Reduction Of Tensile Stress Research Articles

Study on the erosion characteristics of boride coatings by finite element analysis

In this paper, tensile stress distribution in the surface of high-parameter steam turbine blade anti-erosion boride coatings under single particle impact was investigated using a nonlinear finite element software ABAQUS/explicit based on the actual erosion environment in power units. The results showed that the single Fe2B phase coating hardly had stable anti-erosion capability when the coating thickness was less than 20μm under the continuous impacts. For the bilayer boride coating composed of top phase FeB and bottom phase Fe2B, the whole coating began to show the stable anti-erosion performance with top FeB phase coating thickness of 120μm from the standpoint of maximum surface tensile stress reduction. Anti-erosion performance of single Fe2B phase boride coating was 23.15% higher than the bilayer boride coating. Besides, the optimum parameters configurations for boride coatings were studied.

Strut-and-tie model for externally bonded CFRP-strengthened reinforced concrete deep beams based on particle swarm optimization algorithm: CFRP debonding and rupture

A substantial number of studies have been done on the shear strengthening of reinforced concrete (RC) beams with externally bonded carbon fibre reinforced polymers (CFRP). However, in reference to shear, there are still many questions concerning the complexity and nature of the failure mechanism of RC structures strengthened in shear using CFRP. This is particularly true for concrete deep beams because of the nonlinearity of the stress distribution. This study had the goal of developing a simple procedure or model for estimating the shear capacity of RC deep beams strengthened with CFRP sheets. The proposed model was designed based on an extension of the strut-and-tie model (STM) used for un-strengthened RC deep beams to the case of those shear strengthened with CFRP sheets. The technique avoided the traditional trial-and-error procedure for obtaining the unknown coefficients of the proposed model. It utilized a particle swarm optimization algorithm (PSO), in which the optimal STM of an CFRP-strengthened RC beam was determined by searching for the optimum unknown coefficients (stress distribution and concrete tensile stress reduction factors). This model considered the effects of two CFRP failure modes, namely the CFRP debonding and CFRP tensile rupture failure modes. The proposed model was validated using experimental data collected from the current study and existing literature. The hybrid PSO-STM predicted a mean value=1.1, SD=0.098, and coefficient of variation (CoV)=8.9%. These results showed that the proposed model has high accuracy and consistency and it can accurately estimate the ultimate shear strength of CFRP-strengthened RC deep beams.

Development of a Circular Thermoelectric Skutterudite Couple Using Compression Technology

Approximately, 55% of the energy produced from conventional vehicle resources is lost due to heat losses. An efficient waste heat recovery process will lead to improved fuel efficiency and greenhouse gas emissions. Thermoelectric generators (TEGs) are heat recovery devices that are being widely studied by a range of energy-intensive industries. Efficient solid-state thermoelectric devices are good candidates to reduce fuel consumption in an automobile. Thermoelectric materials have had limited automotive applications due to the automotive waste heat recovery temperature range, the rarity and toxicity of some materials, and the limited ability to mass manufacture thermoelectric devices from expensive TE materials. However, skutterudite is one class of material that has demonstrated significant promise in the transportation waste heat recovery temperature domain. Durability and reliability of the TEGs are the most significant concerns in the product development process. Cracking of the materials at hot-side interface is found to be a major failure mechanism of TEGs under thermal loading. Cracking affects not only the structural integrity but also the energy conversion and overall performance of the system. In this paper, cracking of thermoelectric material as observed in performance testing is analyzed using numerical simulations and analytic experiments. This paper shows, with the help of finite element analysis (FEA), the detailed distribution of stress, strain, and temperature is obtained for each design. Finite element (FE)-based simulations show the tensile stresses as the primary factor causing radial and circumferential cracks in the skutterudite. For a TEG design, loading conditions and closed-form analytical solutions of stress/strain distributions are derived. Scenarios with minimum tensile stresses are sought. These approaches yield the minimum of stress/strain fields which produce cracks. Finally, based on these analyses and computational fluid dynamics (CFD) studies, strategies in tensile stress reduction and failure prevention are proposed.

