Produce Stress Concentration Research Articles

Investigation of Deep Mine Shaft Stability in Alternating Hard and Soft Rock Strata Using Three-Dimensional Numerical Modeling

The problem of shaft instability has always been a major difficulty in deep mining practices. The shaft fracture has a high probability of being located near the aquifers and the soft–hard rock contact zone. This paper describes the deformation and stress characteristics of surrounding rock and the shaft lining under the interactive geological conditions under soft and hard rock strata in Anju coal mine, Shandong Province, China. Using the Method of Geological Strength Index (GSI ) and considering the rock-softening characteristics of water, the parameters of rock mass are calibrated. By means of the 3DEC-trigon method, the variation characteristics of surrounding rock and the shaft lining are simulated. After shaft excavation, under the condition of no support, shear failure and tensile failure occur in shallow surrounding rock shafts, and a pressure relief zone is formed. Shear failure is the main destruction mode in deep surrounding rock. Because of the different strengths of the surrounding rock, the deformation of the surrounding rock is significantly different. After the surrounding rock is softened by water absorption, the difference is magnified. The maximum shear stress and plastic zone appear near the interface between soft and hard rock. Under the condition of shaft lining support, uneven deformation of surrounding rock surely leads to nonlinear variation of pressure on the shaft lining. Under the action of an inhomogeneous pressure field, partial shear failure occurs in the shaft lining, and the shear failure area increases after the surrounding rock is softened by water. Because of the nonlinear deformation of the shaft lining, it is easy to produce stress concentration and bending moment near the interface between hard and soft strata. The control methods of advance grouting and pressure relief excavation are proposed to improve the stability of the shaft, and a good effect is gained.

Characterisation of the spatial variability of material properties of Gilsocarbon and NBG-18 using random fields

Graphite is a candidate material for Generation IV concepts and is used as a moderator in Advanced Gas-cooled Reactors (AGR) in the UK. Spatial material variability is present within billets causing different material property values between different components. Variations in material properties and irradiation effects can produce stress concentrations and diverse mechanical responses in a nuclear reactor graphite core. In order to characterise the material variability, geostatistical techniques called variography and random field theory were adapted for studying the density and Young's modulus of a billet of Gilsocarbon and NBG-18 graphite grades. Variography is a technique for estimating the distance over which material property values have significant spatial correlation, known as the scale of fluctuation or spatial correlation length. The paper uses random field theory to create models that mimic the original spatial and statistical distributions of the original data set. This study found different values of correlation length for density and Young's modulus around the edges of a Gilsocarbon billet, while in the case of NBG-18, similar correlation lengths where found across the billet. Examples of several random fields are given to reproduce the spatial patterns and values found in the original data.

Effects of the phase interface on initial spallation damage nucleation and evolution in dual phase titanium alloy

The Ti-6Al-4V dual phase alloy samples were dynamic loaded by one-stage light gas gun experiment and samples with initial spallation were softly recovered. During the loading experiment, the velocity of free surface particles was measured by photonic doppler velocimetry (PDV). The effect of α/β phase interface on nucleation, growth, and coalescence of dynamic damage in Ti-6Al-4V were investigated by 2-D or 3-D testing techniques, such as optical microscopy(OM), x-ray computer tomography(XRCT), and electron backscattered diffraction(EBSD). The results showed that the majority of voids were nucleated within α phases, rather than on the α/β phase interface as predicted by quasi-static damage theory. Due to the effects of reflection and transmission of shock wave at the phase interface, a tensile pulse would be formed within α phase when the shock wave transmit from α phase with high impedance to β phase with low impedance. When this tensile pulse was large enough, voids would be formed within α phase. The analyses of OM and XRCT indicated that the voids at the beginning of nucleation were nearly spherical, then grew up along the direction of 45° with the shock loading direction, and finally the rod-shaped voids were formed. Besides, the voids were not randomly nucleated within α phase, and the EBSD analysis showed that the voids were mainly nucleated at grain boundary triple points which composed of grains with large difference of Taylor Factor(TF) value within α phase. This is because the difference of plastic deformation capacity of this grains is larger, and it is easier to produce stress concentration. Thus these sites became the prior nucleation position of voids.

