Tensile Stress Distribution Research Articles

シンクロトロン放射光によるアルミナのき裂先端の応力遮蔽効果の評価(S06-1 薄膜・微小郎域の応力評価,S06 放射光による応力測定と残留応力評価)

The local stress distributions near the notch and crack tips in Al_2O_3 bend-bar specimens under four-point bending loads were in-situ measured by scanning micro X-ray beams from synchrotron radiation source using sin^2ψ method. The diffraction from Al_2O_3 2.0.14 plane by 8keV radiation of Spring-8 (Super Photon ring-8GeV) was used for X-ray stress measurement. Using a custom-designed bending device, loaded bend-bar specimens were mounted on the X-ray diffractometer. In the case of the notched specimen, grinding residual stress due to notch-machining was found to be remained near the notch. After annealing, the measured local stress distribution near the notch tip agreed very well with the result of a finite element analysis. On the other hand, in the case of the cracked specimen, the measured tensile stress distribution near the crack tip was smaller than the result of the finite element analysis without crack-face bridging. The degradation of about 40 % in the tensile stress singularity was obviously observed near the crack tip. It was concluded that the stress shielding effect due to crack-face bridging could be directly evaluated by the developed X-ray stress measurement using synchrotron radiation source.

Numerical analysis of crack generation in saturated deformable soil under row-planted vegetation

Desiccation cracks play an important role in the drainage of excess water via the subsurface drainage system in clayey puddled paddy fields. The present study focuses on the linear inter-row cracks induced by water absorption from row-planted rice. The shrinkage behavior of clayey puddled soil, which can be regarded as the consolidation of saturated soil subject to pore water suction, was simulated using a numerical model based on the Biot's two-dimensional consolidation theory. The model demonstrates how row-planted crops' absorption of soil water induces the deformation of the soil and the development of tensile effective stress. The model requires parameters to specify the mechanical and hydraulic properties of the soil. These parameters can be determined from the e–log p (void ratio and mean stress) relationship and the e–log k (void ratio and saturated hydraulic conductivity) relationship. The calculations were performed using the finite element method (FEM). The simulation demonstrated a concave distribution of suction between rows and showed the induced deformation of the soil fabric. It showed that the peaks of tensile effective stress are not necessarily located at the center of the rows; the peaks are typically found near the Fronts of a Suction Rise (FSR), where the suction is about to rise. Further numerical experiments demonstrated that the distribution of tensile effective stress is characterized by either single or double peaks, depending on the following conditions. Greater transpiration flux, wider row-spacing, and a thinner soil layer induce the double-peaked distribution of tensile effective stress. This phenomenon corresponds to the field observation that the linear inter-row cracks running parallel to the rows are single in narrower row-spacing, and double in wider row-spacing.

Patterns of Vertical Root Fracture: Factors Affecting Stress Distribution in the Root Canal

Previous studies have indicated that vertical root fracture tends to occur in a buccolingual direction, where dentin thickness is greatest. Factors potentially influencing the location and direction of root fracture include root canal shape, external root morphology, and dentin thickness. In this finite-element study, simulated root sections were varied systematically with respect to canal size and shape, external root morphology, and dentin thickness to determine their relative contribution to vertical root fracture. Similar models were constructed based on cross-sections of human tooth roots that had been fractured clinically or experimentally. Finite-element models demonstrated that canal curvature seems more important than external root morphology, in terms of stress concentration, and that reduced dentin thickness increases the magnitude but not the direction of maximum tensile stress. Models based on actual root fractures showed a strong similarity between tensile-stress distribution and fracture patterns.

