Curing induced residual stresses in laminated cylindrical shells

A viscoelastic finite element analysis is presented to investigate residual stresses occurred in a laminated cylindrical shell during cure. An incremental viscoelastic constitutive equation that can describe stress relaxation during the cure is derived as a recursive formula which can be used conveniently for a numerical analysis. The finite element analysis program is developed on the basis of a 3-D degenerated shell element and the first order shear deformation theory, and is verified by comparing with an one dimensional exact solution. Viscoelastic effect on the residual stresses in the laminated shell during the cure is investigated by performing both the viscoelastic and linear elastic analyses considering thermal deformation and chemical shrinkage simultaneously. The results show that there is big difference between viscoelastic stresses and linear elastic stresses. The effect of cooling rates and cooling paths on the residual stresses is also examined.     

A numerical and experimental study on tubular channel angular pressing (TCAP) process

Tubular channel angular pressing (TCAP) is a new and novel severe plastic deformation (SPD) technique suitable for fabrication of bulk nanograined tubular materials. There are several parameters in the TCAP process. The present paper investigates the effects of curvature angles, deformation ratio (maximum radius during TCAP/initial radius) and deformation direction on the plastic deformation behavior, strain homogeneity and required loads in TCAP processing. The results showed that higher curvature angles (?? ₁ and ?? ₃) leads to better strain homogeneity while have not having any significant effect on the process loads. Also, at the second curvature angle of 90°, the best condition is achieved with good strain homogeneity and lower load requirements rather than lower angles. Investigation of deformation direction showed that inlet deformation direction causes tube thinning at the end of the process, and thus, using the inlet case is not recommended.     

Possibilities of magnetic inspection of plastic deformations preceding failures of low-carbon steels constructions

The possibility of magnetic inspection of plastic deformations preceding the failure of strained steel constructions was studied by locally magnetizing them with an attachable electromagnet. Field dependences of the differential magnetic permeability of a plate made of steel 09Γ2 on the applied and residual stresses were determined. Critical fields of 90° and 180° domain-wall motion at different degrees of deformation were calculated using a model taking into account the contribution of these domain walls to magnetization reversal processes. Dependences of the coercive force and residual magnetization of the plate on the applied and residual stresses, which were measured using a SIMTEST portable magnetic measuring system, are reported.     

Residual stresses in a laminated shell during cure

In this paper, a viscoelastic finite element analysis was performed to investigate residual stresses occurred in a laminated shell during cure. A viscoelastic constitutive equation that can describe stress relaxation during the cure was defined as functions of degree of cure and temperature, and derived as a recursive formula used conveniently for numerical analyses. The finite element program was developed on the basis of 3-D degenerated shell element and the first order shear deformation theory, and was verified by comparing with an exact solution of the one dimensional problem. Effects of chemical shrinkage and stacking sequence on the residual stresses in the laminated shell during the cure were investigated. The results showed that there were big differences between viscoelastic stresses and linear elastic stresses calculated by considering thermal deformation and the chemical shrinkage induced by the degree of cure.     

Tribological Properties and Thermal Stability of Various Types of Polyimide Films

Thermal exposure experiments at 315° and 350°C were conducted on seven different types of polyimide film to determine which was the most thermally stable and adherent. The polyimides were ranked according to the rate of which they lost weight and how well they adhered to the metallic substrate. Friction and wear experiments were conducted at 25°C (room temperature) on films bonded to 440C HT stainless steel. Friction, film wear rates, wear mechanists, and transfer films of the seven films were investigated and compared. The polyimides were found to fall into two groups as far as friction and wear properties were concerned. Group I had lower friction but an order of magnitude higher film wear rate than did group II. The wear mechanism was predominately adhesive, but the size of the wear particles was larger for group I polyimides.     

Tribological Properties of Graphite-Fiber-Reinforced,Partially Fluorinated Polyimide Composites

Graphite-fiber-reinforced polyimide (GFRPI) composites were formulated form three new partially fluorinated polyimides and three types of graphite fiber. Nine composites were molded into pins and evaluated in a pin-on-disk tribometer. Friction coefficients, wear rates, pin wear surface morphology, and transfer film formation were assessed at 25 and 300°C. Also assessed was the effect of sliding distance on friction and the effect of constantly increasing or decreasing temperature on friction. Wear was up to two orders of magnitude lower at 25°C and up to one order of magnitude lower at 300°C than with previously formulated NASA GFRPI composites.     

