Non-uniform Stress Distribution Research Articles

Effect of Substrate Surface Roughness on Microstructure and Mechanical Properties of Cold-Sprayed Ti6Al4V Coatings on Ti6Al4V Substrates

Surface condition, especially surface roughness of substrates, critically influences the adhesion of cold-sprayed titanium alloy coatings. To study this, Ti6Al4V (Ti64) coatings were deposited on Ti64 substrates with increasing surface roughness (Ra) from 0.05 µm (polished surface) to 5.4 µm (water-jet cut surface). It was found that the substrate surface roughness did not significantly affect the porosity, hardness and coating surface roughness because these properties were dependent on the deposition parameters such as propellant gas pressure and temperature and nozzle traverse speed. The adhesion test results showed that smoother substrate surfaces improved the coating bond strength of the cold-sprayed Ti64 coatings from about 7.1 MPa (Ra: 5.4 µm, interface failure) to 68.7 MPa (Ra: 0.05 µm, glue failure). The fracture characteristics of the debonded coating/substrate interfaces revealed that there were more adiabatic shear-induced craters observed on the smoother substrate surfaces. Finite element modeling also showed that the substrate surface features (i.e., peaks and valleys) possibly prevented the intimate contact between the particles and substrate and thus induced the non-uniform distributions of temperature, stress and strain at the particle/substrate interface.

Piezo-phototronic effect in InGaN/GaN semi-floating micro-disk LED arrays

With enhanced light output efficiencies, InGaN/GaN micro-disk LED has received intensive attentions recently. Combining isotropic and anisotropic dry etching processes, an innovative semi-floating InGaN/GaN micro-disk LED array is fabricated, which shows remarkable light intensity enhancement up to 150% compared to the broad-base LED. A systematic study of micro-spectrums and Poisson-Schrodinger coupling self-consistent calculation reveal that there is non-uniform residual stress distribution on the micro-disk LED. Along micro-disk center to micro-disk edge, as the Si substrate is etched off, the in-plane tensile stress in the GaN layer reduces, while the compressive stress in InGaN layer increases gradually. This gradient stress distribution has caused a non-uniform piezo-phototronic effect in the micro-disk LED, which in turn results in a maximum wavelength shift of 16 meV for the light emitted along micro-disk center to micro-disk edge. This study not only opens research of flexo-optoelectronic effect, e.g. non-uniform piezo-phototronic effect, in complex micro/nano optoelectronic/electronic devices, but also provides important guidance for the significant enhancement of light emission efficiency in micro-disk LEDs.

Analytical and numerical modeling of the bond behavior between FRP reinforcing bars and concrete

This study focuses on the modeling of the local bond behavior of Fiber Reinforced Polymer (FRP) reinforcing bars and the surrounding concrete. An analytical bond-slip model is first proposed to describe the interface behavior of the bars embedded in concrete. Then, the parameters of the bond-slip law are identified from experimental data of pull-out tests performed on specimens instrumented with optical fiber sensors. A specific procedure was developed to identify the model parameters. In a second step, the identified analytical law is introduced in a finite element model (cohesive zone damage model) to simulate the pull-out tests, and a complementary model calibration is proposed to account for the non-uniform stress distribution along the interface. Finally, calculated strain profiles at the bar/concrete interface are compared with the experimental profiles measured by optical fiber sensors. The proposed model is found suitable to simulate the bond mechanisms at high load levels, when there is significant interface damage. However, further development of the model is needed to accurately describe the bond stress and slip response at lower load levels and low/moderate interface damage.

Weld fracture under non-uniform stress distribution and its mechanism in CHS-CHS X-joints

