Applying an On-line Crack Detection Technique For Laser Cutting by Controlled Fracture

A new laser cutting system, which uses a laser to induce controlled fracture and combines an image processing technique for crack detection and compensation of the laser path to control crack direction, is proposed in this study. The laser cutting is based on the controlled fracture technique, the laser beam strikes the surface of a thin brittle plate which results in a stable fracture. The cutting of the material results from crack propagation along the cutting path. Because the crack propagation at the crack tip is slower than the moving laser beam, the actual fracture trajectory will deviate from the desired trajectory, for example, when cutting a curve or a right angle, especially at high cutting speed. In order to eliminate the deviation, the actual position of the crack tip must be detected on-line and the path of the laser beam must be modified. The compensation is based on the lagging distance between the crack tip and the laser spot. The laser cutting system comprises a CO2 laser, a digital image-processing system, and an XYZ positioning table. The cutting of right angles for silicon wafers and the cutting of circular arcs for alumina ceramics are carried out to validate the system. The image processing speed of the crack detection system is 0.1 s per detection, which is fast enough for continuous on-line crack detection, so that the method can be applied for any complicated curve cutting.     

Thermal stress fracture mode of CO2 laser cutting of aluminum nitride

A thermal stress fracture mode of material removal by laser cutting was conducted in 1-mm thick wafers of aluminum nitride (AlN) using a continuous wave CO₂ laser with a defocused beam. In this mode, a thin layer (10–20 μm) of AlN surface was melted in an oxygen environment to form aluminum oxide. Solidification of the melt layer coupled with thermal expansion mismatch generated thermal stresses that in turn created a crack along the middle path of the laser beam, resulting in material separation. Thermochemical modeling of laser heating, oxide forming, and subsequent cooling of AlN was performed to validate the formation of cracks as well as material separation through unstable crack propagation. A comparison with the conventional “evaporation/melt and blow” laser cutting method showed that the thermal stress method offers significant benefits such as improved precision, better cut quality, higher cutting speed, and lower energy losses.     

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.     

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.     

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.     

Examination of internal stress by photoelasticity in laser cleaving of glass

Laser cleaving is a glass-cutting technique in which thermal stress induced by laser heating and cooling produces cracks in the glass. Stress measurement during the laser cleaving process is critical in elucidating the crack-propagation mechanism and solving the problems of the laser cleaving method. In this study, we measured the birefringence retardation using a high-speed polarization camera and evaluated the relevance and accuracy of the measured values by comparing them with the results of a numerical calculation. The birefringence retardation at the crack tip was also observed in the experimental process. For the experiment, a soda lime glass was cleaved using CO₂ laser irradiation. Then, the birefringence retardation and azimuthal angle obtained using the polarization camera were compared with the numerical calculation results. The birefringence retardation around the crack tip corresponded with that of the deformation caused by mode I. The crack propagation was arrested when the crack tip approached the edge of the glass. The birefringence retardation observed using the polarization camera confirmed that the mode I deformation decreased as the crack approached the edge.     

单晶硅片在脉冲激光作用下的断裂行为

基于脆性材料在激光辐照下的断裂行为,将可控断裂激光切割技术应用于脆性材料的加工.为了分析脉冲激光辐照脆性材料过程及脉冲激光扫描过程中产生的断裂行为机理,采用数值计算方法建立了含有裂纹的三维有限元热弹计算模型.分析了脉冲激光辐照单晶硅片过程中温度场和热应力场的变化情况,并模拟计算了硅片边缘含有裂纹时裂纹尖端应力强度因子的...     

Fatigue Crack Growth Rate of Ti-6Al-4V Considering the Effects of Fracture Toughness and Crack Closure

Fatigue fracture is one of the main failure modes of Ti-6A1-4V alloy,fracture toughness and crack closure have strong effects on the fatigue crack growth(FCG)rate of Ti-6A1-4V alloy.The FCG rate of Ti-6A1-4V is investigated by using experimental and analytical methods.The effects of stress ratio,crack closure and fracture toughness on the FCG rate are studied and discussed.A modified prediction model of the FCG rate is proposed,and the relationship between the fracture toughness and the stress intensity factor(SIF)range is redefined by introducing a correcting coefficient.Notched plate fatigue tests(including the fracture toughness test and the FCG rate test)are conducted to investigate the influence of affecting factors on the FCG rate.Comparisons between the predicted results of the proposed model,the Paris model,the Walker model,the Sadananda model,and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region,and the corresponding calculated fatigue life is also accurate in the same regions.By considering the effects of fracture toughness and crack closure,the novel FCG rate prediction model not only improves the estimating accuracy,but also extends the adaptability of the FCG rate prediction model in engineering.     

TiN涂层刀具的抗热冲击性能

采用激光照射的方法研究了TiN涂层刀具的抗热冲击性能,并用有限元的方法计算了TiN涂层刀具在激光照射过程中产生的应力分布.结果表明:利用激光照射的方法研究涂层刀具的抗热冲击性能是可行的,将激光的临界功率密度作为评价涂层刀具抗热冲击性能的参数;涂层在激光照射过程中产生的压应力作用下发生失稳翘曲,进而导致裂纹的产生.试验结果和有限元计算结果吻合良好.     

