Induced Stress Field Research Articles

Prediction of machining induced residual stresses in turning of titanium and nickel based alloys with experiments and finite element simulations

Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components and the residual stresses induced by machining are very critical due to safety and sustainability concerns. This paper presents experimental investigations and finite element simulations on turning of Ti–6Al–4V titanium alloy and IN100 nickel based alloy with uncoated and TiAlN coated tools. Face turning of Ti–6Al–4V and IN100 using uncoated tools with various edge radii and TiAlN coated carbide tools is conducted; and residual stresses are measured in radial and circumferential directions using X-ray diffraction technique. 3-D finite element (FE) modeling is utilized to predict forces and machining induced stress fields. The feasibility and limitations of predicting machining induced residual stresses by using viscoplastic finite element simulations and temperature-dependent flow softening constitutive material modeling are investigated. A friction determination method is utilized to identify friction coefficients in presence of tool edge radius. The predicted stress fields are compared against measured residual stresses. Effect of tool edge radius and coating on the predicted stress profiles is also investigated. The results are found useful in predicting machining induced surface integrity that is critical to determine the fatigue life of nickel and titanium alloy components.

Boundary element analysis of nanoinhomogeneities of arbitrary shapes with surface and interface effects

In this paper, a boundary element method (BEM) is proposed to analyze the stress field in nanoinhomogeneities with surface/interface effect. To consider this effect, the continuity conditions along the internal interfaces between the matrix and inhomogeneities are modeled by the well-known Gurtin–Murdoch constitutive relation. In the numerical analysis, the interface elastic moduli and the geometry of the nanoscale inhomogeneity are varied to show their influence on the induced stress field. The interaction between nanoscale inhomogeneities and the effect of different geometric shapes of inhomogeneities, including ellipse, triangle, and square are also investigated for different interface material parameters. It is shown that the elastic field can be greatly influenced by the interfacial energy and geometry of nanoscale inhomogeneities. The proposed BEM formulation is very general, including the complete Gurtin–Murdoch model and is further convenient for arbitrary shapes of inhomogeneity.

Analysis of structure collapse regularity for overlying strata in stope based on microseismic technique

To study the structure collapse regularity for overlying strata in stope under the influence of mining, detect the fracture distribution state of coal and rock mass in working face. A multichannel ESG microseismic monitoring system was established to monitor real‐time and continuous microrupture of overlying strata by the advanced mining dynamic disaster monitoring equipment at the working face 62114(C14) in the Xinzhuangzi mine. A large number of monitoring data under the conditions of working face mining process are obtained. The relationship among 3D microseismic location event clusters, fracture zone and structure collapse regularity can be analysed. The layout plan of methane extraction drainage boreholes are finally determined based on the different methane rich zones. The results indicate that the working face advanced, microevents become more and more, the microseismicity of overlying strata is obvious at different times; mining induced stress field change, the appearance of high stress zone impels the microfracture to germinate, and microfractures as a pathway resulted in methane desorption, migration and interconnection in the goaf. The fracture zone and methane rich zone were located in the goaf, close to the retained gateroad. Therefore, the microseismic technique can provide a new and comprehensive way in researching the evolution rules of covering working face rock.

Multi-scale based stochastic damage evolution

In the present work, the stochastic damage evolution is developed by the random extension of the homogenization based multi-scale damage model. The energy bridging theorem is introduced as the bridging vehicle between micro and macro scales. And then the probability density evolution method (PDEM) and the generalized density evolution equation (GDEE) are introduced to describe the nonlinear transportation of randomness between scales. It is observed from the numerical results that not only the randomness induced stress field oscillation but also the stochastic damage evolution are well reproduced by the proposed framework.

Local shrinkage and stress induced by thermo-oxidation in composite materials at high temperatures

The paper is concerned with the modelling, simulation and experimental characterisation of local shrinkage strains and stresses induced by thermo-oxidation phenomena in the IM7/977-2 carbon/epoxy composite material at elevated temperatures. The oxygen concentration and mechanical fields were established through a coupled model constructed from a unified multiphysical approach and the thermodynamics of irreversible processes. The model was implemented in the ABAQUS ® finite element commercial code. Simulations of thermo-oxidation-induced matrix shrinkage were run at a local scale, i.e., the scale of the elementary constituents of the composite, the fibre and the matrix. The experimental assessment was done at the same scale, and the local matrix shrinkage profiles were measured by confocal interferometric microscopy. A good agreement was found between the simulated and measured profiles, validating the unified model. The thermo-oxidation induced stress field was analysed to understand the influence of the environment on the onset of damage in composite materials at elevated temperature.