Tubular joints are characterized by the non-uniform stress distribution. In order to study the influence of the non-uniform stress distribution on the bearing capacity of the weld for circular hollow section (CHS) joints and the fracture mechanism, monotonic tensile tests for 8 CHS-CHS X-joints were conducted. The specimens were designed utilizing coarse FE models using a triaxiality-and-Lode-angle-related fracture model to ensure the expected failure mode. Refined FE models were also established which were verified by the test. Failure modes of weld fracture and chord punching shear failure were both achieved by FE analysis, and the corresponding fracture evolution were also obtained. The weld fracture initiated from the welding root at the saddle point while the punching shear fracture initiated from the chord surface at the saddle point. It shows that the non-uniform stress distribution has a great impact on the bearing capacity; the non-uniformity of stress distribution is weakened and the weld bearing capacity becomes higher if the joint stiffness is increased; the formula in Chinese code is not safe if the influence of the non-uniform stress distribution is ignored. A simplified mechanical model is established to study the relative stiffness distribution of the joint and subsequent the stress distribution. It was found that the deformable elements of the chord resist the brace force mainly through extrusion deformation but not the shear deformation, and this is the key reason for the non-uniform stress distribution; the non-uniformity of stress distribution will become higher if the geometric parameter γ and β increase.

A peridynamic mechano-chemical damage model for stress-assisted corrosion

We introduce a mechano-chemical coupled peridynamic model for stress-assisted corrosion. In the model, the activation-controlled anodic dissolution is a function of nonlocal dilatation of material near the corrosion front. We use a new experimental design to measure anodic dissolution rate for copper under activation-controlled and applied tensile stress. The experimental results are used to calibrate and then validate the new model. A case for stress-assisted corrosion in a complex geometry with non-uniform stress distribution is included to demonstrate the model's capabilities.

Molecular-dynamics study of the carbon nanotube mechanical metahelix

Hierarchical helical structures extensively exist in both natural and artificial systems and hold remarkable mechanical properties. Here, tensile mechanical properties of metahelix designed by twisting multiply twisted helical carbon nanotube ropes are investigated by coarse-grained molecular dynamic simulations. One-level metahelix are slightly mechanically strengthened with increasing twist angle α. However, two-level ones are more sensitive to the twist operation angles α and β, with maximum reduction in strength and Young's modulus by 64% and 87%, respectively. Three distinct failure modes are identified, although all metahelix show brittle failure under tension. For type I fracture mode, regardless the twist angle in metahelix, stress is uniformly distributed, resulting in simultaneous breakage of bonds at a cross-section. The type II and III failure modes are featured by stepwise localized failures, resulting from non-uniform stress distribution along each filament and identical cross-section of metahelix. This work provides molecular insights into optimal mechanical performance of CNT-based hierarchical helical yarns.

Prediction of bond performance of tension lap splices using artificial neural networks

Recently, machine learning has been widely used in civil engineering, because better design can be achieved using the advanced computer intelligence and test results. In the present study, to improve design reliability and to extend design application range for the development and lap splice lengths, an artificial neural network model (ANN) was presented using 1008 existing experimental studies for splice tests. Although some of the test parameters are out of the limitations of design codes, all test results were used for the ANN to extend design application range considering present-day construction materials and practices. From a parametric study with the ANN, the effect of design variables was investigated, and predictions by the ANN were compared with existing design equations. Finally, based on the parametric study result, modifications were proposed for existing design equations to consider the effect of non-uniform bond stress distribution and the effect of cover concrete and transverse bars, as well as to extend design application range. Comparisons showed that the modifications improved the accuracy of the design methods. The high accuracy to the large number of existing test results confirms that the modifications based on the ANN can improve design reliability and also can extend design application range for the development and lap splice lengths.

Computational hemodynamics study of anterior communicating artery aneurysm associated with fenestration: A case report

Association of intracranial aneurysms with fenestration is of great clinical interest. Hemodynamic characteristics of an intracranial aneurysm associated with fenestration have been rarely reported. We performed a computational hemodynamics analysis of an anterior communicating artery (ACoA) aneurysm associated with fenestration to understand the influence of fenestration on aneurysmal hemodynamics.A patient-specific 3-dimensional geometry of an ACoA aneurysm associated with fenestration was created from the computed tomography (CT) angiography images of a 52-year-old male patient who suffered a sudden loss of consciousness. Computational hemodynamic analysis was performed and some interesting hemodynamic characteristics were found. Fenestration behaved like a flow divider, which led to highly complex blood flow patterns in aneurysm. A pressure elevation and non-uniform wall shear stress (WSS) distribution on aneurysm sac were found. High WSS gradient on the fenestration surface was observed for the first time.Fenestration may significantly influence the hemodynamic characteristics of aneurysms.

