Nd: YAG激光烧蚀裂解加工技术模拟分析与实验研究

激光裂解技术能够极大改善发动机缸体主轴承座的加工质量并显著提高加工效率. 为探寻Nd:YAG激光烧蚀球墨铸铁材料裂解槽的裂解性能, 本文基于有限元法成功构建了发动机缸体主轴承座激光裂解加工过程仿真模型, 针对QT500-7球墨铸铁主轴承座的裂解参数进行了仿真分析. 研究结果表明: 在影响裂解质量的三个裂解槽几何参数中, 槽深较张角及曲率半径对裂解载荷的影响效应更为明显; 裂解载荷随槽深的增加而迅速降低, 随槽张角和曲率半径的增加而升高; QT500-7球墨铸铁发动机缸体主轴承座激光裂解加工优化参数应为裂解槽深选为0.5 mm, 裂解槽张角选为60^o, 裂解槽半径选为0.2 mm. 有限元模拟分析结果得到了单向拉伸实验结果的验证. 本工作通过ABAQUS仿真模拟及大量裂解载荷试验确立了裂解槽几何形状的优化参数, 为显著降低裂解载荷和优化裂解工艺提供了数值参考, 有利于实现发动机缸体加工的快速发展, 从而促进汽车工业实现绿色制造.     

Laser cutting silicon-glass double layer wafer with laser induced thermal-crack propagation

This study was aimed at introducing the laser induced thermal-crack propagation (LITP) technology to solve the silicon-glass double layer wafer dicing problems in the packaging procedure of silicon-glass device packaged by WLCSP technology, investigating the feasibility of this idea, and studying the crack propagation process of LITP cutting double layer wafer. In this paper, the physical process of the 1064 nm laser beam interact with the double layer wafer during the cutting process was studied theoretically. A mathematical model consists the volumetric heating source and the surface heating source has been established. The temperature and stress distribution was simulated by using finite element method (FEM) analysis software ABAQUS. The extended finite element method (XFEM) was added to the simulation as the supplementary features to simulate the crack propagation process and the crack propagation profile. The silicon-glass double layer wafer cutting verification experiment under typical parameters was conducted by using the 1064 nm semiconductor laser. The crack propagation profile on the fracture surface was examined by optical microscope and explained from the stress distribution and XFEM status. It was concluded that the quality of the finished fracture surface has been greatly improved, and the experiment results were well supported by the numerical simulation results.     

Mixed mode I/II brittle fracture in V-notched Brazilian disk specimens under negative mode I conditions

The well-known round-tip V-notched Brazilian disk specimen is utilized for conducting mixed mode I/II fracture tests on PMMA under negative mode I conditions for different notch angles and various notch radii with the aim to measure experimentally the fracture load and the fracture initiation angle. It is shown by the finite element analysis that although the notch is under negative mode I loading, one side of the notch border still experiences tensile tangential stresses suggesting that fracture would take place from the same side of notch border. Experimental observations also indicated that fracture occurs from the tensile side of the notch border confirming the finite element results. The experimental results are then theoretically estimated by means of two stress-based brittle fracture criteria, namely the round-tip V-notch maximum tangential stress and the mean-stress criteria. It is shown that both criteria provide very good predictions to the experimental results obtained under negative mode I conditions.     

Comparison of 1D transfer matrix method and finite element method with tests for acoustic performance of multi-chamber perforated resonator

Multi-chamber perforated resonator (MCPR) is a kind of typical silencer element which can both attenuate broadband noise and satisfy specific installation requirements. The one-dimensional transfer matrix method (TMM) and finite element method (FEM) are widely used to predict the transmission loss of the resonators. This paper mainly focuses on the comparison between 1D TMM and FEM in which detailed perforation modeling is applied for the acoustic modeling of MCPRs. Five resonators with different acoustic attenuation frequency ranges are built for simulation and test. In order to verify the results of the above methods, a transmission loss test facility is designed based on two-load method. Through adjusting the distance between microphones, the facility’s effective measurement frequency can be changed. The results show that despite of the complex modeling and calculation, FEM with detailed perforation modeling shows good consistency with test results in both frequency and amplitude within entire frequency range. In comparison, TMM is limited by the cut-off frequency when calculating transmission losses. Besides, accuracy of TMM in low frequency range is also affected by perforation conditions. However, TMM is time-saving in calculation and structure optimization. In MCPRs’ development process, TMM can be used to quickly design and optimize structure parameters while FEM can be used to verify the acoustic performance before prototyping.     

