Radial Tensile Stress Research Articles

Delamination analysis of the epoxy impregnated REBCO racetrack coil under thermal stress based on a 3D model

Superconducting coils made of Rare-Earth-Barium-Copper-Oxide (REBCO) coated conductor (CC) exhibit superior electromagnetic performance. Employing epoxy impregnation can improve the structural integrity of the superconducting coils. However, the delamination behavior is observed in the epoxy impregnated REBCO coil when the environment temperature cool from the room temperature to 77 K. In previous studies, there is a few researches on the delamination and mechanical behavior of the epoxy impregnated racetrack coil. Therefore, this study proposes a three-dimensional (3D) mechanical-thermal model which incorporates the cohesive zone material (CZM) to investigate the delamination mechanisms in epoxy impregnated REBCO racetrack coils during cooling. We found that the coil experienced a higher tensile radial stress at the semicircular part than the straight part during the cooling process. This leads to that the delamination area tends to appear initially in the semicircular part with large tensile radial stress. And the stress concentration generated at the edge of the delamination area in the semicircular part can cause the extension of the edge of the delamination area to the straight part. In addition, the influences of the thermal expansion coefficient (CTE) of the mandrel and overband on the coil delamination behavior are studied in this paper. It is found that the radial stress, the initial position of the delamination, and the degree of delamination are affected by the CTE of the mandrel and overband. And the delamination of the coil can be avoided by reducing the tensile radial stress of the coil through reducing the CTE of the mandrel or increasing the CTE of the overband. And the prevention of the delamination in the semicircular part can obviously avoid the occurrence of the delamination in the straight part of the racetrack coil.

Formation mechanism of radial and circular cracks promoted by delamination in drying silica colloidal deposits.

Cracks with radial and circular patterns are appealing in nature and industry. Although morphologies and propagation conditions of cracks are extensively studied, the formation mechanism of crack pattern by the interaction of channel fracture and interfacial delamination remains elusive. Here, we present the transition of radial to coexisting radial and circular crack patterns when the thickness of colloidal deposits on both hard and soft substrates exceeds a critical value, through the colloidal volume fraction dependence. In addition, a thickness-dependent phase diagram from radial crack to coexistence of radial and circular cracks was constructed with respect to the radius and the volume fractions of silica colloidal deposits. A phase-field fracture model is developed to elucidate how the formation of radial cracks is facilitated by simultaneous delamination. The warping-induced radial tensile stress at the bottom surface of the striped deposit is proportional to the thickness. It leads to subsequent nucleation and growth of circular cracks in thick deposits. This work provides insight into the formation mechanism of complex crack patterns in drying colloidal deposits and revolutionizes the design space of crack-based micro-nano structures.

Modeling 3D TGO dynamic growth and stresses and the resulting modes of buckling failure in thermal barrier coatings

AbstractBased on experimental data, this study presents a model of the dynamic growth of a thermally grown oxide (TGO) in thermal barrier coatings (TBCs) during thermal cycling. The model was solved using the finite element method. The thermal stress results of the model were compared with experimental results to verify the accuracy of the model. The stress analysis revealed that the combined effect of radial tensile stress and circumferential compressive stress can lead to the delamination of TBCs in a buckling form, which can induce the formation of cracks in TBCs along the axial direction. This observation was consistent with the conclusions drawn from the experimental results. Crack initiation and propagation result from the action of the maximum shear and radial stresses. In addition, the relationship between buckling failure and microscopic cracks was investigated. Thermal cycling induces radial and circumferential cracks, leading to buckling, delamination, and ultimate coating failure as the radial and circumferential cracks interact at the interface.

Numerical Simulation of Geothermal Energy Development at Mount Meager and Its Impact on In Situ Thermal Stress

The Meager Mountain Geothermal Project stands as one of the pioneering geothermal energy initiatives in its early stages of resource development. Despite its abundant geothermal heat resources, no prior studies have systematically evaluated the potential of implementing coaxial borehole heat exchangers on site. This study addresses this research gap by presenting a comprehensive heat transfer model for an underground closed-loop geothermal system utilizing a single coaxial well. Finite element analysis incorporated fluid and solid heat transfer, as well as solid mechanics. The results obtained facilitated the construction of the temperature and thermal stress profiles induced by the cooling effects resulting from years of heat extraction. After 25 years of operation, the outlet temperature has reached approximately 74 °C, and the maximum radial tensile thermal stress amounts to ~47 MPa. Furthermore, the analysis demonstrates that higher fluid velocities contribute to more perturbed temperature and stress distributions. The study attained maximum thermal and electric power outputs of 208 kW and 17 kW, respectively. This research also underscores the significant impact of geothermal gradient and well length on BHE design, with longer wells yielding more power, especially at higher injection velocities.