Finite element optimization of diamond tool geometry and cutting-process parameters based on surface residual stresses

In this paper, a coupled thermo-mechanical plane-strain large-deformation orthogonal cutting FE model is proposed on the basis of updated Lagrangian formulation to simulate diamond turning. In order to consider the effects of a diamond cutting tool’s edge radius, rezoning technology is integrated into this FE based model. The flow stress of the workpiece is modeled as a function of strain, strain rate, and temperature, so as to reflect its dynamic changes in physical properties. In this way, the influences of cutting-edge radius, rake angle, clearance angle, depth of cut, and cutting velocity on the residual stresses of machined surface are analyzed by FE simulation. The simulated results indicate that a rake angle of about 10° and a clearance angle of 6° are the optimal geometry for a diamond tool to machine ductile materials. Also, the smaller the cutting edge radius is, the less the residual stresses become. However, a great value can be selected for cutting velocity. For depth of cut, the ‘size effect’ will be dependent upon it. Residual stresses will be reduced with the decrement of depth of cut, but when the depth of cut is smaller than the critical depth of cut (i.e., about 0.5 μm according to this work) residual stresses will decrease accordingly.     

Prediction of residual stress distribution in multi-stacked thin film by curvature measurement and iterative FEA

In this study, residual stress distribution in multi-stacked film by MEMS (Micro-Electro Mechanical System) process is predicted using Finite Element method FEMi. We develop a finite element program for residual stress analysis (RESA) in multi-stacked film. The RESA predicts the distribution of residual stress field in multi-stacked film. Curvatures of multistacked film and single layers which consist of the multi-stacked film are used as the input to the RESA. To measure those curvatures is easier than to measure a distribution of residual stress. To verify the RESA. mean stresses and stress gradients of single and multilayers are measured. The mean stresses are calculated from curvatures of deposited wafer by using Stoney’s equation. The stress gradients are calculated from the vertical deflection at the end of cantilever beam. To measure the mean stress of each layer in multi-stacked film, we measure the curvature of wafer with the left film after etching layer by layer in multi-stacked film.     

Springback evaluation in hot v-bending of Ti-6Al-4V alloy sheets

In this paper, v-bending of Ti-6Al-4V alloy sheet was conducted from room temperature to 850 °C at a fixed velocity of 0.1 mm/s. Punches with punch radii of 1, 2, 4, and 6 mm, as well as several holding times were used. V-bending and springback behaviors were numerically analyzed with an isotropic hardening model that considered rate-dependent effects. Using a punch radius of 1 mm always leads to negative springback in the temperature range of 550–750 °C. This behavior occurs because an arc formed in the transition side near the end of bending and flattened at the end of bending, leading to an internal bending moment which causes specimen to bow inward after unloading. At a punch radius of 2 mm, positive springback occurs at 300–650 °C, while negative springback occurs at 700–750 °C. At punch radii of 4 and 6 mm, positive springback occurs at 600–750 °C, and the angle decreases as temperature increases. At 850 °C, negative springback occurs at a punch radius of 4 mm due to the decrease in yield strength. At a punch radius of 1 mm, cracking occurs at room temperature and 500 °C, while at 2 mm, it occurs only at room temperature. This discrepancy is ascribed to the greater plastic deformation caused by the smaller punch. As holding time increases, the shape of the deformed specimen more closely matches the desired shape.     

Inspection of the residual curvature in an anisotropic electrical steel strip and reasons for its formation

The residual curvature of strips of anisotropic electrical steel (AES) is a defect of its geometry. This defect results in deterioration in magnetic properties of steel products in a transformer due to their stressed states. The residual curvature also leads to a slowdown in labor productivity during punching of items in automated lines. The main reason for the formation of a residual curvature in AES is attributed to a difference in elastic tensile stresses caused by the coating on different sides of a strip in the case of polythickness of the coating. The obtained minimal values of residual curvature are reached when the difference in the coating thickness on two sides of the strip is limited within 1 μm. The indicators of residual curvature are the bending radius (an absolute indicator) and the deviation of the free end of a vertically suspended specimen (a relative indicator). Laboratory techniques of determining the degree of bending and the methods of determining the residual curvature of the AES strip used at OOO VIZ-Stal’ are presented.     