Finite element simulation and microstructure analysis of nickel based alloy for additive manufacturing

In this paper, the finite element simulation of GH4169 high temperature alloy by selective laser melting was carried out, and the microstructure was analyzed by experiments. The results show that the shape of the temperature field cloud formed by the laser heat source is different from the shape of the theoretical model, but is in the shape of the ellipse. The temperature gradient at the front end of the molten pool is larger than that of the back end of the molten pool, and the isotherm of the front end of the molten pool is more intensive. The temperature of the substrate is less affected by the temperature gradient. The temperature gradient of the front end of the melting pool is larger than the back end of the molten pool, and the temperature field of selective laser melting is like a meteor with trailing tail. In the laser heat source, the temperature isotherm is the most dense and the temperature gradient is maximum. The relative effect of mechanical properties of δ phase is very complex. When the phase is precipitated by widmanstatten structure, it is easy to produce stress concentration as a source of cracks

11.48: An experimental study on welded overlap core: Steel encased buckling‐restrained braces

ABSTRACTBuckling restrained braced frames (BRBFs) can be used as a lateral load resisting system in seismic regions. A typical buckling restrained brace (BRB) is composed of a core segment, debonding material and a buckling restraining mechanism (BRM). The cross‐section of the core segment can be changed along the length to constrain yielding in a limited domain. Most of the BRBs utilize a rectangular core segment which is reduced in width throughout yielding length. The gradual reduction in the width of the core segment produces stress concentrations and employs costly CNC cutting. A novel type of BRB named the welded overlap core steel encased BRB was developed to provide an alternative that does not require CNC cutting of the core plate. This type of a BRB is formed by welding flat bars with different widths to each other. The yielding and non‐yielding segment lengths and areas can be easily tailored to meet the stiffness and strength requirements. An experimental program has been undertaken to study the performance of welded overlap core steel encased BRBs. Pursuant to this goal four BRBs were manufactured and subjected to sub‐assemblage testing. All specimens were tested using the loading protocol recommended in the AISC Seismic Provisions for Structural Steel Buildings (AISC341‐10). Welding detail used to connect the built‐up BRM members was considered as the primary test variable. The experimental results indicated that the proposed core segment concept can be an alternative to more traditional core segment designs. The weld configurations used to connect the buckling restraining mechanisms affected the response significantly. According to the results, properly detailed and inspected welded overlap core BRBs can provide acceptable performance by sustaining cumulative strains in excess of 400 times the yield strain.

Effects of heat treatment on the wear behavior of surfacing AZ91 magnesium alloy

Abstract

Pseudo-static Analysis of Fabricated Steel Cage Concrete Frame Structure

In this paper, the stress bearing behavior of three different frame structure groups (allcast- in-place RC, all-cast-in-place steel cage concrete and fabricated steel cage concrete) which under horizontal low cyclic reciprocating force will be analyzed theoretically. Simulation results show that, in these three structures, all-cast-in-place steel cage concrete structure has the maximum bearing capacity, and fabricated steel cage concrete structure is next to it. However, fabricated structure will produce stress concentration phenomenon in its nodes. So, strengthening the connection parts has to be considered. Meanwhile, in these three structures, beam end is where maximum stress can be produced, which illustrates that the damage of concrete is first occurred at beam end and meets the needs of “strong column and weak beam” seismic design standards.

Failure analysis of a first stage IN738 gas turbine blade tip cracking in a thermal power plant

This research paper deals with the investigation of causes of failure of the first stage gas turbine blade of 30MW gas turbine having tip cracks at the trailing as well as leading edge. This blade is made of nickel based super alloy IN738LC, having directional solidified grain structure and contains aluminide coating (Pt–Al2) by diffusion process. The main focus of the paper is evaluation of tip crack at the trailing edge and further degradation of blade coating. The investigations included the visual observations, material composition analysis, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). During examination of the failed blade, it is found that the blade surface is completely degraded due to overheating. Corrosion pits are formed on the blade surface, these pits act as a notch to produce stress concentrations and cracks is initiated due to fatigue which further propagate due to hot corrosion. It is also reported that the Al-Pt coating is completely degraded, heavily oxidized. From these investigations, it is concluded that the failure of gas turbine blade takes place due to the combined effect of surface degradation caused by overheating, oxidation, hot corrosion and degradation of coating heavily oxidized.

