Showing papers on "Residual stress published in 1991"

Passive material properties of intact ventricular myocardium determined from a cylindrical model.

Abstract: The equatorial region of the canine left ventricle was modeled as a thick-walled cylinder consisting of an incompressible hyperelastic material with homogeneous exponential properties. The anisotropic properties of the passive myocardium were assumed to be locally transversely isotropic with respect to a fiber axis whose orientation varied linearly across the wall. Simultaneous inflation, extension, and torsion were applied to the cylinder to produce epicardial strains that were measured previously in the potassium-arrested dog heart. Residual stress in the unloaded state was included by considering the stress-free configuration to be a warped cylindrical arc. In the special case of isotropic material properties, torsion and residual stress both significantly reduced the high circumferential stress peaks predicted at the endocardium by previous models. However, a resultant axial force and moment were necessary to cause the observed epicardial deformations. Therefore, the anisotropic material parameters were found that minimized these resultants and allowed the prescribed displacements to occur subject to the known ventricular pressure loads. The global minimum solution of this parameter optimization problem indicated that the stiffness of passive myocardium (defined for a 20 percent equibiaxial extension) would be 2.4 to 6.6 times greater in the fiber direction than in the transverse plane for a broad range of assumed fiber angle distributions and residual stresses. This agrees with the results of biaxial tissue testing. The predicted transmural distributions of fiber stress were relatively flat with slight peaks in the subepicardium, and the fiber strain profiles agreed closely with experimentally observed sarcomere length distributions. The results indicate that torsion, residual stress and material anisotropy associated with the fiber architecture all can act to reduce endocardial stress gradients in the passive left ventricle.

Theoretical analysis of the fiber pullout and pushout tests

Abstract: The fiber pullout and pushout tests have been analyzed to predict the load-displacement behavior in terms of fiber/matrix interface parameters. The effects of residual axial strain in the fiber and fiber surface topography were included. The residual axial strain was found to be a significant parameter. It is shown that the interface failure can be progressive or catastrophic. In the case of a progressive failure of the interface, the load-displacement curve is nonlinear. The portion of the curve from above the first nonlinearity to near the peak load can be predicted in terms of parameters of the interface, viz., the friction coefficient, the radial stress at the interface, the fracture toughness of the interface, and the residual axial strain in the fiber. Values for these parameters can be obtained from a single loaddeflection curve. The peak load and load drop, which are usually reported, are found not to be directly relatable to any interface property, since the length of the last portion of the fiber to debond is influenced by end effects and hence not easily predicted. However, for data which describe the peak load as a function of initial embedded length, that factor can be eliminated and the data reduced to yield the relevant interface parameters. In pullout, the peak and friction loads saturate with large specimen thickness. Catastrophic failure is favored when the debond initiation load is high or when residual stress is low. Finally, a methodology to extract interface parameters from experimental data is suggested.

What are the residual stresses doing in our blood vessels

Abstract: We show that the residual strain and stress in the blood vessels are not zero, and that the zero-stress state of a blood vessel consists of open-sector segments whose opening angles vary along the longitudinal axis of the vessel. When the homeostatic state of the blood vessel is changed, e.g., by a sudden hypertession, the opening angle will change. The time constant of the opening angle change is a few hours (e.g., in the pulmonary artery) or a few days (e.g., in the aorta). From a kinematic point of view, a change of opening angle is a bending of the blood vessel wall, which is caused by a nonuniformly distributed residual strain. From a mechanics point of view, changes of blood pressure and residual strain cause change of stress in the blood vessel wall. Correlating the stress with the change of residual strain yields a fundamental biological law relating the rate of growth or resorption of tissue with the stress in the tissue. Thus, residual stresses are related to the remodeling of the blood vessel wall. Our blood vessel remodels itself when stress changes. The stress-growth law provides a biomechanical foundation for tissue engineering.

