Regions Of Stress Concentration Research Articles

High temperature creep property of a novel porous double layer cooling structure for gas turbine blades

This article investigates the high-temperature creep macroscopic and microscopic fracture mechanism of the porous double layer (PDL) cooling structure through experiments, and predicts the creep life of the structure using the back propagation (BP) neural network model based on the bone point stress method. The high temperature creep fracture of the PDL cooling structure is determined by the macro structure characteristics and the microstructure properties. Impingement holes and effusion holes, as stress concentration region, provide a place for crack initiation. The crack growth rate of effusion plate is greater than that of impingement plate. When the creep crack extends to a certain length, the effusion plate breaks first, and then the impingement plate breaks instantaneously. At the interface between carbide and matrix, microcracks are prone to occur due to stress concentration and inconsistent deformation, thereby promoting the initiation and propagation of macroscopic cracks. The BP neural network model based on the bone point method has a good prediction result for the creep life of the PDL cooling structure, and the maximum prediction error is 9%.

FINITE ELEMENT ANALYSIS AND TOPOLOGY OPTIMIZATION OF A HIGH-PERFORMANCE LOWER PROSTHETIC LIMB

Due to the recent development of additive manufacturing technologies, it is now possible to achieve rapid fabrication of fully functional customized products with high geometric complexity and without the constraints of traditional manufacturing techniques, such as machining. In the current paper, a lower prosthetic limb with the knee joint was designed according to the international standards and the recommendations from existing literature. Finite element analyses were performed to investigate the mechanical behavior of the prosthetic limb and detect the stress concentration regions under realistic operation conditions. For this analysis, Nickel alloy 718 was employed as a construction material. The applied loads were derived for a human body weight of 75kg after an extensive literature review. Then, design optimization was applied, employing design for additive manufacturing (DfAM) techniques in order to minimize the number of assembly parts and improve the shape of the object. Moreover, topology optimization processes were performed in order to develop a lightweight prosthetic limb and achieve a mass reduction of 67% compared to the original design. It is worth mentioning that, this remarkable weight reduction was attained without compromising the structural integrity of the part, as the factor of safety was calculated at 1.22.

Mechanical Behavior of One-Piece and Two-Piece Tapered Prosthetic Abutments with 11.5 and 16 Degrees.

The objective of this study is to examine the mechanical behavior of two-piece abutments (Morse taper with 16° internal angulation and Morse taper with 11.5° internal angulation) before and after cyclic fatigue testing, following ISO 14801:2016 guidelines. The specimens were divided into three groups: a modified Morse taper with a taper angle of 16° (GM group), a conventional Morse taper (taper angle of 11.5° deg) with a two-piece (CMt group), and one-piece abutments (CMo group). Each experimental group was formed by ten implants and ten abutments (n = 10) for a total of 30 specimens (n = 30). The abutments were tightened and loosened, and a fatigue test was applied with 15 Hz and 5 × 106 cycles. Subsequently, the abutments were loosened, and a pull-out test was performed on the CMt group. Finite element analysis (FEA) was conducted on stress concentration regions. The statistical analysis of the loosening test was performed using two-way ANOVA and Tukey's tests (p < 0.05) to compare screw loosening within each group and between the groups with and without mechanical fatigue. Significant differences were found among the three groups in the loosening test when analyzing the values with and without fatigue (p < 0.001) within each group. When the groups were compared with each other, there was also a significant difference between them (p < 0.001), except between groups GM and CMt without fatigue (p = 0.840). In the pull-out test of the CMt group, the sample exhibited frictional locking only after fatigue (mean = 94.2 N). The FEA demonstrated a varied stress distribution in all groups. The stress was found to be more concentrated in the upper third and middle third regions of the implant, as well as in the opposite region of the load application for all three groups. Although the CMo group showed lower rates of loosening, it displayed a poorer stress distribution in comparison to the GM and CMt groups. On the other hand, the CMt group exhibited a satisfactory frictional lock after undergoing the fatigue tests.