Investigation of the fracture characteristics of the interfacial bond between bone and cement: experimental and finite element approaches

This study integrated the finite element method, fracture mechanics, and three-point bending test to investigate the fracture characteristics of the interfacial bond between bone and cement. The fracture tests indicated that the interfacial fracture toughness of the bone/cement specimens was 0.34 MN/m^3/2, with a standard deviation of 0.11 MN/m^3/2, which was in good agreement with the experimental data available in the literature. A finite element model of the experimental testing specimen was used to predict the critical stress intensity factor (SIF) at the fracture load by the proposed fracture analysis method. The critical SIF of the opening mode of the interface crack was 0.392 MN/m^3/2, which was slightly higher than the fracture toughness obtained in the experiment. Additionally, considering the coupled effects of the crack opening mode and shearing mode, the critical effective SIF was 0.411 MN/m^3/2, with a phase angle of 17.2°. Comparisons of the results obtained from the bending test and numerical analysis made it obvious that the fracture characteristics of the bonded interface between the bone and cement could be accurately predicted by the proposed model. With this analysis model, a realistic investigation on the debonding behavior of cemented artificial prosthetic components is highly expected.     

Investigation of constantly and transiently propagating cracks in functionally graded materials by dynamic photoelasticity

In this study, the stress intensity factors (SIFs) for steady and transient propagation of cracks in transparent homogeneous functionally graded materials were analyzed by using the photoelasticity technique. The fracture analysis was carried out for the cracks propagating from a region with high elasticity towards low elasticity, as well as the cracks propagating from a region with low elasticity towards high elasticity. The analysis includes cracks propagating (1) at an almost steady speed, and (2) with the rapid increase, followed by a decrease in speed. For cracks with almost constant velocity, the SIFs were greater when a crack started from a high elasticity region, as compared to the cracks which initiated from a low elasticity region. For cracks propagating with rapid acceleration and deceleration, when the strain energy accumulated in the material due to an increase in load or stress was released at the moment of crack propagation, the SIF was momentarily lowered by approximately 45 %–50 % of the static SIF(before crack initiation), which subsequently increases by approximately 30 % eventually, the crack acceleration approaches nearly zero; the SIF decreases and increases respectively as the crack propagates in a material with decreasing and increasing modulus of elasticity.

     

Effects of crack geometries on reliabilities using probability theories

The methodologies to estimate the reliability of cracked structures using the fracture toughness and the SIF are developed. The probability theories such as the FORM and the SORM are utilized to calculate the failure probability. It is found that the failure probability increases with the increase of the crack size and the applied stress, and the decrease of the fracture toughness. It is also found that the failure probabilities obtained by the FORM and the SORM are similar each other for the through crack,the edge crack, and the single and the double crack emanated from a hole, and turn out different for the elliptical and semi-elliptical surface cracks. It is noted that the tensile stress affects significantly on the failure probability among the other random variables on the various crack geometries.     

Mechanical property measurements of nanoscale structures using an atomic force microscope

This paper describes nanometer-scale bending tests of fixed single-crystal silicon (Si) and silicon dioxide (SiO2) nanobeams using an atomic force microscope (AFM). The technique is used to evaluate elastic modulus of the beam materials and bending strength of the beams. Nanometer-scale Si beams with widths ranging from 200 to 800 nm were fabricated on a Si diaphragm using field-enhanced anodization using an AFM followed by anisotropic wet etching. Subsequent thermal oxidation of Si beams was carried out to create SiO2 beams. Results from the bending tests indicate that elastic modulus values are comparable to bulk values. However, the bending strength appears to be higher for these nanoscale structures than for large-scale specimens. Observations of the fracture surface and calculations of the crack length from Griffith's theory appear to indicate that the maximum peak-to-valley distance on the beam top surfaces influence the values of the observed bending strengths.     

厚硅片的高速激光切片研究

基于激光切片原理的分析,给出了厚硅片的高速激光切片方法,采用平凸腔补偿工作物质的热透镜效应,利用Nd∶YAG棒本身的自孔径选模作用,获得了光束质量因子M²等于4.19的50 W 1.064 μm激光输出。选取合适的扩束倍数、重复频率和出气孔直径,当切割0.75 mm厚的硅片时,切片速度达400 mm/min;当切割两层叠放的0.75 mm厚的硅片时,切片速度达到100 mm/min。切片的切口光滑,切缝较窄,重复精度高,切片质量好,达到用传统方法难以达到的切片效果。     

Dynamic crack growth in TDCB specimens

Dynamic crack propagation in tapered double cantilever beam (TDCB) specimens is analysed via beam theory and the finite element method. Steady state and transient solutions of the energy release rate G are given for various load conditions. Finite element analysis is performed to obtain the dynamic G at given crack speed or the crack history for a given fracture toughness. The stress wave effects on the dynamic G are discussed. The beam solutions are compared with the finite element results and some experimental phenomena are explained.     

