Showing papers on "Residual stress published in 1993"

Quantitative measurement of residual biaxial stress by Raman spectroscopy in diamond grown on a Ti alloy by chemical vapor deposition

Abstract: Raman spectroscopy is used to study residual stress in diamond grown on Ti--6Al--4V by chemical vapor deposition. A general model is developed to use Raman spectroscopy to measure biaxial stress in polycrystalline, diamond-structure films. The as-grown film has 7.1 GPa of residual compressive stress, consistent with the difference in thermal-expansion coefficients between the diamond film and the substrate. Examination of the Raman spectra of the film in the vicinity of Brale indentations reveals that residual stresses in the film of up to approximately 17 GPa can be accommodated in the film before delamination occurs.

Stress Measurement in Single-Crystal and Polycrystalline Ceramics Using Their Optical Fluorescence

Abstract: A general methodology is developed for determining the state of stress and the numerical value of the stresses from observed shifts and broadening of optical fluorescence lines. The method is based on the piezospectroscopic properties of single crystals. We present general relationships between the measured fluorescence shifts and the stress state for a number of illustrative cases, pertinent to both polycrystalline and single-crystal ceramics under stress. These include measuring the stresses applied to polycrystalline ceramics, the residual stress distribution due to crystallographically anisotropic thermal expansion, and the stresses applied to single crystals. Using the recently implemented technique of performing the fluorescence measurements in an optical microprobe, we also provide experimental tests of the relationships derived.

Finite element analysis of thermal residual stresses at graded ceramic‐metal interfaces. Part I. Model description and geometrical effects

Abstract: An elastic‐plastic finite element method numerical model has been formulated to study residual stresses developed at graded ceramic‐metal interfaces during cooling. The results were compared with those obtained for sharp (nongraded) interfaces to assess the potential for achieving residual stress reductions. Analyses were conducted for various axisymmetric cylindrical specimen geometries relevant to structural joining, coating, and thick film applications. The graded microstructure was treated as a series of perfectly bonded layers, each having slightly different properties. Constitutive relations for the interlayers were estimated using a modified rule‐of‐mixtures approximation, and strain and stress distributions were calculated for simulated cooling from an assumed fabrication temperature. The results demonstrate the importance of accounting for plasticity when comparing graded and nongraded interfaces. Significant geometrical effects on peak stresses were observed in the graded materials. It is shown ...

Cure Cycle Optimization for the Reduction of Processing-Induced Residual Stresses in Composite Materials

Abstract: The control and reduction of processing-induced residual stresses has been investigated by modifying processing conditions for a graphite/BMI composite material. The effects of dwell temperature, dwell time, cool-down rate, cool-down pres sure, and postcure on residual stresses were investigated using unsymmetric cross-ply laminates. The effects on transverse mechanical properties were also measured. Experi mental results have shown that residual stresses can be reduced by as much as 25-30% while retaining or enhancing transverse mechanical properties by curing at lower tempera tures for longer times or utilizing an intermediate low-temperature dwell in three-step cure cycles. Overall process cycle times are not lengthened for three-step curing.

Effects of thermal residual stresses and fiber packing on deformation of metal-matrix composites

Abstract: The combined effects of thermal residual stresses anmd fiber spatial distribution on the deformation of a 6061 aluminum alloy containing a fixed concentration unidirectional boron fibers have been analyzed using detailed finite element models. The geometrical structure includes perfectly periodic, uniformly spaced fiber arrangements in square and hexagonal cells, as well as different cells in which either 30 or 60 fibers are randomly placed in the ductile matrix. The model involves an elastic-plastic matrix, elastic fibers, and mechanically bonded interfaces. The results indicate that both fiber packing and thermal residual stresses can have a significant effect on the stress-strain characteristics of the composite. The thermal residual stresses cause pronounced matrix yielding which also influences the apparent overall stiffness of the composite during the initial stages of subsequent far-field loading along the axial and transverse direction. Furthermore, the thermal residual stresses apparently elevate the flow stress of the composite during transverse tension. Such effects can be traced back to the level of constraint imposed on the matrix by local fiber spacing. The implications of the present results to the processing of the composites are also briefly addressed.

