Dimensional Stress State Research Articles

Research on the Influence of Conical Anvil on Upsetting Process of Forging by Finite Element Simulation

The upsetting process of cylindrical forging was investigated using finite element method. The influence of conical anvil on deformation rule of metal and distribution of strain and stress was studied using software MSC. Marc. The simulated results indicated that it had a smaller rigid zone and more homogeneous deformation zone in the billet during upsetting process with conical anvil compared with flat anvil. The critical reduction in height which could assure the three dimensional compressive stress states for the upsetting with conical anvil was smaller than that for the flat one.

Axisymmetric Punching Capacity Analysis for Concrete Slabs

The axisymmetric punching analysis for concrete slabs is researched with the triple-shear unified failure criterion, which can explain the failure properties rationally for concrete under the three dimensional stress states, especially the failure properties of the intermediate principal stress effects. Moreover, a new intermediate principal stress parameter is proposed to interpret the different stress states in failure zones for different concrete. It is shown that the coefficients of b and m, which interprets the effects and the magnitudes of the intermediate principal stress have significant impacts on the axisymmetric punching capacity of concrete slabs. For a given thickness of the concrete slabs, the bearing capacity increases with the b and reaches its maximum value approximately when m=1. Using the Mohr-Coulomb failure criterion to analyze the problem of axisymmetric punching of concrete slabs often underestimate the actual ultimate punching capacity significantly.

Integrated Application of Three <i>In Situ</i> Stress Measurement Techniques

The CSIRO overcoring stress relief and hydraulic fracturing methods are the most popular methods used for the measurement of in-situ stress at depth. One major advantage of the CSIRO overcoring stress relief method is that the three dimensional state of stress can be obtained, but the measurement must be done in an excavated tunnel[1]. Hydraulic fracturing method can be carried out on the ground surface, but it assumed that one of the principal stresses direction is vertical[2,3]. In terms of the disadvantages of the two methods, the techniques based on core orientation and acoustic emission behavior of rocks are incorporated in the in-situ stress measurement in order to obtain the in-situ stress conditions at depth in Shuichang Iron Mine. According to the comparison of the measurement data obtained from the acoustic emission test in the laboratory and CSIRO overcoring stress relief measurement in the field, effectiveness of the acoustic emission test is confirmed. In addition, the relationships between in-situ stresses and tectonic settings are analyzed. Finally, the distribution of in-situ stresses in Shuichang Iron Mine is given, which provides a meaningful guideline for the following mining and design.

Damage prediction of RC containment shell under impact and blast loading

There is world wide concern for safety of nuclear power installations after the terrorist attack on World Trade Center in 2001 and several other civilian structures in the last decade. The nuclear containment structure in many countries is a double shell structure (outer shell a RCC and inner a prestressed concrete). The outer reinforced concrete shell protects the inner shell and is designed for external loading like impact and blast. A comparative study of non-linear response of reinforced concrete nuclear containment cylindrical shell subjected to impact of an aircraft (Phantom) and explosion of different amounts of blast charges have been presented here. A material model which takes into account the strain rate sensitivity in dynamic loading situations, plastic and visco-plastic behavior in three dimensional stress state and cracking in tension has been developed earlier and implemented into a finite element code which has been validated with published literature. The analysis has been made using the developed software. Significant conclusions have been drawn for dissimilarity in response (deflections, stresses, cracks etc.) of the shell for impact and blast loading.

