Determination of Discrete-Structure Parameters of Coatings with Allowance for Residual Stresses

The authors suggest that the dimensions of a discrete section of a coating should be chosen on the basis of calculation of the critical crack spacing in cohesion cracking. This approach allows for residual stresses in the coating. The reliability of the calculation method has been verified experimentally by direct measurement of the crack spacing.     

Determination of Residual Stresses in Electro-Spark Deposited Coatings

We propose a method of determining axial residual stresses in prismatic coated rods of rectangular cross section, which provides a means to study the stress state in both homogeneous and inhomogeneous coatings.     

Residual Stresses in Plasma-Sprayed Coatings Al 2 O 3

Residual stresses affect significantly the quality and performance of thermally sprayed coatings. The residual stresses in alumina coatings were investigated by means of X-ray diffraction with Cr- and Cu-radiations. The sin² method could be applied to the evaluation of the stress fields. A very small stress gradient was indicated in surface layers of alumina coatings.     

Thermal Residual Stresses in Multilayered Coatings

The mechanical integrity and reliability of coated devices are strongly affected by the residual stresses in thin films and coatings. However, due to the metallurgical complexity of materials, it is rather difficult to obtain a closed-form solution of residual stresses within multilayered coatings (e.g. functionally graded coatings, FGCs). In this paper, an analytical model is developed to predict the distribution of residual stresses within multilayered coatings. The advantage of this model is that the solution of residual stresses is independent of the number of layers. Specific results are obtained by calculating elastic thermal stresses in ZrO2/NiCoCrAIY FGCs, which consist of different material layers. Furthermore, the residual stress distribution near the edges and the stress-induced failure modes of coating are also analyzed. The topics discussed provide some insights into the development of a methodology for designing fail-safe coating systems.     

金属基复合材料界面残余应力的研究进展

综述了金属基复合材料界面残余应力的各种影响因素、残余应力的实验测试方法和理论分析方法及残余应力对复合材料宏观力学性能的影响,分析讨论了目前研究中存在的问题和不足之处,并指出了今后工作的重点与方向.     

等离子喷涂热障涂层残余应力的实验与模拟研究

分别采用真空和大气等离子体喷涂工艺在GH3128镍基高温合金基材表面制备CoNiCrAlY结合层和氧化钇部分稳定的氧化锆陶瓷层组成热障涂层。采用有限元模拟计算了涂层的残余应力, 研究了基材预热对打底层与陶瓷层界面应力分布的影响规律。结果表明, 预热基材可以显著地降低陶瓷顶层内部产生的残余拉应力。采用钻孔法测量了涂层中的残余应力并与模拟结果作定量比较, 结果表明: 有限元模拟计算结果与实验测量结果能较好吻合。     

Residual Stresses in Short-Fibre Reinforced Injection-Moulded Thermoplastic Parts

Having the prediction of the warpage and shrinkage behaviour of injection-moulded short-fibre reinforced thermoplastics in mind, a model is developed to calculate the residual stresses. In accordance with the structure and the behaviour of these materials, a layerwise orthotropic, viscoelastic constitutive equation is used. As a cause for the stress development, a process determining the history of cooling under pressure is considered. The volume expansion of the matrix is described using a time-dependent viscoelastic free volume concept. From this, the viscoelastic functions for thermal expansion are derived as well as a time-dependent temperature shift factor. The material law of the composite is developed with the help of micromechanical considerations assuming viscoelastic matrix and elastic fibre behaviour. The residual stresses are calculated for a specially designed testing device which possesses regions of different fibre orientation. Measured fibre orientation tensors are used as simulation input. In the following, the calculated stress profiles were compared with measured values, which were determined using the layer removal technique. The calculated and measured profiles are in good agreement and show a strong influence of the fibre orientation state. The highest occurring stresses are in the same order of magnitude as the long-term strength of these materials.     

Determination of Residual Stresses in a Boron Fibre/Aluminium Matrix Composite

In the present work residual stresses in a boron fibre/aluminium matrix composite are determined through an etching method. The inhomogeneous removal of surface layers, due to the different sensitivity of boron and aluminium to alkaline environment, requires the application of a modified technique for the estimation of deformations and the calculation of stresses. The results obtained indicate considerable deviations in both distribution and magnitude of residual stresses in the same MMC, due to different fabrication methods. N. P. Andrianopoulos: Corresponding author.     

