残余应力测定的围箍压痕法模拟

本文从同工量测方法得到启发，提出了一种新型的残余应力测试方法－围箍压痕法。采用轴对称弹塑性有限元计算模拟了围箍压痕法测量结构残余应力的力学过程，得到了围箍压痕时不同残余应力程度下材料的变形，塑性区和接触应力分布。     

基于轮廓法测量钛合金激光熔覆中的残余应力

由于激光熔覆过程中有温度变化快、局部温度梯度大和伴随固液相快速转化等特性,在熔池和热影响区会产生很大的残余应力和变形,不利于金属增材制造和局部修复的精确可控。本文对Ti-6Al-4V(TC4)合金开展激光熔覆实验,采用轮廓法测量了激光熔覆所产生的残余应力;通过三维热力耦合有限元模型计算了温度场和残余应力场,仿真计算结果与轮廓法测量的残余应力趋势一致。结果表明,残余应力在熔覆位置有最大拉伸应力,随着远离焊缝急剧变成压缩应力并逐渐减小。上述工作有利于研究熔覆过程中的温度分布规律及残余应力的生成机制,可为熔覆过程的工艺控制提供参考。     

硅基底多层薄膜结构材料残余应力的微拉曼测试与分析

针对MEMS器件制备中两种典型的硅基底多层薄膜结构的残余应力问题,本文提出了利用微拉曼光谱技术测量其残余应力的方法,分析并给出了硅基底多层薄膜结构中的残余应力分布规律。实验结果表明,在硅基底和薄膜内存在较大的工艺残余应力,残余应力在基底内靠近薄膜两侧部分呈非线性变化,在基底内主要呈线性变化,并引起基底整体翘曲。基于实验结果分析,提出了硅基底多层薄膜结构的分层结构模型。本文工作表明微拉曼光谱技术是测量与研究硅基底多层薄膜结构残余应力的一种有力手段。     

基于FIB的微观变形载体制作技术研究及应用

变形载体包括光栅、散斑和标记点等,是光学变形测量时的重要载体,关系到测试的成败.基于聚焦离子束(FIB)微加工平台和技术,介绍了微观变形载体的设计、制作方法、程序等要点,并且利用FIB刻蚀微型孔、槽等实现材料表面微观残余应力的测量.分析和讨论了FIB制作变形载体对原有结构的影响和由此引起的残余应力测试误差.结果表明,FIB刻蚀作为一种新型的直写微纳米加工技术,结合高倍显微镜对视场的切换,可以在关键微区准确定位,并制作变形载体和实现变形测量,尤其对残余应力的测量特别有效.     

钻孔法测量残余应力中几个问题的探讨

本文针对钻孔法测量残余应力中一些不能用“释放”理论解释的问题,通过对钻孔法基本测量原理的分析和与其它传统电测方法对比,认为常用于测量残余应力的电测方法中钻孔法应定义为“干扰法”,切条法和套孔法为“释放法”,Sach＇s镗削法则为“干扰-释放法”.根据实验结果推荐公式=(λ-1)~(1/2)为预测钻孔深度的经验公式.同时应用“干扰”理论分析了压痕法测量残余应力的基本特性,提出压痕法与电测法结合将发展成为一种近似无损的测量残余应力的新方法.     

低温数字图像相关方法中的制斑技术及其相关应用

数字图像相关技术(Digital Image Correlation, DIC)是一种基于散斑载体的全场变形测量技术。散斑载体的制备是实现变形测量的关键,目前已有喷涂、刻蚀、转印等一系列制斑方法。低温实验测试表明,常温制斑工艺在低温下会出现脆化效应,导致载体承受大变形时易开裂和脱落,使得变形测量产生较大误差,此外测量结果也受到结霜和气流扰动等问题影响,因此迫切需要开发低温散斑制备工艺。结合相关需求,本文发展了刻蚀喷涂法和基于聚四氟乙烯(PTFE)衬底的喷涂法2种低温制斑工艺,由此制得的散斑在低温条件下具有良好的稳定性,低温验证实验中,试件直至断裂前均未发生衬底开裂和散斑脱落;使用13X分子筛干燥方法解决了低温测量中的结霜问题,减少了结霜引起的图像噪声。将开发的低温制斑工艺成功应用于焊接件的低温力学性能测试和断裂力学参数反演,常温和-180℃的拉伸实验测试结果表明,低温下材料的弹性模量呈递增趋势,应力强度因子(Stress-intensification Factor, SIF)呈递减趋势。     