Finite element analysis of thermal residual stresses at graded ceramic‐metal interfaces. Part II. Interface optimization for residual stress reduction

Abstract: An elastic‐plastic finite element method numerical model previously developed (see Part I of this article) for predicting thermal residual stresses at graded ceramic‐metal interfaces has been applied to determine interface conditions favorable for achieving residual stress reductions. Using Al2O3‐Ni as a model system, and for a fixed specimen geometry, a study was performed to investigate the effects of different interlayer thicknesses and nonlinear composition profiles on strain and stress distributions established during cooling from an assumed elevated bonding temperature. For each interface condition, relative stress reductions were evaluated by comparing the magnitude of specific stress and strain components important for controlling interface failure with those predicted for a sharp (nongraded) interface. For the geometry considered, stress was reduced by thick graded interlayers and nonlinear composition profiles that distributed the largest property changes over the interlayer region having low elastic modulus and high plasticity. In contrast to the Part I results for a linear composition profile, the optimized interlayer condition effectively reduced the peak near‐surface axial stress component.

Particulate fracture during deformation of a spray formed metal-matrix composite

Abstract: The mechanisms of deformation and failure in a 2618 Al alloy reinforced with 15 vol pct SiC particilates were studied and compared with those of the unreinforced alloy, processed by spray forming as well. Tensile and fracture toughness tests were carried out on naturally aged and peak-aged specimens. The broken specimens were sliced through the middle, and the geometric features of fractured and intact particulates were measured. The experimental observations led to the conclusion that failure took place by the progressive fracture of the particulates until a critical volume fraction was reached. An influence of the particulate size and aspect ratio on the probability of fracture was found, the large and elongated particulates being more prone to fail, and the fracture stress in the particulates seemed to obey the Weibull statistics. The dif- ferences in ductility found between the naturally aged and peak-aged composites were explained in terms of the number of broken particulates as a function of the applied strain. Numerical simulations of the deformation process indicated that the stresses acting on the particulates are higher in the peak-aged material, precipitating the specimen failure. Moreover, the compressive residual stresses induced on the SiC during water quenching delayed the onset of particulate breakage in the naturally aged material.

Statistical Properties of Residual Stresses and Intergranular Fracture in Ceramic Materials

Abstract: The problem addressed in this paper concerns the statistical characterization of the state of residual stress generated in polycrystalline ceramics during cooling from the fabrication temperature. Detailed finite element simulations are carried out for an ensemble of large numbers of randomly oriented, planar hexagonal grains with elastic and thermal expansion anisotropy, and brittle grain interfaces. The calculations show that the distribution of normal and shear tractions induced by thermal contraction mismatch among grains is gaussian and that these tractions are statistically independent random variables. Although the gaussian nature of the distributions remains unaffected by the introduction of elastic anisotropy, the results indicate that elastic anisotropy has a significant effect on the residual stresses for finite departures from isotropy. When the hexagonal grains are randomly distorted, the magnitude and distribution of residual stresses are found to be insignificantly altered. Spontaneous microfracture due to the generation of internal stresses is also simulated in the analysis by allowing for the nucleation and growth of intergranular microcracks when the fracture energy along the grain facets exceeds a certain critical value. When such microcracking is incorporated into the computation, the levels of residual stress are markedly reduced as a consequence of stress dissipation. The dependence of intergranular microcracking on grain size and temperature variation is examined and the predicted trends on material degradation or the complete suppression of microfracture are discussed in the light of available experimental results.