Thermal Residual Stresses in SiCw/Al Composite

The SiCw/6061Al composites were fabricated by squeeze casting method. Varia-tions of thermal residual stresses with quenching temperature, cooling manner, aging time and thethermal-cold cycle process in thin specimens,and the distributions of thermal residual stresses alongthe distances from the surface and changes with heating temperatnres in thick specimens were stud-ied by means of X-ray diffraction (XRD). The effects of residual stresses on the mierostructure, di-mensional stability and age-hardening behavior were studied by SEM, TEM observations, and tensiletest. The results showed that there existed macrostress, microstress and thermal mismatch stress inSiCw/Al compo-site,and the presence of microstress and thermal mismatch stress has no influenceon the measurement of macrostress, but the macrostress can affect the measured value of thermalmismatch stress. Thermal res dual stress induced during the composite fabrication process, will be further in-creased when the composite were subjected to the e     

Error and Uncertainty Analysis of the Residual Stresses Computed by Using the Hole Drilling Method

Abstract: The hole‐drilling method is one of the most used techniques for the experimental analysis of the residual stresses in mechanical components. For both through‐thickness uniform and non‐uniform residual stress distributions, its application is standardised by the ASTM E837‐08. In accordance with the ASTM limitations, the analysis of uniform residual stresses, to which the present work deals with, leads in general to results with a maximum bias of about 10%. Unfortunately, in general the user does not have appropriate procedures to estimate the actual stress error; consequently, if one or more of the experimental influence parameters fall out of the corresponding standard limitations, the computed residual stresses have to be considered as qualitative results. In order to overcome such drawbacks as well as to permit in general the estimation of the stress uncertainty, in the present work the procedures for the correction of the effects of the main error sources and for the stress uncertainty estimation, are proposed. The practical application of such procedures is carried out by using a simple calculation code properly implemented in Matlab environment. Also, the use of this tool allows the user to highlight the relative error and the stress uncertainty contribution of each influence parameter.     

残余应力对陶瓷/硬质合金复合片硬度的影响

分析了残余剪应力对材料硬度的影响,导出了残余剪应力与硬度压头下最大剪应力的关系式．分析表明,残余剪应力与残余正应力不同,无论其方向如何,总是使材料硬度下降．通过实验证明,陶瓷／硬质合金热压复合片界面附近,残余剪应力使硬度下降的值大于由于残余压应力使硬度升高的值．据此分析认为,当陶瓷层热膨胀系数比硬质合金层小,而且太薄时,虽然可以在陶瓷层形成较大的残余压应力,但因残余剪应力的影响,表层的硬度不仅不会升高,反而会降低．     

X-ray Analysis of Residual Stresses in C/SiC Composites

The residual stress state of C/SiC-composites, which are manufactured by a pyrolytical process is analysed by X-ray diffraction. The residual stress state in the composites results from the superposition of the shrinkage of the matrix material during the pyrolytical process and from effects of the thermal mismatch between fibres and matrix. The dependence of the residual stress state on the fibre coating and the process parameters of the pyrolytical process is determined. Furthermore, the change of the stress state in the samples submitting them to mechanical and thermal loads is analysed.     

Measurement of Residual Stresses with Optical Techniques

Abstract:  The goal of this work was the development of other experimental techniques to measure residual stresses, as an alternative to the hole-drilling method with strain gauges. The proposed experimental techniques are based on the use of Moiré interferometry and in-plane electronic speckle pattern interferometry (ESPI). Both are field techniques allowing the assessment of in-plane displacements without contact and high resolution. Grating replication techniques were developed to record high-quality diffraction gratings onto the specimen's surfaces. An optical set-up of laser interferometry was developed to generate the master grating (virtual). An in-plane ESPI set-up was also designed and implemented to measure displacements in one direction. The stress relaxation was promoted by the blind hole-drilling and the obtained fringe patterns (Moiré and speckle) were video-recorded. Image processing techniques were applied to assess the in-plane strain field. A finite-element code ( ansys ®) was used to simulate the stress relaxation process, whose values were compared with the experimental data, and to calculate the hole-drilling calibration constants.     

金属基复合材料残余应力测定

分析了金属基复合中热残余应力的形成机理及表现特点，并对几种常用的测定金属基复合材料残余应力的方法进行了分析比较，普通Ｘ射线法虽然简便、经济、但穿透能力有限，所测值仅反映试样表面的残余应力状态；中子衍射的区域较大，测出的仅是残余应力平均值。Ｘ射线能量衍射法虽然穿透能力较强，能测出试样内部较深范围的残余应力，但由于产生衍射法穿透力强，分辩率高，能测出复合材料内部的残余应力场梯度，是测定金属基复合材料残     