Hydroforming of anisotropic aluminum tubes: Part II analysis

Part I presented an experimental investigation of hydroforming of Al-6260-T4 tubes and a simple two-dimensional model of the process. Relatively long, extruded circular tubes were formed against a square die with rounded corners, with simultaneous application of axial feeding. Localized wall thinning was reported to occur at mid-span which, accentuated by friction, led to burst. Part II presents fully 3D models of the process that include friction as well as more advanced constitutive models shown in previous studies to be essential for simulation of burst in free hydroforming of aluminum alloy tubes. The models are used to simulate several of the experiments of Part I, emphasizing the prediction of all aspects of the forming process, including wall thinning and its localization that lead to rupture. A shell element model is shown to capture the majority of the structural features of the process very successfully. However, even with the implementation of advanced constitutive models, it fails to reproduce correctly the localization of wall thinning. It is demonstrated that switching to solid elements coupled to non-quadratic yield functions results in accurate predictions of all aspects of the problem, including the onset of rupture. Apparently, slow growing depressions that develop at the interface between the flattened part of the cross section that is in contact with the die and the rounded part that is not, have a complex three dimensional stress state requiring accurate modeling offered by solid elements. Furthermore, the evolution of these depressions is only reproduced with accuracy when in addition non-quadratic yield functions are adopted.

Effect of three dimensional stress state on unstable fracture condition and crack opening level in a new crack growth model

Effect of three dimensional stress state on unstable fracture condition and crack opening level in a new crack growth model

Mechanical Behaviour Of Polycarbonate Sheet Under Punch Loading Deformation

(2006). Mechanical Behaviour Of Polycarbonate Sheet Under Punch Loading Deformation. Materials Technology: Vol. 21, No. 2, pp. 74-79.

Spatio-temporal features of the Portevin-Le Chatelier effect

We show that an extension of Ananthakrishna’s (AK) model exhibits all the features of the Portevin-Le Chatelier (PLC) effect including the recently observed crossover from a low dimensional chaotic state at low and medium strain rates to a high dimensional power law state of stress drops at high strain rates. The model also displays three types of bands. The dislocation configurations are shown to be in the pinned stated in the chaotic regime (i.e., the region of type C and B bands) and at the edge of unpinning in the power law regime where propagating bands are seen.

Determination of Stress Characteristics at Connections of Welded Members Using Finite Element Method

Tubular members are used in various structural members. They have a pleasant effect and good aesthetic values. They can be used in large roof structures, offshore rigs, pressure vessels, etc. The connection between tubular members normally involves cutting one of the members to obtain an exact fit, followed by welding of the two. This intersection becomes a point of high stress concentration, because of the different weld material present as well as the angle of intersection between the different elements. It can be a point of weakness especially in relation to fracture and fatigue. The state of stress here has to be obtained very accurately in order to design the joint accordingly. Finite element analysis provides one such tool. This paper attempts to provide an insight into how to obtain the three dimensional state of stress existing in the welded connection between two tubular members in an offshore structure. It is shown that results from finite element submodelling analysis are in good agreement with those from conventional parametric equations for the determination of stress concentration factor in tubular joints. Journal of Civil Engineering Research and Practice Vol.1(1) 2004: 21-31

円錐孔底ひずみ法におけるコアディスキング現象に関する基礎的研究

Core disking is sometimes observed in high stress region where rock stress measurements are performed by the Compact Conical-ended Borehole Overcoring (CCBO) Technique, which is one of the stress relief methods. When the tensile stress induced on the borehole bottom surface becomes larger than the tensile strength of the rock, a failure from the borehole bottom surface will be imitated. Therefore, if the stress changes on the borehole bottom surface can be analyzed during overcoring, the rock stress prior to boring may be estimated.In this paper, assuming that the tensile stress on a borehole bottom surface induced due to overcoring is one of factors of generating core disking, the mechanism of the core disking in the CCBO is discussed as well as the location of failure initiation and the type of failure. Firstly, the characteristics of core disking in the CCBO is shown. Secondly, simulating the overcoring of the CCBO under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the failure initiated on the borehole bottom surface during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, the criterion of core disking is suggested and the applicable limitation of the CCBO is made clear. Furthermore, the X-ray CT method is applied to the recovered core with core disking, and then the state of failure within the core is visualized. Finally, it is discussed that the rock stress prior to boring is estimated by the suggested criterion of the core disking, comparing the neighboring rock stress measurement and tensile strength obtained from cores.