Analysis of Strong Stress Concentrations of Key Joints for Super-Spanned Steel Truss Bridges

In this paper, the strategy of three scales was proposed to analyse strong stress distributions of key joints during the incremental launching construction. Firstly, the internal forces and reaction forces were obtained by the finite element method (FEM). Secondly, some key joints were separated from the whole structure. The displacements and boundary conditions of the separated joint were taken from the results of the FEM. Then the stress distributions of the separated joint were successfully obtained by using the FEM. Thirdly, the local stress concentration areas were separated from the key joint structure. The high stress distributions of the local stress concentration region were successfully obtained by using the extended boundary element method (XBEM). On the other hand, the stress of some joints were measured through mechanical test on the construction site. And calculation results of the FEM and XBEM were in good agreement with test results. A multi-scale method is developed to obtain the strong stress concentration during the construction of the super-spanned steel truss bridge accurately, which enables the safety of the construction. In addition, the present method can be further used to guide the design of super-tall building, super-spanned bridge, etc.

Biomechanics of calcaneus impacted by talus: a dynamic finite element analysis

This paper aimed to investigate the biomechanical changes during the talus impact with the calcaneus at varying velocities. Various three-dimensional reconstruction software was utilized to construct a finite element model that consisted of the talus, calcaneus, and ligaments. The explicit dynamics method was used to explore the process of the talus impacting on the calcaneus. The velocity of impact was altered from 5 m/s to 10 m/s with a 1 m/s interval. Stress readings were collected from the posterior, intermediate, and anterior subtalar articular (PSA, ISA, ASA), calcaneocubic articular (CA), Gissane Angle (GA), calcaneal base (BC), medial wall (MW), and lateral wall (LW) of the calcaneus. The changes in the amount and distribution of stress in the different regions of the calcaneus that varied with velocity were analysed. The model was validated through comparison with findings from the existing literature. During the process of impact between the talus and calcaneus, the stress in the PSA reached its peak first. Notably, stress was concentrated mainly in the PSA, ASA, MW, and LW of the calcaneus. At varying impact velocities of the talus, the mean maximum stress of the PSA, LW, CA, BA, and MW exhibited statistically significant differences (P values were 0.024, 0.004, <0.001, <0.001, and 0.001, respectively). However, the mean maximum stress of the ISA, ASA, and GA was not statistically significant (P values were 0.289, 0.213, and 0.087, respectively). In comparison with the velocity at 5 m/s, the mean maximum stress increases in each region of the calcaneus at a velocity of 10 m/s were as follows: PSA 73.81%, ISA 7.11%, ASA 63.57%, GA 89.10%, LW 140.16%, CA 140.58%, BC 137.67%, MW 135.99%. The regions of stress concentration were altered, and the magnitude and sequence of peak stress in the calcaneus also varied according to the velocity of the talus during impact. In conclusion, the velocity of the talus during impact had a significant influence on the magnitude and distribution of stress within the calcaneus, which was a crucial factor in the development of calcaneal fractures. It was possible that the magnitude and sequence of stress peaks played a vital role in determining the emergence of fracture patterns.

Effect of thermal barrier coatings and integrated cooling on the conjugate heat transfer and thermal stress distribution of nickel-based superalloy turbine vanes

Based on the comprehensive design of the thermal barrier coatings (TBCs) and advanced integrated cooling systems for the Ni-based superalloy turbine vanes. This study employs a multidisciplinary method to research the aerothermal performance and elastic thermal stress of the first stage Ni-based superalloy (DS GTD 111) turbine guide vane cooled by an integrated cooling system and TBC thickness of 0.5 mm. The results show that the highest temperature and average temperature on the substrate under TBC decreased by 126 K and 110 K, respectively. For the vanes without and with TBC, the thermal stresses are concentrated on the midchord region of the suction side, the upper edge of film holes outflow, and junction regions between the vane root, tip, and endwalls, which bear high-temperature gradient and high temperature. However, TBC reduces stress value by 50%∼95% in thermal stress concentration regions of the substrate, which significantly reduces the slip systems activated regions on the substrate. In addition, the activation of the dominant slip system (DSS) is a further optimization objective for cooling turbine vanes. This study provides macroscopic and microscopic perspectives for improving integrated cooling structures of actual vanes in the design process by comprehensively considering aerothermal and thermomechanical performance.