Tensile performances of mechanically fastened repairs of composite laminates

Composite laminates with a damage hole,whose strength will reduce by about 55% under tensile loads,need to be repaired in order to recover their mechanical properties and meet the usage requirements. Mechanically fastened repair projects of composite laminates with a circular hole were designed and the repair effects were evaluated by tensile tests. Finite element models were established according to test condition,from which the effects of different repair projects on the failure loads,failure modes,stress distributions,and bolt load distributions were analyzed. The results of tests and finite element analysis( FEA) show that the recovery rate of failure strength of bolted repaired laminates will reach about 55%- 60%. The areas beside the outermost bolt holes in the repair area of the laminates mainly produce stress concentration. The final failure mode of the laminates is breaking along the outermost bolt holes in the repair area. Methods such as double patch repair,adding fastened bolts,using a metal patch and increasing the thickness of patch properly can reduce stress concentration,improve bolt load distributions and enhance the recovery rate of structural failure strength.

By Using Software of Finite Element Analysis on the Joint Effect of the Direct Bolt Stress

the lining of shield lining masonry joint part of the stress analysis by the finite element software ABAQUS, obtained the conclusion proved that,although the bolt for joint area is relatively small, but by the positive momentwhen the bolt hole will produce stress concentration phenomenon more obvious, this situation should be given more attention in practical engineering response.

Sensitivity Analysis for the Dependence of Solder Joints Fatigue Life

In electronic package technology, solder joint is not only used for the electrical connection and mechanical connection, but also offering the channel of heat dissipation. As chip package had developed from pin through-hole to pin surface mount, until the current no-pin ball array surface mount. Electronic package is developing in the direction of smaller ball grid array pitch, smaller chip size and more number of solder balls. The fact makes the reliability problem of solder joint become more and more outstanding. Studies have shown that 70 percent of electronic devices’ failure was caused by the failure of package assembly, and the failure of solder joint is the main reason in electronic package. The main failure mechanism of solder joint is low cycle thermal fatigue, and failure reason is the coefficient of thermal expansion (CTE)’s mismatch between components and circuit boards, then the thermal stress and strain don’t match in solder joint. The uncoordinated thermal strain will produce stress concentration, leading to the crack initiation and propagation, finally the solder joints’ failure.

Finite Element Analysis of Aluminum Alloy Pitting Stress Concentration

Based on measurement data and the specific morphology of corrosion pit, three-dimensional finite element model of the corrosion damage specimen is established. Then the local stress condition of corrosion pits is given making use of finite element method, the results of stress concentration due to corrosion pits are also obtained, we can compare the stress concentration arised from the corrosion pit on aluminum alloy specimens in different corrosion time. Results show that the corrosion pits produce stress concentration and become the source of fatigue crack initiation and propagation. With the increasing of corrosion time, the pits get deeper and deeper, and the stress concentration gets more serious near the pits.

A Comprehensive Numerical Stress—Strain Analysis of Laser Beam Butt-Welded Titanium Compared with Austenitic Steel Joints

Experimental tests on laser beam butt-welded joints made of commercially pure titanium and Ti—6Al—4V alloy showed a fatigue behaviour quite different from that usually observed in the case of AISI 304 joints connected with the same welding technique. In fact, fatigue failures of titanium joints were observed at the level of the base material but not in the notch region, as have been found for austenitic steel. In order to explain this experimental evidence, very detailed finite element models are developed in this paper. The models reproduce the real cord profiles of Ti—6Al—4V and AISI 304 steel welded joints detected by a high-precision coordinate measuring machine. Furthermore, mechanical properties given as input to the finite element models correspond to the real joint microstructure revealed by micro-hardness tests carried out in the different regions of the specimen. Finally, finite element models reproduce the distribution of porosity detected via microscopic inspection of the fractured surfaces. Interactions of weld geometry, material properties, presence/absence of porosity, and level of applied stress are considered in this study, which includes nine different finite element models of titanium and austenitic steel joints. A relevant complication in the analysis of titanium joints is represented by the small thickness and stress concentration effects. Numerical results show that the stress field in titanium joints is sensitive to the combined effect of weld seam geometry and presence of welding defects (pores). The latter produces stress concentration much more severe than that caused by the weld cord. Microstructural support length evaluated near the pores of titanium joints is about the same as that computed near the notch of austenitic steel joints (i.e. near 0). Furthermore, the Topper parameter evaluated for the titanium joints in the presence of pores is much smaller than for the stainless steel joints. Microstructure modifications must be accounted for in order to assess correctly the fatigue behaviour of austenitic steel joints.

