High Interfacial Shear Stress Research Articles

The Role of Typha angustifilia Fiber-Matrix Bonding Parameters on Interfacial Shear Strength Analysis.

The microbond test of natural fibers tends to produce scattered interfacial shear stress (IFSS) values. The sources of this scattering are known, but the roles they play in producing high IFSS scattering remain to be investigated. In this study, a numerical method was used to simulate microbond testing and to examine the experimental parameters in a microbond test of Typha angustifolia fiber/epoxy. Three parameters were considered: fiber diameter, fiber length embedded in the epoxy, and the distance between the vise and the specimen. The geometries were modeled and analyzed by ABAQUS software using its cohesive zone model features. There were two types of contact used in this analysis: tie constraint and surface-to-surface. The results showcased the roles of the following experimental parameters: a larger fiber diameter from a sample increased the IFSS value, a longer embedded length reduced the IFSS value, and a shorter vise–specimen distance increased the IFSS value. The IFSS scattering in the microbond test could have originated from the interaction between these parameters. Of the three parameters, only the vise–specimen distance was found to be able to be reasonably controlled. When the IFSS value was atypically large, fiber diameter and/or embedded length potentially drove the scattering. This study advises further compilation and classification of the role of each experimental parameter in modulating the IFSS value.

Mechanical behavior of SiC/SiC composites reinforced with new Tyranno SA4 fibers: Effect of interphase thickness and comparison with Tyranno SA3 and Hi-Nicalon S reinforced composites

The development and availability of a new 3rd generation SiC fiber, the Tyranno SA4 (SA4), are promising for the processing of higher neutron and/or corrosion resistant SiC/SiC composites. Despite its promising properties, especially the higher crystallinity and thermal conductivity than the Hi-Nicalon S fiber, the previous Tyranno SA3 (SA3) reinforcement leads to low damage tolerant SiC/SiC composites, restraining its use as a reinforcement. This is the consequence of very high interfacial shear stress, whatever the pyrocarbon interphase thickness. In this work, tubular samples where produced with both reinforcements and with two different pyrocarbon interphase thicknesses for tensile mechanical characterizations to access the potential benefit of the new SA4 fibers. The tensile mechanical properties of SA4 composites are highly enhanced compared to SA3 composites. The low damage tolerance drawback of SA3 composites is solved with higher failure strain for SA4-based composites. Tensile mechanical tests also highlight an unusual influence of pyrocarbon interphase thickness on the composites tensile modulus and proportional limit stress. The thinner interphase (≈ 60 nm) is the most interesting for repeatable mechanical properties and induces high proportional limit stress. Unloading – reloading cycles during tensile mechanical tests also highlight the benefit of this new fiber compared to Hi-Nicalon S. This work demonstrates that the substitution of SA3 by the new SA4 SiC fiber reinforcement in the processing of SiC/SiC composites is a great opportunity for the ceramic matrix composites development and especially for nuclear applications.

Flexural strengthening of reinforced concrete beams through externally bonded FRP sheets and near surface mounted FRP bars

Regarding the high interfacial shear stresses at end-plate, and end-plate cover separation in externally bonded methods, bar end-interfacial debonding and bar-end cover separation in near-surface mounted technique, a hybrid strengthening method called combined externally bonded near-surface method was proposed. In this method, the externally bonded and near-surface mounted techniques complement each other and mutually overcome their limitations. The purpose of this study is to numerically investigate the flexural strengthening efficiency of reinforced concrete beams with combined externally bonded FRP sheets and near-surface mounted FRP rods. The numerical analyses were conducted with finite element software ABAQUS 6.11, which can accurately simulate the experimental investigations on the flexural behavior of reinforced concrete beams strengthened with FRP composites. Validation of finite element simulation was confirmed first by making a comparison with the experimental study presented in the literature for both un-strengthened and strengthened beams with FRP materials. The verified model of the un-strengthened beam, which serves as a control beam, was used to simulate reinforced concrete beam strengthened with externally bonded FRP sheets and combined externally bonded near-surface mounted technique. The numerical results of mid-span bending moment deflection, ultimate bending moment, failure deflection, and ductility index were reported. Based on the results of this study, it is concluded that the developed finite element models for the externally bonded, near-surface mounted, and combined externally bonded near-surface techniques can be used by structural engineers as an alternative solution in design-oriented parametric studies of strengthened reinforced concrete elements. The performance of the combined externally bonded near-surface mounted technique was confirmed by making the comparison between the results of the intended method with other strengthening techniques.

Sodium Hydroxide and Potassium Permanganate Treatment on Mechanical Properties of Coconut Fibers

The purpose of this research is to determine the tensile stress and interfacial shear stress of coconut fiber that has been treated Sodium Hydroxide and Potassium Permanganate. This research is divided into three stages: soaking for 3 hours, testing, and data analysis. Coconut fiber is soaked in a solution of 5%, 10%, 15%, and 20% sodium hydroxide solution. Then, soaking in a solution of Potassium Permanganate with concentrations of 0.25%, 0.50%, 0.75%, and 1.00%. After that, the coconut fiber is dried in the furnace at 90°C for 5 hours. Then, coconut fiber was tested for tensile stress and interfacial shear stress. Based on the results of the test can be concluded that the treatment of Sodium Hydroxide and Potassium Permanganate affect tensile strength, and the shear strength of coconut fiber. The largest tensile stress was obtained in the treatment of Sodium Hydroxide with a concentration of 20% ie 289.94 N/mm2, while the highest interfacial shear stress was obtained in the treatment also on Sodium Hydroxide with a concentration of 20% ie 3.09 N/mm2.

Design Rules for Enhanced Interfacial Shear Response in Functionalized Carbon Fiber Epoxy Composites.

Carbon-fiber reinforced composites are ideal light-weighting candidates to replace traditional engineering materials. The mechanical performance of these composites results from a complex interplay of influences operating over several length and time scales. The mechanical performance may therefore be limited by many factors, one of which being the modest interfacial adhesion between the carbon fiber and the polymer. Chemical modification of the fiber, via surface grafting of molecules, is one possible strategy to enhance interactions across the fiber-polymer interface. To achieve systematic improvements in these modified materials, the ability to manipulate and monitor the molecular structure of the polymer interphase and the surface grafted molecules in the composite is essential, but challenging to accomplish from a purely experimental perspective. Alternatively, molecular simulations can bridge this knowledge gap by providing molecular-scale insights into the optimal design of these surface-grafted molecules to deliver superior mechanical properties. Here we use molecular dynamics simulations to predict the interfacial shear response of a typical epoxy/carbon-fiber composite for both pristine fiber and a range of surface graftings. We allow for the dynamic curing of the epoxy in the presence of the functionalized surface, including cross-link formation between the grafted molecules and the polymer matrix. Our predictions agree with recently reported experimental data for these systems and reveal the molecular-scale origins of the enhanced interfacial shear response arising from functionalization. In addition to the presence of interfacial covalent bonds, we find that the interfacial structural complexity, resulting from the presence of the grafted molecules, and a concomitant spatial homogeneity of the interphase polymer density are beneficial factors in conferring high interfacial shear stress. Our approach paves the way for computational screening processes to design, test, and rapidly identify viable surface modifications in silico, which would enable rapid systematic progress in optimizing the match between the carbon fiber treatment and the desired thermoset polymer matrix.

3D statistical failure analysis of monolithic dental ceramic crowns

For adhesively retained ceramic crown of various types, it has been clinically observed that the most catastrophic failures initiate from the cement interface as a result of radial crack formation as opposed to Hertzian contact stresses originating on the occlusal surface. In this work, a 3D failure prognosis model is developed for interface initiated failures of monolithic ceramic crowns. The surface flaw distribution parameters determined by biaxial flexural tests on ceramic plates and point-to-point variations of multi-axial stress state at the intaglio surface are obtained by finite element stress analysis. They are combined on the basis of fracture mechanics based statistical failure probability model to predict failure probability of a monolithic crown subjected to single-cycle indentation load. The proposed method is verified by prior 2D axisymmetric model and experimental data.Under conditions where the crowns are completely bonded to the tooth substrate, both high flexural stress and high interfacial shear stress are shown to occur in the wall region where the crown thickness is relatively thin while high interfacial normal tensile stress distribution is observed at the margin region. Significant impact of reduced cement modulus on these stress states is shown. While the analyses are limited to single-cycle load-to-failure tests, high interfacial normal tensile stress or high interfacial shear stress may contribute to degradation of the cement bond between ceramic and dentin. In addition, the crown failure probability is shown to be controlled by high flexural stress concentrations over a small area, and the proposed method might be of some value to detect initial crown design errors.

Experimental study and molecular dynamics simulation of dynamic properties and interfacial bonding characteristics of graphene/solution-polymerized styrene-butadiene rubber composites

The effect of the vinyl content on microstructure, thermodynamics and dynamics properties of graphene/SSBR composites was investigated. SSBR with the highest vinyl content has the highest interfacial shear stress by pullout simulation.

Innovative hybrid bonding method for strengthening reinforced concrete beam in flexure

This paper presents the test results of the experimental behavior of reinforced concrete (RC) beams strengthened with the newly proposed hybrid bonding technique, which is the wise combination of plate bonding and near surface mounting (NSM) technique. Many studies have focused on the experimental behavior of RC beams strengthened with externally bonded steel plates. However, the presence of high interfacial shear stresses at the end of the plate may reduce the resistance to failure of the strengthened structure. Recently, to a certain extent, the development of the near surface mounted strengthening technique can reduce the problem of this premature end debonding failure. However, the application of this method may be limited due to the absence of sufficient width and clear cover of the existing unstrengthened deficit beam. The hybridization bonding method could eliminate the limitations of these two methods to some extent. As data on the performance of structures strengthened with the hybrid bonding method are not available, the main purpose of this paper is to investigate the experimental behavior of RC beams strengthened with the hybrid bonding method. Seven medium-sized beams (one control and six strengthened) were tested. Steel plates and bars were used for strengthening. The failure load, failure mode, deflection, strain and cracking behavior are discussed. The test results confirm that the proposed hybrid bonding is the most effective strengthening technique without significantly hampering the ductility and its performance is 32% higher than that of corresponding plate bonding method through improvement of the bond performance.

New method for interfacial evaluation of carbon fiber/thermosetting composites by wetting and electrical resistance measurements

Interfacial properties were evaluated for carbon fiber (CF) with different thermosetting polymeric matrices in composites. CF tow was wet by phenolic or epoxies, and the interfacial adhesion evaluated by electrical resistance changes. The interfaces between two types of CF tows with phenolic resin and three types of epoxies were investigated. The change in electrical resistance was found to depend on the wettability of CF by the polymer resins, with the more obvious resistance changes being associated with better wettability. The electrical resistance changes were measured 20 min after the polymer resin was dropped on the CF tow. To confirm the relationship between changes in resistance and interfacial properties, both interfacial shear stress (IFSS) and interlaminar shear stress (ILSS) were also measured. The results of these mechanical measurements were generally consistent with the electrical resistance measurements in that the materials with high electrical resistance also exhibited high IFSS and ILSS.

Hybrid carbon fibre–carbon nanotube composite interfaces

Abstract Both low and high modulus carbon fibres are coated with carboxylated single wall carbon nanotubes (SWNTs). It is shown that it is then possible to follow, for the first time, the local deformation of low modulus carbon fibres and composite interfaces using Raman spectroscopy. By deforming coated single carbon fibre filaments in tension, and following the shift in the position of a band located at ∼2660 cm−1 (2D band) it is possible to calibrate the local stress state of a fibre embedded in an epoxy resin. To follow the interface between the fibres and the epoxy resin, a thin film model composite is used. Point-to-point variation of stress along a single fibre, both inside and outside the resin, is recorded and stress transfer models are used to determine the interfacial shear stress (ISS). Values of the ISS (∼20 MPa) are obtained for the thin film model composites for untreated high modulus fibres. A beneficial interfacial effect of the presence of SWNTs on the surface of the high modulus carbon fibre samples is demonstrated resulting in an increase in the maximum ISS (>30 MPa) compared to uncoated samples. Similarly coated low modulus fibres exhibit a very high ISS (>50 MPa). These increases are attributed to an enhanced contact between the resin and the fibres due to an increased surface area as a result of the nanotubes and additional bonding caused due to the presence of carboxylate groups.

Analysis of Annular and Stratified Flows for Vapor Condensation Heat Transfer in Horizontal Tubes

The greater part of the horizontal condenser tube is occupied by the stratified and annular flows, which play important roles in condensation heat transfer coefficients. Both a volume of fluid (VOF) interface tracking method and k-ε two equations model was applied to analyze the characteristics of the stratified and annular flows for horizontal tubes, obtaining distribution of velocity, contours of both temperature and local condensation heat transfer coefficients. The computation shows that an annular flow having thinner liquid film attached on the inner surface of tubes appears at the inlet of the horizontal condenser tube because vapor exerts a higher interfacial shear stress on the gas-liquid interface, therefore, there are a higher local condensation heat transfer coefficients there. Next, as vapor quality decreases and condensate gravity increases along the condenser tube length, the condensation flow pattern transforms from the annular flow into the stratified flow in which local condensation heat transfer coefficients decreases and distributes unevenly along the circumference as liquid film at the bottom of the condenser tube become thicker. In addition, in the stratified flow, a wave structure is formed in the middle of the condensate pool at the bottom of the condenser tube because condensate on the top of the condenser tube slides into the bottom of the condenser tube and collapse each other. The heat transfer rate calculated by the present method is compared to those predicted from a Shah correlation, the agreement is found to be good, and both errors is within 7%.