Elasto–plastic modelling of confined concrete elements following MC90 equations

The present work considers the use of the elasto–plastic model in the analysis of concrete structures taking into account the confining effect. The MC90 equation for uniaxial compression is used in the model for the approximation of concrete behaviour in hardening and softening. The three dimensional state of stress in concrete is considered by the use of convenient yield conditions. The model is developed for Drucker–Prager and Ottosen yield criteria. The confinement effect is formulated by multiplying the derivative to the stress–strain law by a function of the intermediate principal stress. The stress–strain diagram obtained agrees with the stress–strain law defined in the MC90 for uniaxial compression in non-confined concrete. The numerical results show that the model approximates the experimental results obtained in cylinder specimens.

Experimental analysis of surgically corrected knee joint

AbstractHolographic interferometry, speckle photography and photoelasticity have been employed in the experimental investigations of the knee joint in four conditions ‐ anatomically normal joint, varus joint, joint after Coventry popliteal osteotomy and joint after correction made using a new variety of the Coventry procedure1. The studies included the investigations of the displacements of the bone in the sagittal plane, the displacements of points on the bone's surface in the frontal plane and the three dimensional state of stress. The obtained results show to what degree changes in the lower limb geometry affect the knee joint and in particular, its stability, functionality and the way in which it carries loads.

The effect of fibre surface roughness on the mechanical behaviour of ceramic matrix composites

The interface between the matrix and the fibre plays an important role in controlling the strength and toughness of ceramic matrix composites. It has been found experimentally that depending on manufacturing process, the interface may show substantial surface roughness, which has been modelled analytically with certain degree of success. The analytical models, however, do not take into account the interface geometry. Instead, only the magnitude of the surface undulation is included. In this paper, a direct simulation of fibre–matrix interface roughness by the finite element method is performed on an axisymmetric unit cell with a fully debonded interface. The simulation is employed to account for the three dimensional stress state, surface roughness and interface friction, which are normally simplified or idealised in theoretical studies. The model gives the highly non-uniform interface shear and pressure, which have direct implications on the interface damage and composite behaviour. Under the approximation made in the model, the positive transverse strains does not show up in the simulation despite the fact that two different surface roughness are used.

A seminumerical method to determine the depth profile of the three dimensional residual stress state with X-ray diffraction

A new method to determine, non-destructively, the full three-dimensional stress state with depth resolution has been developed. Conventional X-ray residual stress techniques, using different characteristic radiations and several Bragg peaks, are combined with a seminumerical procedure. The analysis incorporates a model function based on the linear theory of elasticity, under consideration of its conditions of equilibrium as well as the surface boundary condition. A least squares fit of a calculated set of peak positions to the experimental data yields the depth dependence of all six components of the stress tensor. This method has been applied to a shot peened work piece, formed from the titanium based alloy Ti-6Al-4V.

ホローコアディスキングの破壊開始条件に関する解析的研究

Most of the overcoring method for rock stress measurements use a large diameter drilling, what is called the overcoring, to relieve the stress around a pilot hole in which a transducer is installed. When the overcoring is advanced into hard rock subjected to high rock stress, a phenomenon of a core disking is sometimes observed. As the recovered cores which have a constant thickness have a center hole like a doughnut, this core disking is called a hollow core disking. In this paper, simulating the overcoring under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the hollow core disking initiated during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary conditions. Firstly, the surface stress intensity coefficients in the tangential and rotational directions of the borehole are analyzed under the conditions in which each principal stress of initial stress is subjected independently. It is clarified that the coefficients is not changed beyond the advance of the overcoring more than the radius of the borehole. Secondly, using the surface stress intensity coefficients, the surface stresses of the borehole are analyzed under three dimensional initial stress state, then the fracture criterion which represents the fracture initiation due to the tensile stress induced by the overcoring is formulated as a function of the advance of the overcoring. Finally, analyzing the surface stress of the borehole induced by the overcoring, it is made clear that the location of the fracture initiation, the type of the fracture and the thickness of disk are dependent upon initial stress state.