FEM simulations and experimental studies of the temperature field in a large diamond crystal growth cell

We investigate the temperature field variation in the growth region of a diamond crystal in a sealed cell during the whole process of crystal growth by using the temperature gradient method (TGM) at high pressure and high temperature (HPHT). We employ both the finite element method (FEM) and in situ experiments. Simulation results show that the temperature in the center area of the growth cell continues to decrease during the process of large diamond crystal growth. These results are in good agreement with our experimental data, which demonstrates that the finite element model can successfully predict the temperature field variations in the growth cell. The FEM simulation will be useful to grow larger high-quality diamond crystal by using the TGM. Furthermore, this method will be helpful in designing better cells and improving the growth process of gem-quality diamond crystal.     

反射声波成像测井的有限元模拟

应用有限元法对新兴的反射声波成像测井进行了数值模拟研究，计算了井旁附近存在不同倾角声阻抗不连续界面对声波的反射，使用的井内激励源为幅度加权相控线阵声波辐射器。从计算出的声全波波形上可以清楚地看到沿井壁传播的折射波和来自井旁界面的反射纵波。应用偏移叠加等数据处理技术对数值模拟波形进行了处理，从偏移剖面上反演出的反射界面的尺寸、位置和倾角等几何特性与数值模拟输入参数基本一致。     

ANSYS仿真激光切割氧化锌纳米线

 以脉冲高斯激光为光源,直径在500 nm～1 000 nm、长度10 μm的一维氧化锌材料为对象,采用有限元分析软件ANSYS建立了激光切割氧化锌纳米线的热力学仿真模型,并采用生死单元技术对超过熔点的单元进行处理,得到不同激光工作参数和氧化锌纳米线直径下的温度分布场和切割形貌。讨论了纳米线直径和聚焦光斑相对纳米线的位移对切割的影响,纳米线直径越大允许的激光离焦量越小;当无离焦或负离焦较小时,切割纳米线产生的形貌较为理想。     

Influence of laser beam profile on the generation of ultrasonic waves

The different ultrasonic fields generated in metallic materials by a laser beam with flat and Gaussian profile are investigated experimentally and using the finite element method (FEM). A high power laser beam irradiating a solid surface produces elastic waves with a mechanics that depends on many parameters, including the profile of the laser beam. The influence of the beam profile is investigated with the FEM analysis, considering the temperature dependence of material properties.     

Thermal stress cleaving of silicon wafer by pulsed Nd：YAG laser

The applied laser energy absorbed in a local area in laser thermal stress cleaving of brittle materials using a controlled fracture technique produces tensile thermal stress that causes the material to separate along the moving direction of the laser beam. The material separation is similar to crack extension, but the fracture growth is controllable. Using heat transfer theory, we establish a three-dimensional （3D） mathematical thermoelastic calculational model containing a pre-existing crack for a two-point pulsed Nd：YAG laser cleaving silicon wafer. The temperature field and thermal stress field in the silicon wafer are obtained by using the finite element method （FEM）. The distribution of the tensile stress and changes in stress intensity factor around the crack tip are analyzed during the pulse duration. Meanwhile, the mechanism of crack propagation is investigated by analyzing the development of the thermal stress field during the cleaving process.     

激光辐照受拉铝板破坏行为的时空多尺度数值模拟

推导耦合过渡区内参变量信息交换的元/网格动量传递多尺度算法,建立离散元与有限元耦合时空多尺度计算模型,并应用于激光辐照下受拉铝板破坏行为的数值模拟中.通过对比有限元计算模型、空间多尺度计算模型与时空多尺度计算模型在激光辐照下受拉铝板破坏算例的模拟结果,验证离散元与有限元耦合时空多尺度计算模型的准确性和数值计算高效率优势.使用该多尺度计算模型从宏观和细观尺度对铝板破坏行为进行数值模拟,模拟结果与实验结果基本一致.     