双折射主切片法研究热历史对有机玻璃残余应力分布的影响

本文应用双折射主切片法研究了热历史对有机玻璃残余应力分布的影响。研究表明:淬火有机玻璃残余应力的分布规律与玻璃型号、冷却方式无无关;残余应力的数值取决于淬火冷却速率。退火能大幅度降低淬火有机玻璃内的残余应力,消除程度取决于退火温度和保温时间。在测定淬火有机玻璃残余应力方面,本文所推荐的双折射主切片法不仅远优于现有的只能定性和半定量的表面应力测量方法,而且优于能定量的剥层法和双折射次切片法。     

各向同性材料中微裂纹的最大屏蔽效应

通过对微裂纹屏蔽不同来源的分析及计算,发现在各向同性脆性材料中,残余应力释放引起的微裂纹对主裂尖产生最大屏蔽效应时该微裂纹的倾角与最大张应力的方向没有明显的对应关系．在Hutchinson[1]所指出的屏蔽效应的第二个来源中,还应计及微裂纹形成引起的远场应力在微裂纹处产生的应力场的释放从而导致应力场的再分布．     

疲劳裂纹扩展时焊接残余应力分布的测定

本文采用切割法测定了沿焊缝横向残余应力的分布规律．由于切割试样可以近似地模拟疲劳裂纹扩展，通过测量释放的应力，计算出残余应力的分布，可发现残余应力对疲劳裂纹扩展速率的影响规律．     

聚晶金刚石复合片热残余应力分布规律实验研究

利用改进的应力释放法、X射线衍射法以及Raman光谱,对平面界面结构金刚石复合片表面热残余应力分别进行了实验研究,得到了金刚石层表面热残余应力值及其分布规律,同时得到了基体厚度与热残余应力的相关关系.研究结果表明,采用应力释放法、X射线衍射法及Raman光谱法测试PDC表面热残余应力,其测试结果均与有限元分析结果相吻合,证明了这三种方法的有效性.其中,X射线衍射法测试结果的误差最大,应力释放法其次,Raman光谱法最为精确.由于应力释放法应变片尺寸及X射线衍射法光斑照射范围的限制,无法在试样表面上取较多的测试点,因此难以得到理想的热残余应力分布曲线.而Raman光谱法中所采用的激光光斑仅5μm,可以取更多的测试点,因此其结果更能真实的反映金刚石层表面热残余应力的分布规律.本文的研究结果为精确测试PDC热残余应力,从而为优化PDC界面结构、提高PDC使用性能提供了理论和实验依据.     

Whole-field residual stress measurement in rail using moiré interferometry and twyman/green interferometry via thermal annealing

A novel whole-field residual stress measurement technique is developed using moiré interferometry and Twyman/Green interferometry coupled with thermal annealing. The technique is successfully applied to residual stress measurement in rail. In the measurement, a high temperature resistant 1200-lines/mm cross grating is made on a rail transverse slice surface. The whole-field residual stress relief is achieved by thermal annealing. Moiré interferometry and Twyman/Green interferometry are employed to obtain the in-plane and out-of-plane deformations generated by the residual stress relaxation. The whole-field strain redistribution due to the residual stress relief is calculated, and the whole-field residual stress distribution, including the possible stress concentration, is then obtained. Because of the three-dimensional nature of the residual stress relaxation and the measurement, the three-dimensional residual stress reconstruction sometimes becomes possible based on some plausible assumptions. In this paper, the principle of the experimental theory, technique and procedures are described. Three-dimensional residual stress reconstruction in a rail using a transverse slice is shown. Its comparison to the hole-drilling method with moiré interferometry is also presented.     

