Magnitude Of Tensile Stress Research Articles

Computational modelling of the extrusion of an Al-SiC metal matrix composite using macroscale and microscale methods

This paper explores the use of both macroscale and microscale modelling for the analysis of extrusion of an AA2009 + 25%SiCp metal matrix composite (MMC). The performance of a micromechanical model, where the heterogeneous microstructure of the MMC is explicitly modelled, in predicting the tensile stress—strain behaviour of an AA2009 + 25% SiCp MMC is examined. A macroscale modelling approach is used to simulate extrusion of the MMC through two different die designs, where the MMC is modelled as a homogeneous continuum. Firstly, the extrusion results are used to compare the two die designs, to determine which is the more favourable. Secondly, the predicted macroscale plastic strain distributions and pressures are used with the micromechanical model to assess microscale stress states in the material during extrusion with a view to gaining insights into the risk of damage in the material. In this context, pressure is shown to be hugely important in controlling tensile stress magnitude and in reducing microscale damage risk, and essentially ensuring that extrusion can be achieved in practice. However, the results reveal that damage risk is not totally eliminated and that there may still be locations where the material may rupture.

Fabrication and wear of nitrogen-doped diamond microtools

Chemical vapour deposited diamond films have many industrial applications but are assuming increasing importance in the area of microfabrication, most notably in the development of diamond-coated microtools. For these applications the control of structure and morphology is of critical importance. The crystallite size, orientation, surface roughness and the degree of sp³ character have a profound effect on the machining properties of the films deposited. In this paper, experimental results are presented on the effects of nitrogen doping on the surface morphology, crystallite size, and wear of microtools. It is concluded that the magnitude of tensile stresses induced in the diamond grain by grinding forces at the rake face is the best indicator of tool wear during the grinding process.

Tensile strength of 〈1 0 0〉 and 〈1 1 0〉 tilt bicrystal copper interfaces

Molecular dynamics (MD) simulations are used to model dislocation nucleation at or near symmetric tilt bicrystal copper interfaces with 〈1 0 0〉 or 〈1 1 0〉 misorientation axes. MD simulations indicate that orientation of the opposing lattice regions and the presence of certain structural units are two critical attributes of the interface structure that affect the stress required for dislocation nucleation. Boundaries that contain the E structural unit are found to emit dislocations at comparatively low tensile stress magnitudes. A simple model is proposed to illustrate the impact of interfacial porosity and stresses acting on the slip-plane in non-glide directions on tensile interface strength. Accounting for interfacial porosity through an average measure is found to be sufficient to model the tensile strength of boundaries with a 〈1 0 0〉 misorientation axis and many boundaries with a 〈1 1 0〉 misorientation axis.

Dynamic response of an elastic sphere under diametral impacts

This paper presents a new analytical solution for an elastic sphere under a double impact load which applies as a Heaviside step function of time along a diameter. The suddenly applied loads are modeled as either uniform or Hertz contact stress and are applied through two patches on two ends of a diameter of the solid sphere. The method of solution uses decomposition theory, in which the final dynamic solution is decomposed into two auxiliary problems: (I) a static solution of the applied loading; and (II) the free vibration of the sphere subject to an initial deformed shape induced by the auxiliary problem I. Time evolutions of the stress field at selected points along the axis of compression agree with the solution by [Jingu, T., Nezu, K., 1985. Transient stress in an elastic sphere under diametrical concentrated impact loads. Bulletin JSME 28 (245), 2553–2561] when contact zone is very small (i.e. case of suddenly applied point loads). If energy loss is allowed at each boundary reflection, our long term solution converges to the static solution. The size and shape of the compressive cones under the applied loads are relatively insensitive to the size of the contact zone, while the magnitude of tensile stress along the axis of compression decreases with the contact area. The maximum tensile hoop stress along the axis of compression always appears at point r/ a = 0.38 and at time of 2.61 T 1 (where 2 T 1 is the time for P-wave traveling across the sphere). Based on wave interference plots, inferred failure patterns are proposed and compared to those observed failure pattern in our double impact test.

