Residual-stress analysis of an epoxy plate subjected to rapid cooling on both surfaces

This paper presents theoretical and experimental methods of finding the residual stress in an epoxy plate subjected to rapid cooling on both surfaces. The theoretical residual-stress distributions in a plate are calculated by using the fundamental equations based on the linear viscoelastic theory. The specimens in the experiment are subjected to rapid cooling. The residual stresses are measured by the layer-removal method. The theoretical and experimental results are compared and discussed.     

Blind-hole residual stress determination using optical interferometry

The in-plane method and the out-of-plane method are used to analyze blind-hole residual stress as measured by optical interferometry. The in-plane method, which constructs a relation between the in-plane displacement field and the residual stress released from blind-hole drilling, is applicable when the sensitivity vector of the interferometer used in the measuring system is parallel to the object surface. Three in-plane displacements obtained from one interference pattern are sufficient to determine the residual stress. The out-of-plane method, which establishes a new relation between the out-of-plane displacement field and the released residual stress, is suggested when the sensitivity vector is perpendicular to the object surface. Two relative out-of-plane displacements extracted from one interference pattern are sufficient to determine the residual stress. With the adoption of these two methods, interpolating calculation is not needed to determine the fringe order of each data point, since the selections of the required data points are flexible using these two methods. Two experiments, one for the in-plane method and the other for the out-of-plane method, were carried out to illustrate the applicability and usefulness of these two methods.     

Residual Stress Analysis of Orthotropic Materials Using Integrated Digital Image Correlation

Measuring residual stress in an orthotropic material is a difficult task due to the complex behavior of the material. Recently, two different approaches based on Smith’s simplified real value formulation and the general solution developed by Lekhnitskii have been proposed. Both solutions assume the measurement of the displacement field via interferometric optical methods and estimate stress values through solving an inverse problem. However, the high sensitivity to vibrations of interferometric techniques makes their use difficult outside optical laboratories; standard Digital Image Correlation could be used, but its low sensitivity and relatively high standard deviation of displacements severely affect the reliability of estimates. In this work we propose to integrate the residual stress displacement functions related to orthotropic materials into the shape functions of Digital Image Correlation. This makes it possible overcome most of the problems related to low sensitivity and large standard deviation because a single large patch can be used for the measurement, thus providing an accurate and reliable algoritm for the measurement of residual stress.     

Full field methods and residual stress analysis in orthotropic material. II: Nonlinear approach

This paper is the second part of a work that analyses the problem of residual stress determination in an orthotropic material by the hole drilling technique combined with full field optical methods. Due to the complex behaviour of the material, two theoretical formulations capable of describing the displacement field in an infinite plate in tension with a hole have been proposed: a simplified one, which can treat only a subset of all orthotropic materials, and a general, much more complex one. In the first part of this work, it has been shown that by using the simplified formulation, it is possible to develop a linear least square approach to the residual stress identification problem capable of treating a large class of orthotropic materials. This second part shows that the same approach can be extended to the general case providing that a somewhat more complex, nonlinear algorithm, is acceptable.     

A micromechanical study of residual stress and its effect on transverse failure in polymer–matrix composites

Cure residual stress and its effect on damage in unidirectional fibre-reinforced polymer–matrix composites under transverse loading were studied using a micromechanical unit cell model and the finite element method. The overall residual stress introduced from curing was determined by considering two contributions: volume shrinkage of matrix resin from the crosslink polymerization during isothermal curing and thermal contraction of both resin and fibre as a result of cooling from the curing temperature to room temperature. To examine the effect of residual stress on failure, a model based on the Maximum Principal Stress criterion and stiffness degradation technique was used for damage analysis of the unit cell subjected to mechanical loading after curing. Predicted damage initiation and evolution are clearly influenced by the inclusion of residual stress. Residual stress is always detrimental for transverse compressive loading and pure shear loading. For transverse tensile loading, residual stress is detrimental for relatively low resin strength and beneficial for relatively high resin strength. Failure envelopes were obtained for both biaxial normal loading and combined shear and normal loading and the results show that residual stress results in a shifting and contraction of the failure envelopes.     