Prediction of Residual Stresses in Butt Welded Plates Using Inherent Strains

Abstract: The source of residual stresses in the vicinity of a weld may be expressed in terms of inherent strains. The characteristics of the inherent strain distributions in butt welds are investigated. It is found that the patterns vary little with changes in the welding conditions and sizes of the welded plates. With some assumptions, simple formulas are derived for the distribution and magnitude of inherent strain in a butt weld. A method of predicting the residual stress in a butt-welded plate using the characteristics of inherent strain distributions is presented. The validity of the method is confirmed by thermal elasto-plastic analysis using the finite element method (FEM).

Study of interface resistances in epitaxial YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions

Abstract: We have measured interface resistances in YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions with different barrier materials in an edge junction geometry. CaRuO3, La0.5Sr0.5CoO3, Y0.7Ca0.3Ba2Cu3O7−x, YBa2Cu2.79Co0.21O7−x, and La1.4Sr0.6CuO4 have been employed as the epitaxial barrier materials. We observe interface resistances of the order of 1×10−8 Ω cm2 when we use CaRuO3 and La0.5Sr0.5CoO3 barriers. These two barrier materials are cubic perovskites. However, in the case of the layered barrier materials, the measured interface resistances are smaller than 1×10−10 Ω cm2. Our study suggests that the oxygen disorder at the YBa2Cu3O7−x surface, due to stress created by the thermal expansion mismatch between YBa2Cu3O7−x and the barrier, may be the origin of the interface resistances, and that the magnitude of this stress can change the resistance by orders of magnitude.

A theoretical investigation of the column behaviour of cold-formed square hollow sections

Abstract: An experimental programme investigating the column behaviour of four sizes of square hollow section was undertaken at the University of Sydney using Australian produced cold-formed square hollow sections. Stub and pinended column tests were performed and detailed measurements of the yield stress and residual stress taken around the sections. A large deflection elastic—plastic finite strip analysis including the measured distributions of yield stress and residual stress is used to investigate the behaviour of the stub and pin-ended columns. In particular, the influence of the measured through thickness residual stress components on the ultimate load and behaviour of the square hollow section columns is demonstrated. The analysis accounts for plate geometric imperfections, the variation of yield stress around a section, the stress—strain characteristics of the material forming the section and the highly complex patterns of residual stress produced by the cold-forming process. Comparison of the analytical results with the test results is provided.

The influence of crystallographic texture on diffraction measurements of residual stress

Abstract: X-ray or neutron diffraction can be used to measure residual stresses in crystalline materials. Normally, the d -sin 2 Ω-method is used to derive the stress from the diffraction data. However, the method fails if the samples are anisotropic because of a crystallographic texture, The present paper gives an overview of this problem. The theoretical part gives an analysis that sheds some light on the influence of crystallographic texture on diffraction data obtained during stress measurements. It is based on linear elastic models (Reuss and Voigt). The next part reviews the experimental findings of various authors, which shows that linear elastic models are sometimes insufficient to deal with anisotropy, especially in cases when plastic deformation has taken place. Finally, several methods (including some that make use of the ODF) that are used when dealing with textured samples are reviewed.

Flaw-tolerance and crack-resistance properties of alumina-aluminum titanate composites with tailored microstructures

Abstract: The microstructures of alumina-aluminum titanate (A-AT) composites have been tailored with the intent of altering their crack-resistance (R- or T-curve) behavior and resulting flaw tolerance. Specifically, two microstructural parameters which influence grain-localized crack bridging, viz., (1) internal residual stresses and (2) microstructural coarseness, have been investigated. Particulate aluminum titanate was added to alumina to induce intense internal residual stresses from extreme thermal intense internal residual stresses from extreme thermal expansion mismatch. It was found that A-AT composites with uniformly distributed 20-30 vol% aluminum titanate (duplex) showed significantly improved flaw tolerance over single-phase alumina. Coarsening of the duplex microstructure via grain growth scaling was relatively ineffective in improving the flaw tolerance further. Onset of spontaneous microcracking precluded further exploitation of this scaling approach. Therefore, an alternative approach to coarsening was developed, in which a uniform distribution of large alumina grains was incorporated within a fine-grain A-AT matrix (duplex-bimodal), via a powder processing route. The duplex-bimodal composites yielded excellent flaw tolerance with steady-state toughness of [approximately]8 MPa [center dot] m[sup 1/2]. A qualitative model for microstructure development in these duplex-bimodal composites is presented.