Prediction of radial stresses due to prestressing in PSC shells

Large prestressed concrete (PSC) shells, such as silos, containment structures are prestressed along their principal curvatures. Prestressing cables are provided in the thickness of the shell either in one or two directions. The stresses normal to the middle surface are generally neglected in the design of such shell structures. However, the development of radial tensile stresses at the time of prestressing can lead to possible delamination failure, if these stresses are excessive. This paper presents simple procedures to estimate the radial stress distribution due to prestressing. Two methods are presented: ‘equilibrium of slice’ and ‘modified Lamé,’ the latter being more rigorous and applicable to both singly and doubly curved shells. The solutions obtained are in conformity with the results of finite element analysis published earlier. Finally, an attempt is made to determine the local tensile stress distribution at the periphery of the prestressing duct hole, which also contributed to the problem of delamination.

Generation of radial growth stresses in the big rays of konara oak trees

In previous models the distribution of radial tensile stresses in the tree trunk has been explained as a response to the mechanisms of growth stress generation in the longitudinal and tangential directions. We investigated the contribution of ray tissue to growth stress generation in the tree trunk by the origin of the radial stresses during differentiation of parenchyma cells. Measurements on three konara oak trees (Quercus serrata Thunb.) were carried out comparing the radial residual strain of big oak rays with the radial residual strain of the axial tissues (containing only uniseriate rays). The results indicated that the ray tissue generated tensile growth stress in the radial direction of the trunk (the axial direction of the parenchyma cells). In contrast to previous assumptions, the growth stresses seem to be variable in relation to the individual tissue types. The mechanical advantage of a radially prestraining effect of the rays is discussed for the living tree.

Finite element modeling of geomaterials using digital image processing

This paper presents a digital image processing based finite element method for the two-dimensional mechanical analysis of geomaterials by actually taking into account their material inhomogeneities and microstructures. The proposed method incorporates the theories and techniques of digital image processing, the principles of geometry vectorization and the techniques of automatic finite element mesh generation into the conventional finite element methods. Digital image techniques are used to acquire the inhomogeneous distributions of geo-materials including soils, rocks, asphalt concrete and cement concrete in digital format. Digital image processing algorithms are developed to identify and classify the main homogeneous material types and their distribution structures that form the inhomogeneity of a geomaterial in the image. The interfaces of the main homogeneous material types are vectorized to form the internal material geometric structure and sub-regions. The vectorized digital images are used as inputs for finite element mesh generations using automatic mesh generation techniques. Lastly, the conventional finite element methods are employed to carry out the computation and analysis of geomechanical problems by taking into account the actual internal inhomogeneity of the geomaterial. Using asphalt concrete as an example, the paper gives a detailed explanation of the proposed digital image processing based finite element method. The paper further applies the new method to the mechanical analysis of the so-called Brazilian indirect tensile test in rock mechanics and pavement engineering. The numerical results show that this new digital image process based finite element method can take into account the material inhomogeneities in the geomechanical analysis which can have significant effects on the tensile stress distribution along the loading axis of the Brazilian indirect tensile tests.

Relationship Between Grab and Strip Tensile Strengths for Fabrics with Roughly Linear Mechanical Behavior

A theoretical analysis is presented here to study the relationship between strengths tested using grab and strip specimens. The shear and tensile stress distributions at the portion of the grab specimen not directly stretched are derived. The total force carried by this portion is calculated, providing the difference between the test results of the two specimen types. Although the analysis is still primitive, the results nonethel ss establish a foundation for a parametric analysis to show the effects of the factors.

Effect of weld geometry and residual stresses on fatigue in butt-welded joints

This paper describes a mathematical model to predict the effects of important butt weld geometry parameters (such as weld toe radius, weld bead flank angle, preparation angle, and plate thickness), and residual stresses on the fatigue crack initiation (FCI) life of butt-welded joints. An effective procedure is also developed by combining the thermal elasto-plastic theory, finite element and strain-life methods, while simultaneously considering the effect of stress concentration at the critical location (weld toe) and tensile residual stress distribution to predict FCI life in the weldments. The procedure proposed is compared to those of the standard specifications of BS 5400, AASHTO, and experimental results. It is demonstrated that fatigue life estimates made using the novel procedure closely approximate the experimental results. Furthermore, the fatigue life of butt-welded joints subjected to simulated residual stress fields, induced by preheating temperature conditions, is calculated using the proposed procedure.