Tire-Pavement Contact Stress Characteristics and Critical Slip Ratio at Multiple Working Conditions

The contact stress distribution of tire-pavement is generally assumed to be uniformly distributed in circles or rectangles in the pavement design. This simplification can solve some common engineering problems, but it neglects two factors including magnitude and direction of the contact stress. The main purpose of this paper is to analyze the contact stress distribution between the bus tire and the pavement under different driving conditions and pinpoint and compare the critical slip ratio during braking and acceleration. First, a tire-pavement simulated contact model is constructed by ABAQUS. Next, we compare, respectively, contact stresses in the vertical, longitudinal, and transverse directions at various rolling conditions and analyze the longitudinal forces at different slip ratios during braking and acceleration. It is observed that the magnitude, direction, and nonuniform distribution of contact stress are affected by the multiple working conditions while the bus is working. Meanwhile, in the braking and acceleration conditions, the longitudinal force grows rapidly as the slip ratio increases, and then grows slowly and at last decreases when the critical slip ratio is reached. However, the critical slip ratio at the time of braking differs from that at the time of acceleration, and the former is smaller than the latter.

In vivo and in vitro evaluation of a biodegradable magnesium vascular stent designed by shape optimization strategy

The performance of biodegradable magnesium alloy stents (BMgS) requires special attention to non-uniform residual stress distribution and stress concentration, which can accelerate localized degradation after implantation. We now report on a novel concept in stent shape optimization using a finite element method (FEM) toolkit. A Mg-Nd-Zn-Zr alloy with uniform degradation behavior served as the basis of our BMgS. Comprehensive in vitro evaluations drove stent optimization, based on observed crimping and balloon inflation performance, measurement of radial strength, and stress condition validation via microarea-XRD. Moreover, a Rapamycin-eluting polymer coating was sprayed on the prototypical BMgS to improve the corrosion resistance and release anti-hyperplasia drugs. In vivo evaluation of the optimized coated BMgS was conducted in the iliac artery of New Zealand white rabbit with quantitative coronary angiography (QCA), optical coherence tomography (OCT) and micro-CT observation at 1, 3, 5-month follow-ups. Neither thrombus or early restenosis was observed, and the coated BMgS supported the vessel effectively prior to degradation and allowed for arterial healing thereafter.The proposed shape optimization framework based on FEM provides an novel concept in stent design and in-depth understanding of how deformation history affects the biomechanical performance of BMgS. Computational analysis tools can indeed promote the development of biodegradable magnesium stents.

Effects of mold pattern characteristics on nanoimprinted aluminum investigated using quasi-continuum simulations

The effects of mold pattern characteristics on the mechanical deformation and mechanics of nanoimprinted aluminum are studied using quasi-continuum simulations in terms of atomic trajectories, strain distribution, and the imprinting stress-mold displacement curve. The simulation results show that a mold whose pattern has a larger period (S value) produces a larger imprinting stress on the substrate for deformation. The effect of mold taper angle on imprinting stress depends on the S value. The required imprinting stress for substrate deformation decreases when mold wear occurs, resulting in the appearance of more non-local atoms in the substrate because of a nonuniform stress distribution. The nucleation and propagation of dislocations in the substrate are significantly confined when the S value is very small (e.g., 150 Å).

Experimental and numerical researches of precast segment under radial dislocation conditions

With the increase of tunnels constructed by the tunnel boring machine (TBM), the widely observed cracks of segment lining severely jeopardize the safety and durability of tunnels. This paper is to investigate these cracks on segment linings under radial dislocation conditions using scaled experiments, field observations and numerical simulations. First, three radial dislocation types of segment lining were categorized and their corresponding crack patterns were summarized from the in-situ statistical analysis of 78 damaged segment rings. Secondly, to investigate the influence of radial dislocation on the evolutions of cracks and the crack patterns of segment lining, an experimental platform was originally developed. The program consisted of three specimens under different radial dislocation types, including the non-radial dislocation segment type, radial dislocation on the key segment type and radial dislocation on the standard segment type. Experimental results proved that the radial dislocation had a significant influence on the crack patterns of segment linings, and in particular different radial dislocation types could finally lead to different evolution types of failure and finally different crack patterns of segment lining. Finally, a non-linear 3D finite element model (FEM) was further conducted to explore the formation mechanisms of cracks, and numerical results were in good agreement with experimental data. The numerical analysis demonstrated that once the radial dislocation segment appeared, the bending moment of the segment lining increased sharply at the arch crown. Meanwhile the bending moment crossing phenomenon also occurred on the radial dislocation segment. In addition, this abrupt stress state variation of segment lining caused a non-uniform global stress distribution of segment lining and different types of local stress concentration, which was more likely to cause the cracks on the segment lining. It was worth to emphasizing that the radial dislocation types had a significant influence on the overall stress distributions of the segment lining, which consequently resulted in different crack patterns of segment lining.

