Interfacial Stress Distribution Research Articles

Finite element analysis of TGO thickness on stress distribution and evolution of 8YSZ thermal barrier coatings

AbstractAccording to the experimental research results of the thermally grown oxide (TGO) layered growth during the pre‐oxidation process of 8 wt.% yttria‐stabilized zirconia thermal barrier coating (TBC), a two‐dimensional sinusoidal TC/bonding coat (BC) curve interface model of the longitudinal section of TBCs based on finite element simulation was constructed; the thickness and composition of the TGO layer relative to the TC/BC curve interfacial stress distribution and its evolution during the thermal cycling process were studied. The results show that when the TGO layer uses α‐Al2O3 as the main oxide (black TGO), the thicker the black TGO layer, the more uniform the stress distribution of the TC/BC interface. When the TGO layer is dominated by spinel‐structured Co and Cr oxides (gray TGO), the stress “band” of the TC/BC interface is destroyed; it shows the alternating phenomenon of tensile stress zone and compressive stress zone, and after the rapid random growth of TGO, the concentrated tensile stress increased by a large jump. Affected by the thickness of the prefabricated black TGO layer, there is a limit peak in the thickness of the black TGO layer, the normal stress at the TC/BC boundary is minimized, and the magnitude of the stress change is also minimized.

Phase field study on the effect of roughness on interfacial intermetallic compounds of micro-solder joints under multifield coupling

In order to investigate the influence of roughness on the evolution of interfacial intermetallic compounds (IMCs) and stress states at the micro-solder joints under the strongly coupled thermo-mechanical-electric diffusion, the two-dimensional model of the rough interface for the Cu/Sn/Cu sandwich micro-solder joints was established using phase field method. Parameter A as the evaluation parameter of the interface roughness was used to represent the contour arithmetic mean deviation Ra. The evolutions of the average thickness of the interfacial IMCs and stress distribution at the Cu/Sn/Cu micro-solder joints were investigated with the variation of A. The thickness of interfacial IMCs in anode reached its maximum when A was 0, while obtained the minimum when A was 9. In cathode, the average thickness of the interfacial IMCs at A = 0 was thicker than that at A = 1, 3, 5, but thinner than that at A = 7 and 9. In other words, the growth of the interfacial IMCs was inhibited by the interface roughness in anode. While in cathode, the interfacial roughness inhibited the growth of the interfacial IMCs when 0 ≤ A ≤ 5, but promoted the growth of the interfacial IMCs when 7 ≤ A ≤ 9.

Multi‐dimensional Self‐splitting Behaviors for Improving Wet Attachment on Nonuniform Bioinspired Pillar Surface

AbstractRobust and controllable wet attachment like tree frog toe pads attracts worldwide attention owing to potential applications in wet climbing robots, medical devices, and wearable sensors. Instead of conventional uniform pillars, nonuniform pillar arrays with features of inclination and gradients are discovered as typical structures on tree frog toe pads, whereas their effects on wet friction have been ignored. Micro‐nano in situ observation demonstrates that such a nonuniform pillar surface brings about unique multi‐dimensional self‐splitting behaviors in interfacial liquid films and contact stress distribution to enhance the wet attachment. The self‐splitting of the interfacial liquid film breaks the thick large area liquid film into an immense number of more uniform and robust tiny thin liquid bridges. Furthermore, the contact stress is redistributed by the inclined and gradient pillar array with contact stress self‐splitting, where the peak normal separating stress decreases ≈91% and lateral stress transmission increases ≈63%. Such contact stress self‐splitting further improves the liquid film self‐splitting by forming sturdy thin liquid films even under a larger load, which generates more robust capillarity with enhanced strong friction. Finally, theoretical models are built for the multi‐dimensional self‐splitting enhanced wet attachment, and applications in robotic and medical fields are performed to validate its feasibility.