Analytical solution to the dynamic analysis of laminated beams using higher order refined theory

Analytical solution to the natural frequency analysis of composite and sandwich beams based on a higher order refined theory is presented. This theory incorporates cubic axial, transverse shear and quadratic transverse normal strain components in the basic formulation — thus modelling the warping of cross section accurately and eliminating the need for a shear correction coefficient. Also, it considers each layer of the lamina to be orthotropic and in a two dimensional state of plane stress. The equations of equilibrium are derived using Hamilton's principle. Numerical experiments are carried out and from the results of thick and thin sections, conclusions are drawn.

円錐孔底ひずみ法の不連続性岩盤への適用と考察

The combination of the 16-elements conical-ended borehole technique and the compact overcoring method is an excellent method to measure the three dimensional state of stress by means of stress reliving in hard rock. In the present paper, this method has been applied to the discontinuous rock and the characteristics of stress distribution on the periphery of faults have been examined by the systematic stress measurement at multipoints in a single borehole. Subsequently, it has been discussed that numerical simulation is indispensable to grasp macroscopic initial stress field.Firstly, it has been shown that the in situ stress measurement performed at 18 adjacent points by means of the combination of the 16-elements conical-ended borehole technique and the compact overcoring method within diorite and granodiorite, and it has been discussed that the present method is promising method to measure the absolute stress. Moreover, it has been clarified that the number of measuring points can be drastically increase by this method.Secondly, from comparison of the measured value with the results by the displacement discontinuity method (DDM) analysis, it has been clarified that the faults are the geological primary factor controlling the initial stress distribution and the cracks are the secondary cause of disturbance of the initial stress distribution.

Two Dimensional Study of Hydraulic Fracturing Criteria in Cohesive Soils

Based on the shear failure mechanism, hydraulic fracturing criteria are extended to three dimensional stress state. According to the situation of the directions of borehole and major principal stress axes, three equations can be derived for three dimensional hydraulic fracturing problems. By comparing these equations, a single criterion is selected for hydraulic fracturing pressure in cohesive soils. The criterion is a function of maximum principal stress, minimum principal stress and soil parameters in UU conditions. The equation indicates that with any increase in maximum principal stress, hydraulic fracturing pressure decreases. In order to prove the integrity of the criteria, laboratory tests are performed on compacted cubical specimens using true a triaxial apparatus. The shape and direction of fractures are determined by injecting colored water after fracture initiation. It is found that the direction of fractures are perpendicular to the σ1 plane.

Three dimensional stress analysis of symmetric composite laminates under uniaxial extension and in-plane pure shear

This paper presents the stress and strain distribution obtained numerically in the thickness direction in the central region of symmetric composite laminates under uniaxial extension and in-plane pure shear loading. It is shown that in the symmetric angle-ply laminates, near the interlaminar surface, there is an interlaminar boundary-layer effect. The abrupt change of the fiber orientation at the interlaminar surface causes a high concentration of stresses and a local three dimensional stress state.

ボーリングコア側面の非弾性ひずみと初期応力の関係について

At three boring sites, principal strain orientations obtained from anelastic strain recovery on core surface were compared with in-situ stress states determined by overcoring or hydraulic fracturing method. Orientations of time-dependent maximum expansion of a core taken from the vicinity of a large mined goaf indicated that they presumably reflected the maximum stress state when the core was retrieved and they were totally different from the stress orientations determined by overcoring technique. Three dimensional principal strain orientations obtained from anelastic strain recovery on surface of granodiorite cores (150mm in diameter) were in good agreement with the in-situ stress state predicted by overcoring method. And anelastic strain recovery measurements on Mannari granite were carried out immediately after the 38 mm diameter cores were removed from 300 m deep borehole, where none of three principal axes of strain coincided with the borehole axis. Laboratory measurements of compressional velocity on relaxed Mannari cores showed that the direction of the slowest velocity correlated with the one of maximum expansion and the strike of microcracks in quartz crystals. Then it is suggested that three dimensional stress state can be determined by combination of hydraulic fracturing test at single borehole section and observation of anelastic strain variations of the core taken from the borehole.