Two-step Method of Designing Distribution of Material Stiffness in Perpetual Pavement to Prevent Structural Rutting and Fatigue

A two-step method for stiffness distribution design with application to perpetual pavements is proposed. In the first step resultant stiffness of asphalt concrete layers is estimated using nomograms obtained via solution of three-dimensional problem of a layered half-space. As controlling parameter vertical strain in the soil base is used. In the second step simplified model of composite beam is applied to predict distribution of stiffness within the asphalt layers. In order to derive design formulas, criterion of equality of stiffness of composite beam's cross-section and stiffness of a homogenous reference layer being a part of layered system, and criterion of horizontal tensile stress reduction at critical locations are used. Obtained formulas require input of thicknesses of individual layers and some material properties, including strain endurance limits.

Microstructure and mechanical properties of SiO2-BN ceramic and Invar alloy joints brazed with Ag–Cu–Ti + TiH2 + BN composite filler

Ag–Cu–Ti + TiH2+BN composite filler was prepared to braze SiO2-BN ceramic and Invar alloy. The interfacial microstructure, mechanical properties, and residual stress distribution of the brazed joints were investigated. The results show that a wave-like Fe2Ti–Ni3Ti structure appears in the Invar substrate and a thin TiN–TiB2 reaction layer forms adjacent to the SiO2-BN ceramic. The added BN particles react with Ti to form TiN–TiB fine-particles, which is beneficial to refine the microstructure of the brazing seam and to greatly inhibit the brittle compounds formation. The interfacial microstructure at various brazing temperatures was analyzed, and the mechanism for the interfacial reactions responsible for the bonding was proposed. The maximum shear strength of the joints brazed with the composite filler at 880 °C for 10 min is 39 MPa, which is 30% greater than that brazed with Ag–Cu–Ti alloy. The improvement of the joint strength is attributed to the variation of joint microstructure and the reduction of tensile stresses induced in the SiO2-BN ceramic. The finite element analysis indicates that the peak tensile stress decreases from 230 to 142 MPa due to the addition of BN particles in the ceramic.

Linear and nonlinear characterization of low-stress high-confinement silicon-rich nitride waveguides.

In this paper we introduce a low-stress silicon enriched nitride platform that has potential for nonlinear and highly integrated optics. The manufacturing process of this platform is CMOS compatible and the increased silicon content allows tensile stress reduction and crack free layer growth of 700 nm. Additional benefits of the silicon enriched nitride is a measured nonlinear Kerr coefficient n(2) of 1.4·10(-18) m(2)/W (5 times higher than stoichiometric silicon nitride) and a refractive index of 2.1 at 1550 nm that enables high optical field confinement allowing high intensity nonlinear optics and light guidance even with small bending radii. We analyze the waveguide loss (∼1 dB/cm) in a spectrally resolved fashion and include scattering loss simulations based on waveguide surface roughness measurements. Detailed simulations show the possibility for fine dispersion and nonlinear engineering. In nonlinear experiments we present continuous-wave wavelength conversion and demonstrate that the material does not show nonlinear absorption effects. Finally, we demonstrate microfabrication of resonators with high Q-factors (∼10(5)).

Method of X-ray residual stress measurement for phase transformed welds

In this study, we attempted to evaluate the residual stress distribution in welds accompanied by phase transformation using X-ray stress measurement. First, relationship between the phase transformation behavior and the residual stress of welded specimens with different phase transformation was discussed. In the specimen with martensite in the weld, reduction in tensile stress due to martensite transformation shown in residual stress distribution followed conventional behavior. However, in the specimen with bainitic ferrite in the weld, the residual stress in the transverse direction was almost the same as the residual stress in the longitudinal direction, and the residual stress did not follow conventional behavior. Next, X-ray elastic constant in the weld was measured, and then the residual stress was reevaluated. In the specimen with bainitic ferrite in the weld, X-ray elastic constant had anisotropy, and the reevaluated residual stress followed conventional behavior. In conclusion, it was shown that depending on the phase transformation behavior, it is difficult to use X-ray elastic constant estimated by Kröner model; however, if we use the measured value, we are able to evaluate the residual stress more accurately.

A study of plastic deformation behaviour of Ti alloy during equal channel angular pressing with partial back pressure

Equal channel angular pressing with partial back pressure (ECAP PBP) accumulates large strains in a processed sample, while extruding it through a die with a modified geometry. In order to investigate the deformation behaviour of Ti6Al4V in detail, an ECAP PBP three-dimensional finite element model has been developed using FORGE software. The analysis included the influence of geometry and heterogeneity of plastic deformation. Numerical results showed that partial back pressure, as well as the other modelled factors, significantly affects size and homogeneity of the imposed strain, temperature and stability of material plastic flow. Average value of imposed strain was ∼1.58. Strain distribution was the most homogenous for the higher friction coefficient; its homogeneity was confirmed by micro-hardness testing. The results show the lowest temperature increase of the billet during ECAP PBP process. Using the partial back pressure, dead zone was eliminated. It also featured the highest reduction of tensile stress when compared to other processes. During the process with partial back pressure, the punch experienced 20% lower load compared to conventional ECAP with back pressure.