Cracking and stress redistribution in ceramic layered composites

Abstract: Problems are analyzed that have bearing on cracking and survivability in the presence of cracking of layered composite materials composed of brittle layers joined by either a weak interface or a thin layer of a well-bonded ductile metal. The problems concern a crack in one brittle layer impinging on the interface with the neighbouring brittle layer and either branching, if the interface is weak, or inducing plastic yielding, if a ductile bonding agent is present. For the case of a weak interface, the effect of debonding along the interface is analyzed and results for the stress redistribution in the uncracked layer directly ahead of the crack tip are presented. Debonding lowers the high stress concentration just across the interface, but causes a small increase in the tensile stresses further ahead of the tip in the uncracked layer. A similar stress redistribution occurs when the layers are joined by a very thin ductile layer that undergoes yielding above and below the crack tip, allowing the cracked layer to redistribute its load to the neighbouring uncracked layer. The role of debonding and yielding of the interface in three-dimensional tunnel cracking through an individual layer is also discussed and analyzed. Residual stress in the layers is included in the analysis.

Transverse cracking in fiber-reinforced brittle matrix, cross-ply laminates

Abstract: The topic addressed in this paper is transverse cracking in the matrix of the 90° layers of a cross-ply laminate loaded in tension. Several aspects of the problem are considered, including conditions for the onset of matrix cracking, the evolution of crack spacing, the compliance of the cracked laminate, and the overall strain contributed by residual stress when matrix cracking occurs. The heart of the analysis is the plane strain problem for a doubly periodic array of cracks in the 90° layers. A fairly complete solution to this problem is presented based on finite element calculations. In addition, a useful, accurate closed form representation is also included. This solution permits the estimation of compliance change and strain due to release of residual stress. It can also be used to predict the energy release rate of cracks tunneling through the matrix. In turn, this energy release rate can be used to predict both the onset of matrix cracking and the evolution of crack spacing in the 90° layers as a function of applied stress. All these results are used to construct overall stress-strain behavior of a laminate undergoing matrix cracking in the presence of initial residual stress.

Experimental and finite element predictions of residual stresses due to orthogonal metal cutting

Abstract: The development and implementation of a finite element method for the simulation of plane-strain orthogonal metal cutting processes with continuous chip formation are presented. Experimental procedures for orthogonal metal cutting and measurement of distributions of residual stresses using the X-ray diffraction method are also presented. A four-node, eight degree-of-freedom, quadrilateral plane-strain finite element is formulated. The effects of elasticity, viscoplasticity, temperature, friction, strain-rate and large strain are included in this formulation. Some special techniques for the finite element simulation of metal cutting processes, such as element separation and mesh rezoning, are used to enhance the computational accuracy and efficiency. The orthogonal metal cutting experiment is set-up on a shaper, and the distributions of residual stresses of the annealed 1020 carbon steel sample are measured using the X-ray diffraction method. Under nominally the same cutting conditions as the experiment, the cutting processes are also simulated using the finite element method. Comparisons of the experimental and finite element results for the distributions of residual stresses indicate a fairly reasonable level of agreement. The versatility of the present finite element simulation method allows for displaying detailed results and knowledge generated by orthogonal metal cutting processes, such as the distribution of temperature, yield stress, effective stress, plastic strain, plastic strain-rate, hydrostatic stress, deformed configuration, etc. Such knowledge is useful to provide physical insights into the process as well as to better design the process for machining parts with improved performance.

