Regions Of Stress Concentration Research Articles

Multiscale study on the mechanical behaviour of catalyst layer under tensile and humidity cyclic loading

The durability and life of proton exchange membrane fuel cells (PEMFCs) decrease as the mechanical damage of the catalyst layer (CL) is aggravated due to mass and electronic transport constraints. Owing to the challenge of real-time monitoring of internal stress and microstructure changes through experiments, developing an effective modelling technique to capture the onset of cracks and delamination is extremely urgent. For this purpose, this work focused on probing the effect of stress concentration regions on the mechanical behaviour of CL under tensile and humidity cyclic loading. Based on the measurement of adhesive force between Pt/C and Nafion, we constructed an interfacial model through molecular dynamics simulation and correlated it with the cohesive zone model in the micro-mechanical model. Finite element simulation was employed to predict the stress–strain response and onset of interface debonding. Some results have shown good agreement with experimental data from the literature. This model induced by interfacial strength, porosity, and ionomer expansion curve can be used to examine the damage degree of CL during the PEMFC durability testing process.

Anatomical variation in intracortical canal network microarchitecture and its influence on bone fracture risk

Intracortical canals are a major contributor to cortical bone porosity and influence its mechanical response. Canal networks act as stress concentrators and the magnitude of which depends on the size and spatial distribution of canals. In the present study, we investigated site-dependent variation in intracortical canal network morphological indices and their effect on the mechanical response of bone. For this, mid-diaphysis of rat tibia bones were scanned using high-resolution micro-CT and morphological indices were measured for four main anatomical sites-anterior, posterior, medial and lateral. Further, a micro-finite element (μFE) model was developed to quantify the stress concentration regions in different cortices. The fracture risk was assessed using an effective strain approach. Results show that canal porosity, canal orientation and canal length are site-dependent whereas canal diameter and canal number density are independent of the site. The lateral cortex has significantly higher porosity compared to the posterior cortex (p < 0.05). The orientation of canals is found significantly different between endosteal and periosteal regions for anterior and medial quadrants. Canals are inclined at higher angles with bone axis in the endosteal region as compare to the periosteal region. The μ-FE results show that the regions with higher effective strain are concentrated around the canals. Further, failed element volume per unit bone volume is found highest for medial cortex whereas lowest for posterior cortex. The higher failed volume is associated with more radial canals in the medial cortex as compare to other cortices. The linear regression analysis shows that the volume of overstrained elements strongly depends on canal orientation (R2 = 0.73, p < 0.0001) and canal porosity (R2 = 0.61, p < 0.0001). The findings from this study suggest that along with vascular canal porosity, canal orientation and canal diameter can further improve the bone fracture risk assessment.

Parametric study of solid propellant slotted grain

One of the goals in solid rocket motor design is to have as large volumetric loading as possible keeping the basic requirements unaffected. Slotted grain can achieve this goal as it has the advantages of sliver-free and no stress-concentration regions that occur in other internal burning grains as star grain and wagon wheel grain. It has the disadvantage of exposing the motor wall to hot gases. In this paper, the geometry of slotted grain is discussed and the effect of design parameters (e.g., number of slots, dimensions of the slot, etc.) of slotted grain on grain burn back is explained. Also, a comparison between results and experimental data is performed.

A Numerical Study on the Control of Horizontal Cracking at the Ends of BS22 Hollow-type PC-girders Utilizing Midas FEA

When the prestressing forces transfer from PC-strands to concrete, a region of stress concentration develops at the ends of pretensioned girders, which often results in horizontal cracking during or just after the detensioning process. In this study, a hollow PC-girder was modeled utilizing a Finite Element Analysis software Midas FEA to identify the horizontal cracking locations in terms of the principal stresses at the end-zone of the hollow PC-girder. Strand-debonding and placing end-zone reinforcements were hired in this work by introducing four cases. The only strand-debonding method could not prevent horizontal end crack penetration. Though the end-zone reinforcements were placed alongside the strand-debonding, this combination could reduce principal stresses to a level that could bring the crack size to a negligible range.