Analysis of the reduction of tensile stress by post‐growth annealing methods in multicrystalline silicon wafers produced by the RST process

AbstractIn this work, we analyze the residual stress, in both as‐grown and post‐growth annealed silicon ribbon on a sacrificial carbon template, by the simulation of the thermo‐mechanical behavior of the cooling ribbon using the Abaqus software, and the measurement of stress on the free silicon surface by µ‐Raman spectroscopy. The results of the simulations gave limits within which flat ribbons could be obtained for a combination of silicon thickness, ribbon width and pull rate. Stress relaxation by off‐line thermal treatments is used to optimize a post‐growth annealing furnace in line with the ribbon growth system. The fragile behavior of the ribbon during laser cutting can be considerably reduced by a short relaxation of the ribbon at medium temperatures around 850 °C, which were observed to give the best behavior in terms of residual tensile stress relaxation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental Study on Cracking, Reinforcement, and Overall Stability of the Xiaowan Super-High Arch Dam

The Xiaowan super-high arch dam has faced challenging construction problems. Here, we provide a scientifically-based reference for applying geomechanical model testing to support the nonlinear design of super-high arch dams. We applied experimental similarity theory and techniques. Based on four 3D geomechanical model tests, the dam stress characteristics, deformation distribution, and the safety factors of the dam foundation were identified and compared. We also analyzed cracking characteristics of the up- and downstream dam surfaces and induced joints in the dam heel, the rock mass failure process of the dam-foundation interface, and the abutments. We propose foundation reinforcement measures for weak rock masses, alteration zones, and other faults in the abutments based on the 3D and plane tests each at a different elevation. The results show that all dam deformations remained normal with no yielding or tensile cracking under a normal water load. The reinforced rock mass increased the crack initial safety in the dam heel and toe by ~20 %. The minimum crack initial safety factor (K 1) of the dam heel was 1.4. The induced joint in the dam heel contributed to a reduction in tensile stress at the upstream dam heel, improving K 1. Compared with similar projects following reinforcement measures, the abutment stiffness and overall stability of the Xiaowan arch dam satisfy operational requirements. Four years of monitoring operations show that key areas near the dam remained normal and the dam foundation is functioning well. Our results may also be applicable to the design and construction of similar projects worldwide.

Fracture of mobile unicompartmental knee bearings: A parametric finite element study

Cases of fractured mobile unicompartmental knee bearings have recently been reported. The purpose of this study was to understand the mechanics behind these fractures and to examine the influence of different design modifications. A parametric finite element model was used to examine the influence of different geometrical factors on the stresses within the bearing. Crack initiation occurred clinically in the centre of the bearing; this correlated with the position of the maximum von Mises stress. Tensile stresses, thought to propagate the fatigue crack, were maximal at the medial-lateral sides of the bearing, and the tensile vectors were normal to the fracture direction observed clinically. Fully congruent femoral articulation on the bearing, use of a thicker bearing size, and minimising wear of the component reduced the risk of fracture. For example, an unworn 6.5-mm-thick bearing (no clinical fractures reported) had 21.6% lower medial-lateral tensile stress compared to an unworn 3.5 mm bearing (five clinical fractures reported). In turn, an unworn 3.5 mm bearing had 34.3% lower tensile stress compared to a 3.5 mm bearing after 1.9 mm wear (average linear wear reported for clinically fractured bearings). The fracture risk was also reduced when the radio-opaque marker wire was positioned further from the centre of the bearing, and when marker balls were used instead of marker wires (19% reduction in tensile stress in some regions). These results indicate the importance of minimising component wear; the data also support the current component design which uses posterior marker balls instead of marker wires, and the continuing use of a congruous femoral component.