Interfacial Stress Analysis of PVD Thin Film Sensor Based on Finite Element

The ability to monitor fractures is based on the integration of PVD thin film sensors and substrates, and the interface stress distribution of the sensor under load which directly impacts the bonding performance. This research analyzes the impact of film elastic modulus and thickness on film-substrate interface stress using the Abaqus software to investigate the influence of PVD film sensor material properties on the film/substrate interface stress distribution. The higher the concentration of interfacial tension, the thicker the layer. The sensor's structural parameters were optimized and significantly enhanced crack detecting sensitivity. The results reveal that the PVD film sensor material can detect structural defects efficiently. The conclusions established in this work have some implications for the optimal design of PVD thin film sensors.

Interfacial Behaviour of Shield Tunnel Segment Strengthened by Thin Plate at Inner Surface

The strength and stiffness of a shield tunnel segment can be improved significantly by bonding a steel plate at its inner surface. In this kind of strengthened segment, interface debonding is usually the controlling failure mode, and it strongly depends on the interfacial stresses of the adhesive layer between the segment and the steel plate. To deepen the understanding of the interfacial behaviour, this study proposes a three-dimensional fine finite element (FE) model regarding the interfacial stresses. An existing full-scale experimental result is then employed to confirm the feasibility and accuracy of the proposed model. Further, the fine finite element model is used to calculate the interfacial stress distributions and to evaluate the structural parameters on the interfacial behaviour of the strengthened segment. A high concentration of interfacial stresses exists at the vicinity of the steel plate ends and the joints, which might result in premature failure at these locations. Both the normal and shear stresses at the interface are significantly influenced by the structural parameters. The findings in this study can provide guidance for the optimal design of strengthened shield segment that can prevent premature interfacial debonding.

Simulation Study on Mechanical Characteristics of Rock Bolt in Rock Mass with Bedding Separation Based on the Nonlinear Bond Slip Relationship

The mechanical equation on the mechanical characteristics of rock bolt in rock mass with bedding separation is established, based on the bond slip relationship of anchoring interface by the load transfer method. The validity of the numerical simulation is verified by the analytical solution of mechanical equation, when the bond slip relationship is linear. Further, the nonlinear bond slip relationship of anchoring interface is input into Flac3D by FISH. The mechanical characteristics of rock bolt are studied with the single and multiple bedding separation by numerical simulation. The results show that the axial force and the interfacial shear stress distribution curves on both sides are symmetrically distributed with the single bedding separation in rock mass, when the bedding separation is located in the center of the anchoring segment. When the bedding separation positions are not located in the center of the anchoring segment, the bedding separation value at different positions versus the axial force curves shows a single peak distribution. The axial force of rock bolt is mainly controlled by the side with the shorter anchoring length. Compared with the single bedding separation in rock mass, the additional stress of rock bolt occurred by the multiple bedding separation will superimpose each other. This paper leads to a better understanding for the load transfer mechanism for grouted rock bolt systems in rock mass with bedding separation and provides a reference for scientific support design and evaluation method.

The effect of inclusions embedded in the adhesive on the stress distribution and strength of single-lap adhesive joints: analytical and numerical analysis

In this study, an analytical analysis and a finite element method (FEM) study of the overall strength and interfacial stress distribution of single lap joints with an embedded inclusion in the adhesive layer were performed. The stress analysis model for the single lap adhesive joints including inclusion layers was firstly established, and the effects of the thickness and stiffness of the inclusions on their mechanical strength were discussed in detail. The results show that the bond strength of the adhesive joints is highly dependent on the geometric parameters and properties of the inclusions, and increases with the inclusion thickness and stiffness. When the stiffness of inclusions is lower than that of the adhesive layers, their introduction degrades the bond strength of the single lap adhesive joints. The interface stress distribution and failure mechanism caused by the inclusion layers are intuitively analyzed by FEM, which is in good agreement with the analytical analysis.

Estimation of Bond Strength and Effective Bond Length for the Double Strap Joint between Carbon Fiber Reinforced Polymer (CFRP) Plate and Corroded Steel Plate.