Rolling shear performance of cross-laminated bamboo-balsa timber panels

This research aimed to develop a set of practical and straightforward test methods to evaluate the rolling shear properties of cross-laminated bamboo and timber (CLBT) panels. Meanwhile, firsthand experimental data on the shear performance of panels laminated with unidirectional engineered bamboo panels and balsa wood lumbers were reported. The mechanical test results for balsa wood used in the hybrid CLBT panels were given. The bonding line's shear test between the out-layer bamboo panels and core wood was performed through the oblique compression shear test. The rolling shear tests of CLBT panels were performed with the two-plate planer shear test method and short-span bending shear test method. The full strain fields of specimens near ultimate loading conditions were given based on the two-dimensional Digital Image Correlation (DIC) method. This study showed that the rolling shear performance of core wood is determined by the annual ring geometrical parameters of wood products in the RT plane. The mean rolling shear moduli were 43 MPa and 48 MPa for two-plate specimens measured with dial gauges and strains, which is estimated through the Digital Image Correlation (DIC) method. The mean shear strength was 0.96 MPa obtained through two-plate shear tests and 2.50 MPa through the short-span bending shear test. The force corresponding to first-crack development instead of the maximum load is suggested to be adopted to calculate the shear strength of CLBT and other hybrid cross-laminated panels. Relatively small modulus and strength values were noticed for the hybrid panels due to the limited contribution of bamboo layers; thus, reinforcement at the stress-concentration regions was suggested for further researches on such lightweight hybrid cross-laminated panels.

Investigation of thermal-mechanical effects on salt cavern during cycling loading

Traditional stability analysis for salt cavern as UGS overlooks the thermal effects induced by cycling loading and cannot predict the potential risk. In this study viscoelastic WIPP model is employed to describe the temperature-dependent creep behavior of rock salt. A thermal-mechanical simulation framework is established to couple the effect of mechanical stress and thermal stress. A coupling thermal-mechanical numerical model implemented by FLAC3D in combination with the WIPP creep model is proposed. A field case, Cavern L, in Jintan, Jiangsu Province of China is demonstrated in this paper to explore the thermal response to gas injection-and-withdrawal process. The results show that there is tensile stress concentration region at the cavern roof. The dynamic process of thermal damage evolution and block falls on cavern roof is also investigated. The disintegration of rock salt mass at cavern roof occurs after the most severe gas depressurization period, and the retain of low pressure after gas withdrawal aggravate the thermal damage. It indicates that the thermal-induced tensile stresses are critical for salt cavern stability assessment. The feasibility and accuracy of the proposed thermal-mechanical modeling is verified by the field case, and the simulation results of thermal damage are coincided with the sonar results. This analysis confirmed the influence of thermal-mechanical effects on salt cavern wall. The approach used in this study provides an improved method for the stability evaluation and optimal operation parameters for salt caverns.

Structural Design and Dynamic Compressive Properties of Ti–6Al–4V Hollow Lattice Structures

Herein, three groups of Ti–6Al–4V hollow lattice structures with different strengthening structures are designed and manufactured using a selective laser melting method. A basic structure (specimen BS), a basic structure with strengthening structures at joints (specimen SJ), and a basic structure with a strengthening structure on tube walls (specimen SW) are compressed by a split Hopkinson pressure bar system. The potential influence of strengthening structures and strain rates on dynamic compressive properties is investigated. The results indicate that the strengthening structures change the structural weaknesses and stress concentration regions inside the basic structures; thus, specimens BS and SJ collapse on the struts, whereas specimen SW breaks at the joints. The strengthening structures of specimen SJ reinforce the nonstructural weaknesses inside the basic structure, resulting in the mechanical properties showing limited improvement or even decrease. However, the strengthening structures of specimen SW strengthen the structural weaknesses inside the basic structure, so the collapse strength, specific collapse strength, energy absorption (EA), and specific EA (SEA) of specimen SW, respectively, increase by 46.1%, 8.4%, 42.0%, and 6.5%, which shows that specimen SW exhibits excellent mechanical properties. Moreover, strain rate sensitivity of specimen SW is dominated by the strain rate effect of Ti–6Al–4V material.

