Stress Discontinuities Research Articles

Stability analysis of seabed strata and casing structure during the natural gas hydrates exploitation by depressurization in horizontal wells in South China Sea

Natural gas hydrates (NGHs) are a new type of clean energy with great development potential. However, it is urgent to achieve safe and economical NGHs development and utilization. This study established a physical model of the study area using the FLAC^3D software based on the key parameters of the NGHs production test area in the South China Sea, including the depressurization method, and mechanical parameters of strata, NGHs occurrence characteristics, and the technological characteristics of horizontal wells. Moreover, this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well. The results are as follows. With the dissociation of NGHs, the overburden strata of the NGHs dissociation zone subsided and formed funnel-shaped zones and then gradually stabilized. However, the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress. Specifically, distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well, which was thus prone to suffering shear failure. Moreover, apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure. Therefore, it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.

A Spatial Geometric Nonlinearity Spline Beam Element With Nodal Parameters Containing Strains

Many slender robs in engineering can be abstracted as Euler-Bernoulli beams. When analyzing their dynamic behavior, it is necessary to establish the dynamic model of the flexible multi-body systems. Geometric nonlinear elements with absolute nodal coordinates solved a large number of dynamic problems of flexible beams, but it still faces such problems as shear locking, discontinuity of nodal stress and low computational efficiency. In this paper, based on the theory of large deformation beam’s virtual power equation, the functional formula between displacements and rotation angles located at the nodes were established, which can satisfy the deformation coupling relationships and the generalized strains that can describe geometric nonlinear effects in this case were derived. Some parameters of boundary nodes were replaced by axial strain and sectional curvature to obtain a more accurate and concise constraint method for applying external forces. In order to improve the numerical efficiency and stability of the system’s motion equations, a model smoothing method was used to filter out high frequencies from the model. The rationality and effectiveness of the proposed unit were verified.

On the construction of stress discontinuity lines for a two-dimensional plastic region

On the construction of stress discontinuity lines for a two-dimensional plastic region

Sliding Wear Behavior of High-Temperature Vacuum-Brazed WC-Co-NiP Functional Composite Coatings.

The present study aimed to investigate the tribological behavior of high-temperature vacuum-brazed WC-Co-NiP functional coatings deposited on 16MnCr5 case hardening steel. Dry sliding wear resistance was evaluated using a non-conformal ball-on-disk arrangement, at room temperature against 100Cr6 and WC-Co static partners, respectively. Morphological, microstructural, and chemical composition analyses showed a complex, phased structure composed of tungsten carbide, nickel, and hard cobalt-based η-structure. In the testing conditions, the coefficient of friction against 100Cr6 and WC-Co counterparts entered a steady-state value after approximately 1000 m and 400 m, respectively. The wear track analysis revealed phenomena of particles trapped between the sliding bodies, as well as gradual removal of asperities. The calculations of the wear rates proved that the values were strongly influenced by properties of the sliding system, such as crystal structure, stress discontinuities, hardness, and material homogeneity.

The solution of shear stress discontinuity in film drainage model

The Stokes–Reynolds–Young–Laplace (SRYL) model of film drainage for pure liquid and surfactant solution systems have been numerically solved with a simplified Young-Laplace equation in several literatures. However, with this simplification, the calculated profile of the tangential shear stress τb at the gas sides had an unphysical increment at the boundary of the computational domain, which violated the continuity in shear stress especially when inhibitory effect of surfactants on film drainage was considered. In this work, an improved model of film drainage was proposed by modifying the simplified Young-Laplace equation and artificial initial shape of the liquid film. The underlying reason for unreasonable τb caused by the simplified Young-Laplace equation was analyzed. This improvement avoided the unphysical sharp increase of τb at the boundary of the computational domain for both pure liquid and surfactant solution systems, which was critical to extend the SRYL model of film drainage to some complex systems.

Analysis of generated shear wave due to stress discontinuity in a monoclinic layered structure

The present study articulates the generation of SH-wave due to sudden stress discontinuity which moves at the monoclinic crustal layer of finite thickness overlying a monoclinic half-space. Free surface displacement field are obtained in exact form by the method due to Cagniard which is modified by De Hoop for different cases of stress discontinuities at the interface of the layered structure, i.e. when the stress discontinuity is created at certain point and then expands with uniform velocity U in z-direction (i.e. Case I); when a dynamic region of discontinuous stress moves with uniform velocity (i.e. Case II); when the stress discontinuity is created in a static region and then expands with uniform velocity (i.e. Case III); and when an impulsive stress discontinuity occurs only at point z = Ut (i.e. Case IV). The numerical calculations and graphical illustrations have been carried out in order to reveal the effect of anisotropy (monoclinic and transversely isotropic) on the free surface displacement field which give some peculiar results and serve as major highlights of the present study. Additionally, as a special case, the deduced results are matched with the isotropic case, which validates the undertaken work.

