Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations. (C) 1998 Elsevier Science Ltd.

Deflections and curvatures of a film-substrate structure with the presence of gradient stress in MEMS applications

The close form solutions of deflections and curvatures for a film-substrate composite structure with the presence of gradient stress are derived. With the definition of more precise kinematic assumption, the effect of axial loading due to residual gradient stress is incorporated in the governing equation. The curvature of film-substrate with the presence of gradient stress is shown to be nonuniform when the axial loading is nonzero. When the axial loading is zero, the curvature expressions of some structures derived in this paper recover the previous ones which assume the uniform curvature. Because residual gradient stress results in both moment and axial loading inside the film-substrate composite structure, measuring both the deflection and curvature is proposed as a safe way to uniquely determine the residual stress state inside a film-substrate composite structure with the presence of gradient stress.

Stress analysis of ZrO2/SiO2 multilayers deposited on different substrates with different thickness periods

The effects of repeating thickness periods on stress are studied in ZrO2/SiO2 multilayers deposited by electron-beam evaporation on BK7 glass and fused-silica substrates. The results show that the residual stress is compressive and decreases with an increase of the periods of repeating thickness in the ZrO2/SiO2 multilayers. At the same time, the residual stress in multilayers deposited on BK7 glass is less than that of samples deposited on fused silica. The variation of the microstructure examined by x-ray diffraction shows that microscopic deformation does not correspond to macroscopic stress, which may be due to variation of the interface stress. (c) 2005 Optical Society of America.

Y2O3 stabilized ZrO2 thin films deposited by electron-beam evaporation: Optical properties, structure and residual stresses

This paper describes the preparation and the characterization Of Y2O3 stabilized ZrO2 thin films produced by electric-beam evaporation method. The optical properties, microstructure, surface morphology and the residual stress of the deposited films were investigated by optical spectroscopy, X-ray diffraction (XRD), scanning probe microscope and optical interferometer. It is shown that the optical transmission spectra of all the YSZ thin films are similar with those of ZrO2 thin film, possessing high transparency in the visible and near-infrared regions. The refractive index of the samples decreases with increasing of Y2O3 content. The crystalline structure of pure ZrO2 films is a mixture of tetragonal phase and monoclinic phase, however, Y2O3 stabilized ZrO2 thin films only exhibit the cubic phase independently of how much the added Y2O3 content is. The surface morphology spectrum indicates that all thin films present a crystalline columnar texture with columnar grains perpendicular to the substrate and with a predominantly open microporosity. The residual stress of films transforms tensile from compressive with the increasing Of Y2O3 molar content, which corresponds to the evolutions of the structure and packing densities. (C) 2008 Elsevier Ltd. All rights reserved.

An investigation into the residual stresses in an aluminium 2024 test weld

This paper uses finite element (FE) analysis to examine the residual stresses generated during the TIG welding of aluminium aerospace alloys. It also looks at whether such an approach could be useful for evaluating the effectiveness of various residual stress control techniques. However, such simulations cannot be founded in a vacuum. They require accurate measurements to refine and validate them. The unique aspect of this work is that two powerful engineering techniques are combined: FE modelling and neutron diffraction. Weld trials were performed and the direct measurement of residual strain made using the ENGIN neutron diffraction strain scanning facility. The predicted results show an excellent agreement with experimental values. Finally this model is used to simulate a weld made using a "Low Stress No Distortion" (LSND) technique. Although the stress reduction predicted is only moderate, the study suggests the approach to be a quick and efficient means of optimising such techniques.

Characterization of stress induced in SOS and Si/gamma-Al2O3/Si heteroepitaxial thin films by Raman spectroscopy

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si/gamma-Al2O3/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si/gamma-Al2O3/Si thin film is much smaller than that of as-grown SOS and SPE upgraded SOS films. The stress decrease for double heteroepitaxial film Si/gamma-Al2O3/Si mainly arises from the smaller lattice mismatching of 2.4% between silicon top layer and the gamma-Al2O3/Si epitaxiial composite substrate, comparing with the large lattice mismatch of 13% for SOS films. It indicated that gamma-Al2O3/Si as a silicon-based epitaxial substrate benefits for reducing the residual stress for further growth of silicon layer, compared with on bulk sapphire substrate. (c) 2005 Elsevier B.V. All rights reserved.

In-situ Boron-doped Low-stress LPCVD Polysilicon for Micromechanical Disk Resonator

Polycrystalline silicon (polysilicon) has been used as an important structural material for microelectro-mechnical systems (MEMS) because of its compatibility with standard integrated circuit (IC) processes. As the structural layer of micromechanical high resonance frequency (high-f) and high quality factor (high-Q) disk resonators, the low residual stress and low resistivity are desired for the polysilicon thin films. In the present work, we investigate the effect of deposition and annealing conditions on the residual stress and resistivity for in-situ deposited low pressure chemical vapor deposition (LPCVD) polysilicon films. Low residual stress (-100 MPa) was achieved in in-situ boron-doped polysilicon films deposited at 570 degrees C and annealed at 1000 degrees C for 4 hr. The as-deposited amorphous polysilicon films were crystallized by the rapid thermal annealing and have the (111)-preferred orientation, the low tensile residual stress is expected for this annealed film, the detailed description on this work will be reported soon. The controllable residual stress and resistivity make these films suitable for high-Q and bigh-f micro-mechanical disk resonators.