In this paper, we examine the development of the estimation models of bond strength and effective bond length for a double strap joint between carbon fiber reinforced polymer (CFRP) plate and corroded steel plate. The experimental study on the bond behavior between CFRP plate and corroded steel plate is summarized first and the analytical interfacial bond–slip model for CFRP plate externally bonded to corroded steel plate is proposed. Based on the theoretical stress analysis for the CFRP plate–corroded steel plate double-lap joint, the piecewise expressions of the interfacial shear stress and the normal peel stress of the interface between CFRP plate and corroded steel plate were established. The estimation models of the bond strength and the effective bond length for the double strap joint between the CFRP plate and the corroded steel plate were consequently developed on the basis of interfacial stress distribution equations and the stress boundary conditions. The comparison between the predicted and experimental results indicated that the proposed models could be adopted to predict the bond strength and effective bond length for the CFRP plates externally bonded to corroded steel substrates with reasonable accuracy. The proposed estimation models are expected to provide meaningful references and essential data for the reliable application of CFRP strengthening system to the performance improvement of corroded steel structures.

Numerical Investigation of Hydraulic Fracture Propagation Morphology in the Conglomerate Reservoir

The random distribution of gravels makes the conglomerate reservoir highly heterogeneous. A stress concentration occurs at the gravel-matrix interfaces owing to the embedded gravel and affects the local mechanical response significantly, making it difficult to control and predict hydraulic fracture (HF) propagation. The mechanism of HF propagation in conglomerate reservoirs remains unclear; thus, it is difficult to effectively design and treat hydraulic fracturing. Based on the global pore-pressure cohesive zone element (GPPCZ) model method, a two-dimensional (2D) fracture propagation model with flow-stress-damage (FSD) coupling was established to investigate HF nucleation, propagation, and coalescence in conglomerate reservoirs. This model was experimentally verified, and fractal theory was introduced to quantify the complexity of fracture morphology. The microscale interactions of the gravel, matrix, and interface have been taken into consideration during simulating HF propagation accurately in macroscale. The influence of the mechanical properties of gravel, matrix, matrix-gravel interface, and reservoir stress distribution state, on HF morphology (HF length, stimulated reservoir square, and HF complexity morphology), was investigated. Finally, the main factors affecting fracture propagation were analyzed. It was revealed that the difference between the mechanical properties of the gravel and the matrix in the conglomerate rock will affect the geometry of HF to varying degrees. The local behavior of fracture propagation is obviously dominated by the elastic modulus, tensile strength, and the strength for the matrix-gravel interface. However, the propagation of HF at the whole scale is mainly dominated by the horizontal stress state, including the minimum horizontal stress and horizontal stress difference. In addition, the difference in horizontal stress significantly affects the fracturing patterns (deflection, bifurcation, and penetration) when HF encounters gravel. In this study, a simulation method of HF propagation in conglomerate reservoirs is introduced, and the results provide theoretical support for the prediction of HF propagation morphology and plan design of hydraulic fracturing in conglomerate reservoirs.

Mushroom-Shaped Micropillar With a Maximum Pull-Off Force

Abstract The bioinspired structure of the mushroom-shaped micropillar has been considered a blueprint of functionalized adhesives due to its prominent dry adhesive performance. Among the design strategies, the geometrical parameters of the stalk and tip are of significance for improving their adhesion performance. In this study, mushroom-shaped micropillars in different diameters of the stalk and tip are fabricated by a new fabrication approach, and the adhesion measurements are performed to study the influences of loading conditions and geometrical parameters on the pull-off force. The experimental and numerical results suggest that the stalk and tip diameters strongly affect the interfacial detachment behavior and the pull-off force. Two detachment modes are distinguished by the positions of the crack initiation. Finite elemental analyses reveal the detachment mechanisms by the interfacial stress distribution and damage evolution. According to the detachment mechanisms, a structure design strategy for mushroom-shaped micropillar with maximum pull-off force is proposed. The present studies provide a fresh insight into the adhesion behaviors of mushroom-shaped micropillars and contribute to the future adhesive design.