A simplified testing method for stress-induced of tunnel surrounding rock

A simple testing method for secondary or induced stress of surrounding rock was presented by laboratory experiment, numerical simulation and in situ testing based on the basic principle of the historical stress restoring method. First, stress equivalent coefficient, which key coefficient of stress restored testing, was obtained by laboratory experiment. Second, experimental results were examined using 3D finite element numerical analysis and the influence factors were analyzed by 2D finite element numerical analysis. The correctness of induced stress measuring results in situ for highway tunnel were proved by elastic mechanics theory solution and single-hole stress rescission method. The new simple method of induced stress measuring has important practical value for underground engineering induced stress field analysis.

3D finite element modeling of the thermally induced residual stress in the hybrid laser/arc welding of lap joint

In this study, a three-dimensional (3D) finite element model is developed to investigate thermally induced stress field during hybrid laser–gas tungsten arc welding (GTAW) process. In the hybrid welding case, we focus on the GTAW process sharing common molten pool with laser beam and playing an augment role in the hybrid welding system. An experiment-based thermal analysis is performed to obtain the temperature history, which then is applied to the mechanical (stress) analysis. A modified material model is used to consider the influence of face-to-face contact between the top and bottom metal sheets in the thermo-mechanical analysis of welding lap joints. Results show that the normal stress components prevail in the weld zone during hybrid welding process, and maximum thermal stress exceeding the yield point of material exists at the heat affected zone (HAZ) near the weld pool. Increasing the welding speed causes the penetration and width of weld bead to decrease, and the thermal stress concentration at the welded joint is also reduced accordingly. After welding and cooling down, longitudinal tensile stress (SZ) and transverse compressive stress (SX) are retained in the formed weld, and the higher longitudinal compressive stress exists around the weld bead. In addition, a series of experiments are performed to validate the numerical results, and a qualitative agreement is achieved. Compared to the welded joint obtained by GTAW and laser welding alone, the residual stress concentration in the weld joint obtained by hybrid laser–GTAW is the minimal one.

Study on the Law of the Movement and Damage to Slope with the Combination of Underground Mining and Open-Pit Mining

Under circumstances in which both underground mining and open-pit mining are employed, the mining effects of two approaches will be superposed and the mining slope will receive several induced stress fields, which makes the sliding mechanism and deformation law of slope rock mass more complicated. This paper, targeting at the east slope of Antaibao Mine with the joint employment of underground mining and open-pit mining, aims to study the moving law of the slope rock mass and the damage mechanism to the overburden of the goaf by numerical simulation. It is supposed that models of possible damage to the slope could be explored for guidance to safety-production of the mine.

Al-doping effect on structural, transport and optical properties of ZnO films by simultaneous RF and DC magnetron sputtering

Transparent and conductive Al-doped ZnO films have been prepared by simultaneous RF and DC magnetron sputtering. In order to study the properties of the Al-doped ZnO films, we performed X-ray diffraction, X-ray absorption spectroscopy, temperature dependence of electrical resistance and Hall measurements, as well as optical transmission spectroscopy. The results revealed that all the samples were polycrystalline with a strong preferential c -axis orientation. A minimum resistivity of 7.13 × 1 0 − 3 Ω cm was obtained, and a metallic-type conducting behavior was observed for the film at 50 W. Our present work suggests that the electrical transport property of the Al-doped ZnO films is closely related to the crystallinity. A large number of defects due to poor crystallinity and the induced stress field are able to immobilize the free carrier thereby reducing the conductivity.

Proof of the Strong Eshelby Conjecture for Plane and Anti-plane Anisotropic Inclusion Problems

Based on the Stroh formalism for anisotropic elasticity and the complex variable function method, we prove in this paper that the strong Eshelby conjecture holds for simply-connected anisotropic inclusion problems under plane or anti-plane deformation. The interfaces can be either perfect or dislocation-like. For these inclusion problems, if the induced stress field inside the inclusion is uniform for a single uniform eigenstrain, the inclusion is of the elliptic shape. Thanks to the generality of the proof method, we obtain also alternative proofs of the strong Eshelby conjecture for isotropic inclusion problems, which are given in the Appendix.