Optical Characterization of GaAs/Si Layers Grown by the Conformal Method (Confined Lateral Epitaxial Growth)

Optical studies of conformal GaAs layers grown on silicon substrates were carried out by cathodoluminescence, photoluminescence imaging, and micro-Raman spectroscopy. These techniques revealed, in the conformal GaAs layers, local variations of the luminescence intensity with the shape of stripes both parallel and perpendicular to the GaAs seed, associated with local variations of tensile stress. The cathodoluminescence and micro-Raman spectra suggest that this distribution of tensile stress plays an important role in the formation of mid-gap states responsible for the variations of the luminescence intensity. The high luminescence emission of the conformal GaAs layers compared to the emission of the GaAs seed grown directly on the Si substrate is consistent with the high quality of the conformal layers.

A calculation of stress distribution in metal-porcelain crowns by using three-dimensional finite element method.

The objective of this study was to calculate stress distribution in a maxillary second premolar tooth which occurred by the mastication force. The tooth model was crowned with Au-Pd alloy, Ni-Cr alloy and porcelain. A load of 450 N, at an angle of 45 degrees to the longitudinal axis was applied on the occlusal margin of the crown tooth. The tooth was assumed isotropic, homogenous, elastic and unsymmetrical. This numerical study was carried out using three-dimensional finite element models and calculation programs were prepared by the authors using FORTRAN 77. The distribution of compressive, tensile and shear stress were plotted for the dentine, dentine-metal and metal-porcelain interfaces. The highest stress values were observed when Ni-Cr alloy and porcelain was used.

Damage in a rock core caused by induced tensile stress and its relation to differential strain curve analysis

Abstract A finite element analysis was carried out to analyze the distribution of tensile stress within and below a long HQ core stub for 77 in situ stress conditions. The maximum tensile stress experienced by the core along the axis during boring under in situ stress was accumulated in an equal-area stereonet for a central part of the cross-section. The maximum tensile stress accumulated for a central area of less than about 60% of the total cross-sectional area was concentrated in a certain direction, which was nearly the same direction as the minimum principal stress for all of the stress conditions, except those in which the minimum principal stress ( σ 3 ) was equal to the intermediate principal stress ( σ 2 ). When σ 2 = σ 3 , the direction of the cumulative maximum tensile stress lay approximately in the plane of σ 2 = σ 3 , which is perpendicular to the maximum principal stress. Based on the assumption that a penny-shaped crack is produced normal to the maximum tensile stress in proportion to the magnitude of such stress, the crack density in the core was analyzed by calculating strain under hydrostatic pressure as in differential strain curve analysis (DSCA). The maximum principal crack density in the central part of the core was much greater than the intermediate and minimum principal crack densities, excluding special cases in which σ 2 = σ 3 . The direction of the maximum crack density was similar to that of the accumulated maximum tensile stress. Thus, the direction of the maximum crack density obtained by DSCA predicts the direction of the minimum principal stress rather than that of the maximum principal stress, if the distribution of pre-existing microcracks before stress relief is isotropic and if additional microcracks are produced only by tensile stress during boring under in situ stress. To verify this, crack parameters were measured by DSCA for two cores of quartz-diorite, which were taken by overcoring when the hemispherical-ended borehole technique was used to measure in situ stress. The directions of the maximum crack parameters measured by DSCA were nearly the same as that of the minimum principal stress for one of the cores. For the other core, for which the magnitudes of the intermediate and minimum principal stresses were close to each other and, accordingly, the direction of the minimum principal stress was uncertain, the direction of the maximum crack density estimated by damage analysis under the assumption that σ 2 = σ 3 coincided with the directions of the maximum crack parameters measured by DSCA.