Finite element modelling of the single fibre composite fragmentation test with comparison to experiments

This paper develops a finite element (FE) model of the single fibre fragmentation test designed for direct comparison with experimental results on an E-glass/epoxy system by McCarthy et al. (2015). Interface behaviour is modelled via a cohesive surface, and stochastic Weibull fibre strengths (determined by independent experiments) assigned at random to the elements along the fibre. Predictions from the model agree with experiment for a range of outputs: The evolution of the number of fibre breaks with strain is similar and breaks occur at random locations as required. The model also captures a transition to a Uniform (rather than Weibull) statistical distribution of break locations at later stages of the test consistent with recent experiments. The evolution of the cumulative distribution of fragment lengths is also similar to that of the experiment. In addition, fibre axial stress and interfacial shear stress distributions conform with experimental observation. Correct model predictions of break locations confirm the approach taken on assigning stochastic (Weibull) strengths along the fibre. The effectiveness of the FE model in capturing a number of key aspects of the fragmentation phenomenon suggest its usefulness as a tool in analysing and interpreting fibre fragmentation tests, including back-calculation of interfacial shear strength.

Interfacial Fracture Energy Between New Translucent Zirconias and a Resin Cement.

To determine the interfacial fracture energy (IFE) and stress distribution of Brazil-nut-shaped specimens made of translucent zirconia and resin cement. Three types of translucent zirconia were used: 3Y-TZP (high, Vita YZ HT), 4Y-TZP (super, Vita YZ ST), and 5Y-TZP (extra, Vita YZ XT). The adhesive surfaces were air abraded and 10-MDP-based resin cement was used. The cemented Brazil-nut-shaped specimens, with an elliptical defect in the center (as in real Brazil nuts), were thermally aged (5°C-55°C; 40,000 cycles). The IFE test was conducted with a piston to apply compression on the specimen, while the adhesive interface was positioned at four different angles (0, 10, 20, and 30 degrees) to measure the IFE during tensile, shear, and mixed failure modes. All adhesive interfaces were observed to determine failure patterns. The finite element analysis (FEA) was used to calculate tensile and shear stress distributions according to inclinations. Statistical analysis was conducted using the Kruskal-Wallis and Dunn's post-hoc tests (95%), as well as the Mann-Whitney test (95%) was applied to compare each group regarding the aging factor. According to Kruskal-Wallis and Dunn's post-hoc tests, there were no statistically significant differences between non-aged (p > 0.05) and aged materials (p > 0.05). However, there was a significant difference between aged and non-aged materials for all inclinations (p < 0.05) (Mann-Whitney test). According to the FEA, the compressive loading of Brazil-nut-shaped specimens at different angles showed a predominance of tensile stress at 0 degrees and shear stress at 30 degrees. The IFE under predominantly shear stresses is higher than when specimens are subjected only to tensile stresses, which allows the interpretation that failures in the oral environmental will probably occur preferentially under tensile stresses, because less energy is needed. All translucent zirconia bonded to resin cement has similar IFE, and thermal aging negatively affects these bonding interfaces.

Numerical simulation of the interfacial stress between epoxy resin and metal conductor of power equipment during epoxy curing

There are many metal and epoxy resin interfacial structures in power equipments, which can generate interfacial stress concentrations and may induce accidents due to epoxy curing. In this paper, through simplifying interface structure into a coaxial cylindrical model, the formation mechanism of interfacial stress concentration of this interface structure was studied during epoxy resin curing process. The strain and temperature at the interface were measured by the strain and temperature measurement system. Then a model for calculating the interface stress distribution was established, including the heat conduction module, curing dynamics module and curing deformation module. The validity of the model was verified by comparing the calculated results with the measured results. In addition, the air gap defects at the interface, appearing after the curing process, were simulated. The results show that, after the curing process, the first principal stress at the centre of the interface gradually increases from the centre to the edge, reaching 41.06 and 40.20 MPa at the upper and lower edges respectively. The existence of air gap leads to partial discharge at low voltage. The calculation model can be extended to the stress prediction between the metal and epoxy resin in other power equipments.

