Interfacial Frictional Stress Research Articles

Elucidating the in-process interfacial friction regime and thermal responses during inertia friction welding of dissimilar superalloys

Inertia friction welding (IFW) is a robust joining technique, particularly for hybrid structures composed of similar or dissimilar alloys. The highly transient thermal responses during friction heating produce variations in heat generation and temperature distribution, which are crucial for designing and controlling the IFW process. This study establishes a finite element (FE) model of the IFW process for dissimilar superalloys and investigates the transition of the interfacial friction regime and its impact on the transient thermal responses. The in-process state variables, including the frictional stress and heat flux, along with their spatial distributions at the welding interface, are obtained through FE simulations. The predicted results indicate that the transient transition of the friction regime produces two Coulomb friction zones accompanied by a shear friction zone. To capture the transient evolution of the friction regime, a novel phenomenological formula is developed. This model accurately predicts the shrinkage and disappearance of the two Coulomb friction zones and the enhancement of the shear friction zone from the initial 0.42 R0–0.87 R0 (where R0 is the workpiece radius) to the entire interface during the IFW process. The variations in interfacial heat flux, frictional stress, and temperature caused by the transition of the friction regime are comprehensively analyzed. Our results reveal that the transition from the Coulomb friction regime to shear friction regime at the interface is beneficial for temperature homogenization at the welding interface. The FE simulation results are validated against published experimental measurements, which confirms the accuracy and reliability of the FE model.

A nonlinear crack bridging model for the composites with curved reinforcement

Both artificial nanocomposites and biological materials typically contain nanofillers with curved forms, which allows them to develop excellent mechanical characteristics and functionalities. In order to measure the fracture toughness of the composites contributed by the bridged reinforcement with curved morphology, a theoretical model based on micromechanics and taking into account the reinforcement's curvature is developed in this study. It is found that the fracture toughness of the composites is more sensitive to the reinforcement waviness rather than its curvature. Larger waviness contributes to higher fracture toughness. By increasing platelet length, volume fraction, and modulus ratio, as well as increasing the interfacial frictional stress after debonding, the fracture toughness of composites is dramatically improved. This study expands our understanding of how reinforcement morphology affects fracture behaviors of composites and offers theoretical insights for elucidating the structure–property relationships of complex micro-structured advanced composites.

Synergistic effects of silica fume and slag on the microstructure and mechanical behaviors of ultra-high-strength and high-ductility cementitious composites

This work evaluated the synergistic effects of silica fume (SF) and ground granulated blast furnace slag (GGBS) binders on the microstructure and mechanical behavior of Ultra-High-Strength and High-Ductility Cementitious Composites (UHS-HDCC). The potential mechanisms of the synergistic effects of GGBS and SF on the fresh flow, mechanical properties, and microstructure of UHS-HDCC were investigated. The results indicated that the use of GGBS instead of SF could effectively improve the flowability of fresh mortar and UHS-HDCC mixtures. The hydration process of cement and the heat of hydration were delayed and reduced when the content of GGBS was below 25 %. Moreover, reducing the SF while increasing the GGBS reduced the compressive strength of the composites but had little effect on the flexural strength. The compressive strength of all specimens remained above 120 MPa. All specimens achieved strain-hardening behavior after the initial cracking of the matrix in uniaxial tension and four-point bending, and had a variety of cracking characteristics. Furthermore, the range of ultimate tensile strain capacity and mid-span deflection were 3.26 %–5.79 % and 16.73 mm–33.05 mm, respectively. The S10G30 specimen reached a maximum tensile strain energy of 414.6 kJ/m3. As the level of GGBS replacing SF increased, the number of cracks in the tensile region or the compressive bending surface of UHS-HDCC specimens increased, while the crack width and crack spacing decreased. The relatively smooth sidewalls of the fiber tunnels and fiber exit points meant that matrix fragments attached to the fiber surfaces were caused by matrix fragments generated throughout the matrix cracking process. Besides, the interfacial frictional stress and slip hardening coefficient acquired from the single fiber pullout test reduced with increasing levels of GGBS replacing SF.

Study on stick-slip vibrations of gelatin hydrogel in dry and wet conditions

High deformability, softness, adhesiveness and swelling ability make hydrogels highly suitable for biomedical and pharmaceutical applications like tissue regeneration, drug delivery system, wound dressing and plastic surgery. Direct shear sliding experiments are conducted on gelatine hydrogel using low velocity linear tribometer under the dry conditions and in presence of water. Sliding experiments are conducted for 10% concentration at sliding velocities of 0.075 to 0.1 mm/sec at normal stresses of 0.613–1.839 KPa. Interfacial frictional shear stress (ι) of the sliding interfaces is measured using Slide-Hold-Slide experiments in stick–slip regime. Effects of sliding velocity(v), normal stress (σn) and the lubrication medium on shear stress, stick–slip behaviour and frequency are studied in detail. Presence of water at the interface reduces the shear stress substantially for all the velocity ranges. Frequencies of stick–slip cycles were relatively higher in presence of water when compared to that of dry condition. Increase in the normal stress leads to decrease in the vibration frequency of stick–slip cycles.

