propagating crack tip is very useful for reducing damage during accidental fracture. Therefore, establishing a criterion for crack propagation with large-scale yielding is very important for not only science fields but also some industrial fields. For fractures with large-scale yielding, micro- or mesoscale damage processes in the crack tip vicinity have to be considered.In this study, Gurson's constitutive model for void occurrence and growth was introduced into the finite element method to discuss failure behavior in the crack tip vicinity. Fast crack propagation behavior under high-speed deformation was simulated using the moving finite element method based on the Delaunay automatic triangulation. The excellent far-field integral path independence of the T* integral was verified for pure mode I fast crack propagation and non-straight crack propagation under mixed mode conditions. The void growth conditions near the crack propagation path were evaluated.     

Photoelastic observation of stress distributions in laser cleaving of glass substrates

Photoelastic observations of thermal stress were conducted during the separation of glass substrates by thermal stress cleaving with a laser. A polariscope system was built in the laser cleaving system for observation during laser cleaving, and isochromatic and isoclinic fringe patterns were studied to analyze the thermal stress induced by laser irradiation. The proposed method allows for the visualization of stress asymmetry that decreases the process accuracy, and for the detection of crack stagnation which decreases the process reliability.     

电磁热效应止裂效果与电流通路尺寸关系的研究

从理论、试验和数值模拟三方面研究利用电磁热效应技术对具有单边裂纹的导体进行止裂时，由于裂纹尺寸不同使得导体中电流通路尺寸不同，从而导致裂纹尖端的温度场、温度梯度场分布状态的不同。理论分析、试验研究和数值模拟结果均表明：由于电流产生的焦耳热源的作用，能够在裂纹尖端处很小的范畴内熔化形成焊口，遏制裂纹的扩展；导体中裂纹的长度（即导体中电流通路尺寸）是影响裂纹尖端温度场和温度梯度场的主要因素。     

ANALYSIS OF CUTTING CHARACTERISTIC OF POLYCARBONATE SHEET SUBJECTED TO WEDGE INDENTATION BY KNIFE EDGE AND GROOVED PLATE

Asymmetric plunge cutting by using a grooved knife edge plate was proposed for advanced die cutting of a polycarbonate (PC) sheet. By varying the tip angle α_U and the tip thickness w _U of knife edge in the counter plate, and combining a standard steel cutting rule of 42° center bevel blade, a cutting load response of PC sheet was experimentally investigated, and the deformation was also observed by a CCD camera. Furthermore, an FEM simulation was carried out by varying the lower tip thickness for revealing the effect of interference of lower grooved counter plate. Through this work, the following were found: 1) there were two deformation modes of sheared profile; 2) the lower tip thickness w _U was a primary factor, compared to the lower tip angle of α_U; 3) the equivalent wedge angle effect was revealed with respect to the upper/lower angles α and α_U, while the PC sheet was bent up by the lower knife indentation.     

Experimental and numerical investigations of mode I delamination behaviors of woven fabric composites with carbon, Kevlar and their hybrid fibers

This paper studied the effect of the hybridization of carbon and Kevlar fibers on mode I interlaminar fracture toughness and crack propagation behaviors with double cantilever beam (DCB) tests. The crack propagation characteristics, crack growth trend and rate, and fracture surfaces were observed using an optical microscope and SEM micrographs for the three different types of materials. Moreover, details of the stress distribution around the crack tip and the crack propagation pattern across the width of the DCB specimen were investigated using the finite element method, including a cohesive element. The mode I interlaminar fracture toughness of carbon-Kevlar hybrid/epoxy was nearly average for carbon/epoxy and Kevlar/epoxy. The maximum load predicted by the numerical method showed good agreement within an error of 5% with the experimental results.     

Measurement of phase retardation of waveplate online based on laser feedback

Polarization flipping accompanying with intensity transfer between two eigenstates of one laser mode happens when waveplate is placed in external cavity. The position of polarization flipping of two eigenstates is a function of phase retardation of waveplate. Phase retardation of waveplate is measured through analyzing the position of polarization flipping of two eigenstates. The measurement accuracy of phase retardation is 0.22°. A new structure of optical cement tray which can eliminate stress birefringence from optical cement process is invented. The accuracy of waveplate manufacture can be improved greatly based on the work of this article.     

Effect of constraint on fracture behavior of welded 17mn4 and aisi304 steels

In this study, 17Mn4 (P295GH) pressure vessels steel and AISI304 stainless steel were welded with ER309L austenitic consumable. In experimental part of the study, tensile tests were conducted on welded plates and variation of hardness values along specimen was measured. J-integral fracture toughness values were investigated for different crack locations. In order to determine the regions where plastic deformation did not take place due to constraint, uni-axial tensile test was performed on welded tensile specimen after attaching strain gauges. In numerical part of the study, finite element (FE) analyses were conducted by fixing 2-D models precracked on different locations by using ANSYS software. In these models, stress triaxiality and plastic deformation characteristics around crack tip were determined for each crack locations after stress — strain analyses. The limitation on the extension of plastic deformation at diffusion line causes extra increase in stress triaxiality at crack tip.     