Acoustoelastic determination of residual stress with laser doppler velocimetry

The main object of this study is to develop a new technique for stress nondestructive measurement. A noncontact measurement technique of ultrasonic wave velocity is proposed. In the measurement system, a laser Doppler velocimeter, which is noncontact, is used to detect wave motions due to Rayleigh waves instead of a piezoelectric transducer. The noncontact measurement technique is applied to determine the stress-acoustic coefficient of Rayleigh waves for aluminum 5052 and a structural steel, and the results are in good agreement with those obtained using knife-edge piezoelectric transducers. The technique is also used to evaluate residual stress existing in an H-section rolled beam of the structural steel. The distribution of residual stress is reasonable.     

Micron-Scale Residual Stress Measurement by Micro-Hole Drilling and Digital Image Correlation

This paper reports a new technique, namely the incremental micro-hole-drilling method (IμHD) for mapping in-plane residual or applied stresses incrementally as a function of depth at the micron-scale laterally and the sub-micron scale depth-wise. Analogous to its macroscale counterpart, it is applicable either to crystalline or amorphous materials, but at the sub-micron scale. Our method involves micro-hole milling using the focused ion beam (FIB) of a dual beam FEGSEM/FIB microscope. The resulting surface displacements are recorded by digital image correlation of SEM images recorded during milling. The displacement fields recorded around the hole are used to reconstruct the stress profile as a function of depth. In this way residual stresses have been characterized around a drilled hole of 1.8microns. diameter, enabling the profiling of the stress variation at the sub-micron scale to a depth of 1.8 microns. The new method is used to determine the near surface stresses in a (peened) surface-severe-plastically-deformed (S²PD) Zr₅₀Cu₄₀Al₁₀ (in atomic percent, at.%) bulk metallic glass bar. In plane principal stresses of -800 MPa ± 90 MPa and −600 MPa ± 90 MPa were measured, the maximum compressive stress being oriented 15° to the axis of the bar.     

Assessment of Primary Slice Release Residual Stress Mapping in a Range of Specimen Types

This paper further explores the primary slice removal technique for planar mapping of multiple components of residual stress and describes application to specimens with a range of alloys, geometries, and stress distributions. Primary slice release (PSR) mapping is a combination of contour and slitting measurements that relies on decomposing the stress in a specimen into the stress remaining in a thin slice and the stress released when the slice is removed from a larger body. An initial contour method measurement determines a map of the out-of-plane stress on a plane of interest. Subsequently, removal of thin slices and a series of slitting measurements determines a map of one or both in-plane stress components. Four PSR biaxial mapping measurements were performed using an aluminum T-section, a stainless steel plate with a dissimilar metal slot-filled weld, a titanium plate with an electron beam slot-filled weld, and a nickel disk forging. Each PSR mapping measurement described herein has one (or more) complementary validation measurement to confirm the technique. Uncertainty estimates are included for both the PSR mapping measurements and the validation measurements. Agreement was found between the PSR mapping measurements and validation measurements showing that PSR mapping is a viable technique for measuring residual stress fields.     

Image-transfer and optical technique for residual-strain measurement

The image-transfer and optical technique that was developed for the measurement of residual strain (permanent set) is herein described. This technique is mainly being used for the measurement of residual strains in metallic structures after they have been loaded at elevated temperature. Some evaluation tests on a beam and shell at room temperature and on a bar at elevated temperature are discussed. These tests indicate that the method is adequate for its intended use.     