Nanomechanical characterization of multilayered thin film structures for digital micromirror devices

The digital micromirror device (DMD), used for digital projection displays, comprises a surface-micromachined array of up to 2.07 million aluminum micromirrors (14 μm square and 15 μm pitch), which switch forward and backward thousands of times per second using electrostatic attraction. The nanomechanical properties of the thin-film structures used are important to the performance of the DMD. In this paper, the nanomechanical characterization of the single and multilayered thin film structures, which are of interest in DMDs, is carried out. The hardness, Young's modulus and scratch resistance of TiN/Si, SiO₂/Si, Al alloy/Si, TiN/Al alloy/Si and SiO₂/TiN/Al alloy/Si thin-film structures were measured using nanoindentation and nanoscratch techniques, respectively. The residual (internal) stresses developed during the thin film growth were estimated by measuring the radius of curvature of the sample before and after deposition. To better understand the nanomechanical properties of these thin film materials, the surface and interface analysis of the samples were conducted using X-ray photoelectron spectroscopy. The nanomechanical properties of these materials are analyzed and the impact of these properties on micromirror performance is discussed.     

Uncertainty of residual stresses measurement by layer removal

A model to evaluate the uncertainty in the measurement of the through-thickness residual stress distribution in plates by the layer removal technique is presented. Thin layers were chemically etched from a stripe on rectangular specimens cut from a low carbon cold-rolled steel sheet. Phase shifting laser interferometry was used to measure the ensuing curvature. Polynomials were least-squares adjusted to the curvatures as a function of the etched depth. The polynomials were inserted into an integro-differential equation relating the curvature to the residual stresses, which were assumed to be a function of depth only. A comparison with X-ray diffraction measurement of the surface residual stresses showed good agreement. The uncertainty was found to increase steeply at the surfaces and to depend mainly on the assumed value for the modulus of elasticity, on the curvature fit, and on the depth of etching.     

Sensitivity analysis of plating conditions on mechanical properties of thin film for MEMS applications

In this paper, we report the detailed investigation into the effects of plating temperature and applied current density upon the mechanical properties of plated nickel film such as Young’s modulus and residual stresses. This method uses the resonance method of atomic force microscope, which does not require specially microfabricated cantilevers and additional experimental set-up. Thin layers of nickel are electroplated onto the tip surface of AFM cantilevers and plating thicknesses were measured at the end of each plating step. The self-deformation of the released AFM cantilever is also measured as a function of the plated nickel thickness, which is converted into the quantitative residual stress by appropriate mechanics. The measured Young’s modulus is as high as that of bulk nickel at low plating temperature and at low applied current density, but drastically drops at high temperature or current density. The dependence of Young’s modulus on the plating thickness is negligible in thin film less than few microns. The residual stress is also a strong function of the process conditions, and decreased with the elevation of the current density and plating temperature. And the intrinsic and extrinsic stresses of plated nickel are separated from the measured residual stress, and correlated with plating conditions. Dependence of the plated thickness on Young’s modulus and residual stress is also considered.     

The use of post‐mortem Raman spectroscopy in explaining friction and wear behaviour of sintered polyimide at high temperature

Due to their thermal stability and high strength, polyimides are an aromatic type of polymer that is used in sliding equipment functioning under high loads and elevated temperature. However, its tribological behaviour under high temperature and atmospheric conditions is not fully understood. It has been reported that a transition from high towards lower friction occurs ‘somewhere’ in the temperature region between 100°C and 200°C; however, a correlation with changes in the polyimide molecular structure remains difficult to illustrate and it is not certain whether or not this transition is correlated to lower wear. In the present work sliding experiments under controlled bulk temperatures between 100°C and 260°C are performed. A transition is observed in both friction and wear at 180°C which is further explained by microscopic analysis of the transfer film on the steel counterface and Raman spectroscopy of the worn polymer surfaces. A close examination of the spectra reveals transitions in relative intensity of certain absorption bands, pointing to different orientation effects of the molecular conformation at the polymer sliding surface at 180°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Cavitation resistance of Cr–N coatings deposited on austenitic stainless steel at various temperatures

Cr–N coatings were deposited on austenitic stainless steel, X6CrNiTi18-10, by means of the cathodic arc evaporation method at three substrate temperatures: 200 °C, 350 °C and 500 °C. All coatings were found to have a composition of Cr(N), CrN and Cr₂N. The substrate temperature was found to have an influence on the hardness and Young's modulus of the Cr–N coatings. The investigation of nanocrystalline Cr–N coatings resistance to cavitation was performed in a cavitation tunnel with a slot cavitator and tap water as the medium. The estimated cavitation resistance parameters of the coatings were the incubation period of damage and total mass loss. It was found that the optimal coating cavitation resistance was deposited at 500 °C. The incubation period for the 500 °C deposition coating was the same as that of the uncoated X6CrNiTi18-10 steel, but the total mass loss was significantly lower than on the uncoated specimen. The scanning electron microscope analysis indicated that the damage process of the Cr–N coating mainly originates from the plastic deformation of the steel substrate–hard coating system, which appears by “micro-folding” of the surface. An increase of tensile stresses at the top of micro-folds initiates micro-cracks and delamination of Cr–N coating. The results of the investigation and the analysis indicate that the factors mainly responsible for cavitation resistance of the steel substrate/hard coating system are resistant to plastic deformation of the total system and coating adhesion.     