Impact of Air Gap Defects on the Electrical and Mechanical Properties of a 320 kV Direct Current Gas Insulated Transmission Line Spacer

Air gap defects inside a spacer reduce its insulation performance, resulting in stress concentration, partial discharge, and even flashover. If such gap defects are located at the interface between the insulation and conductor, a decrease in mechanical stress may occur. In this work, a finite element method-based simulation model is developed to analyze the influence of gap defects on the electrical and mechanical properties of a ±320 kV direct current gas insulated line (DC GIL) spacer. Present findings reveal that a radially distributed air gap produces a more significant effect on the electric field distribution, and an electric field strength 1.7 times greater than that of the maximum surface value is observed at the air gap. The axial distribution dominates the distortion of the surface stress by generating a stress concentration region in which the maximum stress of the air gap is twice the pressure in the surrounding area.

Microleverage Mechanism Assisted Novel Biaxial Piezoresistive Micro Accelerometer With Improved Sensitivity

A novel structure was presented for a microelectromechanical system (MEMS) piezoresistive accelerometer. Taking advantage of the effects of tiny piezoresistive beam (TPB) to generate stress concentration region and microleverage mechanism to amplify the force, a piezoresistive accelerometer was proposed with improved sensitivity. The measured sensitivity was determined by the amplified inertia force applied to the TPBs. Based on the model analyses, the force amplification factor was influenced by the beams’ spring constants and the distance between support beam and TPB. The simulations were further carried out to verify the effects of amplifying force and find out the effects of dimensional parameters on amplification factor. The results suggested that the amplification factor was influenced by support and hinge beam widths dramatically, and also affected by the distance between support beam and TPB. Based on the simulation findings, three prototypes (denoted as XJTU_11, XJTU_12 and XJTU_32) were proposed with different support and hinge beam widths along with the distance between support beam and TPB to verify the amplification effect. Furthermore, the prototypes were successfully fabricated by MEMS technology with a size of 4.4 mm × 4.4 mm and packaged for experimental tests. The sensitivities were determined to be 1.03 mV/g/5V, 0.871 mV/g/5V and 0.736 mV/g/5V for XJTU_11, XJTU_12 and XJTU_32, respectively. The sensitivity of XJTU_11 increased by 18.4% and 40.1% compared with XJTU_12 and XJTU_32 respectively. The results indicated that the higher amplification factor was beneficial to the device to obtain the higher sensitivity. The figure of merit showed that proposed accelerometer obtained a more favorable comprehensive performance. This new design provided a good strategy and revealed the promising potential in improving the accelerometer’s sensitivity.

Development of biodegradable customized tibial scaffold with advanced architected materials utilizing additive manufacturing

In the last decade, the development of customized biodegradable scaffolds and implants has attracted increased scientific interest due to the fact that additive manufacturing technologies allow for the rapid production of implants with high geometric complexity constructed via commercial biodegradable polymers. In this study, innovative designs of tibial scaffold in form of bone-brick configuration were developed to fill the bone gap utilizing advanced architected materials and bio-inspired diffusion canals. The architected materials and canals provide high porosity, as well as a high surface area to volume ratio in the scaffold facilitating that way in the tissue regeneration process and in withstanding the applied external loads. The cellular structures applied in this work were the Schwarz Diamond (SD) and a hybrid SD&FCC hybrid cellular material, which is a completely new architected material that derived from the combination of SD and Face Centered Cubic (FCC) structures. These designs were additively manufactured utilizing two biodegradable materials namely Polylactic acid (PLA) and Polycaprolactone (PCL), using the Fused Filament Fabrication (FFF) technique, in order to avoid the surgery, for the scaffold's removal after the bone regeneration. Furthermore, the additively manufactured scaffolds were examined in terms of compatibility and assembly with the bone's physical model, as well as, in terms of mechanical behavior under realistic static loads. In addition, non-linear finite element models (FEMs) were developed based on the experimental data to accurately simulate the mechanical response of the examined scaffolds. The Finite Element Analysis (FEA) results were compared with the experimental response and afterwards the stress concentration regions were observed and identified. Τhe proposed design of scaffold with SD&FCC lattice structure made of PLA material with a relative density of 20% revealed the best overall performance, showing that it is the most suitable candidate for further investigation (in-vivo test, clinical trials, etc.) and commercialization.