Microstructure evolution and twinning behavior of AZ31 magnesium alloy sheets with bimodal texture during cold deep-drawing deformation

Cold deep drawing experiments were conducted on AZ31 magnesium (Mg) alloy sheets with bimodal texture, which were prepared by the newly developed equal channel angular rolling and continuous bending and subsequent annealing (ECAR-CB-A) process. The results showed that the sheet could be successfully deep-drawn at a limiting drawing ratio (LDR) of 1.6, and the forming performance was better than that of the as-rolled base-texture sheet at room temperature. The microstructure after deep-drawing deformation was investigated by X-ray diffraction (XRD), optical microstructure (OM), and electron backscatter diffraction (EBSD). A large number of {10−12} extension twins (ETs) are observed in the outer shoulder region. Due to the bimodal texture, the activated {10−12} ET variants and the basal slip within the matrix have a higher Schmid Factor (SF) value when the outer shoulder region is under radial tensile stress, this enables the shoulder of the drawn cup to maintain good deformability. In addition, some {10−11} contraction twins (CTs) and {10−11}-{10−12} double twins (DTs) can be observed in this region, which is activated by the local strain accommodation between the twins and the slip systems, and the multiaxial stress. Due to different stress conditions, the volume fractions of {10−12} ETs in the inner shoulder region are less than that in the outer region. The stress concentration caused by {10−11}-{10−12} DTs that occurred in the edge region is the reason for the fracture of the drawn cups at a drawing ratio(DR) of 1.7.

Effect of structure of unfluxed burnt titanomagnetite pellets on strength under static compression

Burnt pellets must retain their strength from the moment they are taken out of an induration machine until they are loaded into a blast furnace. One of the indicators of the burnt pellets’ strength is the compressive strength, i.e. the ultimate force. In experiments to determine compressive strength, the main type of fracture is occurrence and development of cracks that pass through the core center of pellets (where the maximum radial tensile stresses present) or near it. The paper presents the requirements for static compression strength imposed by blast furnace production to iron ore pellets. Using an optical and scanning electron microscope equipped with an energy-dispersive microanalyzer, we analyzed the relationship of structural components and pores in the core of burnt unfluxed iron ore titanomagnetite pellets with the ultimate force under static compression. By scanning electron microscopy and X-ray spectral microanalysis, it was established that the core of pellets is a multiphase material, and its main phases are titanomagnetite, magnetite, titanohematite, hematite and aluminosilicate binder. Optical microscopy made it possible to establish the microstructure of the pellet core, which has three types of microstructures: non-oxidized core (magnetite or titanomagnetite), partially oxidized core – around (magnetite or titanomagnetite) hematite grains (titanohematite) and oxidized core (hematite and titanohematite). The main factors for obtaining pellets with an ultimate force of more than 2.5 kN/pellet according to the requirements of blast furnace production are: the number of closed macropores and the number of large grains in the core. It is shown that with an increase in the number of closed macropores and the number of large grains in the core, the ultimate force is reduced from 3.5 kN to 0.87kN/pellet.

3D numerical investigation on delamination behavior of the epoxy impregnated REBCO pancake coil