A predictive 2D finite element model for modelling FRP-to-concrete bond behavior

External bonding of fiber-reinforced polymer (FRP) plates (including both wet layup and pultruded plates) has been widely used to enhance the flexural or shear strength of reinforced concrete (RC) members. The effectiveness of this technique highly depends on the bond behavior of FRP-to-concrete interfaces, which has been investigated commonly using a bond test. Finite element modelling on the bond behavior of FRP-to-concrete bonded joints is superior to tests in obtaining detailed information for the in-depth understanding of the failure process. This paper presents a truly predictive 2D FE model for simulating the bond behavior of FRP-to-concrete bonded joints. The effective width of concrete prism is introduced to consider the non-uniform distribution of concrete stress in the out-of-plane direction of the bonded joint, and is derived using a rigorous method. With input of the effective width as the depth of concrete elements, the proposed FE model is able to produce predictions close to their test counterparts for specimens with not only debonding failure, but also concrete prism failure, thus making it a truly predictive model, which does not need to select out specimens only with debonding failure for simulation.

Simulation of the stress regime during grain filling in bamboo reinforced concrete silo

Design and development of low-cost farm silos call for a strong understanding of its structural performance and loads. Limited studies address the effect of stored grains on silos. A farm level bamboo reinforced concrete (BRC) silo with a flat bottom has been designed for the storage of rough rice. A full-scale 3D finite element (FE) model of the BRC silo has been developed and the grain filling in progressive layers simulated in the ANSYS® software. The stored grain and silo body interactions have been modeled considering the characteristic properties of both rough rice as well as the BRC, with minimal simplifications. The numerical results have been compared with the outcomes of classical theories (Jannsen’s and Reimbert’s) and design code (IS 4995-1974). While a significant difference was observed between the lateral stress magnitudes predicted by the numerical and analytical methods, the axial wall stresses from FEM approach deviated from the IS code values by 16%. Contrary to the analytical approaches, FEM predicted non-uniform stress distribution due to the bulk grain at the silo bottom. The numerical approach could also identify the localized peak pressures and stress distribution patterns within the grain layers, which is usually beyond the scope of analytical techniques. Possible reasons for fluctuations in stress patterns are discussed in detail. The study unveiled the intricacies involved in the FEM and the analytical outcomes while predicting the stresses in small and medium scale silos intended for use on farms.

Real-time observation of stress-induced domain evolution in a [011] PIN-PMN-PT relaxor ferroelectric single crystal

This paper reports on the real-time observation of mechanical stress-induced micro and nano domain evolution in a single crystal relaxor ferroelectric [011] poled 24PIN-PMN-PT “3-2 mode” nano-sized lamella. Mechanical loading in the [100] direction was applied to lamella in situ within a transmission electron microscope, with a [01¯1] viewing direction selected for the recording of real-time videos of the domain evolution within the lamella. The observed dominant behaviour under loading is a reversible response composed of the movement of microdomains and the disappearance of nanodomains. Changes in the selected area diffraction pattern down the [01¯1] zone axis are reported, and provide evidence of a significant increase in crystal symmetry under compression. A qualitative insight into the behaviour of the lamella's mechanical response is obtained via predictions of the non-uniform stress distribution within the lamella found using simple finite element analysis. The correlation between the observed morphology changes, diffraction pattern changes, and the known mechanical stress induced polydomain-rhombohedral to monodomain-orthorhombic phase transition of bulk [011] poled 24PIN-PMN-PT “3-2 mode” single crystal is discussed.