Shear waves in a Piezo-Fiber-Reinforced-Poroelastic composite structure with sandwiched Functionally Graded Buffer Layer: Power Series approach

Love wave sensors are highly sensitive devices owing to the concentration of acoustic energy within a few wavelengths of the surface. The basic model of these high-performance devices is layered structures composed of highly sensitive composite and functionally graded materials. Thus the present work aims to establish an analytical framework to investigate the propagation characteristics of horizontally polarized shear waves (SH waves) in a composite structure comprised of a Functionally Graded (FG) buffer layer sandwiched between an overlying transversely isotropic Piezo-Fiber-Reinforced Composite (PFRC) material guiding layer and an underlying transversely isotropic Piezo-Poroelastic (PP) half-space. The motive considering the gradient materials, especially at the interface adjoining the layers, is to avoid the discontinuity of the inter-laminar stress and high local stress fields, which can arise around the edges of the laminates due to material mismatch across the interface. The functional gradient of the buffer layer is configured in terms of volume fractions of the guiding layer and the half-space material. The power series technique is employed to solve the governing equations of the FG-buffer layer. For the propagating SH wave, the dispersion relation relating the wave number with phase velocity through the mechanical and electrical parameters of the PFRC and PP materials, the gradient parameter, and the widths of the guiding and buffer layer is presented for the electrically open (EO) as well as electrically shorted (ES) conditions. Furthermore, mass loading sensitivity is analyzed by coating a fine layer on the free surface of the structure. The obtained results on the dispersion relations are compared and found to be in well-agreement with some of the outcomes available in the extant literature, thus validating the present study. To graphically delineate the impact of all influencing parameters, a layered structure of ZnO-Epoxy (PFRC)/ FG-buffer Layer/PZT-1 (PP) is considered. Also, the mass loading sensitivity is graphically illustrated for the coating of a fine layer of SiO2 on the free surface. The present study may be applied for a myriad of engineering applications like manufacturing and optimizing sensors, actuators, transducers, etc. The obtained results are specifically useful for developing and enhancing the performance of surface acoustic wave (SAW) devices/Love wave sensors.

Generation and Propagation of SH Waves Due to Shearing Stress Discontinuity in Linear Orthotropic Viscoelastic Layered Structure

Generation and Propagation of SH Waves Due to Shearing Stress Discontinuity in Linear Orthotropic Viscoelastic Layered Structure

Physics-informed deep neural network enabled discovery of size-dependent deformation mechanisms in nanostructures

The surface effects based on the Gurtin-Murdoch interface model are incorporated into a physics-informed deep neural network (DNN) to enable the discovery of the size-dependent mechanical response in nanoscale structures for the first time. The DNN directly deals with the minimization of a cost function with contributions from total potential energies and penalties representing the displacement boundary conditions. Following this spirit, the Gurtin-Murdoch interface model is directly implemented into the DNN cost function through additional surface energies associated with the surfaces of the nanostructures without resorting to the implementation of Young-Laplace equations explicitly employed in the analytical approaches. The DNN technique is first validated on both displacement and stress fields vis-à-vis the analytical solution for the modified Kirsch problem with the Gurtin-Murdoch interface under far-field shear loading. Then, the DNN technique is critically and vigorously assessed against the Q4, Q8, and Q9 finite-element results of nanocylinders under diametrical loading. Notably, the accuracy of the DNN is demonstrated to be comparable to the analytical, Q8 and Q9 finite element-based results without the apparent stress discontinuities noticeable in the Q4 element. The numerical example indicates that the selection of collocation points in the DNN approach can be made completely random using the Monte-Carlo simulation without sacrificing the accuracy of the predicted stress field. An inverse load identification problem is also considered to demonstrate the special strength of the DNN approach, namely, both the forward and inverse problems can be handled using the same framework. The present contribution provides a compelling alternative and independent means of identifying surface elasticity effect and hence is a good tool in assessing the finite-element method as well as other approaches’ predictive capability for this class of materials.