Superconducting coils made of rare-earth-barium-copper-oxide (REBCO) coated conductor (CC) exhibit superior electromagnetic performance. Employing epoxy impregnation can improve the structural integrity and mechanical property of the superconducting coils. However, due to the extreme work environment and weak adhesion strength of REBCO CC, the delamination induced by radial thermal stress and electromagnetic force significantly affects the electromagnetic property and the reliability of the superconducting coil. This study proposes a three-dimensional thermal-electromagnetic mechanical delamination model that incorporates the cohesive zone model to investigate the delamination mechanisms in epoxy impregnated REBCO pancake coils during the cooling and coil operation processes. The simulation employs a three-parameter Weibull distribution to account for the inhomogeneity of transverse tensile strength in the CCs. The delamination behavior and mechanisms of the coils under different conditions are analyzed. The simulation results show that the model considering random adhesion strength proves to be more effective in representing the delamination behavior of the coil. And large tensile radial stresses caused by thermal stresses and electromagnetic forces lead to the delamination behavior of the coil during cooling and operation. The main reason for the tensile radial stress is the mismatch in the thermal contraction among components of the coils during cooling process. Furthermore, we investigate the influence of the thermal expansion coefficient (CTE) and thickness of the mandrel, the CTE and prestress of the overband and the initial localized damage. The results indicate that these factors significantly affect the tensile radial stress and the extent of delamination in the windings. And the extent and distribution of delamination is related to the stress release caused by delamination to a certain degree.

Design and feasibility analysis of a new completion monitoring technical scheme for natural gas hydrate production tests

As a prerequisite and a guarantee for safe and efficient natural gas hydrates (NGHs) exploitation, it is imperative to effectively determine the mechanical properties of NGHs reservoirs and clarify the law of the change in the mechanical properties with the dissociation of NGHs during NGHs production tests by depressurization. Based on the development of Japan’s two offshore NGHs production tests in vertical wells, this study innovatively proposed a new subsea communication technology—accurate directional connection using a wet-mate connector. This helps to overcome the technical barrier to the communication between the upper and lower completion of offshore wells. Using this new communication technology, this study explored and designed a mechanical monitoring scheme for lower completion (sand screens). This scheme can be used to monitor the tensile stress and radial compressive stress of sand screens caused by NGHs reservoirs in real time, thus promoting the technical development for the rapid assessment and real-time feedback of the in-situ mechanical response of NGHs reservoirs during offshore NGHs production tests by depressurization.

A High Performance Insulated REBCO Pancake With Conductive Cooling Capability

REBCO Coated Conductors have high tensile strength along their length. They are however sensitive to delamination when traction force is applied on the tape surface, which in a pancake winding corresponds to radial tensile stress. For this reason, most REBCO magnets are dry-wound (not impregnated). One of the drawbacks of dry winding is that conduction cooling to low temperature (below 20 K) would be inefficient. Edge impregnation structures were developed to mitigate this problem but retaining a soft non-stick insulating layer in between the turn. In this paper, we propose an improved pancake structure with the benefits of edge-impregnation but using Hastelloy tape with thin insulation coating to reinforce the winding. The mechanical behavior of such structure is first estimated then validated with high current tests under large background magnetic field. The insulation performance is then tested by rapid ramping and fast discharge.

Degradation in interfacial shear strength of carbon fiber/ vinyl ester composites due to long-term exposure to seawater using push-out tests

In this study, single fiber (∼7-μm diameter) push-out tests are conducted to evaluate hygrothermal effects on the interfacial shear strength (IFSS) of carbon fiber/vinyl ester (CF/VE) composites. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40 °C for two years. An investigation has been conducted on the preparation, validity, and interpretation of the push-out test results. First, the authors present a polishing methodology that results in thin films of CF/VE composites in the thickness range of 15–120 μm and produces an average 41.2% drop in IFSS due to long-term hygrothermal exposure. Using scanning electron microscopy (SEM), we show that during the push-out tests, the failure initiates locally at the zone of minimum bond strength at the bottom (away from the indenter), then propagates along the length of the interface. The influence of radial tensile stresses originating due to bending is found to be negligible. Using the SEM imaging of the pushed-out fibers, we validate the failure of the interface to be the primary source of failure. The associated results are found to depend on the thickness of the interface. We then reevaluate the results using the Weibull distribution, knowing that the failure mechanism is analogous to the weakest link theory. The results show a 25.5% drop in the IFSS of the CFVE composite, measured at an infinitesimal scale due to long-term to hygrothermal conditioning at 40 °C. A significant drop in IFSS was observed after reheating above glass transition temperature (Tg) and cooling.