Abrasive layer fracture wear of microgrinding tools

Research presented indicates that grain fracture is the most important mechanism of material removal from sharp abrasive wheels during grinding. The relationship between the wear of facets and vertices on the surface of microgrinding tools and the induced stresses in model abrasive grains is described in detail. It is concluded that the magnitude of tensile stresses induced in the abrasive layer depth is the best indicator of microgrinding tool wear during a grinding operation.

Characterization of N-Doped Polycrystalline Diamond Films Deposited on Microgrinding Tools

Chemical vapor deposited diamond films have many industrial applications but are assuming increasing importance in the area of microengineering, most notably in the development of diamond coated microgrinding tools. For these applications the control of structure and morphology is of critical importance. The crystallite size, orientation, surface roughness, and the degree of sp 3 character have a profound effect on the tribological properties of the films deposited. In this article, we present experimental results on the effects of nitrogen doping on the surface morphology, crystallite size, and wear of microgrinding tools. The sp 3 character optimizes at 200 ppm nitrogen, and above this value the surface becomes much smoother and crystal sizes decrease considerably. Fracture-induced wear of the diamond grain is the most important mechanism of material removal from a microgrinding tool during the grinding process. Fracture occurs as a consequence of tensile stresses induced into diamond grains by grinding forces to which they are subjected. The relationship between the wear of diamond coated grinding tools, component grinding forces, and induced stresses in the model diamond grains is described in detail. A significant correlation was found between the maximum value of tensile stress induced in the diamond grain and the appropriate wheel-wear parameter (grinding ratio). It was concluded that the magnitude of tensile stresses induced in the diamond grain by grinding forces at the rake face is the best indicator of tool wear during the grinding process.

Dynamic Focusing Effects of Piezoelectric Fiber-Reinforced Laminated Composites System Subjected to Thermal Shock

ABSTRACT This article reveals some phenomena of the dynamic focusing effects of laminated composites with piezoelectric fiber and matrix layer, subjected to thermal shock of a transitory temperature change, by means of an exact analytical solution and the corresponding numerical example. The dynamic focusing effects give rise to much higher tensile stress magnitudes and electric potential at the center or at near the center of the piezoelectric fiber-reinforced composites system, which easily results in breakage along the axial direction of the piezoelectric fiber at the higher tensile stress region. From the analytical expression and numerical example for the piezoelectric fiber reinforced composites system PZT-4/Matrix, it is important to know the mechanism of the dynamic focusing effect of the piezoelectric fiber reinforced composites system and to evaluate the dynamic strength and electric signal of the laminated composites with piezoelectric fiber.

Microstructure-dictated resistance properties of some Indian dinosaur eggshells: finite element modeling

Finite element modeling (FEM) has been used to evaluate microstructure-controlled stability of selected eggshells of Indian dinosaurs. Our study suggests that under static load the eggshell microstructure of Megaloolithus cylindricus displays a low magnitude of tensile stress over most of the spherolith. The magnitude of this tensile stress is lower than that displayed in M. jabalpurensis, M. baghensis, and Subtiliolithus kachchhensis. In M. cylindricus, a shell thickness matching the length of the spheroliths prevents the failure of eggshells, whereas in M. jabalpurensis and M. baghensis, which have thinner shells, the development of additional subspheroliths compensates for the relatively higher magnitude of tensile stresses. Extremely thin eggshell in S. kachchhensis shows a still higher magnitude of tensile stresses, thereby making it prone to cracking, but the propagation of cracks is apparently checked and stability reinforced by wider spacing of pore canals.

Spatially resolved Raman spectroscopy evaluation of residual stresses in 3C-SiC layer deposited on Si substrates with different crystallographic orientations