Conformable Tire Patch Loading for FRP Composite Bridge Deck

Fiber-reinforced polymer (FRP) composites are increasingly being used in bridge deck applications. However, there are currently only fledgling standards to design and characterize FRP deck systems. One area that should be addressed is the loading method for the FRP deck. It has been observed that the type of loading patch greatly influences the failure mode of a cellular FRP deck. The contact pressure distribution of a real truck loading is nonuniform with more concentration near the center of the contact area as a result of the conformable contact mechanics. Conversely, the conventional rectangular steel patch on a FRP deck act like a rigid flat punch and produces stress concentration near the edges. A proposed simulated tire patch has been examined for loading a cellular FRP deck with the load distribution characterized by a pressure sensitive film sensor and three-dimensional contact analysis using ANSYS. A loading profile is proposed as a design tool for analyzing FRP deck systems for strength and dur...

STRESS CONCENTRATION FACTORS FOR PRESSURIZED ELLIPTIC CROSSBORES IN BLOCKS-PART II-EFFECT OF BLOCK GEOMETRY

The stress concentration point at the intersection of elliptical crossbores in rectangular blocks has been investigated. Again, the results showed that elliptical crossbores intersection produce stress concentration factors lower than those found at circular crossbores intersections. These findings corroborate with Faupel’s3 results concerning stress concentration factors for elliptic side holes in cylinders and infinite plates. Furthermore, this study showed that stress concentration factors of elliptical crossbores in rectangular blocks are lower than those found in cubical blocks.1 Specifically, this reduction is pronounced for W/a ⩾ 2.5 over the whole range of a2/a1 and are compressive for θ = 60°.

Computational Determination of the Critical Microcrack Size That Causes a Remodeling Response in a Trabecula: A Feasibility Study

Bone is a living tissue, which undergoes continuous renewal to repair local defects. Two separate processes, adaptation and remodeling, are involved when a defect appears. The defect produces stress concentrations that provoke regional adaptation, and is gradually repaired, first by resorption and then by deposition of new bone. Using a mathematical formulation of the adaptation mechanism in trabeculae of cancellous bone, we hypothesize that in some cases, where a microcrack is small enough relative to the dimensions of the trabecula, the adaptation response of the whole trabecula may be sufficient to regain homeostatic mechanical conditions (with no need for a remodeling process). The simulation results showed that for trabeculae with nominal length of 900 microm and nominal thickness of 80-800 microm, a microcrack with minimal length of 48 microm and minimal depth of 13% of the trabecula's thickness was required to initiate a remodeling process. A longer (100 microm) but shallower (depth of 7% of the trabecula's thickness) crack also triggered remodeling. These computational results support our hypothesis that when a microcrack small enough relative to the dimensions of the trabecula occurs, adaptation of the whole trabecula may be sufficient to regain homeostatic mechanical conditions with no need for a local remodeling process.