Failure analysis of the edge-notched beam test on fluid-exposed Berea Sandstone

Injection of fluids during wastewater disposal and geologic carbon sequestration causes induced stress and changes in rock’s elastic and failure properties, such as elastic moduli and fracture toughness. An accurate understanding of such changes in the response requires modeling and analysis of fluid-induced changes in rock’s stress and deformation states for which core-scale mechanical loading tests are often employed. Using experiments and simulations of Single Edge Notched Beam (SENB) test on water-exposed and supercritical (sc) CO_2-exposed Berea sandstones, we quantify the effect of pore fluid on tensile crack propagation, spatial distributions of shear and tensile stresses, and fracture toughness and J-integral properties. We use Digital Image Correlation (DIC) to monitor the changes in local stress and deformation fields during crack initiation and propagation. A representative numerical simulation model of dynamic crack propagation under SENB loading is built using the extended finite element method. Results are analyzed using the stress path analysis to identify shear and tensile stress concentration regions, which provide precursory information for failure events during the test. Stress profiles along and across the growing crack are analyzed and compared with Irwin’s classical solutions to understand the impact of SENB loading. Evolution in stress intensity factor and crack length shows subcritical crack growth prior to Griffith-like failure at the structure scale.

Post-repair residual stresses and microstructural defects in wind turbine blades: Computational modelling

Wind turbine blades require regular maintenance and repair. The post-repair reliability and lifetime depend on the quality and method of repair. Residual stresses present in the repaired sections of the blades are one of the reasons for reduced lifetime and premature failure of repaired blades. Residual stresses occur due to the thermal cycle used to cure the composite repair patch or adhesive and chemical shrinkage during curing. The presence of voids further aggravates the magnitude of the residual stresses. To investigate these effects, analytical models are used to study the cure kinetics and void evolution while the development of residual stresses during the post-gelation period of curing and the effect of the voids are obtained using coupled temperature-displacement analysis in ABAQUS. User subroutines (UEXPAN, HETVAL and UMAT) are used to define the chemical shrinkage, heat generation due to curing and cure-dependent modulus. Both hard and soft patch repair models are considered. It is found that the temperature cycle used affects both the time taken for complete cure and the residual stresses developed in the adhesive. Increasing the first dwell temperature decreases the time to cure but increases the residual stresses. Also, the presence of voids is found to cause stress concentration regions, with stresses increasing up to 2 to 2.5 times the average stresses. Therefore, a balance is needed to minimise the time taken for the repair process while maintaining residual stresses as low as possible, especially in wind turbine repair applications.

Numerical simulation of fracture mechanisms due to directional shrinkage in natural composites

The use of natural composites in modern engineering applications has numerous benefits owing to their excellent sustainable attributes and negligible environmental impacts. The full commercialization of natural composites as alternative materials to conventional ones is undermined by environmental factors such as moisture content leading to varied shrinkage and swelling behaviors. Uncontrolled shrinkage behavior can often lead to premature split in natural composites. The aim of this study is to investigate the fracture mechanisms due to directional shrinkage in natural composite material through numerical simulation by considering the finite element modelling software of LS-DYNA. From numerical results, directional shrinkage could be seen to clearly affect the fracture patterns in natural composites as compared to the control one. The fracture patterns, which indicated a narrowed region of stress concentration in the material, is assumed to emanate from internally induced forces in the material, namely tensile and compressive forces. This specific pattern is associated to longitudinal split failure typically observed in orthotropic materials. Results of this study can be useful to further investigate the complex fracture behavior in natural composite materials to better understand their durability limitations.