Modeling of energy carrier in solar-driven calcium-looping for thermochemical energy storage: Heat-mass transfer, chemical reaction and stress response

The solar-driven calcium looping process (CaL) poses a great potential for thermochemical energy storage. The calcium-based particle, a core energy carrier for CaL, however, is prone to fragmentation, significantly reducing the efficiency and stability of energy storage. In this work, a particle scale model for core–shell structured energy carrier that considers the heat transfer, hierarchical reaction, mass transport and thermal stress processes is proposed to study the characteristics of energy storage performance. The multiprocess model confirms that stress failure at the particle center is the main reason of energy carrier fragmentation, resulting from the exceedingly high radial tensile stress. To improve the energy storage performance, the operation conditions and modified particle properties of CaL are investigated. It was found that the thermal stress can be relieved with the reduction of energy carrier temperature, while energy storage efficiency is decreased. In addition, the thermal stress can also be reduced by an appropriate increase in the airflow temperature without sacrificing the efficiency. Moreover, energy carrier can yield a high energy storage efficiency and cycle stability when the radius falls into 300–500 μm.

Prediction of ring crack initiation in ceramics and glasses using a stress–energy fracture criterion

AbstractCrack initiation in brittle materials upon spherical indentation is associated with the tensile radial stresses during loading. However, location of crack onset often differs (offset) from the site of maximal stress. In addition, experiments reveal a strong dependency of crack initiation forces on geometrical parameters as well as the surface condition of the sample. In this work, a coupled stress–energy fracture criterion is introduced to describe the initiation of ring cracks in brittle materials, which takes into account the geometry of the contact and the inherent strength and fracture toughness of the material. Several experiments reported in literature are evaluated and compared. The criterion can explain the location offset of the ring crack upon loading, as observed in various ceramics and glasses. It also predicts the ring crack initiation force upon contact loading, provided that surface compressive stresses, introduced during grinding or polishing processes, are taken into account. Furthermore, the stress–energy criterion may be employed to estimate the surface residual stress of ceramic parts, based on simple contact damage experiments.

Rheological analysis and molecular dynamics modeling of Ultra-High Performance Concrete for wet-mix spraying

The Sprayed Ultra-High Performance Concrete (SUHPC) is one of the most promising methods for the rapid construction and stabilization of concrete structures. In this work, viscosity-modified agents (VMA) are used to adjust the rheological parameter of Ultra-High Performance Concrete (UHPC) slurry for wet-mix. The obtained results show that 0.5‰ hydroxypropyl methylcellulose (HPMC) doped SUHPC exhibits the most suitable plastic viscosity and static yield stress without significant reduction in mechanical properties. Subsequently, the spraying technique is employed to produce SUHPC blocks, and about 16% improvement in mechanical properties can be observed in this study compared to the conventional casting UHPC. The X-ray CT results show that the fiber in SUHPC blocks tends to be parallel to the sprayed surface and exhibits a lower porosity, which is beneficial for withstanding radial tensile stress. Additionally, to further understand the effect of moisture on the viscosity regulation mechanism of HPMC in this study, the molecular dynamics method is employed, which demonstrates that the formation of water clusters and intermolecular hydrogen bonds between HPMC molecular chains can slightly increase the yield stress of HPMC at low moisture content.

Progressive Damage Analysis for Spherical Electrode Particles with Different Protective Structures for a Lithium-Ion Battery.

Charge-discharge in a lithium-ion battery may produce electrochemical adverse reactions in electrodes as well as electrolytes and induce local inhomogeneous deformation and even mechanical fracture. An electrode may be a solid core-shell structure, hollow core-shell structure, or multilayer structure and should maintain good performance in lithium-ion transport and structural stability in charge-discharge cycles. However, the balance between lithium-ion transport and fracture prevention in charge-discharge cycles is still an open issue. This study proposes a novel binding protective structure for lithium-ion battery and compares its performance during charge-discharge cycles with unprotective structure, core-shell structure and hollow structure. First, both solid and hollow core-shell structures are reviewed, and their analytical solutions of radial and hoop stresses are derived. Then, a novel binding protective structure is proposed to well-balance lithium-ionic permeability and structural stability. Third, the pros and cons of the performance at the outer structure are investigated. Both analytical and numerical results show that the binding protective structure serves with great fracture-proof effectiveness and high lithium-ion diffusion rate. It has better ion permeability than solid core-shell structure but worse structural stability than shell structure. A stress surge is observed at the binding interface with an order of magnitude usually higher than that of the core-shell structure. The radial tensile stress at interface may more easily induce interfacial debonding than superficial fracture.