Nucleation rate reduction through stress relief of thermally annealed hydrogenated amorphous silicon films

The effect of film stress on crystallite nucleation is investigated in 0.11 μm thick, thermally annealed hydrogenated amorphous silicon films. Using a recently developed optical method, the crystallite density is measured as the films are isochronally annealed at 600 °C, which enables the determination of the crystallite nucleation rate. This rate is significantly suppressed around scratches, cleaved film edges, and laser ablated areas, extending laterally as much as 100–150 μm from these regions where the film connectivity is disrupted. μ-Raman measurements of the transverse optical mode of Si demonstrate an accompanying reduction in tensile stress in the regions where nucleation is suppressed. The first measurements of nucleation rate in stress and in stress relieved areas in the same film are presented.

Tailoring the perpendicular exchange bias in [Pt/Co/CoO]n multilayer by tensile stress on curved substrate

The effects of the tensile stress on exchange bias of [Pt/Co/CoO]n multilayer are investigated by depositing the magnetic film onto the ordered curved substrate composed of a polystyrene nanosphere monolayer film. The square ratio of the loop decreases first and increases then with the reduction of tensile stress when the nanosphere size increases. The square ratio and exchange bias field are enhanced significantly when 60 nm polystyrene nanosphere arrays are chosen as the substrate, which is ascribed to the increased interfacial uncompensated antiferromagnetic spins created by the periodical knots between neighbor spheres and the formation of the entire domain wall. In addition, the easy axis of [Pt/Co/CoO]n multilayer is tailored when the CoO sublayer thickness changes, which is due to the gradual development of a tilted anisotropy. When CoO is 1.9 and 2.6 nm, the exchange coupling with tilted magnetic easy axis is obtained in the 45° direction.

Shape optimization of concrete buried arches

Shape optimization in connection with numerical modelling is used to reduce bending and associated flexural stresses in buried concrete arches. Modelling of the arch is carried out via a nonlinear finite element model that accounts for soil constitutive relations, soil–structure interactions, sequential construction stages and soil compaction. Centre line of the arch is parameterized by Bézier curve with three degrees of freedom that are subjected to optimization by genetic and Levenberg–Marquardt algorithm.The paper presents a parametric study which aims to determine the optimal shapes for buried arches of various span/rise ratios, backfill depths and foundation soil types. In the second part of the paper it shows a theoretical reduction in tensile stresses obtained by shape optimization of concrete arch culvert with a 9.4m span tested at the University of Massachusetts at Amherst.

Enhancement of fatigue life of net section in fitted bolt connections

The basic idea of this paper underscores enhancement of fatigue life of the net section in bolted connections by means of the developed method whereby beneficial residual hoop compressive stresses, distributed almost uniformly along the hole axis, are created around the bolt hole. Since the method includes bolt hole cutting up to a precise size after preliminarily cold hole expansion, it is especially appropriate for fitted bolt connections. In the case of conventional cold hole expansion the residual compressive hoop stresses are characterized by significant axial gradient and a tensile field sometimes arises on the hole entrance face. The proposed method homogenizes the compressive field around the bolt hole in an axial direction by means of residual hoop stress redistribution. These stresses significantly reduce the operating tensile stresses at the net section points. Due to the tensile operating load, the resultant hoop normal stresses (superposition from residual compressive and operating tensile stresses) at the net section points are significantly smaller in comparison with the conventional case. The developed method has been studied both experimentally, by X-ray diffraction technique, and numerically, by finite element (FE) simulations. Four FE models have been developed for investigation and optimization of the proposed approach. Application of this approach enhances the fatigue life of the net section in bolted connections due to operating tensile stress reduction.

Effect of Graded AlxGa1-xN Layers on the Properties of GaN Grown on Patterned Si Substrates

2.2-µm-thick crack-free GaN films were grown on patterned Si substrates. The crack-free GaN films were obtained by patterning Si substrate and optimizing the graded AlxGa1-xN layers. With the increase of the graded AlxGa1-xN layer thickness, the GaN crystal quality improved as judged from the X-ray diffraction data. By applying multi-AlxGa1-xN layers on the patterned Si substrate, a 31% reduction of tensile stress for the GaN film was obtained as measured by micro-Raman. For the AlGaN/GaN high electron mobility transistor grown on 1×1 cm2 larger patterns, the device exhibits maximum drain current density of 776 mA/mm and maximum transconductance of 101 mS/mm.