Effect of crystallographic orientation on mechanical properties of single-crystal CoCrFeMnNi high-entropy alloy

Using molecular dynamics (MD) simulation, the influence of crystallographic orientation on the mechanical properties and microstructure evolution of the single-crystal CoCrFeMnNi high-entropy alloy as a face-centered cubic (FCC) metal is explored and quantified by nanoindentation. At the elastic stage of P-h curve, the results of MD-simulation are almost consistent with that of Hertz contact theory fitting, which ensures the accuracy of our results. The plastic deformation is studied by correlating the P-h curve with the instantaneous defect structure, and dominated by nucleation of Shockley partial dislocations or the movements of stacking faults. Furthermore, we discuss the effects of crystallographic orientations [ 001 ] , [ 110 ] , and [ 111 ] on the mechanical response of materials and the microstructure evolution. The slip mode and the stress-concentrated region are controlled by the crystallographic orientations, and an obvious pop-in behaviors are viewed in [ 111 ] -oriented sample, without appearing in other samples. The microstructure evolution exhibits distinct anisotropy causing the discrepancy of dislocation density and hardness. By analyzing the evolution of dislocation density and hardness, the linear relationship between the square root of dislocations density and hardness is revealed, and in good agreement with the classic Taylor hardening expression where the pre-factor is strongly dependent on the crystallographic orientation. In the study, the atomistic results are consistent with those predicted by the classic continuum mechanical theory, which contributes to the application of classic mechanical theory in molecular dynamics simulation.

Study on deformation mechanism of Ti-2Al-2.5Zr alloy tube in the flattening test

The flattening test, using one pair paralleled plates to flatten tube structure radially, is generally employed to examine the ductility of tube used in once through steam generator (OTSG). This study focuses on the stress condition and deformation mechanism analysis on the concentrated stress regions of Ti-2Al-2.5Zr alloy tube during the flattening test. Firstly, the finite element analysis was performed using the commercial software ABAQUS to determine the stress condition. Secondly, the Electron Back-Scattered Diffraction (EBSD) was implemented to observe the microstructure evolution of Ti-2Al-2.5Zr alloy. Finally, Schmid law was employed to analyze the activated deformation mechanism under condition of the complex stress. It was found that the condition of the complex stress in stress concentration regions, including tension and compression regions, can be simplified into two directional stresses condition. In grains whose c-axis is nearly towards TD and ND, the strain is mainly accommodated by prismatic slip, whereas in grains whose c-axis deviates about 45° from ND to TD mainly by basal slip. The {10–12} extensive twin and corresponding parent grain orientation mainly relied on the stress condition. Additionally, the intergranular twin pairs connected at common grain boundaries (GBs) with high strain compatibility were found at low angle GBs, which were activated under a combination of macro stress and strain compatibility effect.

FEM analysis of 3D lattice structures of ABS material

FDM is a typical additive manufacturing method. Since FDM is a method of stacking layers one by one, it generally has a flat lattice structure. In this study, by checking the distribution of stress and deformation for several lattice structures made of ABS material, it is intended to find a structure with better mechanical properties with less material. Several three-dimensional lattice structures are modeled using parametric modeling. Subsequently, a constant pressure is applied to the same area to check the stress and strain distribution. A structure with a low maximum stress value in the stress concentration region and a small amount of deformation will have the best mechanical properties. To do this, parametric modeling is performed using Inventor to model four three-dimensional lattice structures. Afterwards, use Ansys Workbench to check the stress and deformation distribution. Looking at the stress distribution, stress concentration occurred in the truss supporting the upper surface of the SC structure. In the BCC and PTC structures, stress concentration occurred at the point where the upper surface and the truss met. In the FCC structure, it can be seen that the load is distributed throughout the truss structure. Looking at the deformation distribution, both the SC and BCC structures show similar amounts of deformation. It was confirmed that the FCC structure had less maximum deformation than the PTC structure with the thickest truss. Unlike previous studies, it was confirmed that the higher the internal filling rate, the better the mechanical properties may not come out. The FDM method can obtain different mechanical properties depending on the internal lattice structure as well as the internal filling rate. In a later study, we will find a new calculation algorithm that applies variables by FDM characteristics using the data obtained by printing the actual specimen.