Raman scattering studies were performed on hot-wall chemical vapor deposited (heteroepitaxial) silicon carbide (SiC) films grown on Si substrates with orientations of (1 0 0), (1 1 1), (1 1 0) and (2 1 1), respectively. Raman spectra suggested that good quality cubic SiC single crystals could be obtained on the Si substrate, independent of its crystallographic orientation. Average residual stresses in the epitaxially grown 3C-SiC films were measured with the laser waist focused on the epilayer surface. Tensile and compressive residual stresses were found to be stored within the SiC film and in the Si substrate, respectively. The residual stress exhibited a marked dependence on the orientation of the substrate. The measured stresses were comparable to the thermal stress deduced from elastic deformation theory, which demonstrates that the large lattice mismatch between cubic SiC and Si is effectively relieved by initial carbonization. The confocal configuration of the optical probe enabled a stress evaluation along the cross-section of the sample, which showed maximum tensile stress magnitude at the SiC/Si interface from the SiC side, decreasing away from the interface in varied rate for different crystallographic orientations. Defocusing experiments were used to precisely characterize the geometry of the laser probe in 3C-SiC single crystal. Based on this knowledge, a theoretical convolution of the in-depth stress distribution could be obtained, which showed a satisfactory agreement with stress values obtained by experiments performed on the 3C-SiC surface.

Stress wave propagation in piezoelectric fiber reinforced laminated composites subjected to thermal shock

This paper presents an analytical solution for stress wave propagation in piezoelectric fiber reinforced laminated composites subjected to thermal shock loading. The thermoelectrodynamic equation for each separate material layer is solved by means of finite Hankel transforms and Laplace transforms. Then by using the interface continuity conditions between the piezoelectric fiber layer and the matrix layer, and the boundary condition at the external surface of the piezoelectric fiber reinforced laminated composite the unknown constants are determined. Thus, an exact solution for stress wave propagation in piezoelectric fiber reinforced laminated composites subjected to thermal shock loading is obtained. From the exact analytical expression and the corresponding numerical example, this paper reveals some phenomena of the dynamic focusing effect of a piezoelectric fiber reinforced composite laminated cylinder subjected to thermal shock of a transitory temperature change. The dynamic focusing effect gives rise to very higher tensile stress magnitudes and electric potential at the center of the piezoelectric fiber, which easily results in a fracture along the axial direction of the piezoelectric fiber at the higher tensile stress region. It is important to know the mechanism of the dynamic focusing effect of the piezoelectric fiber reinforced laminated composites to evaluate the dynamic strength and electric signal of the composite systems.

Magnetic Properties of Iron-Based Amorphous Metal Wires

It has been studied how the conditions of machining and the elastic tensile stresses affect the magnetic properties of amorphous metal wires of composition Fe75Si10B15 produced by drawing from a melt. The magnetic characteristics of wires subjected to both thermal treatment and treatment with a continuous electric current of different magnitude have been investigated. The residual induction of wires is their magnetic parameter most sensitive to the conditions of treatment. The dependences of the residual induction on temperature and on the magnitude of the treating electric current are qualitatively similar. The greatest changes in residual induction are observed in the range of treating electric currents from 0.5 to 0.8 A, which can be associated with the processes of structural relaxation and crystallization occurring in the wires. The run of the dependence of the residual induction on the magnitude of tensile stresses is nonmonotonic in character and is determined by the level of internal hardening stresses of the test wires.

Determination of three dimensional in situ stress from core discing based on analysis of principal tensile stress

To determine all of the components of in situ stress from core discing, both the directions and magnitudes of the principal in situ stresses must be determined for a disc of a given thickness. In this study, we analyzed the direction and magnitude of tensile stress below an HQ core stub for 11 core lengths using stress conditions under which core discing is likely to occur. First, based on an analysis of the direction of tensile stress below the core stub, we propose a method for determining directions of in situ stress from the height distribution at the periphery of the end surface of a disc. This method can be used with a disc of any thickness. Next, based on an analysis of the magnitude of tensile stress in the central part of a core, we propose a linear criterion for core discing, which can be applied to a core of any length. This criterion was in good agreement with an empirical formula obtained previously in laboratory experiments. By combining information on the direction of in situ stress and the linear criterion for core discing, we propose a method for determining all of the components of in situ stress from core discing under the assumption that vertical stress is given by the overburden stress . Finally, these methods were applied to discs obtained from a field where hydraulic fracturing was performed to measure horizontal stresses. The results showed that the azimuths of the principal stresses estimated from core discing were consistent with those of the principal horizontal stresses determined by hydraulic fracturing and that while the magnitudes of the principal horizontal stresses estimated from core discing showed a large scatter, they were similar to those determined by hydraulic fracturing.