Material model calibration against axial-torsion-pressure experiments accounting for the non-uniform stress distribution

The need to calibrate material models towards complex multiaxial stress states has received much attention in the last decades. Such stress states are often obtained by axial, torsion and pressure experiments on tubular test bars. These experiments are typically simulated by considering the behavior in a single material point, using the thin-walled assumption of uniform stresses and strains. In this paper, a new simulation methodology that does not rely on the thin-walled assumption has been developed. The accuracy improvements are compared with experimental uncertainties for tubular test bars. Compared to using ABAQUS for equivalent simulations, a speed increase of 100–200 times was found. This simulation methodology has been implemented in an open source software material calibration software called matmodfit. The calibration of material parameters is first demonstrated for cyclic ratcheting axial-torsion experiments using thin-walled test bars. Thereafter, calibration of material parameters from experiments on solid test bars subjected to very large shear deformation under axial compression is performed. This demonstrates a key advantage with the proposed method: Thick-walled or even solid test bars can be modeled without loss of simulation accuracy at a low computational cost.

Elastic Visco-Plastic Model for Binary Sand-Clay Mixtures with Applications to One-Dimensional Finite Strain Consolidation Analysis

AbstractThe pore water dissipation of sand–clay mixtures is significantly affected by the sand fraction due to nonuniform stress distribution. On the basis of the elastic visco-plastic modeling con...

Welding-induced buckling prediction for large thin-walled cylindrical structures with non-uniform stress fields by friction stir welding

The non-uniform welding stress distribution in large thin-walled cylindrical structures easily causes the buckling distortion due to weak structural stiffness and nonlinear geometric deformation. The non-uniform temperature field model induced by friction stir welding is analytically developed and is used to theoretically deduce the non-uniform stress model in the weld region. Both the two models are verified by the experimental data. A prediction model of critical buckling load is established for large thin-walled cylindrical structures considering the non-uniform welding stress distribution. The deflection function for buckling analysis is developed by using polynomials and the Chebyshev polynomials of the first kind that satisfy the boundary conditions along the four edges of the structure. The critical buckling loads are calculated for various cylindrical welded shell structures, which obviously decrease with the increase of the diameter-thickness ratio and the length-diameter ratio. The mitigation of buckling distortion is achieved by the correlation of the buckling load and the welding stress with various welding process parameters. The results are beneficial for the determination of geometric configuration and welding process parameters for large thin-walled cylindrical structures in fabrication of propellant tanks of launch vehicles.

Structural failure test of a 52.5 m wind turbine blade under combined loading

Blade is one of the basic and key components in wind turbines. To get closer to the actual loading condition of blade under complex and extreme wind conditions to more accurately analyse the structural failure characteristics of large wind turbine blades. The structural collapse test of a 52.5 m wind turbine blade under combined loading was carried out. The entire process of damage under ultimate load was recorded and analysed in detail. The joint analysis of the failure region after the blade collapse and the failure mode during the test process were performed. The research shows that the geometrical nonlinear buckling of spar cap and shear web, the delamination result from non-uniform stress distribution and further fracture of Aft panel cause catastrophic failure of the blade. The torsional moment cause oblique cracks and oblique bulges, aggravates the expansion of the internal and external composite cracks of blade, affects the final failure mode after blade collapses (critical failure mode), and make the spar cap at suction side to clockwise twist from blade tip to root. The research results lay a theoretical foundation for subsequent model establishment and simulation analysis of large wind turbine blades under combined loading.

Out-of-plane distortion prediction of large thin-walled structures induced by friction stir welding

ABSTRACTA numerical method is developed to predict the out-of-plane distortion for large thin-walled structures induced by friction stir welding. Considering the constraint of non-welded region in a large structure, the non-uniform stress distribution along the weld is accurately computed with a small structure. The welds in the small and large structures are divided into the beginning, middle, and end regions due to the non-uniform stress situation. By using the interpolation function, the stress components in various regions of the small structure are mapped to the corresponding locations in the large thin-walled structure. Then, the out-of-plane distortion of the large structure is computed when the clamps are released. The predicted residual stress and distortion for the large welded structure are in accordance with the experimental data.