Two-field formulations for isogeometric Reissner\u2013Mindlin plates and shells with global and local condensation

In this paper, mixed formulations are presented in the framework of isogeometric Reissner–Mindlin plates and shells with the aim of alleviating membrane and shear locking. The formulations are based on the Hellinger-Reissner functional and use the stress resultants as additional unknowns, which have to be interpolated in appropriate approximation spaces. The additional unknowns can be eliminated by static condensation. In the framework of isogeometric analysis static condensation is performed globally on the patch level, which leads to a high computational cost. Thus, two additional local approaches to the existing continuous method are presented, an approach with discontinuous stress resultant fields at the element boundaries and a reconstructed approach which is blending the local control variables by using weights in order to compute the global ones. Both approaches allow for a static condensation on the element level instead of the patch level. Various numerical examples are investigated in order to verify the accuracy and effectiveness of the different approaches and a comparison to existing elements that include mechanisms against locking is carried out.

Enhancement behavior of reinforced concrete beam with transverse holes under combined twisting and bending using steel fiber

Openings in beams cause load path and stress discontinuities. Several research experiments have been conducted in recent years to improve the area surrounding the opening using various techniques. In this research investigate the effect of hybrid concrete (normal concrete and steel fiber reinforced concrete) on the beams with transfer opening subject to combined loading (torsion and bending). Six specimens of reinforced concrete beams with the same dimension and reinforcement were tested. The variable where distance of hybrid area around opening (150, 225, 450 and 1500) mm. The opening in reference concrete beams was decrease in the ultimate twisting moment (18.28%) with respect to solid one (without opening). The ultimate twisting moment increases by (32-68%) as steel fiber reinforced concrete is used around the opening. Simultaneously, the twist angle was increased by about (27.6- 46.7%). In addition, relative to the control specimen, the first crack was delayed by bout (41.6-75) %.

A study on the optimization of residual stress distribution in the polyethylene and polyketone double layer pipes

Pipes are generally used in the variety of industries such as water purification, automobiles and air ventilation and so on which is commonly made from various materials like stainless steel, plastic and iron. Recently, plastic and copper pipes were widely used in the water industries due to their higher corrosive resistivity and durability. However, the mechanical defects are the major problem in the usage of the plastic pipes. The plastic pipe wall thickness and stress distribution are key components for the plastic deformation in the pipe. Among them, the residual hoop stress is the mainly affects the life time of the pipe. The present study investigates the effect of the inner and outer wall thickness on the pipe defects is explored by finite element analysis. As a result of observation, a discontinuity of stress is observed at the PK/PE interface of the double-multiple pipe due to the void value, and it is judged that the PK pipe has superior tensile strength due to its high mechanical strength and excellent physical properties such as yield stress and stiffness compared to PE pipe. It is judged that the conditions in the real environment can be analyzed by verifying the peeling phenomenon due to thermal fatigue due to different coefficients of thermal expansion between the heterogeneous resins and observing the peeling phenomenon due to local stress concentration due to the thickness non-uniformity of the bonding interface of the heterogeneous resins. The simulation and experimental results confirmed PK pipe has superior tensile strength due to its high mechanical strength and excellent physical properties such as yield stress and stiffness compared to PE pipe.Capsule: This work investigates the residual stress distribution in the polyethylene and polyketone double layer pipe.

Study on the shear performance of adhesive shear connectors in push-out tests

This paper introduces the adhesive shear connector aiming to remedy the defects of traditional mechanical shear connectors, such as stress concentration, discontinuous shear stress transmission and welding residual stress. To study the shear behavior of such connectors, the push-out tests of two groups of adhesive shear connector specimens (6 in total) were designed and completed, in which the adhesive materials are epoxy resin and magnesium phosphate cement (MPC) respectively. During the whole process of loading, the stress–strain curve, stress-slip curve and failure modes were obtained. In addition, the calculation method of predicting the ultimate shear bearing capacity of adhesive shear connectors was also explored. The results show that the shear strength of epoxy resin shear connector is higher than that of MPC one. The average shear strength of epoxy resin shear connectors is about 4.07 MPa, which is higher than that of the traditional stud shear connectors which is 3.0 MPa, indicating that epoxy resin shear connectors have the same interface shear capacity as traditional stud shear connectors. The failure mode of epoxy resin shear connectors is mainly the strength failure of concrete or adhesive layer, while the failure mode of MPC shear connectors is mainly the debonding failure between binder and steel. Suggestion for further study is that metal shear connectors can be used in combination with the adhesive shear connectors so as to make full use of the advantages of both.