An X-ray study of creep-deformation induced changes of the lattice mismatch in the γ′-hardened monocrystalline nickel-base superalloy SRR 99

Abstract: X-ray line profile measurements were performed on monocrystalline specimens of the γ′-hardened nickel-base superalloy SSR 99 with axes close to the [001] direction. The aim was to measure the local lattice parameters in the γ-matrix and in the γ′-particles and to obtain information on the lattice mismatch and internal stresses. For this purpose, a special high-resolution double crystal diffractomoter with negligible instrumental line broadening was used. Measurements were performed on specimens in the initial state with cuboidal γ′-particles and on creep-deformed specimens containing the so-called γ/γ′-raft structure. In several cases the line profiles were measured as a function of the rocking angle, and the intensity distributions were mapped in reciprocal space around the (002) and (020) Bragg reflections. In the undeformed state these intensity distributionsindicate that the local lattice parameter varies spatially in the γ-phase. The line profiles of specimens in the initial state were asymmetric. A remarkable result obtained on creep-deformed specimes was that, whereas the asymmetry of the (020) line profiles was enhanced to the extent that a hump or a second peak appeared, the asymmetry of the (002) line profiles was reversed in sign. A quantitative evaluation yielded mean values of the constrained lattice mismatch which, in the case of creep-deformed specimens, differ significantly for the (001) and (010) lattice plane spacings. It is concluded that the orientation-dependent lattice spacings represent a triaxial state of residual stress which has its origin in the superposition of the originally present coherency stresses and the deformation-induced internal stresses. All observed features could be explained in detail in terms of a composite model of plastic deformation, which takes into account the dislocation networks deposited at the γ/γ′-interfaces during deformation.

Micromechanical modeling of fiber/matrix interface effects in transversely loaded SiC/Ti-6-4 metal matrix composites

Abstract: The transverse tensile behavior of a composite composed of unidirectional silicon-carbide fiber (Textron SCS-6) in a Ti-6AL-4V matrix is examined with emphasis on the effects of fiber-matrix interface strength. The residual stresses as a result of a mismatch in the coefficients of thermal expansion of silicon carbide and titanium are estimated analytically and compared with measurements made using X-ray diffraction techniques. Idealizing the composite as a regular rectangular array of fibers in an elasto-plastic matrix, the transverse tensile stress-strain behavior is predicted under the assumptions of an infinitely strong interface as well as an interface without tensile strength. These results are compared with experiments conducted at three different temperatures. The agreement between experiment and predictions based on an interface without tensile strength is extremely close. The modeled stress-strain curves predict a well-defined knee in the transverse tensile stress-strain curve associated with the separation of fiber and matrix at their interface. The same stress-strain behavior is observed experimentally. Results of edge replica experiments and mechanical unloading from stress levels above the knee are also presented as additional evidence of the association of fiber-matrix separation with the knee in the transverse tensile stress-strain curve.

Calculation of residual stresses in injection molded products

Abstract: Both flow- and thermally-induced residual stresses which arise during the injection molding of amorphous thermoplastic polymers are calculated in the filling and post-filling stage. To achieve this, a compressible version of the Leonov model is employed. Two techniques to calculate flow-induced residual stresses are investigated. First, a direct approach is developed where the pressure problem is formulated using the viscoelastic material model. Second, generalized Newtonian material behavior is assumed in formulating the pressure problem, and the resulting flow kinematics is used to calculate normal stresses employing the compressible Leonov model. The latter technique gives comparable results, while reducing the computational cost significantly.

Finite element visualization of fatigue crack closure in plane stress and plane strain

Abstract: Elastic-plastic finite element simulations of growing fatigue cracks in both plane stress and plane strain are used as an aid to visualization and analysis of the crack closure phenomenon Residual stress and strain fields near the crack tip are depicted by both color fringe plots and x-y graphs Development of the residual plastic stretch in the wake of a growing plane stress fatigue crack is shown to be associated with the transfer of material from the thickness direction to the axial direction Finite element analyses indicate that crack closure does occur under pure plane strain conditions The development of the residual plastic stretch in plane strain is shown to be associated with the transfer of material from the in-plane transverse direction to the axial direction This in-plane contraction also leads to the generation of complex residual stress fields The total length of closed crack at minimum load in plane strain is shown to be a small fraction of the total crack length, especially for positive stress ratios This suggests that experimental measurement of plane strain closure would be extremely difficult, and may explain why some investigators have concluded that closure does not occur in plane strain

Fatigue of Yttria-Stabilized Zirconia: II, Crack Propagation, Fatigue Striations, and Short-Crack Behavior

Abstract: Fatigue crack propagation in 3Y-TZP was investigated using controlled surface flaws. A unique growth law strongly dependent on the maximum stress intensity factor and quadratically dependent on the amplitude of the range of stress intensity factor was established. This growth law was found to apply for both surface flaws and internal flaws and could be used to predict fatigue lifetime. The presence of residual stress altered the growth mechanics so that an inverse growth rate dependence on the applied stress, reminiscent of the so-called "short-crack behavioe was manifested. Fatigue striations resulting from alternate overload fracture and fatigue fracture during stress cycling were observed. The appearance of striations varied with the R ratio and was very sensitive to the loading condition and crack geometry.