Effect of rapid thermal annealing on both the stress and the bonding states of a-SiC:H films

Abstract: The stress evolution of plasma enhanced chemical vapor deposition a‐SiC:H films was studied by increasing the annealing temperature from 300 to 850 °C. A large stress range from −1 GPa compressive to 1 GPa tensile was investigated. Infrared absorption, x‐ray photoelectron spectroscopy, and elastic recoil detection analysis techniques were used to follow the Si‐C, Si‐H, and C‐H absorption band evolutions, the Si2p and C1s chemical bondings, and the a‐SiC:H film hydrogen content variations with the annealing temperatures, respectively. It is pointed out that the compressive stress relaxation is due to the hydrogenated bond (Si—H and C—H) dissociation, whereas the tensile stress is caused by additional Si—C bond formation. At high annealing temperatures, a total hydrogen content decrease is clearly observed. This total hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. The results are discussed and a quantitative model correlating the intrinsic stress variation to the Si...

Finite element analysis of the rake angle effects in orthogonal metal cutting

Abstract: The plane-strain finite element method is developed and applied to model the orthogonal metal cutting of annealed low carbon steel with continuous chip formation. Four sets of simulation results for cutting with −2°, 0°, 5°, and 15° rake angle are summarized and compared to analyze the effects of rake angle in the cutting processes. The initial and deformed finite element meshes, as the cutting reaches steady-state condition, are first presented. Simulation results of the cutting forces and residual stresses, along with the X-ray diffraction measurements of the residual stresses generated using a worn cutting tool with 5° rake angle, are used to identify the influences of the rake angle and tool sharpness. Elements are selected to represent three sections along the shear and contact zones and under the cut surface. The normal and shear stresses, distributions of parameters along these three sections, and contours of temperature, plastic strain, and effective stress are then presented. Limitations of the finite element method for metal cutting simulation are discussed.

Fracture energy release and size effect in borehole breakout

Abstract: SUMMARY The paper presents a simple approximate analytical solution of the remote stresses that cause the collapse of a borehole or other circular cylindrical cavity in an infinite elastic space. Regions of parallel equidistant splitting cracks are assumed to form on the sides of the cavity. Their boundary is assumed to be an eIlipse of a growing horizontal axis, the other axis remaining equal to the borehole diameter. The slabs of rock between the splitting cracks are assumed to buckle as slender columns, and their post-critical stress is considered as the residual stress in the cracked rock. The buckling of these slab columns is assumed to be resisted not only by their elastic bending stiffness but also shear stresses produced on rough crack faces by relative shear displacements. The energy release from the infinite medium'-caused by the growth of the eIliptical cracking region is evaluated according to Eschelby's theorem. This release is set equal to the energy dissipated by the formation of alI the splitting cracks, which is calculated under the assumption of constant fracture energy. This yields the collapse stress as a function of the elastic moduli, fracture energy, ratio of the remote principal stresses, crack shear resistance characteristic and borehole diameter. The collapse stress as a function of crack spacing is found to have a minimum, and the correct crack spacing is determined from this minimum. For small enough diameters, the crack spacing increases as the (4/5)-power of the borehole diameter, while for large enough diameters a constant spacing is approached. In contrast to plastic solutions, the breakout stress exhibits a size effect, such that for small enough diameters the breakout stress decreases as the ( - 2/5)-power of the borehole diameter, while for large enough diameters a constant limiting value is approached. Finally, some numerical estimates are given and the validity of various simplifying assumptions made is discussed.

A Room Temperature Chemical Vapor Deposition SiOF Film Formation Technology for the Interlayer in Submicron Multilevel Interconnections

Abstract: A new interlayer dielectric film formation technology for multilevel interconnection by catalytic chemical vapor deposition has been developed. This technique utilizes fluorotriethoxysilane and water vapor as gas source. The films deposited at 25°C have remarkably good properties, such as tightly bonded Si‐O networks with no OH radicals, large density value (2.20 g/cm3), small residual stress (50 MPa), low leakage current, and small dielectric constant (3.7), although the film contains residual fluorine and carbon atoms with , respectively. Based on the film characterization results, we speculate that the reaction sequence for the film deposition is: hydrolysis of fluorotriethoxysilane monomers, formation of siloxane oligomers with reaction by‐product (alcohol), adsorption of the oligomers to the wafer surface, and then polymerization. The electrical conduction mechanism study revealed that the Schottky emission was dominant for the electric conduction through the film. It also has clarified that the deposition film thickness has no dependence on Al wiring widths, and is completely isotropic with no crack or keyhole in the film.