In-process estimation of fracture surface morphology during wheel scribing of a glass sheet by high-speed photoelastic observation

Defect-free glass separation techniques are in strong demand in glass processing industries. In this study, we intended to observe the internal stress field during/after wheel scribing of a glass sheet using the photoelastic method. First, we visualized the crack propagation behavior in a 0.7-mm-thick non-alkali glass sheet during mechanical scribing with a 2.0-mm-diameter serrated diamond wheel using high-speed imaging techniques. The observation results under various applied load conditions showed that the crack propagation behavior changed dramatically at a load of approximately 9–10 N; the generated crack hardly propagated in the thickness direction under lower load conditions, in contrast to the rapid propagation under higher load conditions. The fracture surface morphology that was observed after cleavage also changed, from damaged to defect-free surfaces with increments in the applied load around the transition point (9–10 N). This result indicated that the fracture surface morphology was determined by the crack propagation behavior. Second, the birefringence phase difference was measured from the upper side of the glass sheet to enable understanding of the stress fields induced by scribing wheel indentations. As a result, the phase differences that were distributed along the scribe line were shown to differ depending on the applied loads; the phase difference changed little under lower load conditions, but vanished immediately under higher load conditions. Therefore, these differences were dependent on whether or not rapid crack propagation occurred. The measured phase difference distribution thus included information about the crack propagation behavior, and this information could be used as a criterion for estimation of the fracture surface morphology. An in-process estimation method for the fracture surface morphology during mechanical wheel scribing was therefore developed based on high-speed polarization imaging techniques.     

Mode I fracture toughness analysis of a single-layer grapheme sheet

To predict the fracture toughness of a single-layer graphene sheet (SLGS), analytical formulations were devised for the hexagonal honeycomb lattice using a linkage equivalent discrete frame structure. Broken bonds were identified by a sharp increase in the position of the atoms. As crack propagation progressed, the crack tip position and crack path were updated from broken bonds in the molecular dynamics (MD) model. At each step in the simulation, the atomic model was centered on the crack tip to adaptively follow its path. A new formula was derived analytically from the deformation and bending mechanism of solid-state carbon-carbon bonds so as to describe the mode I fracture of SLGS. The fracture toughness of single-layer graphene is governed by a competition between bond breaking and bond rotation at a crack tip. K-field based displacements were applied on the boundary of the micromechanical model, and FEM results were obtained and compared with theoretical findings. The critical stress intensity factor for a graphene sheet was found to be K _IC = 2.63 ~ 3.2MPa \(\sqrt m \) for the case of a zigzag crack.     

氢对裂纹尖端塑性变形及裂纹扩展影响的TEM观察

利用透射电镜原位动态拉伸方法观察、分析氢对裂纹尖端塑性变形及裂纹扩展过程的影响。结果表明，氢促进裂纹尖端的局部塑性变形，改变裂纹扩展方式，并使裂纹以“Ｚ”字型的途径扩展。     

V形切口尖端裂纹起裂方向的主应力判别准则及其验证

围绕V形切口尖端裂纹起裂方向,分析了V形切口尖端裂纹应力场、位移场、应力强度因子,提出了裂纹起裂方向的主应力判别准则。首先,详细给出了V形切口尖端应力应变场的求解方法,通过裂纹尖端场本征值的三次线性拟合及误差分析,确定了V形切口尖端裂纹位移场;然后,建立了V形切口尖端的数值分析模型,运用数值计算方法确定了应力强度因子和切口强度因子,提出了V形切口尖端裂纹起裂方向的主应力判断准则,给出了外推法求解分析过程;最后,以LY8为试验材料,在张角2β＝60°的V形切口情况下,对提出的V形切口尖端裂纹起裂方向计算方法与判别准则进行了试验验证。     

Numerical calculation and experimental research on crack arrest by detour effect and joule heating of high pulsed current in remanufacturing

The remanufacturing blanks with cracks were considered as irreparable. With utilization of detour effect and Joule heating of pulsed current, a technique to arrest the crack in martensitic stainless steel FV520B is developed. According to finite element theory, the finite element（FE） model of the cracked rectangular specimen is established firstly. Then, based on electro-thermo-structure coupled theory, the distributions of current density, temperature field, and stress field are calculated for the instant of energizing. Furthermore, the simulation results are verified by some corresponding experiments performed on high pulsed current discharge device of type HCPD-I. Morphology and microstructure around the crack tip before and after electro pulsing treatment are observed by optical microscope（OM） and scanning electron microscope（SEM）, and then the diameters of fusion zone and heat affected zone（HAZ） are measured in order to contrast with numerical calculation results. Element distribution, nano-indentation hardness and residual stress in the vicinity of the crack tip are surveyed by energy dispersive spectrometer（EDS）, scanning probe microscopy（SPM） and X-ray stress gauge, respectively. The results show that the obvious partition and refined grain around the crack tip can be observed due to the violent temperature change. The contents of carbon and oxygen in fusion zone and HAZ are higher than those in matrix, and however the hardness around the crack tip decreases. Large residual compressive stress is induced in the vicinity of the crack tip and it has the same order of magnitude for measured results and numerical calculation results that is 100 MPa. The relational curves between discharge energies and diameters of the fusion zone and HAZ are obtained by experiments. The difference of diameter of fusion zone between measured and calculated results is less than 18.3%. Numerical calculation is very useful to define the experimental parameters. An effective method to prevent further extensi     