底部气流对CO2激光切割硅钢切口质量的影响

激光切割相对冲压及线切割工艺具有自由度大、加工效率高、不会影响铁心质量等优点,但由于残渣及切口粗糙问题一定程度上影响了激光切割硅钢片的质量和应用范围.通过采用在工件底部增设辅助侧吹喷嘴,控制熔渣流向,保证成品切割质量激光切割工艺,试验证明,合理控制工艺参量,可获得良好效果.利用有限元法进一步对工件底部气流状况进行了数值模拟,分析了流动过程中在不同的角度和流速下光斑移动和气流场变化的情况,初步确定工件底部侧吹气流以20°吹入的辅助侧吹工艺,为进一步合理控制熔渣流向获得光滑的精细切口提供了实践和理论依据.     

The finite element method for micro-scale modeling of ultrasound propagation in cancellous bone

Quantitative ultrasound for bone assessment is based on the correlations between ultrasonic parameters and the properties (mechanical and physical) of cancellous bone. To elucidate the correlations, understanding the physics of ultrasound in cancellous bone is demanded. Micro-scale modeling of ultrasound propagation in cancellous bone using the finite-difference time-domain (FDTD) method has been so far utilized as one of the approaches in this regard. However, the FDTD method accompanies two disadvantages: staircase sampling of cancellous bone by finite difference grids leads to generation of wave artifacts at the solid–fluid interface inside the bone; additionally, this method cannot explicitly satisfy the needed perfect-slip conditions at the interface. To overcome these disadvantages, the finite element method (FEM) is proposed in this study. Three-dimensional finite element models of six water-saturated cancellous bone samples with different bone volume were created. The values of speed of sound (SOS) and broadband ultrasound attenuation (BUA) were calculated through the finite element simulations of ultrasound propagation in each sample. Comparing the results with other experimental and simulation studies demonstrated the capabilities of the FEM for micro-scale modeling of ultrasound in water-saturated cancellous bone.     

A finite-element method using dispersion reduced spline elements for room acoustics simulation

This paper presents a finite element method (FEM) using hexahedral 27-node spline acoustic elements (Spl27) with low numerical dispersion for room acoustics simulation in both the frequency and time domains, especially at higher frequencies. Dispersion error analysis in one dimension is performed to increase the accuracy of FEM using Spl27 by modifying the numerical integration points of element stiffness and mass matrices. The basic accuracy and efficiency of the FEM using the improved Spl27, which uses modified integration points, are presented through numerical experiments using benchmark problems in both the frequency and time domains, revealing that FEM using the improved Spl27 in both domains provides more accurate results than the conventional method does, and with fewer degrees of freedom. Moreover, the effectiveness of FEM using the improved Spl27 over that using hexahedral 27-node Lagrange elements is shown for time domain analysis of the sound field in a practical sized room.     

Finite element acoustic simulation based shape optimization of a muffler

This paper describes a methodology which combines finite element analysis and Zoutendijk’s feasible directions method for mufflers shape design. The main goal is to obtain the dimensions of the acoustic muffler with the transmission loss (TL), being maximized in the frequency range of interest. The improved four parameters method is used for TL evaluations and the Helmholtz’s equation is solved numerically with the finite element method (FEM). The quadratic triangular finite element meshes are adequately constructed to control the pollution error and the optimization problem is solved using the Zoutendijk’s feasible directions method due to robustness and efficiency for problems with nonlinear constraints. Numeric experiments performed with circular expansion chambers with extended inlet and outlet show results for constrained and unconstrained shape optimization.     

Semi-analytical model for quasi-double-layer surface electrode ion traps

To realize scale quantum processors,the surface-electrode ion trap is an effective scaling approach,including singlelayer,double-layer,and quasi-double-layer traps.To calculate critical trap parameters such as the trap center and trap depth,the finite element method(FEM) simulation was widely used,however,it is always time consuming.Moreover,the FEM simulation is also incapable of exhibiting the direct relationship between the geometry dimension and these parameters.To eliminate the problems above,House and Madsen et al.have respectively provided analytic models for single-layer traps and double-layer traps.In this paper,we propose a semi-analytical model for quasi-double-layer traps.This model can be applied to calculate the important parameters above of the ion trap in the trap design process.With this model,we can quickly and precisely find the optimum geometry design for trap electrodes in various cases.     