Machine learning-based optimization of the design of composite pillars for dry adhesives

A machine learning-based optimization strategy was used to find optimal designs for adhesive composite pillars with either high adhesion strength or high adhesion tunability. Neural networks were trained with data generated by finite element analysis to predict the adhesion strength of composite pillars with different designs; an average prediction error of less than 1% was achieved. Through a sensitivity study with the trained neural networks, it is found that the geometry of the stiff core above a critical cut off height has no effect on the interfacial stress distribution and the adhesion of the pillar. A randomly initialized constrained optimization algorithm was then applied to the trained neural networks to find the optimal composite pillar design. A composite pillar design with a stiff core that has an enlarged tapered flat end is optimal for realizing robust and high adhesion, since it can achieve high adhesion under different loading and contact conditions. The optimized pillar has a critical normal detachment force that is nearly 11 times that of a homogenous pillar and 1.7 times that of a composite pillar with a simple wide rectangular core under normal loading. A composite pillar with a thin flat stiff core shows the highest effective adhesion difference between being loaded with a normal force and being loaded with a moment.

Interface stress analysis and fatigue design method of steel-ultra high performance concrete composite bridge deck

Orthotropic steel-UHPC composite deck consists of orthotropic steel deck and ultra-high performance concrete (UHPC) layer. The shear force transferred by shear studs at the interface between steel deck and the UHPC layer is a key in fatigue design. Based on the finite element (FE) whole model analysis of composite deck, the detailed submodels of composite deck segments with single stud and single row studs were established respectively. It aimed to investigate the interface stress distribution and slip behavior of steel-UHPC composite deck under negative bending moment, and explore the failure mechanism of short headed stud. In term of shear force relationship, considering likely influences such as location and pattern of loading, flexural stiffness of the composite deck and stud arrangement, the whole model analysis results were compared with the submodel analysis results, and a fitting formula of shear relationship between the two models was proposed. A formula for fatigue design of short headed stud in orthotropic steel-UHPC composite deck was also proposed. In respect to the longitudinal and transverse shear force distribution of studs, composite deck stiffness, hot spot stress of fatigue-prone details of steel bridge deck and the maximum stress level of UHPC layer, the evaluation method of stud arrangement was proposed. The calculation results were assessed and compared with the stud layout schemes of three actual steel-UHPC composite decks, and the effectiveness of the design formula was validated.

A comparative study of bone remodeling around hydroxyapatite-coated and novel radial functionally graded dental implants using finite element simulation

This comparative study simulates bone remodeling outcome around titanium dental implants and compares the final bone configuration with the one around novel implants composed of radial functionally graded materials (FGMs) and the titanium implants with hydroxyapatite (HA) coating. A dental implant system embedded in 3D mandibular bone with masticatory loading was simulated by the finite element method. A bone remodeling algorithm was applied to cancellous and cortical bones. Young's modulus and von Mises stress were obtained to ensure bone homeostasis and evaluate the final bone configuration. Local stress distribution in the bone-implant interface was analyzed before and after bone remodeling. The average final Young's modulus of cancellous bone reached 2.68, 2.49, and 2.32 GPa for the FGM, HA-coated, and the titanium models, respectively. These values for cortical bone were 17.75, 16.86, and 17.20 GPa in the same order. Radial FGM implants generated the highest remodeling stimulus and bone density. Their superiority over the HA-coated models was confirmed by four implant surface stiffness values (10, 20, 30, and 40 GPa). Remodeling increased bone density around the implant, consistent with clinical data and reduced stress concentration in the cortical neck. The stress values were in the safe zone regarding overload-induced bone resorption. The findings of this study were substantiated by clinical images and bone density values from previous literature.

Flexural behavior of single-lap joints of similar and dissimilar adherends

During the past decades adhesively bonded joints have been intensively studied because of their extensive use from the simplest to the most complex constructions. The lap joint has become a standard test specimen with a view to assessing the strength and the properties of joints and for that reason numerous analytical relations have been proposed in order to predict the distribution of stresses within the joint as well as its strength. The present study concerns the experimental and analytical investigation of adhesively bonded single-lap joints between similar and dissimilar adherends in a three-point bending (3PB) arrangement. The experimental investigations include the effect of overlap length on the mechanical response and the strength of the joint. Single-lap joint coupons were manufactured from wood, Poly Vinyl Chloride (PVC) and titanium sheets and the corresponding joints were tested up to failure. The experimental results are grouped into two mains categories depending on the similarity of the adherends. The interfacial stress distribution within the adhesive, was computed using an appropriate analytical model and compared with Finite ElementAnalysis (FEA) results for the symmetric lap joints with very good correlation. The analytical model was used in order to assess the effect of different geometric and material parameters on joint stiffness and maximum normal and shear adhesive stresses at the ends of the overlap.