Stress and resistivity controls on in situ boron doped LPCVD polysilicon films for high-Q MEMS applications

The simultaneous control of residual stress and resistivity of polysilicon thin films by adjusting the deposition parameters and annealing conditions is studied. In situ boron doped polysilicon thin films deposited at 520 ℃ by low pressure chemical vapor deposition (LPCVD) are amorphous with relatively large compressive residual stress and high resistivity. Annealing the amorphous films in a temperature range of 600-800 ℃ gives polysilicon films nearly zero-stress and relatively low resistivity. The low residual stress and low resistivity make the polysilicon films attractive for potential applications in micro-electro-mechanical-systems (MEMS) devices, especially in high resonance frequency (high-f) and high quality factor (high-Q MEMS resonators. In addition, polysilicon thin films deposited at 570 ℃ and those without the post annealing process have low resistivities of 2-5 mΩ·cm. These reported approaches avoid the high temperature annealing process (> 1000℃), and the promising properties of these films make them suitable for high-Q and high-f MEMS devices.

Effect of Silicon-on-Insulator Substrate on Residual Strain in 3C-SiC Films

One group of SiC films are grown on silicon-on-insulator （SOI） substrates with a series of silicon-overlayer thickness. Raman scattering spectroscopy measurement clearly indicates that a systematic trend of residual stress reduction as the silicon over-layer thickness decreases for the SOI substrates. Strain relaxation in the SiC epilayer is explained by force balance approach and near coincidence lattice model.

Geometrical effects on residual stresses in 7050-T7451 aluminum alloy rods subject to laser shock peening

Laser shock peening (LSP) is an emerging surface treatment technology for metallic materials, which appears to produce more significant compressive residual stresses than those from the conventional shot peening (SP) for fatigue, corrosion and wear resistance, etc. The finite element method has been applied to simulate the laser shock peening treatment to provide the overall numerical assessment of the characteristic physical processes and transformations. However, the previous researchers mostly focused on metallic specimens with simple geometry, e.g. flat surface. The current work investigates geometrical effects of metallic specimens with curved surface on the residual stress fields produced by LSP process using three-dimensional finite element (3-D FEM) analysis and aluminium alloy rods with a middle scalloped section subject to two-sided laser shock peening. Specimens were numerically studied to determine dynamic and residual stress fields with varying laser parameters and geometrical parameters, e.g. laser power intensity and radius of the middle scalloped section. The results showed that the geometrical effects of the curved target surface greatly influenced residual stress fields.

Geometrical effects on residual stresses in 7050-T7451 aluminum alloy rods subject to laser shock peening

Laser shock peening (LSP) is an emerging surface treatment technology for metallic materials, which appears to produce more significant compressive residual stresses than those from the conventional shot peening (SP) for fatigue, corrosion and wear resistance, etc. The finite element method has been applied to simulate the laser shock peening treatment to provide the overall numerical assessment of the characteristic physical processes and transformations. However, the previous researchers mostly focused on metallic specimens with simple geometry, e.g. flat surface. The current work investigates geometrical effects of metallic specimens with curved surface on the residual stress fields produced by LSP process using three-dimensional finite element (3-D FEM) analysis and aluminium alloy rods with a middle scalloped section subject to two-sided laser shock peening. Specimens were numerically studied to determine dynamic and residual stress fields with varying laser parameters and geometrical parameters, e.g. laser power intensity and radius of the middle scalloped section. The results showed that the geometrical effects of the curved target surface greatly influenced residual stress fields.

Development of an inverse routine to predict residual stresses in the material based on a bending test

Bending and reverse bending are the dominant material deformations in roll forming, and hence property data derived from bend tests could be more relevant than tensile test data for numerical simulation of a roll forming process. Recent investigations have shown that residual stresses change the material behavior close to the yield in a bending test. So, residual stresses introduced during prior steel processing operations may affect the roll forming process, and therefore they need to be included in roll forming simulations to achieve improved model accuracy. Measuring the residual stress profile experimentally is time consuming and has limited accuracy while analytical models that are available require detailed information about the pre-processing conditions that is generally not available for roll forming materials. The main goal of this study is to develop an inverse routine that determines a residual stress profile through the material thickness based on experimental pure bend test data. A numerical model of the skin passing (temper rolling) process is performed to introduce a residual stress profile in DP780 steel sheet. The skin passed strips are used in a pure bending simulation to record moment-curvature data and this data is then applied in an inverse analysis to predict the residual stress profile in the material. Comparison of the residual stress profile predicted by the inverse routine with that calculated by finite element analysis (FEA) indicates an inverse approach combined with pure bend test may present an alternative to predict residual stresses in sheet metals.

Application of acoustoelasticity to measure the stress generated by milling in ASTM A36 steel plates

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Numerical analysis of different methods to calculate residual stresses in thin films based on instrumented indentation data

In this work, different methods to estimate the value of thin film residual stresses using instrumented indentation data were analyzed. This study considered procedures proposed in the literature, as well as a modification on one of these methods and a new approach based on the effect of residual stress on the value of hardness calculated via the Oliver and Pharr method. The analysis of these methods was centered on an axisymmetric two-dimensional finite element model, which was developed to simulate instrumented indentation testing of thin ceramic films deposited onto hard steel substrates. Simulations were conducted varying the level of film residual stress, film strain hardening exponent, film yield strength, and film Poisson's ratio. Different ratios of maximum penetration depth h(max) over film thickness t were also considered, including h/t = 0.04, for which the contribution of the substrate in the mechanical response of the system is not significant. Residual stresses were then calculated following the procedures mentioned above and compared with the values used as input in the numerical simulations. In general, results indicate the difference that each method provides with respect to the input values depends on the conditions studied. The method by Suresh and Giannakopoulos consistently overestimated the values when stresses were compressive. The method provided by Wang et al. has shown less dependence on h/t than the others.