Effect of TGO thickness, pores, and creep on the developed residual stresses in thermal barrier coatings under cyclic loading using SEM image-based finite element model

Thermal barrier coatings (TBC) allow the metallic internal components of gas turbine engines to operate at elevated temperatures near its melting points. Formation of thermally grown oxide (TGO) layers at the top coat (TC) and bond coat (BC) interface induces cracks in the TC that may lead to complete TBC failure due to spallation. An SEM image-based finite element (FE) model is developed using commercial finite element package ABAQUS to investigate the development of residual stresses resulting from cyclic loading of TBCs. The model includes thermo-mechanical material properties and considers the real interface between the coating layers. The model includes real pores based on an SEM image, taking advantage of image processing techniques. Effect of TC surface roughness and pores on the developed residual stresses during thermal cycling is investigated with respect to different TGO thicknesses. The analysis shows that presence of TC roughness causes stress concentration sites during heating that may force horizontal cracks to initiate and propagate with stress values that are indifferent to the TGO thickness. The pores are found to shift stress concentration regions from the TC/TGO interface to the vicinity of the pores during cooling, and that may cause horizontal cracks to start from within the TC with stresses that increase with TGO thickness. Moreover, the effect of creep for all layers on the generated residual stresses is studied. Considering creep gives lower stresses at the end of cooling, however, stress distribution remains the same with and without creep.

Stacking sequence optimization of laminated plate structures using the boundary element method

In the present paper, we implement a pure boundary element method for the static analysis of 2D and 3D structures composed of thin, unsymmetric laminated composite plates. The domain integrals related to distributed loads are transformed into boundary integrals using the radial integration method. Furthermore, we apply an equilibrium-based procedure to directly recover the out-of-plane stress state from the classical laminated plate theory. Some benchmark examples verify the accuracy and efficiency of the present formulation, highlighting its steady convergence in regions of stress concentration near holes with sharp corners. To exploit the computational performance of the resulting boundary element code, we use the resulting code as the analysis step of an optimization scheme, taking fiber orientation angles as design variables.

Thermal and Vibration Mechanical Reliability of Multicoat Compound Thin-Film Thermocouple

For the thin-film thermocouple with ITO/In2O3 as the functional layer, the mechanical reliability of their composite multilayer membrane structures under high temperature and random vibration conditions are discussed. Using the elastic plasticity model based on the finite element simulation method, the stress and strain distribution patterns caused by the thermal mismatch and vibration of multilayer film materials are systematically analyzed, and the distribution of the stress concentration and strain accumulation regions that may lead to fatal reliability problems are characterized. When a thermal load of 1000 °C is applied, the protective layer of the structure is vulnerable to severe stress environment. In addition, plastic deformation occurs in the adjacent layers of the functional layer, making it a weak reliability area. The stress generated by the vibration load is much smaller than the thermal stress, which mainly occurs at the roots of the structure, where prolonged loading may lead to fatigue failure.

A mesoscale finite element modeling approach for understanding brain morphology and material heterogeneity effects in chronic traumatic encephalopathy

Chronic Traumatic Encephalopathy (CTE) affects a significant portion of athletes in contact sports but is difficult to quantify using clinical examinations and modeling approaches. We use an in silico approach to quantify CTE biomechanics using mesoscale Finite Element (FE) analysis that bridges with macroscale whole head FE analysis. The sulci geometry produces complex stress waves that interact with one another to create increased shear stresses at the sulci depth that are significantly larger than in analyses without sulci (from 0.5 to 18.0 kPa). Sulci peak stress concentration regions coincide with experimentally observed CTE sites documented in the literature. Highlights Sulci introduce stress localizations at their depth in the gray matter Sulci stress fields interact to produce stress concentration sites in white matter Differentiating brain tissue properties did not significantly affect peak stresses

Hydrodynamic analysis of an underwater glider wing using ANSYS fluent as an investigation tool