Calculation of Moisture Distribution in Early-Age Concrete

The moisture content in concrete pores is a critical parameter for most of the degradation processes suffered by concrete, such as concrete shrinkage and related cracking. The objective of this paper is to present the formulation of a general moisture distribution model for young-age concrete. In the modeling, both cement hydration and moisture diffusion resulted humidity variations are taken into account synchronously. The effect of initial water distribution (after concrete casting) on the development of moisture distribution is considered by introducing a critical time parameter. The simulation of humidity reduction produced from self-desiccation is based on cement hydration degree that taking the effect of temperature into account by using the equilibrant age concept. During modeling the moisture diffusion, a multilinear model was adopted to simulate the moisture dependent diffusivity. The developed model and finite deferential method can well predict the moisture distribution as well as its variations with time. Good agreement between model predictions and experimental results is found. These results can subsequently be used in shrinkage induced stress field analyses, and further be used for cracking control of concrete structures.

Influence of transparent coating thickness on thermoelastic force source and laser-generated ultrasound waves

A numerical model is established to investigate the influence of transparent coating thickness on the laser-generated thermoelastic force source and ultrasound waves in the coating–substrate system by using the finite element method (FEM). Taking into account the effects of thermal diffusion, the finite width and duration of the laser source, as well as the temperature dependence of material properties, the transient temperature distributions are obtained firstly. Applying this temperature field to structure analyses as thermal loading, the thermoelastic stress field and laser-generated ultrasound wave in the specimen are obtained. The generation and propagation of the laser thermoelastically induced stress field and ultrasonic waves in coating–substrate system are presented in detail. The influence of transparent coating thickness on the transient temperature distribution, thermoelastic force source and the laser-generated ultrasound waveforms is investigated. The numerical results indicate that the thermoelastic force source and laser-generated ultrasound waveform are strongly affected by the coating thickness due to the constraint of coating. This method can provide insight into the generation and propagation of the laser-generated stress field in coating–substrate system consisting of a transparent coating and an opaque metallic substrate. It provides theoretical basics to optimize ultrasonic signal generation in particular applications and invert the physical and geometrical parameter of the coating–substrate system more accurately in the experiment.

Non-Uniform Eigenstrain Induced Stress Field in an Elliptic Inhomogeneity Embedded in Orthotropic Media with Purely Imaginary Roots

Complex analysis of anisotropic elasticity indicates that the degree of anisotropy is characterized by two complex parameters. A closed-form solution has been formulated for the stress field resulting from non-uniform eigenstrains in an elliptic inhomogeneity in elastic media with complex roots. In this paper, corresponding analytical solution for the induced stress field in elastic materials with pure imaginary roots is derived based on the polynomial conservation theorem and conformal transformation. The solution is verified by continuity conditions for stresses at the interface between the inhomogeneity and matrix. In addition, numerical examples are provided to present distributions of stresses and strains.

Thermal Simulation of an Arbitrary Residual Stress Field in a Fully or Partially Autofrettaged Thick-Walled Spherical Pressure Vessel

The equivalent thermal load was previously shown to be the only feasible method by which the residual stresses due to autofrettage and its redistribution, as a result of cracking, can be implemented in a finite element (FE) analysis of a fully or partially autofrettaged thick-walled cylindrical pressure vessel. The present analysis involves developing a similar methodology for treating an autofrettaged thick-walled spherical pressure vessel. A general procedure for evaluating the equivalent temperature loading for simulating an arbitrary, analytical or numerical spherosymmetric autofrettage residual stress field in a spherical pressure vessel is developed. Once presented, the algorithm is applied to two distinct cases. In the first case, an analytical expression for the equivalent thermal loading is obtained for the ideal autofrettage stress field in a spherical shell. In the second case, the algorithm is applied to the discrete numerical values of a realistic autofrettage residual stress field incorporating the Bauschinger effect. As a result, a discrete equivalent temperature field is obtained. Furthermore, a FE analysis is performed for each of the above cases, applying the respective temperature field to the spherical vessel. The induced stress fields are evaluated for each case and then compared to the original stress. The FE results prove that the proposed procedure yields equivalent temperature fields that in turn simulate very accurately the residual stress fields for both the ideal and the realistic autofrettage cases.

Origins of waveguiding in femtosecond laser-structured LiNbO3

Femtosecond laser-induced structural changes in LiNbO3 are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications.