Deformation Properties of Nano-Silica Modified Concrete Mixtures under Uniaxial Compression Loading

The enhancement that nano-silica (NS) constitutes in the microstructure of concrete would affect its deformation behavior. In this work, the pre-peak and post-peak stress–strain diagram characteristics of concrete mixtures with NS (1.5% of cement by wt.) under uniaxial compression were investigated. The results were compared with the mixtures containing micro-silica (MS) (7.3% of cement by wt.). Aggregate characteristics in the range of 16 to 22 mm, as another variable, were also changed by utilizing river gravel (RG II) or crushed sandstone (CS II). Compared to reference concrete, the addition of NS did not improve the compressive strength and modulus of elasticity significantly. However, the addition of only 1.5% NS significantly affected the stress–strain diagram characteristics, such as discontinuity and critical stress limits before the peak load. Relative to the mixtures containing MS, the effect of NS on the pre-peak region was more significant. For example, when the mixtures containing RG II as coarse aggregate are considered, with respect to reference mixture, the increase in normalized discontinuity stress limits at 28 days was found as 52% and 26%, respectively, for the mixtures containing NS and MS. However, in the same order, the increase in 28-day compressive strength was just 8.1% and 5.2%. Post-peak behavior of the mixtures also indicated that the fracture of concrete is more brittle when MS or NS is used. Among these, the effect of NS on the post-peak region was found to be more dominant.

Impact of Probiotics on the Salivary Microbiota and Salivary Levels of Inflammation-Related Proteins during Short-Term Sugar Stress: A Randomized Controlled Trial

Background: The purpose of the present investigation was to characterize the effect of probiotics on the composition of the salivary microbiota and salivary levels of inflammation-related proteins during short-term sugar stress. We tested the hypotheses that consumption of probiotics may partly counteract the detrimental influence of sugar stress on oral homeostasis. Methods: The present study was a five-week, blinded, randomized controlled trial with four study arms—A: sucrose and probiotic (n = 20); B: sucrose and placebo (n = 20); C: xylitol and probiotic (n = 20); D: xylitol and placebo (n = 20). Saliva samples were collected at baseline and after two and five weeks. The salivary microbiota was characterized by means of 16S rDNA sequencing, and sequences were referenced against the Human Oral Microbiome Database (HOMD). Neutrophil gelatinase-associated lipocalin (NGAL) and transferrin levels were quantified using immunoassays. Results: Sugar stress induced a significant increase in the relative abundance of the genus Streptococcus from 29.8% at baseline to 42.9% after two weeks. Changes were transient and were completely reversed three weeks after discontinuation of sugar stress. Xylitol and probiotics alone had no effect on the salivary microbiota, whereas the combination of xylitol and probiotics induced a significant decrease in the relative abundance of Streptococcus species from 37.6% at baseline to 23.0% at week 2. Sugar stress significantly increased salivary transferrin levels, and the effect was partly counteracted by concomitant use of probiotics. Conclusions: The data clearly demonstrate an impact of combined consumption of xylitol and probiotics on the composition of the salivary microbiota. Future studies are needed to evaluate whether the combined use of xylitol and the probiotic strains tested could have clinically protective effects during periods of sugar stress.

Analytical graphic statics

Graphic statics is undergoing a renaissance, with computerized visual representation becoming both easier and more spectacular as time passes. While methods of the past are revived, little emphasis has been placed on studying the mathematics behind these methods. Due to the considerable advances of our mathematical understanding since the birth of graphic statics, we can learn a lot by examining these old methods from a more modern viewpoint. As such, this work shows the mathematical fabric joining different aspects of graphic statics, like dualities, reciprocal diagrams, and discontinuous stress functions. This is done by introducing a new, three dimensional force diagram (containing the old two dimensional force diagram) depicting the three dimensional equilibrium of planar force systems. A corresponding three dimensional “form diagram” (dual diagram) is introduced, in which forces are treated as linear functionals (dual vectors). It is shown that the polyhedral stress function introduced by Maxwell is in fact a linear combination of these functionals; and the projective dualities connecting these three dimensional diagrams are also explained.

A hybrid mimetic spectral element method for three-dimensional linear elasticity problems

We introduce a domain decomposition structure-preserving method based on a hybrid mimetic spectral element method for three-dimensional linear elasticity problems in curvilinear conforming structured meshes. The method is an equilibrium method which satisfies pointwise equilibrium of forces. The domain decomposition is established through hybridization which first allows for an inter-element normal stress discontinuity and then enforces the normal stress continuity using a Lagrange multiplier which turns out to be the displacement in the trace space. Dual basis functions are employed to simplify the discretization and to obtain a higher sparsity. Numerical tests supporting the method are presented.