The analysis of the hydrodynamic behavior of the modified trailing edge of an underwater glider wing was conducted using ANSYS Fluent 18.1. The procedure was carried out using the computational fluid dynamics (CFD) approach. A standard four digits NACA0016 aerofoil profile was selected while the geometry points were extracted from NACA online aerofoil generator using Solidworks, which enabled the creation of the 3D wing profile. Since the focus of the study is more toward hydrodynamic behavior or the influence of the modified round trailing edge as opposed to the normal sharp trailing edge, coefficient of lift (CL), coefficient of drag (Cd), coefficient of pressure (Cp) and the drag polar (Cd/CL) are very crucial parameters and have an influence on the accuracy of the results. The results showed lift and drag coefficient are a function of the angle of attack (AOA) and free stream velocities. The investigation showed the optimal design point of the AOA of 12° at the corresponding speed of 0.26 m/s. The critical AOA matched with the optimal design point of 12°. The results also showed that Cp is higher closer to the fuselage and therefore explains why more structural rigidity must be provided in this region of high stress concentration. The drag and lift curve corresponded to the typical wing characteristics of the published experiment data regardless of the operating conditions.

Analysis of the failure modes and design improvement of an eccentrically loaded connecting rod for a double front axle steering linkage prototype

Failure analysis and optimal redesign study of the connecting rod prototype to be used in the multi-axle steering linkage of an 8x8 Aircraft Rescue and Fire Fighting (ARFF) vehicle is reported. At the prototyping stage, the field tests showed that the connecting rod experiences a plastic deformation during a zero-speed steering manoeuvre. In order to predict the possible causes of the failure, Finite Element (FE) analyses were applied to the complete steering linkage which contains the failed rod design. Primary FE analyses showed that there are critical stress concentration regions on the rod. It was also seen that the maximum stress values at these regions are influenced by the tyre steering angle and the slope of the road surface. Subsequently, critical structural parameters having an influence on the mechanical behaviour of the part were chosen, and a parametric base model was built. Then, using DOE-RSM (Design of Experiments – Response Surface Methodology) based optimisation study, optimal values were selected within the applicable variation range. Nonetheless, by considering the form of the structure together with the loading type, it was decided that the elastic stability should also be taken into account. Although a sufficient improvement was obtained against the overload failure, the elastic stability limit remained exceeded according to the American Institute of Steel Structures (AISC) criteria. Thus, the eccentricity of the connecting rod was also decreased by employing a particular kinematic arrangement in order not to disturb the Ackermann’s rule-based synchronisation ratio of the linkage. Compared to the failed design, it was seen that von-Mises stress can be reduced by up to 66%. The results of this study showed that, factors such as wheel steering angle and road inclination-related axle load variation greatly influence the design load of the steering linkage. Field tests also indicated that, suggested design improvements such as increase in thickness and eccentricity reduction meet the strength requirements of the system.

Steel foil reinforcement for high performance bearing strength in Thin‐Ply composites

This study investigates the influence of local hybridization of Thin and Thick-Ply CFRP laminates on the open-hole and bearing properties. The area weight of the CFRP unidirectional prepregs used is 40 gsm in the case of Thin-Ply layers and 160 gsm in the case of Thick-Ply layers. The steel used is a 1.4310 stainless-steel foil with the same layer thickness as the prepregs. In the hybrid area, 90∘ layers were locally replaced by stainless steel patches. The local metal foil content varies from 6.25%, 12.5% to 25.0%. For notched laminates, the open hole tensile strength is significantly decreased with thinner layer thicknesses. The failure behavior changes from complex delamination dominated failure to brittle failure. By using stainless steel foils in the regions of stress concentrations, energy can be dissipated by plastic deformation of the steel foil and stresses can be deflected to neighbouring areas. For Thin-Ply samples with a local steel content of 25% the open hole tensile strength increases by 64% in comparison to the reference Thin-Ply specimens and the sensitivity towards stress concentrations decreases. The bearing strength of the hybrid CFRP laminates is increased by up to 54.6% in comparison to the reference material, due to the confinement of the steel foil and the resulting higher compressive strength and the plastic deformation at high stresses. The stress–strain diagrams and micrographs of the fibre metal samples reveal that damage is initiated before the maximum bearing strength. However, the damage offset bearing strength increase concerning the specific density of the material significantly.