Interaction of a Dynamic Rupture on a Fault Plane with Short Frictionless Fault Branches

Spontaneous bilateral mode II shear ruptures were nucleated on faults in photoelastic Homalite plates loaded in uniaxial compression. Rupture velocities were measured and the interaction between the rupture front and short fault branches was observed using high-speed digital photography. Fault branches were formed by machining slits of varying lengths that intersected the fault plane over a range of angles. These branches were frictionless because they did not close under static loading prior to shear rupture nucleation. Three types of behavior were observed. First, the velocity of both rupture fronts was unaffected when the fault branches were oriented 45° to the main slip surface and the length of the branches were less than or equal to ~0.75 R0* (where R0* is the slip-weakening distance in the limit of low rupture speed and an infinitely long slip-pulse). Second, rupture propagation stopped at the branch on the compressive side of the rupture tip but was unaffected by the branch on the tensile side when the branches were ~1.5 R0* in length and remained oriented 45° to the principle slip surface. Third, branches on the tensile side of the rupture tip nucleated tensile ``wing tip'' extensions when the branches were oriented at 70° to the interface. Third, when the branches were oriented at 70° to the interface, branches on the tensile side of the rupture tip nucleated tensile ``wing-crack'' extensions. We explain these observations using a model in which the initial uniaxial load produces stress concentrations at the tips of the branches, which perturb the initial stress field on the rupture plane. These stress perturbations affect both the resolved shear stress driving the rupture and the fault-normal stress that controls the fault strength, and together they explain the observed changes in rupture speed.

Roughness of boundary interfaces in polycrystals and its influence on the yield strength of nanocrystalline materials

The distribution in roughness of grain-boundary interfaces in polycrystals is measured metallographically. The result shows that there are high populations of localized interface areas with roughness much lower than the average in polycrystals. Effects of the roughness on the boundary sliding in nanocrystalline materials are discussed, suggesting that the sliding in the localized interface areas produces stress concentrations to cause a reduction in the yield strength of nanocrystalline materials.

Geodynamic implications of xenolith geotherms

Studies of xenolith suites yield depth‐temperature arrays for the lithosphere at the time of eruption. These studies constrain the conductive heat flow through the lithosphere and limit the temperature range across the rheological boundary layer to <300 K. Numerical models of convection illustrate features that may be found within suites of samples. An individual dike may sample the geotherm along a finite length of its strike having variable geotherms, causing scatter in the depth‐temperature array. In a suite from the Slave Province of Canada, local heating within a deep stockwork is a more likely cause of scatter. Plume material ponded beneath a craton may yield a linear array extrapolating to the shallow geotherm without any noticeable kink or curvature. This situation existed beneath Lesotho in South Africa at ∼90 Ma. Convection at the base of the lithosphere may produce stress concentrations that trigger dike formation. Dikes may stagnate and form stockworks in low‐stress regions above the thermal boundary layer.

The development of compression damage zones in fibrous composites

Recent experimental work (Narayanan S, Schadler LS. Mechanisms of kink band formation in graphite/epoxy compsites: a micromechanical experimental study. Comp Sci Technol 1999; 59:2201-13) suggests that kink bands in unidirectional continuous carbon fiber reinforced polymer composites initiate from damage zones formed under axial compressive loads. A damage zone consists of a cluster of locally crushed fibers and broken fibers, that are often fractured at an angle, θ > 0°, normal to the fiber axis. Typically, under compressive loads, fiber breaks in damage zones form roughly along a plane at an angle φ, normal to the fiber axis. These damage zones produce stress concentrations which can lead to instabilities in the nearby fiber and matrix and initiate microbuckling and kink bands. This paper extends a micromechanical influence function technique based on earlier shear lag fiber composite models. Our modified technique calculates the fiber axial and matrix shear stress concentrations due to multiple angled and crushed fibers in arbitrary configurations. Modeling reveals that angled or ‘shear’ breaks ( θ > 0°) can lead to higher shear stress concentrations in the matrix than transverse breaks ( θ=0°). Also we find that the damage zone is more likely to form at an angle φ, which is greater than that of its individual fiber breaks, θ. When φ is slightly greater than θ, the shear stress in the surrounding matrix regions within the damage zone achieves a maximum, potentially weakening the matrix and interface and consequently leading to kink band formation. Monte Carlo simulations incorporating this stress analysis predict that the initiation and propagation of crushed and angled breaks progress roughly along an angle, φ ≈ 17° in a linear elastic system. When possible, our model results are compared to strain measurements of fiber composites under compression obtained by Narayanan and Schadler using micro-Raman spectroscopy (MRS).