Fracture dominated wear of sharp abrasive grains and grinding wheels

Fracture-dominated wear of abrasive grains is the most important mechanism of material removal from an abrasive wheel during the grinding process. Fracture occurs as a consequence of tensile stresses induced in the abrasive grains by grinding forces to which they are subjected. Experimental work is extracted from the literature and compared with a model abrasive grain using a variety of abrasive grain materials such as alumina, silicon carbide, cubic boron nitride and diamond, with loads applied to the apex and the rake face of an abrasive wedge. The relationship between the wear of grinding wheels, component grinding forces and induced stresses in the model abrasive grains is described in detail. A significant correlation is found between the maximum value of tensile stress induced in the abrasive grain material and the appropriate wheel-wear parameter (grinding ratio). It is concluded that the magnitude of tensile stresses induced in the grain material by grinding forces at the rake face is the best indicator of wheel wear during the grinding process.

Multilayer Boundary-Element Method for Evaluating Top-Down Cracking in Hot-Mix Asphalt Pavements

Cracking in hot-mix asphalt (HMA) pavements is a major mode of premature failure. Recent work at the University of Florida has led to the development of a new viscoelastic fracture mechanics-based crackgrowth law called the HMA fracture mechanics law, which is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The successful simulations of crack growth for generalized pavement conditions depend largely on how well the state of stress can be predicted in and around existing cracks in pavements. Previous work has focused on the adaptation of a displacement-discontinuity boundary-element method for predicting stresses in the Superpave® indirect tensile test (IDT), which then were subsequently used to predict the crack initiation and crack growth in simulated IDT tests that used HMA fracture mechanics. The previous displacement-discontinuity boundary-element formulation is here extended into layered materials. Homogeneous layers are stitched together numerically in "welded" contact. The ability of the new numerical formulation to model the effects of temperature-induced stiffness gradients on tensile stresses at the top of two cracked pavement sections in Florida is demonstrated. These pavement sections were modeled with and without temperature-induced stiffness gradients. The introduction of stiffness gradients into the HMA layer is shown to increase the magnitude of tensile stresses at the top of the pavement, which is consistent with previous observations.

Thermal residual stresses in co-continuous composites

The thermal residual stresses in two types of co-continuous composites copper/aluminum oxide (Cu/Al 2O 3) and aluminum/aluminum oxide (Al/Al 2O 3) were measured by neutron diffraction experiments. These stresses were generated during the cooling after high processing temperature. The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases led to significant amount of thermal stresses. In both the composites, the metallic phase was found to be under tension and aluminum-oxide phase under compression. Even though the magnitude of compressive stress in both the composites was similar; the two metal-phases had very different magnitude of tensile stresses. The difference in volume fraction, CTE, elastic stiffness and plastic flow properties led to this difference. The hydrostatic stresses were found to be predominant in both the phases. Finite element simulations were used to predict the stress distributions inside each phase and at the interfaces. A representative unit cell approach was considered to represent the composite. Concept of effective Δ T was utilized to simulate the thermal stress distribution inside the two phases in the unit cell. This model utilized the neutron diffraction measurements to predict the stress distribution inside each phase and at the interface. The simulations showed that significant amount of tensile stresses develop at the metal–ceramic interfaces.

Lower‐Crust Ductile Flow and its Mechanical Relation with Syn‐Collision Crustal Extension in Orogenic Belt