Stress Monitoring of Post-processed MEMS Silicon Microbridge Structures Using Raman Spectroscopy

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated Micro-Electro-Mechanical Systems (MEMS) devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not well controlled during deposition. The residual stress; for example, tends to vary significantly for different deposition methods. Currently, few nondestructive techniques are available to measure residual stress in MEMS devices prior to the final release etch. In this research, micro-Raman spectroscopy is used to measure the residual stresses in polysilicon MEMS microbridge devices. This measurement technique was selected since it is nondestructive, fast, and provides the potential for in-situ stress monitoring. Raman spectroscopy residual stress profiles on unreleased and released MEMS microbridge beams are compared to analytical and FEM models to assess the viability of micro-Raman spectroscopy as an in-situ stress measurement technique. Raman spectroscopy was used during post-processing phosphorus ion implants on unreleased MEMS devices to investigate and monitor residual stress levels at key points during the post-processing sequences. As observed through Raman stress profiles and verified using on-chip test structures, the post-processing implants and accompanying anneals resulted in residual stress relaxation of over 90%.     

极端环境下连续碳纤维增韧的陶瓷基复合材料的力学行为

连续纤维增韧的碳化硅复合材料(以下简称C/SiC),作为超高速飞行器热结构使用时,有可能在高温环境下受到高速撞击的作用,因此,掌握其在极端环境(高温、高应变率)下的力学性能是进行结构安全设计的基础。本文采用具有高温实验能力的分离式Hopkinson杆,在293~1273K温度范围内进行了动态压缩力学性能测试,研究了环境温度和加载速率对材料力学性能的影响。结果表明:C/SiC复合材料的高温压缩力学性能主要受应力氧化损伤和残余应力的共同影响。实验温度低于873K时,应力氧化损伤的影响很小,而由于增强纤维和基体界面残余应力的释放使界面结合强度增大,复合材料的压缩强度随温度的升高而增大;当实验温度高于873K时,应力氧化损伤加剧,其对压缩强度的削弱超过残余应力释放对强度的贡献,材料的压缩强度随温度的升高逐渐降低。由于应力氧化损伤受应变率的影响很大,当温度由873K升高至1273K时,高应变率下压缩强度降低的程度要比应变率为0.0001/s时低得多。     

Determination of Residual Stresses in High Speed Milled Aluminium Alloys Using a Method of Indent Pairs

An improved method of indent pairs is utilised to determine residual stresses in high speed milling specimens of AA 6082-T6 and AA 7075-T6 aluminium alloys. To carry out the measurement procedure, this approach does not need specific equipment but only requires a universal measuring machine and an oven. An indentation device is incorporated to the measuring machine, which allows reducing the absolute error of measurement to just ±0.9 MPa. The geometry of the tool and cutting parameters are selected to evaluate the sensitivity of the method. The residual stress distributions generated by high speed milling are exhaustively evaluated taking into account orthogonal components of cutting speed and tangential force, which are parallel and perpendicular to feed direction.     

Fabrication of nanoscale speckle using broad ion beam milling on polymers for deformation analysis

We first report a fabrication technique of nanoscale speckle patterns on polymers using broad ion beam milling. The proposed technique is simple and low-cost to produce speckles ranging from dozens of nanometers to less than three micrometers in a large area of several millimeters. Random patterns were successfully produced with an argon(Ar) ion beam on the surfaces of four kinds of polymers: the epoxy matrix of carbon fiber reinforced plastic, polyester, polyvinyl formal-acetal, and polyimide. The speckle morphologies slightly vary with different polymers. The fabricated speckle patterns have good time stability and are promising to be used to measure the nanoscale deformations of polymers using the digital image correlation method.     

基于DIC方法的残余应力快速测量系统

结合数字图像相关(Digital Image Correlation,DIC)方法与钻孔法,开发了残余应力快速测量系统。该系统可分为两部分:适用于现场测量的便携式机械系统与针对残余应力测量而改进的基于DIC算法的程序。在四点弯曲加载平台上对工件进行载荷释放前后的残余应力测量试验,通过与应变片测量结果进行对比,该残余应力测量系统的精度达到了应变片测量的同等精度。同时,该测量系统解决了传统应变片测量系统对心误差大、操作繁琐、效率低和测量结果稳定性差等问题,具有较高的工程应用价值。