Residual stresses and their effects on deformation

Abstract: Residual stresses induced by thermal expansion mismatch in metal-matrix composites are studied by three-dimensional (3-D) elastic-plastic finite element analyses. Typically, the stress-free state is 150 to 300 K above room temperature. The coefficient of thermal expansion of the matrix is 3 to 5 times larger than that of the ceramic inclusion, resulting in compressive stresses of order 200 MPa in the inclusions. Both compressive and tensile stresses can be found in the matrix. Since the stress may exceed the matrix yield strength near the particles, plastic flow occurs. The authors find a significant influence of this flow on the elastic and plastic properties of the composite. The calculated residual strains in TiC particles due to thermal expansion mismatch and external loads compare well with recent neutron diffraction experiments (Bourkeet al.) The present work is the first reported three-dimensional analysis of spherical inclusions in different arrays (simple cubic (sc) and face-centered cubic (fcc)) that permit a study of particle interactions.

Origin and development of residual stresses in the NiNiO system: in-situ studies at high temperature by X-ray diffraction

Abstract: In order to characterize the respective importance of the growth stresses, thermal stresses and stress relaxation developed in oxide scales, two high temperature chambers for X-ray diffraction were designed, allowing us to determine the stresses in both the oxide and the substrate with the sin2 ϕ technique, at high temperatures or room temperature and during heating-cooling sequences. It was applied to NiNiO. At room temperature after oxidation, NiO is subjected to compressive stresses whose level depends on the substrate thickness and on the oxidation time and temperature. In the substrate, compressive stresses are mainly due to internal oxidation. During oxidation at 900 °C, the oxide scale is subjected to slight tensile stresses which can be due partially to anionic diffusion, internal oxidation or the heating process. During heating-cooling sequences, the stresses in the scale decrease with increasing temperature and become negligible when the oxidation temperature is reached. The reversibility of the stress-temperature curve indicates that no stress relaxation occurs. The stresses found at room temperature are due only to thermal stresses and fit well the theoretical calculation of thermal stresses in NiO scale based on the newly determined thermal expansion coefficients of Ni and NiO. All these results show that the stresses found at room temperature are mainly generated during cooling and that the effect of the Pilling-Bedworth ratio or of factors playing a role during isothermal growth is negligible.

The effect of residual stresses on hardness measurements

Abstract: The RockwellC hardness,RC, was measured as a function of position on steel rings with different residual-stress profiles through the thickness. An experimental correlation between residual stress andRC was obtained. A relationship between the average pressurep of a spherical indenter, the yield strengthSy and the residual stress of the material was conceived and used in fitting the experimental data.

Relationship of crystallographic orientation and impurities to stress, resistivity, and morphology for sputtered copper films

Abstract: Copper has 44% higher conductivity than aluminum, making copper an attractive interconnect metal for advanced multilevel interconnections in integrated circuits (IC) and multichip modules (MCM). Since sputter deposition is widely used in IC and MCM fabrication, the properties of sputtered copper films are investigated in this study. In particular, MCM technology requires relatively thick coatings (5–8 μm), which can result in high stress. Thin film stress can lead to dielectric cracking, wafer bow, and reduced operating lifetime. This study reports the residual stress, morphology, and electrical resistivity of annealed copper coatings as a function of the thin film crystal orientation determined by x‐ray diffraction. We found that the stress and electrical resistivity of copper thin films can be significantly reduced by controlling the orientation of the copper crystallites.