Nonlinear vibrations of earth structures

This article offers a detailed analysis of the current state of plane structure dynamics, addressing the complexities posed by non-homogeneous materials exhibiting nonlinear elastic and viscoelastic properties during real-life operations. The study proposes a comprehensive mathematical model and algorithm to investigate the dynamic behavior of such structures, employing the non-linear hereditary Boltzmann-Volterra theory to describe viscoelastic material properties accurately. Nonlinear oscillatory systems are analyzed using Lagrange equations based on the d’Alembert principle. The problem is approached through a multi-step process. Initially, the linear elastic problem of the structure’s natural oscillations is solved to determine its natural frequencies and modes of oscillations. Subsequently, these eigenmodes are employed as coordinate functions to address forced nonlinear oscillations in viscoelastic non-homogeneous systems. The complexity of the problem necessitates solving a Cauchy problem comprising a system of nonlinear integrodifferential equations. To illustrate the methodology, the study examines the Gissarak earth dam, considering real operational conditions and nonlinear, viscoelastic material properties near resonant modes of vibrations. Utilizing numerical methods, the dynamic behavior of the structure is analyzed, assessing displacements and stress components at different time points under non-stationary kinematic action. Stress concentration regions within the structure are identified for resonant vibrations, allowing the evaluation of its strength. Furthermore, the impact of nonlinear elasticity and viscoelasticity on the structural dynamics is quantified. This research provides valuable insights into the behavior of plane non-homogeneous structures, considering real-world scenarios and material complexities, ultimately contributing to an improved understanding of structural dynamics and facilitating the identification and mitigation of potential structural challenges.

Influence of ground shaking on the fault: numerical simulation

The paper considers a possible model of the behavior of faults under the influence of external influences. A complication in the form of a step is assumed in the structure of the fault banks. A hook on a step leads to the appearance of two local stress concentration regions. A possible mechanism for slowing down the response to external stimuli is discussed. An external influence can be a shaking of the ground from a neighboring earthquake or a deformation tidal wave. The calculation is carried out within the framework of the elastic-brittle medium model by the finite-difference method supplemented with the ability to calculate cracks with the definition of their faces as free surfaces. As a result of the simulation, the emergence of a region of stress concentration is observed, which may not immediately lead to destruction. There will be an accumulation of damage as a result of a hook on the fault step, and the reaction to external influence will slow down. Over time, the medium exhausts the possibilities for implementing all the mechanisms of accommodation to the load, which leads to a break in the vicinity of the roughness with the emission of the next portion of seismic waves.

A mixed XFEM and CZM approach for predicting progressive failure in advanced SiC/SiC CMC component

This research work focused on simulating different fracture modes in the high-stress concentrated region of an advanced SiC/SiC CMC component at the macroscopic scale in consideration of increasing cost and time during experimental tests of CMCs. A hybrid of extended-finite-element-method (XFEM) and cohesive-zone-modeling (CZM) was utilized to predict mode-I and mode-II fracture behavior, respectively. The cohesive element (CE) layers were used as fiber-tow interfaces in the stress-concentrated region where mode-II failure was supposed to occur. The 1st crack initiation and its further propagation were found to be dependent on XFEM damage criteria, whereas delamination was observed at the damage-front CE layers. The simulated force-displacement relationships were compared with the experimental ones, where stiffness and serrations in the force-displacement curve corresponding to the delamination matched quite well. With concern to predicting the fracture behavior in a complex-shape component of SiC/SiC CMCs, XFEM with CZM methodology can be a reliable method in the near future.

Ductile Tearing or Plastic Collapse?

Abstract Ductile fracture is characterized by a large amount of deformation. Two mechanisms may lead to ductile failure: local tearing may appear in front of a stress concentration region like a crack front or large geometry changes in a weak part of the structure can induce global collapse of the structure. For large cracks such as those considered in Leak Before Break studies, tearing analyses and net-section collapse criteria have been used. The present paper examines the link between these two mechanisms for a through-wall cracked pipe under bending and concludes that the transition from ductile tearing to collapse depends not only on material properties but also on geometrical parameters. The reference stress approach is a promising approach for predicting the transition, provided the transferability issue of j-resistance curve has been solved.