AbstractIn attempt to reveal the mechanical relation between lower‐crust ductile flow and upper‐crust extension in collisional orogenic belt, the present paper makes numerical simulation using the Finite Element Method (FEM), which is based on stress‐strain constitution of incompressional Non‐Newtonian rheology. Results show that, under the tectonic compression and the gravitational loading conditions, the crust in mountain‐building process undergoes complex ductile flow. Kinematics of ductile flow is time‐dependent, and influenced by the shape of the crust transition zone (CTZ) between orogenic belt and frontal stable plate. Under the mechanic balance of tectonic compression and gravitational loading, the ductile flow is facilitated first in mountain root. However after several Maxwell times, it will be localized narrowly to the CTZ, where the flow pattern is featured by a downward flow near the mountain side and an upward flow near stable‐plate side. This flow pattern causes stress partition and displacement differentiation spatially in orogenic belt. In details, the minimum principle stress in upper crust near the mountain center will change from horizontal compression to horizontal tension. The magnitude of tensile stress is mainly related with the height of mountain belt above reference surface. It indicates that crustal extension in syn‐collision orogenic belt has a mechanic relation with gravitational collapse, but the extension is probably intensified further by lower‐crust ductile flow. For this consideration, it is concluded that mechanics of compression‐extension coupling in orogenic belt is derived from the lower‐crust ductile flow that is initiated by the gravitational loading and horizontal tectonic compression. Using model result, the north‐south crustal extension in the southern Qinghai‐Xizang Plateau has been discussed.

The role of oxygen in the intrinsic tensile residual stress evolution in sputter-deposited thin metal films

The intrinsic tensile stress and oxygen content in thin Cr films sputter deposited at high argon pressure are investigated as a function of negative substrate bias. With increasing bias from 0 to −100 V, the tensile stress increases to a maximum while the oxygen content decreases. At bias higher than −100 V, the tensile stress decreases but no significant change is observed in the oxygen content. A similar trend in stress evolution with bias is also observed in Cr films capped with Al to protect from postdeposition oxidation. It is concluded that, for a given deposition condition, the incorporation of oxygen may lower the magnitude of tensile stress without affecting the trends in stress evolution with increasing substrate bias. These effects are discussed using a model that correlates the film microstructure with long range attractive forces predicted from interatomic potentials.

Zero-stress states of human pulmonary arteries and veins

The zero-stress states of the pulmonary arteries and veins from order 3 to order 9 were determined in six normal human lungs within 15 h postmortem. The zero-stress state of each vessel was obtained by cutting the vessel transversely into a series of short rings, then cutting each ring radially, which caused the ring to spring open into a sector. Each sector was characterized by its opening angle. The mean opening angle varied between 92 and 163 degrees in the arterial tree and between 89 and 128 degrees in the venous tree. There was a tendency for opening angles to increase as the sizes of the arteries and veins increased. We computed the residual strains based on the experimental measurements and estimated the residual stresses according to Hooke's law. We found that the inner wall of a vessel at the state in which the internal pressure, external pressure, and longitudinal stress are all zero was under compression and the outer wall was in tension, and that the magnitude of compressive stress was greater than the magnitude of tensile stress.

Analytic Solutions for Diametral Point Load Strength Tests

This paper investigates the tensile stress distribution within a solid circular cylinder of diameter D and length 2L subjected to double diametral indentors or diametral point load strength test (PLST). The contact problem between the indentors and the cylinder is solved analytically, and exact solution for stress distribution is obtained through the use of displacement functions and double Fourier expansion in θ- and z-coordinates. Numerical results show that the tensile stress at the center of the cylinders, in general, decreases with Poisson's ratio ν and increases with 2L/D for 2L/D< 1, but remains roughly constant for 2L/D> 1. This prediction agrees exactly with the shape effect observed experimentally for the PLST for marbles, granite, and tuff found in Hong Kong and with previous published results for other rocks. The magnitude of tensile stress at the center is about three times the prediction obtained by using Wijk's formula, but seems more comparable with experimental results. The size effect of the point load strength index observed experimentally is also predicted by our theory.

Sliding microindentation fracture of brittle materials: Role of elastic stress fields

An analytical model of the stress field caused by sliding microindentation of brittle materials is developed. The complete stress field is treated as the superposition of applied normal and tangential forces with a sliding blister approximation of the localized inelastic deformation occurring just underneath the indenter. It is shown that lateral cracking is produced by the sliding blister stress field and that median cracking is caused by the applied contact forces. The model is combined with measurements of the material displacement around an indentation to show that the relative magnitude of tensile stresses governing lateral crack and median crack growth varies with the magnitude of the applied load. The model also predicts a range of loads at which the lateral crack will grow only after the indenter is removed from the surface. These predictions are consistent with observations of the different regimes of cracking observed under a sliding pyramidal indenter in soda–lime glass and other brittle solids.