Mechanical Stability of the Heterogenous Bilayer Solid Electrolyte Interphase in the Electrodes of Lithium–Ion Batteries

Mechanical stability of the solid electrolyte interphase (SEI) is crucial to mitigate the capacity fade of lithium–ion batteries because the rupture of the SEI layer results in further consumption of lithium ions in newly generated SEI layers. The SEI is known as a heterogeneous bilayer and consists of an inner inorganic layer connecting the particle and an outer organic layer facing the electrolyte. The growth of the bilayer SEI over cycles alters the stress generation and failure possibility of both the organic and inorganic layers. To investigate the probability of mechanical failure of the bilayer SEI, we developed the electrochemical-mechanical coupled model with the core–double-shell particle/SEI layer model. The growth of the bilayer SEI is considered over cycles. Our results show that during charging, the stress of the particle changes from tensile to compressive as the thickness of bilayer SEI increases. On the other hand, in the SEI layers, large compressive radial and tensile tangential stress are generated. During discharging, the compressive radial stress of the bilayer SEI transforms into tensile radial stress. The tensile tangential and radial stresses are responsible for the fracture and debonding of the bilayer SEI, respectively. As the thickness ratio of the inorganic to organic layers increases, the fracture probability of the inorganic layer increases, while that of the organic layer decreases. However, the debonding probability of both layers is decreased. In addition, the SEI covering large particles is more vulnerable to fracture, while that covering small particles is more susceptible to debonding. Therefore, tailoring the thickness ratio of the inorganic to organic layers and particle size is important to reduce the fracture and debonding of the heterogeneous bilayer SEI.

Numerical and Experimental Study on Large-Diameter FRP Cable Anchoring System with Dispersed Tendons

Based on a previously designed variable-stiffness load transfer component (LTC), a novel dispersed-tendon cable anchor system (CAS) was developed to increase the anchoring efficiency of large-diameter basalt-fiber-reinforced polymer (BFRP) cables. The static behaviors of the CAS are then numerically evaluated by a simplified three-dimensional finite-element (FE) model and implemented in a full-scale BFRP cable. The FE results indicated that the accuracy of the simplified dispersed-tendon model could be effectively ensured by dividing the revised compensation factor. The anchor behavior of the dispersed-tendon CAS was superior to that of the parallel-tendon CAS when the same cable was applied. The radial stress and tensile stress difference can be reduced by decreasing the tendon spacing. The testing and simulated results agreed well with the load–displacement relationship and axial displacement. All tendons fractured in the testing section, and the LTC suffered minimal damage. The ultimate force of the cable with 127 4-mm-diameter tendons was 2419 kN, and the corresponding anchoring efficiency was 93%. The cable axial tensile strain in the anchoring zone decreased linearly from the loading end to the free end. The cable shear stress concentration at the loading end can be avoided by employing a variable-stiffness anchoring method.

Numerical Simulation of the Straight-Swirling Integrated Jet and Its Application in Natural Gas Hydrate Drilling

Summary Natural gas hydrate (NGH) is a potential clean energy source and is buried abundantly in seafloor sediments. Waterjet is a key technology involved in both the marine NGH solid fluidization exploitation method and the integrated radial jet drilling and completion method. To improve the efficiency of breaking and extracting NGH through a waterjet, a straight-swirling integrated jet (SSIJ) nozzle is designed based on the convergent-divergent geometry and impeller in this study. With a computational fluid dynamics method, the 3D model of SSIJ is constructed, and the characteristics of velocity field, pressure field, cavitation cloud distribution, and turbulence kinetic energy are analyzed, the results of which are compared with conical jet (CJ), convergent-divergent jet (CDJ), and swirling jet (SJ). Laboratory experiments of gas hydrate-bearing sediments (GHBS) erosion by the four kinds of jets mentioned above are conducted to evaluate the jet erosion performance based on the in-house experimental apparatus for NGH generation and cavitating jet erosion. Results indicate that the SSIJ can significantly enhance the breaking volume and efficiency of waterjet erosion on GHBS compared with the other three methods. The most important driving force for improved efficiency is the 3D velocity, which can induce axial impact stress, radial tensile stress, and circumferential shear stress on the impinged GHBS. Additionally, the insertion of an impeller with the center hole greatly improves the cavitation erosion performance of SSIJ. This paper illustrates the erosion performance of four kinds of waterjets in breaking GHBS and provides preliminary insights into the potential field applications in NGH exploitation.

