Origin Of Residual Stress Research Articles

Theoretical study of the effect of stress-dependent remodeling on arterial geometry under hypertensive conditions

Remodeling of arterial geometry was studied on the basis of a theoretical model. Sustained hypertension was simulated by a step increase in blood pressure. The artery was considered to be a thick-walled two-layer tube made of nonlinear elastic incompressible material. The basic hypothesis is that the artery remodels its zero-stress configuration in such a way that the strain and stress distributions in the arterial wall under hypertensive conditions are the same as under normotensive conditions. Using this hypothesis, a method for determining the geometrical dimensions of the zero-stress configuration of the hypertensive artery was proposed. To ensure uniqueness of the solution, two side conditions on the remodeling process are imposed: (a) the inner radius of the artery in the unloaded state remains unchanged; and (b) the ratio between the thickness of the inner and outer layer of the hypertensive artery in the zero-stress configuration is known. The model predicts that the arterial wall remodeling causes: (i) an increase of the wall thickness both in the unloaded and physiological states; (ii) an increase of the inner diameter of the hypertensive vessel under high pressure compared to the diameter of the normotensive artery under normal pressure; (iii) the opened-up configuration which arises when the unloaded arterial segment is cut radially still contains residual strains and stresses. These results are consistent with published experimental findings. It is speculated that the origin of residual stresses that exist in the unloaded and opened-up configurations is the stress-modulated growth.

Origins of Residual Stress in Mo and Ta Films: the Role of Impurities, Microstructural Evolution, and Phase Transformations

AbstractBoth the sign and magnitude of residual stress can vary with the thickness of sputter deposited films. The origins of this behavior are not well understood. In this work, we consider the correlation between the residual stress behavior and the depth dependence of impurities in thin (2.5 nm - 150 nm) sputtered Mo and Ta films. We also consider the effects of phase transformations and microstructural changes on the stress behavior. Films were deposited onto Si substrates with native oxide. The residual stress observed in the Mo films varied from highly compressive at 2.5 nm film thickness to ∼ 0 ˜ 10 nm thickness. Ta films also exhibited a high compressive stress, which relaxed from highly compressive to tensile between 10 nm and 50 nm film thickness. Impurities in the films may originate from the sputtering targets, the background gases, and the substrate surfaces. Auger Electron Spectroscopy (AES) results showed the presence of O and C contamination near the film/Si interface; these impurities contributed to the compressive stresses in the thinner films. As anticipated, both Mo and Ta films exhibited grain growth as a function of film thickness, which may have contributed to the relaxation in the compressive stress. The Mo films were entirely bcc. The Ta films showed a transformation from the amorphous phase to the β crystalline phase between 2.5 nm and 20 nm film thickness, which contributed to the relaxation in stress observed in that thickness regime.

Stress analysis of elastic streams in continuous forming operations

In continuous forming operations, such as strip rolling and drawing, the stream of material deforms purely elastically while travelling towards or away from a plastic working zone. A rigorous account of the inhomogeneous state of stress in elastic streams appears to be lacking, except for a short paper by Hill (1985, J. Mech. Phys. Solids 33, 315–322). This was concerned with isotropic materials and isochoric flows under conditions of plane strain. The analytical framework is now considerably extended to take account of compressibility and elastic orthotropy. The fundamental character of the basic set of field equations is examined thoroughly, and then general solutions are derived for the various regimes. The origin of residual stresses in formed products is considered finally.

Residual Stress Control by Ion Beam Assisted Deposition

AbstractThe origin of residual stresses were studied in both crystalline metallic films and amorphous oxide films made by ion beam assisted deposition (IBAD). Monolithic films of AI2O3 were deposited during bombardment by Ne, Ar or Kr over a narrow range of energies, E, and a wide range of ion-to-atom arrival rate ratios, R and were characterized in terms of composition, thickness, density, crystallinity, microstructure and residual stress. The stress was a strong function of ion beam parameters and gas content and compares to the behavior of other amorphous compounds such as MoSix and WS12.2 With increasing normalized energy (eV/atom), residual stress in crystalline metallic films (Mo, W) increases in the tensile direction before reversing and becoming compressive at high normalized energy. The origin of the stress is most likely due to densification or interstitial generation. Residual stress in amorphous films (Al2O3, MoSix and WSi2.2) is initially tensile and monotonically decreases into the compressive region with increasing normalized energy. The amorphous films also incorporate substantially more gas than crystalline films and in the case of Al2O3 are characterized by a high density of voids. Stress due to gas pressure in existing voids explains neither the functional dependence on gas content nor the magnitude of the observed stress. A more likely explanation for the behavior of stress is gas incorporation into the matrix, where the amount of incorporated gas is controlled by trapping.