The anisotropic mechanical properties of a Ti matrix composite reinforced with SiC fibers

Abstract: The anisotropic mechanical properties of a Ti alloy composite reinforced with SiC fibers have been investigated and rationalized using analytical models. The appropriate material model for this composite involves the following features: an interface that debonds and slides, a flaw insensitive ductile matrix, and high-strength elastic fibers subject to residual compressive stress caused by thermal expansion mismatch. This, model is broadly consistent with the longitudinal, transverse, and shear properties of the composite.

Elastoplastic finite element analysis of short-fiber-reinforced SiC/Al composites: effects of thermal treatment

Abstract: Elastoplastic finite element analyses of realistic models of short-fiber-reinforced composites were extended to include the effects of prior thermal treatments on predictions of subsequent mechanical properties. Two three-dimensional models were used, one in which the fiber ends were transversely aligned and another in which they were staggered. Both models were found to be necessary for accurate predictions of the behavior of higher volume fraction composites. The temperature dependence of the yield stress of the matrix material was explicitly included in the analysis. The spatial and temporal history of calculated. The room temperature residual stresses were also predicted. Both the plastic deformation and the residual stresses in the matrix were spatially non-uniform and varied rapidly from the regions near the ends of the fiber to those near the midpoint. Predictions of subsequent tensile stress-strain properties were in good quantitative agreement with experiments. The presence of residual stresses and locally deformed regions caused the tensile behavior to differ from the compressive behavior. These differences were complex and depended on the volume fraction and aspect ratio of the reinforcement. The analyses provide detailed insight into the deformation mechanisms of these composites.

The fatigue performance of machined surfaces

Abstract: It is well known that surface condition has a strong effect on fatigue life, and that most surfaces produced by conventional manufacturing operations such as machining and forging have poorer fatigue behaviour than polished surfaces commonly used for laboratory specimens. As yet, there are no reliable quantitative models to predict the behaviour of such surfaces; the problem is a multi-parameter one, involving surface roughness, surface microstructure and residual stress. High-cycle fatigue data was obtained for En19 steel, using four types of machined surface, produced by: polishing, grinding, milling and shaping. Residual stress was eliminated by heat treatment. Fatigue limit data were plotted as a function of roughness parameters using Kitagawa-type diagrams, and compared to simple notch-based and crack-based models. It was found that, whilst both theories tended to be overly conservative, fracture mechanics approaches are useful for relatively low roughness, when the surfaces can be modelled as a series of short cracks. For higher roughness a notch-based approach is appropriate.

Mechanical approach to the residual stress field induced by shot peening

Abstract: A simplified analytical model for calculating the compressive residual stress field due to shot peening is given. For eight shot-peening conditions. experimental verifications were carried out on 40Cr steel with four different heat treatment conditions. The results show good agreement with predictions of the model.

Residual stress measurements of thin aluminum metallizations by continuous indentation and x-ray stress measurement techniques

Abstract: Stress relaxation in aluminum films of several thicknesses was characterized by using both continuous indentation and x-ray diffraction techniques. Results of the indentation and x-ray stress measurements compare closely for films of small thicknesses. Indentation data from thicker films do not compare well to the x-ray data due to the presence of a residual stress distribution.

The influence of residual stress on the yielding of metal matrix composites

Abstract: A systematic numerical study of the effect of residual stresses on the yielding behavior of composites comprised of elastic particles well bonded to a ductile matrix is carried out. The calculations are made within the framework of continuum plasticity theory using cell models. An investigation is made into the roles volume fraction, particle shape, and hardening play in this interaction. A slight transient softening of the composite in both tension and compression is found, but the limit stress of the composite is unaffected by the residual stress. Thus the limit stress-strain response is symmetric in tension and compression for strains greater than a few times the matrix yield strain. A qualitative connection is made between the transient reduction in stiffness and the extent to which there was prior plastic deformation in the matrix due to residual stresses.