Effect of Graded AlxGa1-xN Layers on the Properties of GaN Grown on Patterned Si Substrates

2.2-µm-thick crack-free GaN films were grown on patterned Si substrates. The crack-free GaN films were obtained by patterning Si substrate and optimizing the graded AlxGa1-xN layers. With the increase of the graded AlxGa1-xN layer thickness, the GaN crystal quality improved as judged from the X-ray diffraction data. By applying multi-AlxGa1-xN layers on the patterned Si substrate, a 31% reduction of tensile stress for the GaN film was obtained as measured by micro-Raman. For the AlGaN/GaN high electron mobility transistor grown on 1×1 cm2 larger patterns, the device exhibits maximum drain current density of 776 mA/mm and maximum transconductance of 101 mS/mm.

Resonant characteristics of ultranarrow SiCN nanomechanical resonators

We report the machining of doubly-clamped SiCN nanomechanical resonators as narrow as 16 nm and lengths of up to 10 μm with a yield approaching 100%. The resonators were actuated using a piezoelectric disk, and their resonant response was detected using optical interferometry. Resonators with widths ranging from 16 to 375 nm and lengths from 10 to 50 μm were analyzed at room temperature at pressures ranging from 10 to 50 mTorr. Resonant frequencies in the 4–15 MHz range and quality factors in the 1000–7000 range were measured. We observed a significant decrease in resonant frequency with decreasing resonator width. The results of finite element analysis (FEA) show that this width dependence is mainly due to the resonators vibrating in the horizontal rather than vertical direction. At widths below 50 nm the comparison of experimental and FEA data suggest a gradual tensile stress reduction in the resonators as their width is reduced. Material softening is the most likely cause of this stress reduction. Additionally, the resonant behavior of 16, 55, and 375 nm wide devices was studied as a function of ambient pressure in the 10−5–10 Torr range. Resonance quality becomes dominated by gas damping effects at pressures above a threshold determined by the intrinsic Q-factor of the resonator. The intrinsic Q-factor tended to decrease with decreasing resonator width but was independent of length or resonant frequency. This suggests that surface-related mechanisms dominate the dissipation of energy in these devices.

Nonlinear Finite-Element-Based Investigation of the Effect of Bedding Thickness on Buried Concrete Pipe

The pipe-soil interaction is studied by using the finite-element software ABAQUS/CAE Version 6.5-1 as a symmetric model of embankment installation to study the effect of bedding property and thickness on pipe-soil interaction with increase in the height of fill. A three-dimensional (3D) finite-element method (FEM) model of the concrete pipe and surrounding soil is developed. The FEM model is capable of simulating material, geometric, and contact nonlinearities which employs a nonlinear incremental solution algorithm. Several different element types and mesh sizes were tested to obtain the optimum converged mesh. These elements include eight-noded linear brick (C3D8R) and six-noded linear triangular prism (C3D6) for modeling of the concrete pipe and surrounding soil. The behavior of the 3D model is investigated by varying the pipe diameter, backfill height, bedding thickness, and bedding material. Three material constitutive relationships of soil involving in the model are gravelly sand and sandy silt. To study the effect of bedding thickness on the pipe wall, due to the increment of backfill soil depth, contact elements were employed in the interface between each two regions. The lateral boundaries and model length were also studied for the converged solution. A parametric study was conducted to study the effects of bedding thicknesses. The results showed that the increase in bedding thickness reduces tensile stress at crown, spring line, and especially invert of the pipe wall depending on the material property. This means the change in material property and compaction level has a greater effect on the reduction of tensile stresses than the effect from the variation of bedding thicknesses. Also, materials with lower bedding stiffness (high deformability characteristics) cause greater reduction in induced stresses. This study shows that for commonly used bedding material (coarse grained gravelly sand), the effect of the increase in bedding thickness has a minimal effect on stress reduction of the pipes studied.

Impact stress absorption and load spreading in multi-layered erosion-resistant coatings

The development of high performance erosion-resistant coatings requires understanding of stress propagation upon particle impact. In this work, we apply finite element methodology to enhance and optimize the resistance of protective coatings to erosion by solid particles with appropriate stress management. Controlled distribution of the initial residual stress in the coating was used to counteract impact stress, while Young's modulus distribution was applied to optimize impact energy spreading throughout the coating system. Considering both tensile stress reduction and energy absorption, a multi-layer configuration with specific Young's modulus and residual stress distributions along the coating depth is suggested as an optimal coating architecture. Guidelines and design principles for erosion-resistant coating systems are discussed.