Structural and mechanical study of elastomers under plane deformation

The plane shrinkage of various elastomers [natural rubber, synthetic isoprene rubber, and plasticized poly(vinyl chloride)] at room temperature has been studied via direct microscopic observations. Prior to deformation, the surface of polymer samples is decorated with a thin (several nanometers) metallic layer. Further deformation leads to formation of the surface relief in the coating. An analysis of the formed microreliefs allows one to visualize and characterize the induced stress field in the sample. The shrinkage of poly(vinyl chloride) samples is accompanied by development of the uniform surface relief over the whole surface of the deformed polymer. This fact suggests a homogeneous character of the stress field and, hence, a homogeneous structure of the polymer sample. In the case of crosslinked rubbers (natural rubber and synthetic isoprene rubber), their plane shrinkage leads to the development of an irregular pattern on the polymer surface. In addition to the folded surface relief that is typical of the poly(vinyl chloride) structure, one can observe 20-to 50-μm “islands,” which are characterized by their own morphological features. Information on structural inhomogeneity of rubbers that is obtained by scanning electron microscopy correlates with the data of DSC measurements. The advantages of electron microscopic procedure for studying structural rearrangements in polymers during strain recovery of elastomers are demonstrated.

Non-uniform eigenstrain induced stress field in an elliptic inhomogeneity embedded in orthotropic media with complex roots

This paper deals with an elastic orthotropic inhomogeneity problem due to non-uniform eigenstrains. The specific form of the distribution of eigenstrains is assumed to be a linear function in Cartesian coordinates of the points of the inhomogeneity. Based on the polynomial conservation theorem, the induced stress field inside the inhomogeneity which is also linear, is determined by the evaluation of 10 unknown real coefficients. These coefficients are derived analytically based on the principle of minimum potential energy of the elastic inhomogeneity/matrix system together with the complex function method and conformal transformation. The resulting stress field in the inhomogeneity is verified using the continuity conditions for the normal and shear stresses on the boundary. In addition, the present analytic solution can be reduced to known results for the case of uniform eigenstrain.

Mechanics of tidally driven fractures in Europa's ice shell

A fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. Such penetration is found to be very unlikely in the absence of an ocean. A cycloidal crack would then form as a sequence of near instantaneous discrete failures, each extending roughly the brittle layer thickness in range, linked with a much lower apparent propagation speed set by the moving tidal stress field. The implications of this porous ice fracture model for ice-penetrating radar scattering loss and seismic activity are quantified.

Flow stress behavior of polycrystalline Ni under combined magneto- and acousto-plastic effects

The effects of an alternating magnetic field (50 Hz, 6.4 × 10 4 A/m) and an ultrasonic deformation (2 × 10 4 Hz, 5 × 10 −5) on the flow stress of polycrystalline nickel during unidirectional tension at 290 K have been investigated in detail. Special attention is paid to clarify the mechanism of the magneto-plastic (MPE) and acousto-plastic (APE) joint effects. The combined MPE + APE effect is found to be smaller than the sum of the individual ones. An experimental procedure allowed synchronization and consecutive shifts in phase between beginning of the application of the magnetic field period and the initiation of the ultrasonic deformation. The combined MPE + APE is found to be dependent on the phase shift with extremes at 170° and 350°. Considering both magnetically and thermally activated dislocation motion, a differential equation is proposed describing the deformation process in magnetically and ultrasonically induced stress fields. Calculated dependencies of stress on time and strain correlate well with experimental results and clarify the revealed extremes.

Fluid injection and surface deformation at the KTB location: modelling of expected tilt effects

AbstractThis investigation is indented to explore the relationship between changes in pore fluid pressure and deformation of the land surface induced by a large‐scale injection experiment at the KTB site. Deformation will be monitored by ASKANIA borehole tiltmeters at five locations. During the year 2003, a network of borehole tiltmeters was installed, data transmission links established and tested, and recording of tilt data started. Our first main interest was to receive data sets of all stations well before the injection experiment to start in May 2004, to be able to evaluate local site effects. Thus, the separation of injection‐induced effects will be more reliable. Principal 3D numerical modelling (poro‐elastic modelling and investigations, using the finite element method, FEM) of poro‐elastic behaviour showed that significant tilt amplitudes can be expected during controlled fluid injection. Observed deformation will be investigated within the framework of the fluid flow behaviour and resulting deformation. Two models have been used: a coupled hydro geomechanical finite element model (abaqus) and, as a first step, also a multi‐layered poro‐elastic crust (poel). With the numerical model two effects can be quantified: (i) the deformation of the upper crust (tilt measurements) and (ii) the spatial distribution and the changes of material properties in the KTB area. The main aim of the project is to improve the knowledge of coupled geomechanic–hydraulic processes and to quantify important parameters. Thus, the understanding of fracture‐dominated changes of the hydrogeological parameters will be enhanced, geomechanical parameter changes and the heterogeneity of the parameter field quantified. In addition, the induced stress field variation can be explained, which is believed to be mainly responsible for the increase of local seismic activity. Here, we introduce the tiltmeter array at the KTB site, the modelling for a poro‐elastic crust and the preliminary FEM modelling.