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。     

表面残余应力的仪器化压入检测方法研究进展

针对表面残余应力的仪器化压入检测方法, 阐释利用压入方式检测残余应力的基本原理及其力学机制, 梳理建立残余应力压入检测方法的常用技术路线, 结合残余应力压入检测方法的不同分类, 着重分析6种代表性压入检测方法的优势与局限, 讨论验证压入检测方法可靠性的常用方法, 最后总结残余应力压入检测方法的研究进展, 展望未来的发展趋势为建立无需参考试样一体化检测材料力学参数和非等轴残余应力的仪器化压入分析方法, 探讨建立方法的四个研究要领, 即机制清楚、分析可靠、技术可行、结果可信. 仪器化压入检测技术是郑哲敏先生晚年的研究兴趣之一, 作者在中科院力学所工作期间曾受到郑哲敏先生的热切关注和提问式激励, 谨以此文纪念郑哲敏先生逝世周年.      

冲击载荷下自增强身管残余应力变化规律

基于有限元分析方法,建立速射火炮身管截面的有限元模型,采用直接耦合的方法,模拟了承受热应力和火药燃气压力的自紧身管在先50连发、冷却后再连发10发过程中身管的残余应力,得到并分析了残余应力的变化规律,指出内壁薄层的塑性变形和残余应力的反复变化,不利于内壁的稳定,还提出了减小残余应力变化程度的策略.     

Full field methods and residual stress analysis in orthotropic material. I Linear approach

This work analyzes the problem of residual stress determination in an orthotropic material using the hole drilling technique combined with non-contact, full field optical methods. Due to the complex behavior of the material, first a solution algorithm for the isotropic case is analyzed, then the procedure is extended to solving the more complex problem. In the first part of the work, the simplified Smith–Schajer solution to the through-hole problem for an orthotropic material is analyzed, showing that the same linear least square approach used in the isotropic case applies to a large set of orthotropic materials; based on this analysis a simple residual stress measurement algorithm is developed using either analytical or numerically estimated calibration coefficients.In the second part of the work, the general solution is discussed: since in this case the simplified Smith–Schajer solution cannot be used, the Lekhnitskii’s analysis of the through-hole plate in tension is introduced and extended to handle residual stresses. On this basis a solution algorithm using the nonlinear fit of the theoretical displacement field capable of treating all the orthotropic materials at the cost of a more complex numerical procedure is proposed. The performances of both algorithms are tested against numerically generated noisy fields and experimental ones and show a good reliability and accuracy.     

RECENT PROGRESS IN RESIDUAL STRESS MEASUREMENT TECHNIQUES

Residual stress measurement is of critical significance to in-service security and the reliability of engineering components, and has been an active area of scientific interest. This paper offers a review o[ several prominent mechanical release methods for residual stress measurement and recent developments, focusing on the hole-drilling method combined with advanced optical sensing. Some promising trends for mechanical release methods are also analyzed.     

Residual stresses and warp generated by one-sided quench of an epoxy-resin beam

This paper discusses the warp and accompanying residual stress in a rectangular epoxy beam produced by water cooling its lower surface. First, the theoretical values of this warp and residual stress are obtained by the linear-viscoelastic theory. The specimen is then subjected to quenching. The variations in the warp are observed. After quenching, the residual stress is measured by a layer-removal method. The experimental and theoretical results are then compared and discussed.     

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.     

爆炸法消除焊接接头残余应力的数值模拟

利用非线性动力有限元法对爆炸处理消除焊接接头残余应力的全过程进行了数值模拟。首先，采用温度场与位移场的间接耦合方法计算了钢板对接焊的焊后冷却及残余应力的生成过程，求得焊接接头处由高温冷却到室温由于变形受到阻碍而产生的不均匀的残余塑性变形和应力。然后，在焊缝区引入移动的爆炸载荷，计算了爆炸波作用下该钢板焊接接头附近应力的变化。计算结果表明，爆炸处理可引起板内应力的重新分布，从而有效地释放超过塑性极限的残余应力。利用炸药爆炸消除大型焊接结构残余应力是一种经济有效的方法，本文的数值模拟为研究炸药爆炸消除焊接结构残余应力的机理提供了有力的工具。     

Orthotropic photoelastic analysis of residual stresses in filament-wound rings

In the filament-winding process for fabricating composite materials, winding tension and other factors combine to produce a residual-stress state in the wound structure. Techniques have been developed for producing a transparent composite, and the orthotropic photoelastic method was applied to determine this residual stress in circumferentially wound rings. Results from the photoelastic analysis were compared with two other residual-stress-determination methods: the boring-out and cut-through methods. Satisfactory agreement was achieved on most of the data. However, peak stresses revealed in the photoelastic-analysis method were not indicated by the other two methods.     