等离子喷涂ZrC基涂层逐道逐层沉积残余应力模拟与实验验证

采用商用ANSYS14.5软件, 依据复合梁增层力学模型, 采用逐道逐层累积模型模拟了C/C复合材料表面等离子喷涂ZrC基涂层沉积残余应力的特征, 分析了SiC过渡层、第二相(SiC, MoSi₂)和涂层厚度对ZrC基涂层残余应力的影响, 并进行了实验验证。结果表明, SiC过渡层有效缓解了涂层与基体的热失配应力。涂层体系的应力随着涂层厚度的增加逐渐减小, 符合应力松弛和叠加规律。在涂层内部的径向应力以拉应力为主, 基体中主要为压应力, 且在界面边缘存在压应力集中的极限区域, 易使涂层产生裂纹并沿界面扩展。该模拟采用逐道逐层累积的方法更逼近实际喷涂过程, 能更准确预测涂层的残余应力。     

Determination of Stresses in a Two-Layer Coating

We propose an approach to the analytic evaluation of the distributions of normal and tangential stresses in plates with two-layer coatings. It is demonstrated that, as a result of the optimal choice of the structural and technological parameters of base-coating systems, one can significantly decrease the level of stresses in the coatings. It is also shown that scientifically substantiated recommendations concerning the possibility of decreasing the level of stresses should be used in designing the base-coating systems. __________ Translated from Problemy Prochnosti, No. 4, pp. 121 – 132, July – August, 2005.     

A Simple Method for Estimation of Residual Stresses by Depth‐Sensing Indentation

Abstract: The evaluation of residual stresses is an important aspect in many engineering applications, such as surfaces produced by mechanical or thermal treatment processes or even thin films deposited on substrates. Currently, there are several techniques for residual stress measurement. However, its application is limited by problems associated with the precision and simplicity of measurement, as well as their applicability to a wide variety of materials and situations. Indentation tests are widely used in determining the mechanical properties of materials, so it is very important to assess their sensitivity to the presence of residual stresses. In this context, recourse to numerical simulation of indentation testing proves to be an important tool to study the effect of residual stresses in the determination of hardness and modulus as well as the actual determination of residual stresses. This paper investigates the influence of the presence of equibiaxial residual stresses in the indentation test results. It proposes a methodology for reverse analysis to determine the sign and value of the equibiaxial residual stresses present in the surface of materials, from depth‐sensing indentation results. For applying this methodology, the elastic and plastic behaviour of the material must be previously determined.     

Voxel-Based Full-Field Eigenstrain Reconstruction of Residual Stresses

Inverse eigenstrain (inherent strain) analysis methods are shown to be effective for the reconstruction of residual stresses in plane eigenstrain problems (continuously processed bodies) while conversely residual stress reconstruction in discontinuously processed bodies is extremely challenging and necessitates the use of complex regularizing assumptions. Herein, a new generic inverse eigenstrain method suitable for the reconstruction of residual stresses along with residual elastic strains and displacements in discontinuously processed bodies is introduced. The proposed method uses the superposition of eigenstrain radial basis functions together with a set of limited experimental data for model-free (unconstrained) determination of unknown eigenstrain fields. This approach eliminates the limitations introduced by global basis functions such as polynomials. The novel point of this method is the ability to account for all six components of strain in an isotropic body without using regularizing assumptions. By lifting complex guiding formulation, the fidelity of full-field eigenstrain reconstruction becomes directly related to the quality of experimental data and proper discretisation of the model domain. The FEniCS implementation has been validated using the experimental data of pointwise high-energy synchrotron X-ray diffraction measurements from a bent titanium alloy bar. A hybrid high throughput computing approach is also introduced for effective parallel computing.     

残余应力对RF MEMS开关谐振频率偏移的影响

由沉积工艺产生的残余应力对RF MEMS开关的动态特性有重要影响。利用变量分离法推导出桥模式RF MEMS开关的频率方程组。给出二分法求解频率方程的流程图,并进行实例计算。计算结果表明,残余应力引起的频率偏移随着t/L的减小而增加;残余应力与频率偏移之间呈现出明显的非线性特性。     

涂层基体条件对梯度涂层残余应力影响研究

采用有限元法，对结构一定的Al/Ni-ZrO2梯度涂层在基体条件改变时涂层的残余应力进行了分析，结果表明，基体材料的热膨胀系数对涂层的残余应力有显著的影响，对于基体为圆柱形的涂层，其基体与涂层界面的残余应力梯度，最大轴向拉应力均随热膨胀系数的增大而线性增大，表面纯陶瓷层与次层界面残余应力梯度则随之减小，增大基体的直径和厚度，可缓和涂层残余应力，并在基体直径为36mm,厚度为20mm时各残余应力基本稳定。