Structural behavior of cable anchorage zones in prestressed concrete cable-stayed bridge

Since the cable anchorage zone in a prestressed concrete cable-stayed bridge is subjected to a large amount of concentrated tendon force, it shows very complicated stress distributions which can cause serious local cracks. Accordingly, it is necessary to investigate the parameters affecting the stress distribution, such as the cable inclination, the position of the anchor plate, the modeling method, and three-dimensional effects. The tensile stress distribution in the anchorage zone is compared to the actual design condition by varing the stiffness of spring elements in the local modeling, and an appropriate position for the anchor plate is determined. The results provide elementary data for the stress state in the anchorage zones and encourage more efficient designs.Key words: finite element analysis, bursting stress, spalling stress, cable anchorage zone, cable-stayed bridge.

Investigation of Joints Behavior in Seismic Response of Arch Dams

There has been extensive research to examine the effects of contraction joints opening in the nonlinear seismic response of arch dams. However, there has been less attention on the effects of perimetral joint on the response. In this study, a special finite element program is developed, that is described and verified initially. Based on this program, the nonlinear dynamic behavior of a typical thin arch dam is studied. The perimetral joint, as well as, contraction joints are included in the nonlinear cases analyzed. It is shown that the distributions of the maximum tensile stresses are very sensitive to the properties used for the joints. Moreover, the modified joint model proposed for the modeling of perimetral joint is found to be less sensitive and very effective in seismic analysis of concrete arch dams.

Nonlinear finite element modelling and incremental analysis of a truck tyre

A non-linear finite element (FE) model of a radial truck tyre is developed to analyse the tensile stress distributions, deformation fields and inter-ply shear stresses as a function of the normal load. The tyre model is established based on consideration of the geometry and orientation of the cords in belts and carcass layers, the stacking sequence of different layers and the large deformation of the multiple layers in order to predict the interactions between the plies. The study incorporates the nearly incompressible property of the tread rubber block and anisotropic material properties of the layers. A non-linear incremental analysis of the rubber element is performed using the modified Mooney-Rivlin material model. The non-linear FE tyre model is analysed using commercial software, the ANSYS® program in order to determine the deformation fields, contact patch geometry, contact pressure distribution, principal stress fields, distributions of fibre stresses and inter-ply shear stresses. The influence of normal load and thus the tyre deflection on the above response parameters are discussed. The computed footprint geometry is qualitatively compared in a limited sense with the measured data to examine the validity of the model. It is concluded that the proposed model can provide reliable predictions about the three-dimensional stress, strain and deformation fields in an inflated radial truck tyre based on the known tyre material properties and structural features for given internal air pressures and normal loading conditions.

The analysis of large Barkhausen effect in the FeSiB amorphous wire

Amorphous FeSiB wires with positive magnetostriction are very perspective soft magnetic materials for many applications, e.g. torque, field or current sensors, pulse generators and highly sensitive magnetometers. The appearance of the Large Barkhausen Effect (LBE) during slow magnetization of FeSiB wires is described by means of the core-shell model assuming a residual radial tensile stresses in the as-cast state. In this work, the LBE during magnetization reversal of Fe77.5Si7.5B15 amorphous wire in the as-cast state was analysed. We have studied the kinetics of the reverse domain in the core region of the wire by means of Sixtus-Tonks method of two small pick-up coils placed in an asymmetric way with respect to the ends of the wire. We estimated the velocity of the reverse domain wall and the core region volume of the wire. It was found that the residual radial tensile stress distribution of the shell region strongly influences the magnetization reversal in the FeSiB wire.