Residual stresses in silicon-on-sapphire thin film systems

This paper uses the finite element method to analyse the generation and evolution of residual stress in silicon-on-sapphire thin film systems during cooling. The effects of material properties, thin film structures and processing conditions, on the stress distribution were explored in detail. It was found that under certain conditions, significant stress concentration and discontinuity can take place to initiate crack and/or delamination in the systems. However, these can be minimised by controlling the buffer layer thickness.     

Residual Stress Determination Using Hole Drilling and 3D Image Correlation

In recent years, the hole drilling method for determining residual stresses has been implemented with optical methods such as holographic interferometry and ESPI to overcome certain limitations of the strain rosette version of hole drilling. Although offering advantages, the interferometric methods require vibration isolation, a significant drawback to their use outside of the laboratory. In this study, a 3D image correlation approach was used to measure micron-sized surface displacements caused by the localized stress relief associated with hole drilling. Residual stresses were then found from the displacements using non-dimensional relations previously derived by finite element analysis. A major advantage of image correlation is that it does not require interferometric vibration isolation. Experiments were performed to check the ability of this new approach for uniaxial and equi-biaxial states of stress. Stresses determined by the approach were in good agreement with computed values and those determined by hole drilling using holographic interferometry.     

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.     

Viscoelastic analysis of residual stress in quenched thermosetting resin beams

The residual stress generated by the molding process of thermosetting resins exerts serious influences upon their mechanical properties. This residual stress is generally classified by two groups: one produced by shrinkage in the curing reaction of monomers, the other produced by the nonuniformity of the temperature distribution in the cooling process. This paper is concerned with the theoretical and experimental analysis of the generation of residual stress of the latter type, using examples of rectangular beams of thermosetting resins quenched on both the upper and lower surfaces. First, a viscoelastic model is applied to make a qualitative prediction of the residual stress in quenched beams. Second, using linear-viscoelastic theory, fundamental equations are derived for the residual stress in a viscoelastic rectangular beam, where an unsteady and nonuniform temperature distribution is assumed in the direction of depth. The theoretical values of the residual stress in rectangular beams are calculated under various quenching conditions for two resins having different viscoelastic characteristics, i.e., epoxy and unsaturated polyester. The theoretical residual-stress distributions agree fairly well with the residual stress measured experimentally at every quenching condition for both resins. The qualitative prediction that the residual stress in quenched beams is compressive in the vicinity of the upper and lower surfaces and is tensile in the inner parts is confirmed. The relaxation modulus of epoxy resin changes more greatly with time and temperature than that of unsaturated polyester resin. The theoretical and experimental analysis shows that the residual stress for the former resin is larger than that for the latter. Therefore, it is concluded that the generation of residual stress is more significant where the relaxation modulus of resin changes greatly with time and temperature.     

整体化复合材料结构残余应力实验研究

通过钻孔法测试了T型整体化复合材料结构件在不同固化工艺方案下的残余应力,并研究了多墙类加筋壁板的腹板和蒙皮的残余应力分布,以及不同钻孔深度对结构件内部残余应力测试结果的影响。研究结果表明:共固化方式下产生的残余应力要小于胶结共固化方式下的残余应力。     

膨胀管残余应力测试

膨胀管残余应力是影响膨胀管抗外挤、内压强度的重要因素之一。确定膨胀管的残余应力,测试方法至关重要。在常用测试方法中,机械测量法中的盲孔法体现出测试的优越性。应用盲孔法测试J55、K55两种规格膨胀管的残余应力,并根据实际测试的应力水平和实验误差,采用应变释放系数分级法对试验结果进行修正,以提高测试结果的精度。实验结果表明,膨胀管膨胀后存在环向、轴向的残余应力,且外表面总是压残余应力。将实验结果与仿真结果对比,误差满足工程要求,说明盲孔法测试膨胀管残余应力方便、快捷、准确、有效。