基于断裂力学的Nomex蜂窝复合材料超声切割机理研究

Nomex蜂窝复合材料的超声切割技术克服了传统高速铣削中存在的工件固持困难、加工粉尘大等问题。基于断裂力学研究Nomex蜂窝复合材料的超声切割机理,为超声切割工艺参数优化以及超声波声学主轴的优化设计提供理论依据。根据直刃刀超声切割Nomex蜂窝复合材料加工工艺,建立直刃刀的运动学方程,分析得到超声切割断续加工过程中直刃刀与材料相互作用时间关系;应用断裂力学理论,引入动态应力强度因子建立蜂窝复合材料的断裂韧性模型,研究超声切割作用下蜂窝复合材料的微观断裂过程,根据直刃刀位移和裂纹扩展的关系模型,分析切削力的影响因素,并进行仿真研究。研制了超声切割工艺试验台,对蜂窝复合材料进行了有超声和无超声切割加工的对比试验,试验结果显示超声切割显著地减小了切削力,也证实了冲击产生的微裂纹扩展是蜂窝复合材料在直刃刀超声切割作用下,切割力减小的主要原因。理论分析和试验研究表明基于断裂力学的Nomex蜂窝复合材料超声切割机理研究具有有效性和合理性。     

光弹性法和有限元法对应力强度因子的求解

为了探讨应力强度因子的计算方法,应用光弹性法和有限元法分别对含单边裂纹的紧凑拉伸试件进行实验分析和数值计算,给出有限元计算断裂问题的详细步骤以及光弹性法中通过求形状因子间接得到应力强度因子的实验过程。结果表明实验值与有限元计算值相吻合。有限元计算能够为优化设计提供较便捷的手段,而实验可以校验数值计算的正确性。两者可以取长补短,发挥各自的优势,可以更好地解决工程问题。     

U形切槽梁准脆性材料力学双参数测试法

提出用变化U形切槽根部圆角半径的纯弯曲梁或三点弯曲梁同时测试准脆性材料断裂韧度和抗拉强度的方法.其测试原理为:首先对U形切槽二等分线上的弹性张开应力在断裂过程区长度上积分,再除以该长度,得到平均应力;然后用非局部应力法和平均应力得到含有断裂韧度和抗拉强度的破坏准则;将材料试验机测得的几组U形切槽梁的临界载荷代入平均应力破坏准则,得到有关这两个力学参数的超定方程组,再用恰当的数值方法求解.用此力学双参数测试方法必须知道U形切槽梁相应的应力集中系数,为此分别给出纯弯曲和三点弯曲U形切槽梁在宽范围内的应力集中系数,它们扩充和改进了现有应力集中系数手册中的有关数据.利用相关文献中的U形切槽梁实验数据验证了建议的测试方法是可行的.     

高速铣削航空铝合金7050-T7451时刀具的磨损破损

分析涂层硬质合金刀具高速铣削航空铝合金7050-T7451时的磨损、破损形态,通过正交试验研究了高速铣削航空铝合金的铣削力及其变化规律,提出模拟高速铣削刀具裂纹萌生的脉冲激光热冲击试验方法,研究铣削力和热应力在刀具磨损、破损过程中的不同作用.试验和理论分析证明:高速铣削航空铝合金7050-T7451时,热应力使刀具萌生裂纹,裂纹在热应力和机械应力综合作用下扩展.研究刀具失效机理,证明:高速铣削航空铝合金时,粘结磨损和扩散磨损是主要磨损机理.提出通过提高切削系统稳定性和优化切削参数,可以有效降低机械应力对刀具的冲击作用,并在生产现场收到良好的效果.     

Mechanical characterization of micro/nanoscale structures for MEMS/NEMS applications using nanoindentation techniques

Mechanical properties of micro/nanoscale structures are needed to design reliable micro/nanoelectromechanical systems (MEMS/NEMS). Micro/nanomechanical characterization of bulk materials of undoped single-crystal silicon and thin films of undoped polysilicon, SiO(2), SiC, Ni-P, and Au have been carried out. Hardness, elastic modulus and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Fracture toughness was measured by indentation using a Vickers indenter. Bending tests were performed on the nanoscale silicon beams, microscale Ni-P and Au beams using a depth-sensing nanoindenter. It is found that the SiC film exhibits higher hardness, elastic modulus and scratch resistance as compared to other materials. In the bending tests, the nanoscale Si beams failed in a brittle manner with a flat fracture surface. The notched Ni-P beam showed linear deformation behavior followed by abrupt failure. The Au beam showed elastic-plastic deformation behavior. FEM simulation can well predict the stress distribution in the beams studied. The nanoindentation, scratch and bending tests used in this study can be satisfactorily used to evaluate the mechanical properties of micro/nanoscale structures for use in MEMS/NEMS.