Effective dynamics for low-amplitude transient elastic waves in a 1D periodic array of non-linear interfaces

This article focuses on the time-domain propagation of elastic waves through a 1D periodic medium that contains non-linear imperfect interfaces, i.e. interfaces exhibiting a discontinuity in displacement and stress governed by a non-linear constitutive relation. The array considered is generated by a, possibly heterogeneous, cell repeated periodically and bonded by interfaces that are associated with transmission conditions of non-linear “spring–mass” type. More precisely, the imperfect interfaces are characterized by a linear dynamics but a non-linear elasticity law. The latter is not specified at first and only key theoretical assumptions are required. In this context, we investigate transient waves with both low-amplitude and long-wavelength, and aim at deriving homogenized models that describe their effective motion. To do so, the two-scale asymptotic homogenization method is deployed, up to the first-order. To begin, an effective model is obtained for the leading zeroth-order contribution to the microstructured wavefield. It amounts to a wave equation with a non-linear constitutive stress–strain relation that is inherited from the behavior of the imperfect interfaces at the microscale. The next first-order corrector term is then shown to be expressed in terms of a cell function and the solution of a linear elastic wave equation. Without further hypothesis, the constitutive relation and the source term of the latter depend non-linearly on the zeroth-order field, as does the cell function. Combining these zeroth- and first-order models leads to an approximation of both the macroscopic behavior of the microstructured wavefield and its small-scale fluctuations within the periodic array. Finally, particularizing for a prototypical non-linear interface law and in the cases of a homogeneous periodic cell and a bilaminated one, the behavior of the obtained models are then illustrated on a set of numerical examples and compared with full-field simulations. Both the influence of the dominant wavelength and of the wavefield amplitude are investigated numerically, as well as the characteristic features related to non-linear phenomena.

Mechanisms for the control of the complex propagation behaviour of hydraulic fractures in shale

The complex propagation behaviours of hydraulic fracture (HF) at bedding planes (BPs) and the produced complicated fracture geometry are essential to enhance the production of shale gas reservoirs. To better understand the complex propagation behaviour of HF at BPs with different bond strengths, the propagation behaviour, including arrest, crossing, and deviation, was identified first from post-fracturing shale specimens. The discontinuous stress and displacement fields on both sides of a BP ahead of an HF were then determined using the numerical simulation method. Finally, three mechanisms—principal stress jump, Cook–Gordon debonding or Poisson effect, and elastic dissimilarity—were explored in detail to interpret the complex propagation behaviour. The results revealed that HF is arrested/deviated only at extremely weakly cemented or fully opened BPs, whereas HF crosses strongly cemented BPs. The high heterogeneity of the BPs in cementing strength is responsible for the complex propagation behaviour of HF. The principal stress jump at a BP is caused by the difference in stiffness between the BP and the rock matrix. The maximum tensile principal stress ahead of the HF cannot be transmitted across the weakly cemented or fully opened BPs, suggesting that the HF cannot cross BPs. The principal stresses may rotate at a weakly cemented BP, and the rotated principal stresses tend to terminate or deviate an HF. Because of Cook–Gordon debonding and the Poisson effect, if a weakly cemented BP is present and is roughly normal to an advancing HF, the BP may break at some distance ahead of the fracture tip and induce a secondary fracture along the BP. The HF then reaches the opened BP and deviates towards the BP. The propagation process of L- or T-shaped fractures can be interpreted both by the Cook–Gordon debonding and Poisson effect from the viewpoints of stress and displacement. The three mechanisms often operate together when an HF deviates towards a weak BP; while for a special case, there may be only one dominant mechanism.

Residual stresses evolution during strip clad welding, post welding heat treatment and repair welding for a large pressure vessel

Strip clad welding is a key manufacturing process of large pressure vessel and repair welding was usually used to repair the defects of strip welded structure, which generates very complex residual stresses and have a great effect on structure integrity. Therefore, in this paper, the residual stress distributions and their evolutions during strip clad welding, post-welding heat treatment (PWHT) and repair welding of a large pressure vessel were investigated. A new strip cladding heat source model considering the magnetic pinch effect was proposed. The predicted welding pool size by the proposed heat source model has a good agreement with the experiment results. And the predicted residual stresses in every stage agree very well with those by indention measurement method. The residual stresses after strip clad welding along the through-thickness direction present tension-compression-tension distribution. The PWHT has little eliminating effect on tensile residual stress on cladding layer, and even worsens the discontinuity stress distributions across the interfaces between the clad metal and base metal. After repair welding, local detrimental tensile residual stresses were generated in the repaired zone. A new heat treatment method based on distinct temperature regulation is proposed, which has a good ability to eliminate the tensile stress of the cladding layer to compressive stress and has a good application in engineering.