Effect of matrix yield properties on fragmentation behavior of single fiber composites

Experimental and theoretical investigations have been conducted to study the dependence of fiber fragmentation behavior on matrix yielding properties. The cured Epikote 828 resins with two types of curing agents have almost similar elastic moduli, but different tensile yield strengths. The interfacial chemistry between fiber and epoxy resin is unchanged due to the same constituent of the epoxy resin. The experimental results indicate that the fragmentation behavior of the fibers embedded in the matrix is significantly different for the tested glass fiber treated by γ-glycidoxypropyltrimethoxysilane. The average fragment length decreased with increasing tensile yield strength of resin, which suggests that the interfacial shear strength determined in the fragmentation test should be different depending on the tensile yield strength of resin used. The important phenomenon observed is the transition of the micro-damage mode from matrix crack to interfacial debonding. An elastoplastic shear-lag model was used to calculate the shear stress and fiber tensile stress distributions considering different plastic behaviors of the matrices. The theoretical results indicate that the plastic behavior of the matrix has a large influence on stress transfer. Based on elastic and plastic properties of the matrix, the fiber fragmentation behavior in the matrix is predicted. Experimental and theoretical results are favorably compared.

A composite material model for investigation of micro-fracture mechanism of brittle rock subjected to uniaxial compression

A two-phase model of rock was proposed in order to investigate the mechanism of brittle fracture due to uniaxial compression, in which rock was considered to be a composite material consisting of hard grains and colloids. The stress state in colloid region near grains was calculated using Finite Element Method (FEM). The influence of the tensile stresses on the crack initiation and failure process of brittle rock subjected to uniaxial compression was investigated by numerical experiments. The FE results show that tensile stresses are induced easily in the neighboring area of hard grains with the maximum value near grain boundaries. The distribution of tensile stresses depends on the relative position of hard grains. The cracks initiated just near the boundary area of hard grains, which was governed by tensile stress. These results clearly reveal the micro-fracture mechanism of brittle rock loaded by uniaxial compression. It can be concluded that the failure mode of brittle rock under uniaxial compression is still tensile fracture from the point view of microstructure. However, since the wide colloid region is still under compressive stress state, further propagation of boundary cracks through this region obviously needs more external load, thus causing the uniaxial compressive strength of rock much higher than its tensile strength obtained via Brazilian (splitting) experiment.

Sealants versus gaskets: The development of an interactive design tool

A design software, based on computer-aided design and finite element analysis (FEA) tools, was set up for the parametric analysis of bolted joint connections with adesive sealants; this software can be used for a low-cost evaluation of joint performances. To this aim, the mechanical properties of two typical anaerobic and silicon-based sealants were experimentally obtained by means of an optimized specimen. Two-dimensional and fully three-dimensional finite element (FE) models of bolted connections were developed to take into acount bolt pre-load and position, flange thickness, internal pressure and mechanical properties of sealant material. Non-linear contact analysis was employed for a realistic joint simulation. Results provide the tensile stress distribution in the sealant along the joint middle layer, for any user-specified configuration. A user-friendly software tool was then developed for speedy evaluation of commonly used joint geometries.

Finite element analysis of single layer forming on metallic powder bed in rapid prototyping by selective laser processing

A method for calculating the distribution of temperature and stress within a single metallic layer formed on the powder bed in rapid prototyping with the selective laser melting method is proposed. The solidified layer is assumed to be subjected to plane-stress deformation and the two-dimensional finite element methods for heat conduction and elastic deformation are combined. In the simulation, the finite element mesh is constructed on the surface of the powder bed. The heat caused by laser irradiation is given to the elements under the laser beam. Shrinkage due to solidification is assumed to result in only the change of the layer thickness. In the elastic finite element simulation, the Young's modulus of the solidified part is expressed as a function of temperature. To simplify the calculation, the whole area is treated to be continuous, and the powder bed and the molten part are assumed to have a very small Young's modulus. The heat conduction and the elastic finite element calculations are carried out alternately. The obtained results of deformation and tensile stress distribution show the possibility and places of cracking of the layer during forming.

Effect of Flaw State on the Strength of Brittle Coatings on Soft Substrates

A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ, using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated‐load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack‐wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome.