Interfacial Stress Concentrations Research Articles

Effects of coating on dynamic stress concentration in fiber reinforced composites

Fiber reinforced materials consisting of aligned fibers within a matrix are effective for applications in nature and engineering in which strength and stiffness are required in a dominant direction. The fibers are typically coated. Optimization of these materials is typically based upon choosing elastic properties of phases to reduce static stresses. However, materials with well-matched moduli can have mismatched acoustic impedance, and thus a fiber reinforced material that is strong in quasistatic loading may be weak in dynamic loading. To explore this trade-off, we modeled perfectly bonded, isotropic, linear elastic coated fibers in an infinite, isotropic, linear elastic matrix and calculated dynamic stresses and interfacial stress concentrations induced by continuous and transient waves using the wave function expansion method. Results revealed ways that the physical properties and geometrical dimensions of a coating around a fiber can be tailored to reduce dynamic stress concentration, and point to a pathway for improving the shock resistance of fiber reinforced materials.

Effect of the thickness of Ti intermediate layer on the microstructure and mechanical properties of the W/Ta multilayer composites

Mechanical properties of tungsten (W) multilayer composites can be modified by the toughening layers and interfacial microstructure between W layer and toughening layer. The W/Ta multilayer composite usually shows low tensile strength and toughness because of the low strength of Ta layers and large bonding strength between W layer and Ta layer. In order to improve the strength and toughness of the W/Ta multilayer composite, Ti intermediate layers with different thickness ranging from 30 µm to 100 µm have been added between W layer and Ta layer to get a kind of W/Ti/Ta/Ti multilayer composites. It can be found that the thickness of the Ti layer has a large effect on the microstructure and mechanical properties of the W/Ta multilayer composites. The solid solution with pure β-Ti and β-Ta are formed in the Ti layer when the thickness of Ti layer is lower than 50 µm. Ti layer composed of β-Ti, β-Ta and α precipitates is developed when the thickness of Ti layer is larger than 80 µm. The comprehensive mechanical properties of the W/Ta multilayer composites increase as the thickness of Ti intermediate layer increases. The tensile strength of the W/Ti/Ta/Ti multilayer composites is up to 642 MPa, which is much higher than that of the W/Ta multilayer composites (274 MPa). The tensile strength of these multilayer composites can be evaluated by a modified mix model considering the interfacial stress concentration factor. As the fracture mechanism of the W/Ta multilayer composites is also modified by addition of the Ti layer, the toughness of the W/Ti/Ta/Ti multilayer composites is improved because of the delamination along with the W/Ti interface and plastic deformation of the Ti/Ta/Ti layer in plane stress condition.

Using Quantitative Passive Thermography and Modified Paris-Law for Probabilistic Calculation of the Fatigue Damage Development in a CFRP-Aluminum Hybrid Joint.

Although metal to Carbon-fiber-reinforced-polymer (CFRP) hybrid-joints possess a high lightweight construction potential, their extensive application has to deal with interfacial stress concentrations promoting fatigue damage. Furthermore, the underlying damage processes and their influencing factors are still not completely understood. Besides interfacial property-gradients, generic shapes counteract a precise determination of local stresses or strains, respectively. Hence, new methods are required that combine non-destructive testing and fracture mechanics to account for the fatigue damage. In this work, data of mechanical fatigue testing of an aluminum-CFRP hybrid-structure is presented by means of the dynamic stiffness and the mechanical hysteresis. Additionally, in situ passive thermography allows for capturing the heat development due to delamination growth. Correlating the obtained data implies that faster delamination growth coincides with higher amplitude values of lock-in thermography and higher mechanical hysteresis. Supported by this observation, a model is formulated to calculate the local dissipation per loading cycle. Further integration into a Paris-law like formulation results in a calculation model to account for the mode-I fatigue delamination growth. Additional validation of the model parameters shows good agreement with the experimental data.

Thermoelastic problem of two arbitrarily-shaped inclusions

In this paper, we study the plane thermoelastic problem of an infinite matrix containing two interacting inclusions of arbitrary shapes for a uniform heat flux applied on the matrix remotely. We propose a numerical scheme for calculating the temperature and stress fields in the inclusion-matrix system by using the principle of superposition and the Faber series method. A group of numerical examples is given to demonstrate the stress field around two soft inclusions (softer than the matrix) of triangular and square shapes. The results show that the interfacial thermal stress concentration may be relieved by improving the thermal conductivities of the soft inclusions. In particular, we find that for soft inclusions with higher thermal conductivities (higher than the matrix), the interfacial thermal stresses could even reduce slightly when the inclusions approach each other. Additionally, it is shown that the thermal load-induced interaction between the inclusions decays much slower than the mechanical load-induced interaction between the inclusions as the inclusions move away from each other.

Biomechanical Evaluation of Bone Quality Effect on Stresses at Bone-Implant Interface: A Finite Element Study

The aim of this study is to evaluate the effect of the alveolar bone quality on von Mises stress at the bone-implant interface during occlusal loading. Four (3D) finite element models of fully osteointegrated 3-mm diameter × 11.5-mm length dental implant indifferent alveolar bone with different cortical bone thickness are created, using SolidWorks computer aided design software. The alveolar bone cortical-spongy bone ratio modelled includes I) 90%-10%, II) 60%-40%, III) 40%-60%, and IV) 10%-90%. These models are then exported to ABAQUS software and stress analyses are run under an occlusal load of 70 N acting on the platform face of the dental implant. Results of this study show that the implants are subjected to similar stress distributions in all models; maximum stress values are confined in the outer cervical plate of the cortical bone around the neck. This could explain bone loss and implant de-osseointegration. Peak stresses are lowest in the model with 90% cortical bone (14.2 MPa) and almost doubled in the model with 10% cortical bone (26.6 MPa). The stress values gradually reduce towards the apical area, demonstrating masticatory force transfer from implant to bone. Furthermore, both cortical and spongy bone structures exhibit highest stress values in the model with thinnest cortical layer. The high interfacial stress concentration near the implant-cortical bone junction could lead to bone failure or implant instability induced by fatigue or overload risk. Results of our study could be a first step towards the development of a clinical pre-operative planning tool for dental implantolgy.

Interfacial Stress Concentrations of Vertical Through-plate to H-beam Connections in CFT Column

Interfacial Stress Concentrations of Vertical Through-plate to H-beam Connections in CFT Column

Effect of surface micro-pits on mode-II fracture toughness of Ti-6Al-4V/PEEK interface

Herein, the delamination issue of TiGr(TC4/PEEK/Cf) laminate is addressed by investigating the influence of TC4(Ti-6Al-4V) surface micro-pits on mode-II interfacial fracture toughness of TC4/PEEK interface through experimental and finite element modeling. The micro-pits unit cell, unit strip and the end notched flexure (ENF) models are established based on the finite element simulations and the effect of micro-pit size parameters is studied in detail. The results of micro-pits unit cell model reveal that the presence of micro-pits can effectively buffer the interfacial stress concentration under mode-II loading conditions. Furthermore, the micro-pits unit strip model, with different micro-pit sizes, is analyzed to obtain the interface parameters, which are converted and used in the ENF model. Both the unit strip and ENF models conclude that the presence of interfacial micro-pits effectively improves the mode-II fracture toughness. It is worth mentioning that the utilization of converted interface parameters in ENF model avoided the limitation of micro-pit size and reduced the workload. Finally, the experimental and computational ENF results exhibited excellent consistency and confirmed the reliability of the proposed finite element models. The current study provides useful guidelines for the design and manufacturing of high-performance TC4/PEEK interfaces for a wide range of applications.

Interfacial stresses in reinforced concrete cantilever members strengthened with fibre-reinforced polymer laminates

Fibre-reinforced polymers have been increasingly used to strengthen reinforced concrete structures. However, premature brittle debonding failures may occur at the ends of externally bonded fibre-reinforced polymer laminates due to interfacial stress concentrations caused by stiffness imbalances. Although many studies exist on fibre-reinforced polymer-strengthened simply supported beams and slabs, the interfacial stress distributions in fibre-reinforced polymer-strengthened cantilever members are very different from those in simply supported members. Based on the assumptions of linear elasticity, deformation compatibility and static equilibrium conditions, the interfacial stresses in fibre-reinforced polymer-strengthened reinforced concrete cantilever members under arbitrary linear distributed loads were analysed. In particular, closed-form solutions were obtained to calculate the interfacial stresses under either a uniformly distributed load or a single concentrated load located at the overhanging end of the cantilever member. Existing test results on cantilever slabs strengthened by carbon fibre–reinforced polymer sheets were used to verify the model. According to the parametric analysis, the maximum interfacial stresses can be reduced by decreasing the fibre-reinforced polymer thickness, increasing the fibre-reinforced polymer bonding length and increasing the adhesive layer thickness, and by using less rigid fibre-reinforced polymer laminates with high tensile strengths. These results are useful for engineers seeking to optimize strengthening design parameters and implement reliable debonding prevention measures.

Proteoglycan degradation mimics static compression by altering the natural gradients in fibrillar organisation in cartilage

Structural and associated biomechanical gradients within biological tissues are important for tissue functionality and preventing damaging interfacial stress concentrations. Articular cartilage possesses an inhomogeneous structure throughout its thickness, driving the associated variation in the biomechanical strain profile within the tissue under physiological compressive loading. However, little is known experimentally about the nanostructural mechanical role of the collagen fibrils and how this varies with depth. Utilising a high-brilliance synchrotron X-ray source, we have measured the depth-wise nanostructural parameters of the collagen network in terms of the periodic fibrillar banding (D-period) and associated parameters. We show that there is a depth dependent variation in D-period reflecting the pre-strain and concurrent with changes in the level of intrafibrillar order. Further, prolonged static compression leads to fibrillar changes mirroring those caused by removal of extrafibrillar proteoglycans (as may occur in aging or disease). We suggest that fibrillar D-period is a sensitive indicator of localised changes to the mechanical environment at the nanoscale in soft connective tissues. Statement of SignificanceCollagen plays a significant role in both the structural and mechanical integrity of articular cartilage, allowing the tissue to withstand highly repetitive loading. However, the fibrillar mechanics of the collagen network in cartilage are not clear. Here we find that cartilage has a spatial gradient in the nanostructural collagen fibril pre-strain, with an increase in the fibrillar pre-strain with depth. Further, the fibrillar gradient changes similarly under compression when compared to an enzymatically degraded tissue which mimics age-related changes. Given that the fibrils potentially have a finite capacity to mechanically respond and alter their configuration, these findings are significant in understanding how collagen may alter in structure and gradient in diseased cartilage, and in informing the design of cartilage replacements.

Considerations for readdressing theoretical descriptions of particle-reinforced composite food gels.

The aim of this work was to address the ability of established theoretical models to describe the small deformation mechanical properties of particle-filled food protein gels. To this end, the effect of incorporating glass microspheres on the elastic modulus of heat-set whey protein isolate/xanthan gum gels is reported. Filler size and polydispersity strongly influenced the observed reinforcement with increasing filler content; however, these effects were also strongly correlated to the ionic strength of the gelator phase (0-200 mM NaCl). Fillers with greater polydispersity provided less reinforcement at high filler content, which was associated with improved packing efficiency. Increasing ionic strength reduced the extent of filler/matrix binding, drastically reducing the impact of the smaller glass microspheres (4 μm, 7-10 μm). Larger particles increased the elastic modulus at high salt content due to interfacial stress concentration and particle-particle contacts. Theoretical fits could not satisfactorily describe the general trend in reinforcement observed with increasing filler content, despite employing various methods to account for the effects of filler self-crowding. Using an empirical approach, we propose an alternative functional form which provides improved fits over the entire range of filler content investigated. This general power law (GPL) model provided physically reasonable values for the maximum packing fraction through an empirically-derived expression for the scaling exponent. A weighted average approach was also proposed to incorporate effects of imperfect filler/matrix adhesion. This method incorporates contributions of both bound and unbound fillers, providing a means to model the effect of increasing ionic strength.

New Optimized Numerical Solution of Interfacial Stresses in Steel Strengthened Structures with CFRP

Abstract Fiber-reinforced polymers (FRPs) are widely used in many structural applications, such as rehabilitating and reinforcing different structures that are subjected to risks of external damage, because of their excellent material properties. However, this technique leads to a delamination problem, which is a failure mode that occurs between the FRP patch and the retrofitted structure and is caused by the interfacial stress concentration in the adhesive layer, thereby reducing the effectiveness of this reinforcement technique. Thus, the objective of this research is to investigate the taper effect of multilayer FRP patch on the reduction of the interfacial stresses, including fiber orientation and shear-lag effect, while thermo-mechanical loads are applied. An analytical model is developed to predict the interfacial stresses in the adhesively patched structures with FRP. A numerical simulation using finite element method is also employed to validate the results from the analytical analysis. Finally, a parametric study is carried out in order to determine the influence of each parameter and to determine the optimal configurations.

Voltaglue Bioadhesives Energized with Interdigitated 3D‐Graphene Electrodes

Soft tissue fixation of implant and bioelectrodes relies on mechanical means (e.g., sutures, staples, and screws), with associated complications of tissue perforation, scarring, and interfacial stress concentrations. Adhesive bioelectrodes address these shortcomings with voltage cured carbene-based bioadhesives, locally energized through graphene interdigitated electrodes. Electrorheometry and adhesion structure activity relationships are explored with respect to voltage and electrolyte on bioelectrodes synthesized from graphene 3D-printed onto resorbable polyester substrates. Adhesive leachates effects on in vitro metabolism and human-derived platelet-rich plasma response serves to qualitatively assess biological response. The voltage activated bioadhesives are found to have gelation times of 60 s or less with maximum shear storage modulus (G') of 3 kPa. Shear modulus mimics reported values for human soft tissues (0.1-10 kPa). The maximum adhesion strength achieved for the ≈50 mg bioelectrode films is 170 g cm-2 (17 kPa), which exceeds the force required for tethering of electrodes on dynamic soft tissues. The method provides the groundwork for implantable bio/electrodes that may be permanently incorporated into soft tissues, vis-à-vis graphene backscattering wireless electronics since all components are bioresorbable.

Edge effects in adhesively bonded composite joints integrated with piezoelectric patches

Analytical electromechanical solutions to the interfacial stresses in the adhesively bonded composite joints integrated with piezoelectric actuator are presented in this paper within the framework of full layerwise theory. Two lap joints with and without interfacial void are studied, and the edge effects near the end-points of the bondline as well as around the void are investigated. Sets of fully coupled governing equations of equilibrium are derived using the principle of minimum total potential energy and are simultaneously solved using the state space approach. It was observed that the edge effects results in significant interfacial stress concentrations around the void edges that may cause propagation of microcracks and debonding.

Tensile stress distribution in maxillary central incisors restored with cast-made and prefabricated dental posts.

Abstract: Aim: To analyze and compare the tensile stress distribution in endodontically treated teeth restored with cast-made (Ni-Cr and gold) and prefabricated (titanium and glass fibre) dental posts. Methodology: Four three-dimensional finite element (FE) models of a maxillary central incisor restored with Ni-Cr cast-made (Model Ni-Cr), gold cast-made (Model GO), prefabricated titanium (Model TI) and prefabricated glass fibre (Model FP) posts were constructed. An oblique loading of 100N was applied to each three-dimensional model. Tensile stress distribution within the root dentine and at the post and surrounding structure interfaces were analysed. Results: In all the FE models studied, a higher magnitude of tensile stresses was observed on the palatal aspect of the cervical dentin as compared to the labial aspect and progressively decreases from the outer to the inner part of the root. The gold cast-made and glass fibre post models showed significantly less tensile stress concentration in the post-core component than the other experimental models. The maximum tensile stress was seen on the palatal aspect of the Ni-Cr compared to other posts. The higher magnitude interfacial tensile stress concentration was observed in a pulpless tooth restored with a Ni-Cr cast-made post, followed by titanium and gold cast-made posts, respectively. However, the minimum interfacial tensile stress was noticed in a pulpless tooth restored with a glass fibre post. Conclusion: Glass fibre posts tend to transfer tensile stress more homogenously within the tooth and at interfaces than the other types of investigated posts.

Elastic analysis of interfacial stress concentrations in CFRP-RC hybrid beams: Effect of creep and shrinkage

A simple closed-form solution to calculate the interfacial shear and normal stresses of retrofitted concrete beam strengthened with thin composite plate under mechanical loads including the creep and shrinkage effect has been presented in this paper. In such plated beams, tensile forces develop in the bonded plate, and these have to be transferred to the original beam via interfacial shear and normal stresses. Consequently, debonding failure may occur at the plate ends due to a combination of high shear and normal interfacial stresses. These stresses between a beam and a soffit plate, within the linear elastic range, have been addressed by numerous analytical investigations. Surprisingly, none of these investigations has examined interfacial stresses while taking the creep and shrinkage effect into account. In the present theoretical analysis for the interfacial stresses between reinforced concrete beam and a thin composite plate bonded to its soffit, the influence of creep and shrinkage effect relative to the time of the casting, and the time of the loading of the beams is taken into account. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions.

Low-velocity impact of woven CFRE composites under different temperature levels

Low-velocity impacts are unavoidable during the assembly stages and service life of aircrafts, marine and automobiles composite structures. This work aims to study the damage resistance of woven carbon fiber reinforced epoxy (CFRE) composite laminates subjected to a wide range of low-velocity impacts (15–85J). The composite laminates were fabricated using prepreg technique. The impact tests were carried out using falling weight impact tester at room temperature (RT), 50 °C and 75 °C. The selected test temperature of 50 °C represents the summer midday temperature of some countries, while the highest temperature value (75 °C) was used to study the possible effects of unexpected temperature rise. Compression-after-impact (CAI) were undertaken to measure their residual strength. Specimens that were tested at low impact energies (15 and 35J) as well as at higher temperature level (75 °C) are not penetrated by the impactor. Therefore, part of the kinetic energy is absorbed by the specimen in the form of damages. A second part is stored in the specimen as elastic energy, which is restituted back to the impactor for its rebound. Though no penetration is observed, the highest reduction in the stiffness and strength is observed at 75 °C. This is explained in terms of the softening and plastification of the epoxy matrix, the debonding at the interfacial fiber/matrix and the increase in the interfacial stress concentration of the re-solidified matrix. The threshold energy to initiate damage in the composite plates is measured/estimated using four methods and ranges between 2.8 and 9.6J.

Effects of PCB Pad Metal Finishes on the Cu-Pillar/Sn-Ag Micro Bump Joint Reliability of Chip-on-Board (COB) Assembly

While solder bumps have been used as the bump structure to form the interconnection during the last few decades, the continuing scaling down of devices has led to a change in the bump structure to Cu-pillar/Sn-Ag micro-bumps. Cu-pillar/Sn-Ag micro-bump interconnections differ from conventional solder bump interconnections in terms of their assembly processing and reliability. A thermo-compression bonding method with pre-applied b-stage non-conductive films has been adopted to form solder joints between Cu pillar/Sn-Ag micro bumps and printed circuit board vehicles, using various pad metal finishes. As a result, various interfacial inter-metallic compounds (IMCs) reactions and stress concentrations occur at the Cu pillar/Sn-Ag micro bumps joints. Therefore, it is necessary to investigate the influence of pad metal finishes on the structural reliability of fine pitch Cu pillar/Sn-Ag micro bumps flip chip packaging. In this study, four different pad surface finishes (Thin Ni ENEPIG, OSP, ENEPIG, ENIG) were evaluated in terms of their interconnection reliability by thermal cycle (T/C) test up to 2000 cycles at temperatures ranging from −55°C to 125°C and high-temperature storage test up to 1000 h at 150°C. The contact resistances of the Cu pillar/Sn-Ag micro bump showed significant differences after the T/C reliability test in the following order: thin Ni ENEPIG > OSP > ENEPIG where the thin Ni ENEPIG pad metal finish provided the best Cu pillar/Sn-Ag micro bump interconnection in terms of bump joint reliability. Various IMCs formed between the bump joint areas can account for the main failure mechanism.

Tissue-Engineering Strategies for the Tendon/Ligament-to-Bone Insertion

Injuries to connective tissues are painful and disabling and result in costly medical expenses. These injuries often require reattachment of an unmineralized connective tissue to bone. The uninjured tendon/ligament-to-bone insertion (enthesis) is a functionally graded material that exhibits a gradual transition from soft tissue (i.e., tendon or ligament) to hard tissue (i.e., mineralized bone) through a fibrocartilaginous transition region. This transition is believed to facilitate force transmission between the two dissimilar tissues by ameliorating potentially damaging interfacial stress concentrations. The transition region is impaired or lost upon tendon/ligament injury and is not regenerated following surgical repair or natural healing, exposing the tissue to risk of reinjury. The need to regenerate a robust tendon-to-bone insertion has led a number of tissue engineering repair strategies. This review treats the tendon-to-bone insertion site as a tissue structure whose primary role is mechanical and discusses current and emerging strategies for engineering the tendon/ligament-to-bone insertion in this context. The focus lies on strategies for producing mechanical structures that can guide and subsequently sustain a graded tissue structure and the associated cell populations.

Effect of shear deformation on interfacial stresses in plated beams subjected to arbitrary loading

Bonding a fibre reinforced polymer (FRP) composite or metallic plate to the soffit of a reinforced concrete (RC), timber or metallic beam can significantly increase its strength and other aspects of structural performance. These hybrid beams are often found to fail due to premature debonding of the plate from the original beam in a brittle manner. This has led to the development of many analytical solutions over the last two decades to quantify the interfacial shear and normal stresses between the adherends. The adherends are subjected to axial, bending and shear deformations. However, most analytical solutions have neglected the influence of shear deformation of the adherends. For the few solutions which consider this effect in an approximate manner, their applicability is limited to one or two specific load cases. This paper presents a general analytical solution for the interfacial stresses in plated beams under an arbitrary loading with the shear deformation of the adherends duly considered. The shear stress distribution is assumed to be parabolic through the depth of the adherends in predicting the interfacial shear stress and Timoshenko's beam theory is adopted in predicting interfacial normal stress to account for the shear deformation. The solution is applicable to a beam of arbitrary prismatic cross-section bonded symmetrically or asymmetrically with a thin or thick plate, both having linear elastic material properties. The effect of shear deformation is illustrated through an example beam. The influence of material and geometric parameters of the adherends and adhesive on the interfacial stress concentrations at the plate end is discussed.

Analytical Solution for Fracture Analysis of CFRP Sheet–Strengthened Cracked Concrete Beams

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.