Delamination model of an epoxy-impregnated REBCO superconducting pancake winding

Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-field applications. In epoxy-impregnated REBCO superconducting windings, during the cooling process, the delamination induced by thermal mismatch stress significantly threatens stable operation due to the weak c-axial strength of REBCO CC tapes. In this study, a two-dimensional axisymmetric multilayer delamination finite element model with main layers of REBCO CC tapes and insulation materials was developed based on the bilinear cohesive zone model. Based on the proposed model, the stress distribution and delamination properties of an epoxy-impregnated REBCO winding induced during the cooling process were investigated. Furthermore, the effects on the structural configuration on the delamination and optimisation schemes of fabrication are discussed. The model results indicate that during the cooling process of the REBCO winding, when the radial tensile stress induced by the thermal mismatch stress is greater than the delamination strength, interface cracking will occur. Interface failure occurs in regions containing several turns of coils and not just a single-turn coil. Our analyses indicate that structural failure caused by delamination depends on the radius ratio of the outer to the inner winding, where a small radius ratio is preferred to reduce the risk of delamination. An excessive radius ratio can result in multiple delamination failures during the cooling process. The optimisation scheme, which divides the winding into several sub-windings with a consistently small radius ratio, is an effective method for mitigating the risk of delamination failure, which is consistent with available experimental results. Additionally, by reducing the thermal expansion coefficient of the epoxy resin, the risk of delamination failure during cooling can be significantly reduced. For the winding structure considered in this study, if the coefficient of thermal expansion of the epoxy resin is reduced to 5 ( K−1), then delamination failure caused by thermal mismatch will be eliminated. Our model results are consistent with those of several valuable experimental phenomena and numerical calculated in the literature.

Orientation and Residual Stress Distributions in Thick Melt Spun Polystyrene and Polycarbonate Filaments

Abstract A study of orientation development and residual stresses in thick melt spun polystyrene filaments and polycarbonate filaments described. Linear relationships between spinline stress and mean birefringence under different melt spinning conditions were obtained. These results were used together with models of the dynamics of melt spinning to predict the characteristics of thick melt spun filaments. Birefringence distrbutions and residual stresses were computed. High orientations and compressive circumferential residual stresses were predicted at the larger radii and low orientation and tensile circumferential and radial residual stress in the fiber core are obtained. The distribution of birefringence across the thickness of thick melt spun filaments was measured by determining changes of retardation across a wedge. The birefringence distributions were correlated with severity of quench and model prediction. Thick melt spun fibers were peeled and found to have fibrillar cores.

Deep Drawing Process Using a Tractrix Die for Manufacturing Liners for a CNG High-Pressure Vessel (Type II)

The liner of a CNG pressure vessel is manufactured by a DDI (deep drawing and ironing) process for the cylinder part, which is a continuous process that includes a drawing process to reduce the diameter of the billet and a subsequent ironing process to reduce the thickness of the billet. A tractrix die used in the 1st deep drawing allows the blank to flow smoothly by decreasing the punch load and radial tensile stress occurring in the workpiece. It also increases the draw ratio compared to conventional dies, but it causes forming defects. In this study, a shape coefficient (Sc) is proposed for the tractrix die using the blank diameter (D0), inflow diameter of the workpiece (di), and inflow angle of the workpiece (theta) for design of the tractrix die. The effects of the thickness and inflow angle of the workpiece on wrinkling and folding were investigated through FEA. Also, a discriminant is proposed for the relative radial stress (tilde{sigma }) generated during the deep drawing process using the tractirx die and used to predict fracture. Based on the results, the blank thickness, the draw ratio, and the inflow of the workpiece angle in the 1st deep drawing process are suggested, and the number of operations in the DDI process was reduced from 6 to 4. This improves the productivity and reduces the manufacturing cost.