4D imaging of chemo-mechanical membrane degradation in polymer electrolyte fuel cells - Part 1: Understanding and evading edge failures

This work is a two-part article series on chemo-mechanical membrane degradation in fuel cells, wherein the present work in Part 1 investigates edge failure and Part 2 investigates failure within the active area of an edge-protected cell. Membrane electrode assembly (MEA) edges are sensitive regions that can cause premature failure. Here, two different MEA frame designs are implemented to study their robustness during a combined chemical/mechanical membrane degradation accelerated stress test. 4D in situ visualization by periodic, identical-location X-ray computed tomography is performed to understand and thus mitigate the design issues responsible for edge failures. Interfacial interaction of adhesive-containing polyimide gasket and MEA and non-uniform load distribution along MEA edges are identified as the two key contributors to premature edge failures, which introduce significant cell voltage decay due to permanent membrane deformation. Edge failure mitigation is demonstrated by using a non-adhesive sub-gasket along with increased gasket coverage area, which leads to: (i) delayed onset of edge failure; (ii) five times reduction in edge crack size; (iii) elimination of membrane tearing; and (iv) minimal impact of edge failures on cell performance, thus enabling a robust MEA edge wherein the performance-impacting failure is shifted to the more important active area regions. • 4D in situ visualization scheme is applied to study membrane failure at MEA edges. • Edge crack propagation at identical MEA locations is captured for the first time. • Edge failure occurs primarily due to non-uniform interfacial stress distribution. • A robust MEA frame design is demonstrated that can endure chemo-mechanical stress.

A generalized shear-lag theory for elastic stress transfer between matrix and fibres having a variable radius

A generalized shear-lag theory for fibres with variable radius is developed to analyse elastic fibre/matrix stress transfer. The theory accounts for the reinforcement of biological composites, such as soft tissue and bone tissue, as well as for the reinforcement of technical composite materials, such as fibre-reinforced polymers (FRP). The original shear-lag theory proposed by Cox in 1952 is generalized for fibres with variable radius and with symmetric and asymmetric ends. Analytical solutions are derived for the distribution of axial and interfacial shear stress in cylindrical and elliptical fibres, as well as conical and paraboloidal fibres with asymmetric ends. Additionally, the distribution of axial and interfacial shear stress for conical and paraboloidal fibres with symmetric ends are numerically predicted. The results are compared with solutions from axisymmetric finite element models. A parameter study is performed, to investigate the suitability of alternative fibre geometries for use in FRP.

A nonlinear analytical model for predicting bond behavior of FRP-to-concrete/steel substrate joints subjected to temperature variations

The effective interfacial stress transfer mechanism plays an important role in the mechanical performance of externally bonded fiber reinforced polymer (FRP) strengthened structures. Therefore, a profound understanding of the effect of temperature variation on the bond behavior of FRP-to-concrete/steel joints is helpful to improve the engineering specification of FRP strengthening technology. This paper presented a new analytical approach aimed at predicting the bond and debonding behavior of FRP-to-concrete/steel bonded joints under thermal loading. The proposed analytical solutions were derived based on an exponential temperature-dependent bond-slip model, and these solutions had a rather good prediction accuracy compared with the experimental and finite element numerical results. Some discussions about the bond behavior under temperature variations were carried out with an example. Results of this study showed that with the increase of bond length, there existed an upper limit value for the failure temperature variation, as well as for the maximum FRP stress. Temperature-dependent bond-slip relation was shown to have a great effect on the distribution of FRP stress and interfacial bond stress, especially on the temperature variation–free end slip response.