Fundamentals of Residual Stresses in Joints Between Dissimilar Materials

When a discontinuity in material properties exists across a bonded interface, stresses are generated as a result of any thermal or mechanical loading. These stresses significantly affect strength and failure characteristics and may be large enough to prevent successful fabrication of a reliable joint. The use of an interlayer material to successfully reduce mismatch stresses, thereby preventing joint failure or improving joint strength and reliability, requires knowledge of failure mechanisms and of the effects of interlayer properties on the critical stress components.The origin of residual stresses developed during cooling of a ceramic-metal joint from an elevated fabrication temperature is illustrated qualitatively in Figure 1. Away from edges, the in-plane (parallel to interface) stresses are typically compressive in the ceramic and tensile in the metal. These stresses can cause cracking perpendicular to the interface, leading to spalling or delamination failures. Such failures are frequently observed in thin-film and coating geometries. Where the interface intersects a free edge, large shear and axial (perpendicular to the interface) stresses are generated. The edge stresses are typically tensile within the ceramic and tend to promote crack propagation within the ceramic parallel and adjacent to the interface. This is the most commonly observed failure mode in bonded structural components.

Origin of residual stress in a textured Au thin film on a LiF substrate

The results of residual stress and texture measurements by X-ray diffraction of a Au thin film sputtered on a LiF substrate at 300°C are presented. The residual stress (determined by the “sin 2Ψ“ method) in the film is about -300 MPa and the intra-granular structure is close to the bulk one. A texture and a high crystallinity with a coherency relationship between the 〈 1 01〉 Au and the 〈011〉 LiF directions was observed. These results are compared to those obtained for Au thin films deposited on oxidized silicon. In the case of Au on LiF substrate, the origin of stress generation during the film growth is mainly due to the difference between the thermal expansion coefficients of the film and the substrate and not to a crystallographic coherency relationship.

Investigation on origins of residual stresses in Ni-NiO system by X-ray diffraction at high temperature

To characterize the respective role of oxidation stresses, thermal stresses and relaxation phenomena in the oxide scales, two high temperature chambers for X-ray diffraction have been designed allowing to determine residual stresses in situ, during oxidation of Ni, with the [MATH] technique. At room temperature, the scales are subjected to compressive stresses and compressive stresses are also analyzed in the substrate. During heating-cooling sequences, a reversible variation of the stresses is observed, without relaxation. The stresses determined at room temperature are thermal stresses and theoretical calculation fits well with experimental determination. In situ stress determinations at 900 °C show that slight tensile stresses are then generated in the scale. All these results show that the stresses found at room temperature are mainly generated during cooling, and that the role of Pilling and Bedworth ratio, often considered as the main factor for stress generation in Ni-NiO system, has little effect.

Creep of a Hollow Sphere

Using internal stress arising from a spherically symmetric, finite plastic strain, creep of a hollow sphere subjected to inner and outer pressures, and also thermal stress, is discussed. If computer-aided numerical calculation method is used, creep is easily followed up to a finite plastic strain range including initial transient creep, whatever type of creep law is employed. If assumed in a steady state, creep rate, stress, small plastic strain leading to a stress state in steady creep, and another small plastic strain relaxing thermal stress are analytically obtained. Numerical method is also applicable to creep relaxation. Further, the origin of residual stress after unloading is clarified.

Aspects of various processes for coating and surface hardening

The manufacture of hard surfaces for tribological and many tooling applications normally involves altering the outer microstructure of the original tool or component by introducing a source of strengthening, by effecting a transformation to a harder phase, or by superposing a covalently bonded structure. However, hardened surfaces may not provide the best solution to a wear problem and it may prove preferable to use a coating material in its bulk form, or to use one of the new toughened ceramics. A review is made of relevant processes, the characteristics that provide resistance to specific wear mechanisms, and sources of weakness such as the origin of residual stresses. It is concluded that despite the concentrated effort in research on processes for coatings and for hardening surfaces the understanding of the dependence of mechanical properties on microstructure in very thin coatings is poor. No reliable tests exist for measuring mechanical properties and as a result, attempts at characterizing hard surfaces provide little information to guide a potential user.

Regularities of the origin of residual stresses in nonuniform cooling of parts with boxlike section

1. When local zones of a part are additionally cooled (partial cooling) in the process of normalization, residual compressive stresses are induced in the zones subjected to partial cooling. 2. The residual stresses are the larger, the larger the temperature gradient is between the partially cooled and the surrounding zones after the latter have attained 650°C. Partial cooling is effective if the temperature of the part in cooling during the process of normalization does not attain 650°C. 3. Phase transformations in the partially cooled zone do not affect the residual compressive stresses if the disintegration of austenite in the surrounding zones is of the type of pearlitic transformation. Otherwise variants are possible whereby residual tensile stresses are induced in the zone subjected to partial cooling.