Surface acoustic wave pressure sensor on ScAlN/3C-SiC/Si layer structure incorporating biomimetic microcantilever: modelling and performance enhancement

This paper presents a bio-inspired cantilever-type SAW pressure sensor based on Scandium-doped AlN (ScAlN)/3CSiC/Si layer structure. The proposed biomimetic microcantilever is configured with stress concentration region (SCR) comprising of a series of narrow slits. For a given depth, by properly setting the number and the width of those slits for a given depth, homogeneous stress/strain distribution in vicinity of SCR are achieved. Furthermore, multi-physics finite element modeling for evaluating the pressure coefficient of frequency (PCF) of a one-port SAW resonant sensor is performed and validated. Then the PCF for the proposed SAW pressure sensor with the optimized biomimetic microcantilever is analyzed. The dependence of ScAlN thin film thickness h ScAlN and scandium-doped concentration r of Sc r Al1-r N thin film on PCF for different SAW modes including Rayleigh mode and Sezawa mode are calculated. The investigation results show that the maximum PCF of −111 ppm bar−1 can be achieved for Rayleigh mode, which is 2.4 times larger than that of the previously reported ZnO/Si structure. In addition, its PCF has weak dependence on h ScAlN, which are quite favorable for manufacturing pressure sensors with good reproducibility. The conceived bio-inspired cantilever-type SAW devices based on ScAlN/3 CSiC/Si structure will open the door to develop pressure sensors with a stable and reliable performance and enhanced sensitivity.

Effect of bias yarn on tensile fracture mechanism of multiaxial 3D angle-interlock woven composites

A novel 3D woven structure was designed and developed in this paper, namely multiaxial 3D angle-interlock woven composites (MAWC). In order to determine the specific influence of bias yarns on the tensile properties and failure mechanisms, two MAWCs with different weave patterns and 3D angle-interlock woven composites (3DAWC) were fabricated. Based on the meso-structure characteristics of MAWC, a realistic meso-model was proposed. The tensile behavior of MAWCs and 3DAWC were experimentally characterized, and the full-field strain distributions were recorded by digital image correlation (DIC). Moreover, a progressive damage model along with the X-ray micro-computed tomography (Micro-CT) technique were applied to reveal the damage processes and failure mechanisms of composites. Results showed that the axial tensile strength of MAWCs decreased due to the change of yarn orientation. The stress-strain curves of two MAWCs presented nonlinear behavior, while those of 3DAWC could be seen as linear within the whole strain range. Stress concentration regions of two MAWCs were found to be distributed more dispersedly than those of 3DAWC. The local tow splitting, fiber slippage and interfacial debonding were the primary failure modes of bias yarn in MAWCs under tension.

Method of Evaluating Residual Stresses in the Product Material

The method of evaluating the level of residual surface stresses in the products without the loss of their integrity at elastoplastic strains and under static and cyclic loading by scattering of hardness characteristics is proposed. Residual surface stress levels are assessed not by absolute hardness values but by the state of material structure. Its homogeneity coefficient in the Weibull distribution, characterizing the scattering of large-scale hardness data, was taken as a calculated structure damage parameter and evaluated by the LM-hardness method based on direct multiple independent hardness measurements. The estimation of residual stresses is built upon the correlation relation between the material structure damage and residual stresses. By comparing the measured parameters of structure homogeneity, charactering their damage level in the region of residual stresses, with the structure parameter in the region free from them, their level is determined. In the absence of such a region, residual stresses are evaluated by comparing the homogeneity parameter of the product element in their region with that of a reference sample made of the same material and subject to the same operating loads as the examined product. The product quality can be estimated by the regions of high residual stress concentrations, comparing the parameters obtained in its different characteristic sections. Correlation relations, determining the values of residual stresses, are found from preliminary experimental tests of the material specimens by constructing the structural parameter – stress diagram at a preset loading step. The structure parameter is measured by scattering its large-scale hardness measurements.