A new CFRP anchor for preventing separation of externally bonded laminates from concrete

Twelve identical concrete prisms were strengthened with Carbon Fibre-Reinforced Polymer (CFRP) laminate strips on two opposite faces and the laminates were anchored using the newly developed CFRP π-anchor. Prisms were tested in tension to investigate the effectiveness of the anchor to possibly delay delamination and/or to prevent the complete separation of the laminate from the prism. The salient feature of the anchor is its wide head to resist high interfacial shear stresses and its shanks that were inserted in predrilled holes to provide mechanical anchorage and to resist pull-out. The anchor doubled the tensile load-carrying capacity, effectively delayed delamination and prevented the CFRP laminate from full separation. Furthermore, the strengthened prisms experienced noticeable deformation.

Observations from testing of indeterminate FRP-plated steel beams on flexible supports

This paper discusses the fabrication of, failure testing of and test data analysis from multiple two-span continuous FRP-plated steel I-beams. The study incorporated variation between beams of the layout and amount of FRP plating along the positive and negative moment zones, the novel concept of bonding (rather than welding) the steel plate stiffeners to the beams’ webs and flanges, use of longitudinal strain gauges on the bonded and exposed surfaces of key FRP plates, and elastomeric (hence flexible) pads to distribute stresses into the webs at the supports. The bonded steel stiffeners significantly delayed local buckling of the beams. Use of strains from only the exposed surface of any FRP plate produces erroneous magnitude changes and sign reversal of the high interfacial shear stresses near plate curtailment, relative to the prudent approach of using strain averages from the plate’s bonded and exposed surfaces. Differences between bonded and exposed surface strains enabled quantification of interfacial normal stresses. Relaxation of the rigid support assumption in favour of experimentally determined flexible support stiffnesses significantly nudges the predicted moment distributions toward the test distributions. This enables reliable assessment of the potential for end-peel failure arising from offsets between plate curtailment and contraflexure.

Investigation on Intermediate Crack Induced Debonding Failure of FRP Laminates in Flexurally FRP-Strengthened RC Beams

Debonding failure is commonly observed in FRP-strengthened RC beams. It can be classified into two types: plate end debonding and intermediate crack induced debonding. Intermediate crack induced debonding is the main failure mode in FRP-strengthened RC beams.There is no specialized experiment study on this failure mode. To investigate intermediate crack induced debonding failure of FRP-strengthened reinforced concrete beams, 9 CFRP-strengthened reinforced concrete beams were tested. Based on the investigation of the strain in the FRP laminates, the width of the main crack and tributary crack, as well as the phenomena of the test beams, the whole debonding process can be separated into three stages: initial debonding stage, stable debonding stage, and the final debonding failure. The fundamental reason of intermediate crack indued debonding are the opening-up of intermediate crack and the flexural deformation induced high interfacial shear stress and peeling stress. When the principal tension stress acting on the concrete substrate exceeded the tension stress of concrete, the CFRP laminate would debond from the concrete substrate.

Analysis of active cooling through nickel coated carbon fibers in the solidification processing of aluminum matrix composites

Solidification processing is widely used for synthesizing fiber reinforced aluminum matrix composites. Some recent studies have used cooling through reinforcing carbon fibers to obtain better control over the solidification microstructure of cast metal-matrix composites. The present study is focused on understanding the effect of active cooling and the role of nickel coating on the structure of the composite. Multiphysics analysis is conducted to simulate the composite system and obtain temperature distribution and stresses at various stages of cooling. The analysis is limited to cooling of the liquid melt and the solidification processes is not modeled. Results show that the direction of the heat flux along the fiber can reverse as the system is allowed to cool. The results show that the von-Mises stresses do not have a severe gradient in the nickel coating but the shear stress gradient is very steep. If the structure solidifies with steep heat flux gradient and high interfacial shear stresses then the nickel coating of the fibers can fail. The analysis shows that the failure of the nickel coating can occur under the effect of thermal stresses generated in the system as a result of temperature gradients.