Residual stresses induced by laser-shocks

Abstract: When irradiating a metallic target with a short and intense laser pulse, a high pressure plasma is produced on the surface. An elastic-plastic wave is, then, propagating in the target, creating plastic strains. As a result, a residual stress field is induced. In this paper, by mean of a very simple model, we evaluate the laser produced plasma pressure. The analysis of the resulting wave propagation allows us to compute the plastic strain field. Using these results, we express analytically the induced surface residual stress and the plastically affected depth. We show how this work allows the optimization of the use of laser-shocks as a fatigue surface treatment and an example of application is presented.

Stress relaxation of passivated aluminum line metallizations on silicon substrates

Abstract: In the present study, the Eshelby theory of inclusions is applied to model the stresses arising after heat treatment at 400 °C in aluminum line metallizations, embedded in silicon/passivation matrix. The stresses obtained are about 200 MPa higher than the ones previously reported. Moreover, the stresses in the axial and width directions of the lines are shown to be on the same order, while the normal stress is smaller, especially in the lines of low thickness‐to‐width ratio. A modification of the familiar sin2 ψ method of x‐ray stress measurement is presented to deal more accurately with the [111]‐fiber texture present in the aluminum lines studied. The lateral and normal stresses in the aluminum metallizations after a heat treatment at 400 °C are measured in room temperature by x‐ray diffraction from 4 h after the heat treatment at 400 °C up to 3 months. The experimental results are well in accord with predictions obtained from the Eshelby model. Particularly, the lateral stresses are found to be about equal, while the initial normal stress is smaller, but eventually becomes the largest stress component. Dislocation mechanisms to rationalize the present observations are discussed: at longer times, diffusion‐controlled dislocation climb and void growth connected to it appear to be the most important mechanisms to relieve the stress, while during cooling dislocation glide is also significant.

On the toughening of intermetallics with ductile fibers : role of interfaces

Abstract: The role of fiber debonding and sliding on the toughness of intermetallic composites reinforced with ductile fibers is examined. The toughness is shown to be a function of the matrix/fiber interface properties, residual stresses and the volume fraction, size and flow behavior of the fibers. Mechanical testing and in situ microstructural observations were carried out on a Ti-25at.%Ta-50at.%Al intermetallic matrix reinforced with W-3Re fibers. The fibers were coated with a thin oxide layer in order to induce debonding and prevent interdiffusion between the fiber and the matrix. The ductility, high strength and debond characteristics of coated tungsten-rhenium fibers promote a large increase in toughness. However, the mismatch in thermal expansion coefficients is the source of large residual tensile stresses in the matrix that induces spontaneous matrix cracking. Matrix cracking and composite toughness are examined as a function of the interfacial properties, residual stresses and properties of the fiber.

Control of residual stress of polysilicon thin films by heavy doping in surface micromachining

Abstract: Control of residual stress of polysilicon thin films for use in surface micromachining was experimentally studied. It was found that stress can be controlled by fairly heavy doping and subsequent annealing. Polysilicon films with lower than 50 ppm tensile and -50 ppm compressive strain were fabricated as measured from micromachined test structures. Very low tensile strains of less than 100 ppm were obtained with As/sup +/ implantation doses on the order of 1-3*10/sup 16/ cm/sup -2/ and annealing temperatures of 1050 degrees C. Strains as low as 15-30 ppm were achieved. The problem of sticking of structures to the substrate after wet etching of sacrificial oxide was observed and circumvented. >

An analysis of the effect of residual stresses on deformation and damage mechanisms in AlSiC composites

Abstract: The effect of thermally induced residual stresses on the mechanical properties and ductility of AlSiC composites was investigated numerically. The predicted behavior in uniaxial loading was calculated with and without the inclusion of the residual stresses which result from the mismatch in thermal expansion between aluminum and SiC. In this analysis, void nucleation by interfacial debonding at the whiskers' ends was assumed to be the limiting failure mechanism. Two cases, both with a fiber volume fraction of 20% and fiber aspect ratio of 4, but with different fiber spacings, were considered. The residual stresses had a small effect on the predicted ductility of the composite, even when a relatively weak interface strength was assumed. The residual stresses are shown to redistribute as interfacial failure is approached. A close end-to-end fiber spacing gives a greater flow strength in compression than in tension and the residual stresses which arise during thermomechanical processing tend to enhance this effect.