Analysis of the formation and evolution of oriented microstructures on laser ablated silicon

A theoretical approach and qualitative analysis of the changes induced on the surface morphology and the formation of microstructures on silicon targets irradiated by excimer laser are presented. This study is based on theoretical principles of the laser ablation process, in particular, on the analysis of the contribution of the laser energy density, which involves the laser beam parameters and also the physical properties of the target material. For different laser incident angles, the formation of micro-columns oriented towards the laser incident direction is explained. Moreover, numerical simulations and ablation experiments carried out with an excimer laser corroborate the theoretical analysis.     

Linear least squares approach for evaluating crack tip fracture parameters using isochromatic and isoclinic data from digital photoelasticity

In this work, digital photoelasticity technique is used to estimate the crack tip fracture parameters for different crack configurations. Conventionally, only isochromatic data surrounding the crack tip is used for SIF estimation, but with the advent of digital photoelasticity, pixel-wise availability of both isoclinic and isochromatic data could be exploited for SIF estimation in a novel way. A linear least square approach is proposed to estimate the mixed-mode crack tip fracture parameters by solving the multi-parameter stress field equation. The stress intensity factor (SIF) is extracted from those estimated fracture parameters. The isochromatic and isoclinic data around the crack tip is estimated using the ten‐step phase shifting technique. To get the unwrapped data, the adaptive quality guided phase unwrapping algorithm (AQGPU) has been used. The mixed mode fracture parameters, especially SIF are estimated for specimen configurations like single edge notch (SEN), center crack and straight crack ahead of inclusion using the proposed algorithm. The experimental SIF values estimated using the proposed method are compared with analytical/finite element analysis (FEA) results, and are found to be in good agreement.     

Effects analysis of ambient conditions on process of laser surface melting

In the process of laser surface melting (LSM), ambient conditions around the workpiece have important influences on the processing results. As an effective and feasible method for ambient changing, water-assisted approach can be expected to gain better results such as desired machining goals and reliable service performances. However, the effects of different water ambient on LSM process are needed to be further clarified. To this end, three 3-D transient process models in ambient dry air, water film and water are developed, respectively, using finite element method (FEM); the thermo-mechanical parameters, which depend on temperature, are taken into account and the complex physical essences are integrated. In experimental verification, these three LSM processes on mild steel Q235 are carried on and the computed results are in good agreement with respective measurements. Based on the proposed models, the transient temperature fields and residual stress distributions on workpieces are investigated. The numerical results suggest that the states of temperature and residual stress fields can be improved to different degrees using water film and water ambient.     

A combined three-dimensional finite element and scattering matrix method for the analysis of plane wave diffraction by bi-periodic,multilayered structures

A three-dimensional finite element method (FEM) for the analysis of plane wave diffraction by a bi-periodic slab is described and implemented. A scattering matrix formalism based on the FEM allows the efficient treatment of light reflection and transmission by multilayer bi-periodic structures, and the computation of Bloch modes of three-dimensional arrays. Numerical simulations, which show the accuracy and flexibility of the FEM, are presented.     

Statistical analysis of parameter effects on bending angle in laser forming process by pulsed Nd:YAG laser

In recent years, laser application has been introduced for bending and forming as new processes in manufacturing. The capability of laser bending demands more studies to recognize parameters influencing bending angle of sheet metals. In this study the effects of parameters such as material, laser power, beam diameter, scan velocity, sheet thickness, pass number and pulse duration on bending angle were studied by FEM initially and then followed by experiments. Furthermore, the Taguchi experimental design method was employed to pin point parameters, which significantly affect the bending process of laser bending of St12 and 304 alloy steels, which have a wide range of applications in products manufacturing. A regression analysis was conducted and a closed form equation was derived. The closed form equation can be used in industry to determine which process parameters (factors) enhance the bending angle in laser bending process.