基于傅里叶变换相位提取法的激光回馈应力测量系统

高端玻璃的内部应力精确测量关系其所在系统的安全性和可靠性。本文提出一种基于激光回馈效应的应力测量方法,激光回馈系统由激光器和外部反射镜构成,待测样品放置在回馈外腔中,通过回馈光对激光器内部增益调制产生的偏振跳变现象提取双折射信息,进而获得应力。首先,从理论上分析了回馈系统中激光器输出的正交偏振模式相位与外腔应力双折射的关系;接着,通过傅里叶变换的方式得到双折射外腔激光回馈系统光强调谐曲线的相位信息;然后,采用标准四分之一波片对系统和算法的精度进行了测试。最后,采用激光回馈系统对不同的飞机座舱有机玻璃样品内应力进行了测量,并给出测量结果。实验结果表明:该系统对应力的条纹数测量精度优于8.3×10-4,满足高端玻璃的应力检测需求。     

铝合金板材中心孔裂纹尖端塑性区数值计算

通过有限元对铝合金板材中心孔裂纹尖端处塑性区模拟计算,说明在裂纹尖端产生了较大的塑性区,并相应地计算出塑性区的大小。本文通过有限元模拟2124铝合金板材中心孔裂纹扩展情况。铝合金材料为典型的脆性材料,2124铝合金板材在疲劳加载情况下会先进行弹性形变,达到屈服强度后进行塑性形变。本文对2124铝合金板材进行有限元模拟时,先采用线弹性模型,计算裂纹扩展的应力强度因子,然后采用弹塑性模型,计算裂纹尖端的塑性区大小,从而进一步对裂纹尖端应力强度因子进行修正。在建立有限元模型时,以二维的Ramberg-Os-good（R-O）本构为基础,采用参数化的方式,这样是为了可以更好地对有限元程序进行调试。在有限元网格划分时,由于在相同精度下四边形单元的计算效率是三角形单元的几倍,所以采用四边形单元,提高计算精度。有限元建模时,采用plane42、solid 45和solid 95三种单元,plane42单元用于建立2D网格,solid45单元用于建立3D网格,而solid95单元则是用于引入奇异单元。同时,由于试样模型对称性,所以取1/4模型来进行计算。在计算裂纹尖端应力强度因子及塑性区大小时,采用恒ΔK方式和增ΔK两种加载方式来进行计算。首先在恒ΔK下,计算出相应的应力强度因子,其值和理论值相吻合,同时观察得到的塑性区形状与理论形状相似,计算塑性区尺寸大小,首先证明有限元程序的正确性。进一步有限元模拟计算在增大ΔK情况下不同预裂纹长度下塑性区的变化情况。经过有限元计算得到的塑性区尺寸大小,最后可以近似用经验公式表达。     

The effect of continuous and pulsed beam modes on cut path deviation in diode laser cutting of glass

During laser cleaving of brittle materials, with the controlled fracture technique, thermal stresses are generated which induce the crack and extend it along the cutting path, subsequently causing material separation. One of the problems in laser cutting of glass with this technique is the cut path deviation at the leading and the trailing edges of the glass sheet. Previous work with a continuous beam diode laser has shown the deviation to be partly due the high magnitudes of thermal stresses generated near the edges of the sheet. This paper reports on the effects of using a pulsed diode laser to cut soda lime glass. The effect of pulse parameters and cutting speed on the quality output variables such as cut deviation angle and surface finish are studied. Finite element modelling is also used to simulate the effects of the moving beam on stress generations to facilitate the understanding of the process mechanisms, and the results are compared with the experimental data. This work shows how to minimise the cut path deviation at the edges by reducing thermal stresses using optimum pulsed diode laser parameters and providing additional flexibility to the process.     

Accuracy improvement technique for measuring stress intensity factors in photoelastic experiment

Fracture coefficients, together with the exact origin of the crack, were extracted from data sets produced an overdeterministic system solved by an iterative least squares method. Power series type williams equations were used in the analyses. The accuracy evaluation indicated that the first four terms of williams equations are sufficient to describe the stress field in the vicinity of the crack tip for both mode I and mixed mode cases. Experimental study showed that the first two terms of williams equations, which are the same as the modified westergaard equations, cannot be used to extract mixed mode fracture parameters accurately within the data collection region of 0.07 <r/a <0.30, where r and a are radial coordinate and crack length for an edge crack, respectively.