On the origin of compressive stress in PVD coatings — an explicative model

Abstract: Compressive stress in hard plasma vapor deposition coatings — up to values of several gigapascals — is found to be strongly dependent on the process parameters, especially on the energy of ion bombardment on the growing film. Increasing energy usually causes increasing stress. Internal stress influences all other coating properties, e.g. adhesive strength, microhardness and wear resistance. A detailed knowledge of the origin of this stress is therefore of great significance, particularly for practical applications. Experimental work was performed with various coating and substrate materials and various coating processes (d.c. sputtering, r.f. sputtering, arc ion plating) to determine the interrelationships. The creation of internal stress can be explained by a phenomenological model describing the interaction of bombarding ions in the plasma and coating ions, whose mobility is restricted by their position on the substrate or previously deposited layers of the coating. The validity of the model is demonstrated by good agreement with experimental results for both crystalline and amorphous coatings.

Moiré interferometry determination of residual stresses in the presence of gradients

Abstract: The full-field technique of high-sensitivity moire interferometry in conjunction with a multiple-hole-drilling procedure is applied to residual-stress measurements in the presence of gradients. The method arrives at residual-stress estimates starting from in-plane displacement components. Successful applications of the method to problems simulating the nonuniform transverse residual stresses of welded joints are reported.

Self-adjusting microstructures (SAMS)

Abstract: Composite LPCVD polysilicon/silicon nitride flexures have been fabricated on the sidewalls of previously patterned polysilicon mesas by anisotropic reactive-ion etching Cantilever beams 450 nm thick (150 nm of silicon nitride and 300 nm of polysilicon) and 25 mu m wide (the mesa height) were fabricated Upon release from the sidewall, the cantilever deflects laterally away from the mesa due to a large built-in bending moment arising from the compressive residual stress in the polysilicon layer and the tensile residual stress in the silicon nitride layer End deflections of about 20 mu m are observed for 70 mu m-long cantilevers This self-adjusting microstructure (SAMS) makes use of residual stresses in thin films to reduce intercomponent clearances or to apply preloads in micromechanical systems The authors present a design theory for SAMS, describe the fabrication process in detail, and discuss the results of initial experiments >

Residual Stress in Pzt Thin Films and its Effect on Ferroelectric Properties

Abstract: The residual stress in solution derived Pb(Zr.53Ti.47)O3, PZT 53:47, films was determined by measuring the bending of the substrate due to the stress. The substrate consisted of an oxidized (100) silicon wafer with 300 nm coating of platinum. In all cases the stress was tensile. Films fired at a temperature in the range where pyrochlore formation occurs (500° to 575°C) had the highest residual stresses, 200 to 350 MPa, whereas those fired at higher temperatures, 600° to 650°C, where the perovskite phase forms had stresses of 100 to 200 MPa. Stress measurements made during film firing indicate that the pyrochlore containing films had higher residual stress because their coefficient of thermal expansion was much larger than that of predominantly perovskite films. The effect of the amount of stress on ferroelectric properties was studied by making measurements on a film with and without the application of an external stress. The external stress was applied by bending a circular section of the substrate, which effectively lowered the amount of tensile stress in the film by ~30%. Decreasing the stress in this manner was found to increase the remanent polarization by ~11% and the dielectric constant by ~2%.

Thermal Residual Stresses in Ceramic‐to‐Metal Brazed Joints

Abstract: When a ceramic-to-metal brazed joint is fabricated, residual stresses develop as the material is cooled from the brazing temperature to room temperature. These residual stresses reduce the strength of the brazed joint, and in some cases lead to catastrophic failure at or near an interface, during the brazing process itself. Finite element analysis modeling techniques are presented and used to determine the residual stresses inherent in the bonding process. Elastoplastic material behavior of the active filler metal, including its temperature-dependent stress–strain response, is accounted for. Various combinations of ceramics and metals are considered. Residual stresses are calculated and used to explain joint strength. The effects of the filler metal work-hardening rate and thickness on the levels of residual stresses are investigated. Analysis shows that the conclusions reached from a laboratory-brazed coupon test may not be applicable to the practical case in which ceramic tiles are brazed onto a metal substrate. There is qualitative agreement between analytical predictions and experimental findings regarding failure modes that occur during cooling from the brazing temperature to room temperature.