On the state and severity of frictional sliding contacts between nominally flat metallic surfaces

This paper develops and uses an analytically traceable model to study the state and severity of frictional contacts between nominally flat surfaces of elastic-perfectly-plastic materials. The state and severity of the contact is characterized by four area-of-contact variables. They include the real area of contact, the portion of the real area of contact in full plastic deformation, the portion of the real area of contact with high interfacial shear stresses, and the percentage increase of the real area of contact due to the friction-induced junction growth. Problems are studied ranging from very elastic to very plastic contacts under good-to-poor boundary-lubrication conditions. Dimensionless results are presented in a two-dimensional parameter space of the surface plasticity index and a system friction coefficient. Judged by these dimensionless results, the state and severity of the frictional contact is primarily governed by two model parameters: the plasticity index and the coefficient of friction. It is fairly insensitive to other system parameters such as the applied normal load and height distribution of surface asperities. The results may shed some lights on surface run-in, system operations, and possible friction instability of the tribo-system.

Experimental and Computational Modeling of Low Cycle Fatigue Damage of CFRP Strengthened Reinforced Concrete Beams

Improvement in the short-term behavior of deficient reinforced concrete beams is very well known through extensive testing of reinforced concrete beams strengthened by externally bonded carbon fiber reinforced polymer strips. However, long-term performance of such beams needs to be assessed before the method can gain full acceptance. Prominent modes of failure in strengthened reinforced concrete beams reported in literature are concrete crushing in compression, carbon fiber reinforced polymer plate rupturing in tension and peeling of the carbon fiber reinforced polymer plate due to high interfacial shear and peeling stresses at the plate cut-off point. Modeling of the overall system of strengthened reinforced concrete beams requires modeling of the individual components that ultimately leads to failure i.e., concrete, carbon fiber reinforced polymer and concrete - carbon fiber reinforced polymer. Component elasto-damage constitutive models are developed for concrete crushing, carbon fiber reinforced polymer rupture and concrete-carbon fiber reinforced polymer interface failure, calibrated by data from both suitably designed experiments and from experiments reported in literature, and are utilized to investigate the low cycle fatigue behavior of reinforced concrete beams strengthened by carbon fiber reinforced polymer strips. The predictive ability of the model is tested by comparison to data from two different beam strengthening schemes i.e., with end anchorage and without end anchorage.

FINITE DEFORMATION BEHAVIOR OF A SOFT POLYMER COATING ADHERING TO TITANIUM ALLOY (TI6AL4V)

This article presents the experimental methodology which elucidates the stress-state dependency of the deformation behavior of a soft polymer coating adhering to titanium alloy ( Ti 6 Al 4 V ). The methodology included in situ observations of the evolution of the deformation behavior of elastic film and a rigid substrate under a micro-indenter. The tip of indenter is driven to the polymer film coating at constant displacement rate until interfacial fracture is observed. However, the stress and strain fields which develop in the coating during penetration have been calculated using finite element analysis. For the experimental results, the interfacial stresses are affected by the constraint of the rigid substrate. When the crack was constant, the region in which the stress field increased with time. In addition, the maximum shear stress and the initiation of the interfacial crack were in this region. Furthermore, the high interfacial shear stresses are distributed over a large radial distance, which lead to the fracture initiation in the region of interface.

Geometric edge shape based optimization for interfacial shear stress reduction in fiber reinforced polymer plate retrofitted concrete beams

This paper focuses on the reduction of interfacial shear stresses when using bonded laminates in strengthening existing structures. The presence of high interfacial shear stresses that develop near the end of the composite known as edge effect may compromise the resistance to failure of the strengthened structure. It is known that the reduction in plate thickness decreases the magnitude of stress concentration at the plate extremities. Another way to tackle the problem is proper design of the plate end shape. In this regard in this paper, an analytical solution for reduction of interfacial shear tresses based on edge shape optimization is developed and extended by numerical modal analysis procedure of finite element method (FEM) in retrofitted concrete (RC) beam with fiber reinforced polymer (FRP) plate with different end shapes with different mechanical loading type. The approach allows selecting the proper configuration that reduces considerably the interfacial shear stress concentration.

On the reduce of interfacial shear stresses in fiber reinforced polymer plate retrofitted concrete beams

One major problem when using bonded fiber reinforced polymer (FRP) plate is the presence of high interfacial shear stresses near the end of the composite (edge effect) which might govern the failure of the strengthening schedule. It is known that the decrease of plate thickness reduces the magnitude of stress concentration at plate ends. Another way is to use a plate end tapering. In this paper, the analytical solution of interfacial shear stresses obtained has been extended by a numerical procedure using the modal analysis of finite element method (FEM) in a retrofitted concrete (RC) beam with the FRP plate with tapered end, which can significantly reduce the stress concentration. This approach allows taking into consideration the variation of elastic properties of adhesive and plate as well as the complicated geometrical configurations and effects of thermal loads.