沟曲率半径系数对柔性轴承应力的影响

采用有限元分析方法,使用ANSYS Workbench建立了柔性轴承的参数化接触模型。对不同沟曲率半径系数的柔性轴承进行静力学接触分析,得到了不同参数的柔性轴承内外圈的变形、应力的分布规律。结果表明:内外圈的最大等效应力和最大径向应力发生在长轴处的钢球与内外滚道接触的区域;内外圈最大等效应力、钢球与沟道的最大接触压力随内外圈沟曲率半径的增加呈线性增大趋势。用该方法能准确得到柔性轴承最大的接触应力,可以用来对柔性轴承进行优化设计。     

Tribological Characteristics of NiAl Matrix Composites with 1.5 wt% Graphene at Elevated Temperatures: An Experimental and Theoretical Study

In this study, the tribological properties of NiAl matrix composites with 1.5 wt% graphene nanoplatelets (NAG) at elevated temperatures were simulated and tested. NAG exhibits excellent characteristics at 100 and 200°C due to the formation of metal oxides and graphene nanoplatelets lubrication film. Furthermore, the removed layer thickness, the stress distributions, and the high stresses of the affected layer have been estimated theoretically. At 400°C, the friction coefficient increased due to the absence of the lubrication layer. In addition, the wear rate increased due to the excessive stresses, the increased layer thickness removal, and the propagation of subsurface cracks.     

微桥结构Ni膜杨氏模量和残余应力研究

采用MEMS(MicroelectromechanicalSystems)技术研制了镍(Ni)膜微桥结构试样,应用陶瓷压条为承力单元,与纳米压痕仪XP系统的Berkovich三棱锥压头相结合,解决了较宽Ni膜微桥加载问题。测量了微桥载荷与位移的关系,并结合微桥力学理论模型得到了Ni膜微桥的杨氏模量及残余应力,其值分别为190.5GPa和146MPa,与应用纳米压痕仪直接测得的带有Si基底的Ni膜杨氏模量186.8±7.34GPa相吻合。     

Thermorheological behaviour of a lithium lubricating grease

Thermal-induced changes in the viscous and viscoelastic responses of lubricating greases have been investigated through different rheological techniques in a temperature range of 0–175 °C. Small-amplitude oscillatory shear and viscous flow measurements were carried out on a model conventional lithium lubricating grease prepared by inducing the in situ saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide. The linear viscoelasticity functions dramatically decrease above 110 °C, but not below this critical temperature, which determines the maximum recommended operating temperature in relation to its durability and resistance under working conditions. Two different regions, below and above this critical temperature, in the plateau modulus versus temperature plot have been detected. From this thermal dependence, a much larger thermal susceptibility of the lubricating grease at temperatures above 110 °C is apparent. The thermo-mechanical reversibility of this material has been studied by applying different combined stress–temperature protocols. Regarding the viscous flow, a minimum in the shear stress versus shear rate plots appeared at temperatures above 60 °C, more pronounced as temperature increases, resulting from material instabilities. The experimental results obtained have been explained on the basis of the thermo-mechanical degradation of the lubricating grease microstructure.     

曲率测量技术在微机电系统薄膜残余应力测量中的应用

在比较曲率测量技术常用的Stoney公式及其修正式的基础上,利用有限元分析方法,建立薄膜/基底结构的有限元模型,给出一种薄膜残余应力的等效施加方法,从两个方面详细分析并对比这两个公式在微机电系统(Micro electromechanical systems,MEMS)薄膜残余应力测量中的检测精度.仿真及分析结果表明,修正后的Stoney 公式在很大程度上提高薄膜残余应力的测量精度,使曲率测量技术的适用范围得到较大扩展.但是当薄膜厚度接近于基底厚度或结构处于大变形状态下,修正式的计算精度也将受到较大影响,此时可以采用有限元分析方法来获得临界状态值,以提高残余应力的检测精度.同时,通过有限元分析,证实曲率测量技术应用中存在的另一个问题,即曲率的空间分布不均匀性现象.