Dynamic Response and Seismic Vulnerabilities of the Historic Tophane-i Amire: Dynamic and Seismic Analysis of a Historic Masonry Building

This study deals with the dynamic characterization of the historic Tophane-i Amire building, located in Istanbul. The domed structure was constructed in the 18th century and is known as the first industrial building in the Ottoman times. It was an arsenal building used for the production of cannons and cannon balls. An ambient vibration survey was conducted with the Enhanced Frequency Domain Decomposition method to identify the dynamic characteristics of the structure. Later, a finite element model of the structure was constructed and numerical dynamic identification was pursued. The obtained experimental and numerical results have been compared and the finite element model of the structure has been tuned to obtain a validated tool to represent the real dynamic behavior of the structure. While the tuned model of the structure is intended to contribute to understanding its structural behavior and evaluate the mechanical properties of masonry, it has also been used for the evaluation of its seismic vulnerabilities. For this purpose, the structure has been analyzed under maximum considered earthquake response spectrum, and stress concentration regions have been determined. The results also serve as data for future structural health monitoring studies related to the structure.

Passive Tomographic Study on Velocity Changes in Underground Mines Using Tabular Mesh Grids

Abstract Double difference tomographic inversion on measurements of travelling time and location are performed to analyze the velocity structure within rock mass in underground mining. Residuals of each iteration are estimated to evaluate the conversion of computation. Average wave propagation velocities in tabular areas are assessed to compare the velocity change affected by mainshocks. It is summarized that velocity increases before mainshocks and then reduces with temporal evolution after them. Possible explanations include static stress buildup that enhances the wave propagation before mainshocks and static stress reduction that weakens the waveform propagation. Additionally, wave propagation is attenuated by the dynamic-shaking induced fractures and ruptures within rock masses. Velocity change is shown to be of importance in assessing the stress redistribution and stability of rock masses. It is found that P-wave velocity increased by 1% ~5% before the occurrence of mainshocks. After the mainshocks, the velocity turned into decreasing and eventually dropped to a level that was even lower than the average level before the mainshocks. It can be inferred that stress increased and formed a stress concentration region before the mainshocks. The occurrence of mainshocks caused damage and stress relaxation in the rock mass, leading to a significant velocity decrease.

Ratcheting behavior of non-equiatomic TRIP dual-phase high entropy alloy

Ratcheting behavior of metastable dual-phase (FCC+HCP) Fe50Mn30Co10Cr10 high entropy alloy has been investigated for various combinations of mean stress(σm) and stress amplitude (σa) at ambient temperature. The experimental results show that the accumulation of ratcheting strain and fatigue life primarily depend on the applied mean stress and stress amplitude combination. Detailed microstructure analyses using electron back scattered diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi Diffraction (TKD) revealed the activation of multiple deformation mechanisms, including planar slip, deformation-induced twinning (TWIP), and deformation-induced martensitic phase transformation (TRIP) in both the phases. The synergistic operation of these mechanisms with reverse HCP to FCC transformation at local stress concentration regions in the microstructure promoted cyclic hardening leading to an improved low cycle fatigue behavior of the TRIP DP-HEA under asymmetrical stress-controlled cyclic deformation. In a nutshell, the operation of multiple factors responsible for twinning is unique, providing means to design novel multicomponent alloys.

Degradation mechanism for mechanical property of carbon fiber reinforced epoxy composite under simulated seawater absorption coupled with flexural load

Property evolution of carbon fiber reinforced epoxy composite used in marine environment was investigated in this paper. The moisture absorption and flexural properties of carbon fiber composite laminate were studied under the combined pretreatment condition of simulated seawater and three-point flexural load. An in-situ observation system was used during mechanical test to analyze damage initialization and development on dry and wet composites. Theoretical analysis considering hydro-mechanical coupling was conducted, based on experimental data, moisture simulation and stress analysis. Stress distribution under flexural load was discussed using finite element method, and its effects on moisture diffusion and failure process were analyzed based on the theoretical and experimental results. It demonstrates that moisture absorption of composites is inhibited as flexural load is applied during moisture absorption. It is attributed to compressive stress concentration of composite on the loading region. The retention of flexural property shows slight increase under moisture coupled with flexural load, which is resulted from decreased moisture content. Failure process of composite during three-point flexural test after immersion is different from that of dry composite, especially for the position and number of crack formation and propagation. Weakened interlaminar interface and more large cracks are found after immersion coupled with flexural load, which are explained based on water degradation and initial damage at stress concentration region. It denotes that more attentions should be paid on failure process of wet composite structure in marine field.