Interfacial stress transfer in monolayer and few-layer MoS2 nanosheets in model nanocomposites

Understanding the stress transfer mechanisms from a polymer matrix to two-dimensional (2D) reinforcements is essential for the preparation of high performance nanocomposites. In this study, the interfacial stress transfer from a flexible polymer substrate to monolayer and few-layer molybdenum disulfide (MoS2) under tension has been investigated. Layer-dependent and strain-dependent photoluminescence (PL) spectroscopy were used to examine the stress transfer efficiency. The interlayer stress transfer efficiency of MoS2 was determined to be in the range of 0.76–0.86, higher than that of graphene. The transfer of strain from the polymer substrate to the flakes was derived through strain-dependent band shifts. With progressive loading, the strain distribution in monolayer MoS2 can be described by the shear-lag, partial-debonding and total-debonding models. The interfacial shear and frictional stresses were calculated to quantify the strength of the MoS2/polymer interface. It was found that the strength of the interface is similar to the strength of a graphene-polymer interface. Strain mapping was performed at different strain levels and it was found that the strain distribution in bilayer MoS2 is similar to the case of a monolayer sample. The interfacial shear strength remains almost unaffected, while the stress transfer length increases with increasing layer number.

Thermal-fluid-structure coupling analysis of void defect in friction stir welding

Understanding the void defect formation mechanism and simultaneous predicting the tool service life in friction stir welding are critical for optimizing the welding parameters. However, the void defect formation mechanism in friction stir welding is not yet elucidated. In this study, a novel integrated thermal-fluid-structure coupling model of the friction stir welding process was proposed for simultaneous prediction of the weld formation and tool service life. A new non-uniform distribution model of the tool-workpiece contact pressure was proposed to describe the interaction between the tool and the workpiece. The void defect formation mechanism was quantitatively studied using the proposed integrated thermal-fluid-structure coupling model. The results show that the plastic material flows in the horizontal direction and can completely fill the cavity behind the tool for the welding condition of forming a sound weld. While the tool-workpiece contact interfacial frictional shear stress in the rear of the tool is decreased significantly which leads to a severe decrease in the plastic material flow velocity. Therefore, after bypassing the tool from the retreating side, the plastic material at the bottom of the weld stagnates, and void defect forms in the middle and lower part of the weld at the advancing side. The difference between the maximum and the minimum tool-workpiece contact pressure could serve as a numerical criterion to predict void defects. A sound joint is formed when the difference is lower than the critical value of 15 MPa, while a void defect is formed in the weld if it is higher than this critical value. The maximum equivalent stress acting on the tool is located at the pin root with severe stress concentration at a high welding speed. The front of the tool is subjected to tensile stress while its rear is subjected to compressive stress, therefore the tool is apt to fracture at its root under an inappropriate welding condition. The average normal stress of the tool varies periodically with its period consistent with the rotation period of the tool. The service life of the tool is decreased with the increase in welding speed and the decrease in rotation speed. The model is validated by experimental results.

Transition of interfacial friction regime and its influence on thermal responses in rotary friction welding of SUS304 stainless steel: A fully coupled transient thermomechanical analysis

Rotary friction welding (RFW) has proved to be a successful joining technology in advanced materials manufacturing. The highly transient thermal responses during friction heating, including heat generation and temperature distribution, are essential factors in the design and control of the RFW process. In this study, a 2D axisymmetric finite element (FE) model for the RFW process of SUS304 stainless steel is presented in order to investigate the transition of the interfacial friction regime and its influence on the transient thermal responses. By using the FE simulation, the in-process state variables, i.e., frictional stress and heat flux, and their spatial distribution at the welding interface are obtained. The FE simulation is validated by published experimental results. A new analytical model for the transient evolution of the friction regime is concluded from the FE simulation, which accurately predicts the shrinkage of the Coulomb friction zone while the Shear friction zone builds up from the workpiece periphery. Comprehensive analysis is conducted on the variation of interfacial heat flux, frictional stress and temperature resulted from the transition of friction regime. It is revealed that the transition of the interfacial friction regime from the Coulomb friction regime to the Shear friction regime is preferable for temperature homogenization at the welding interface.

Interlayer properties of tungsten fibre-reinforced composites and their determination by different methods

Tungsten features a unique combination of properties which makes it a candidate for the use as a plasma-facing material in fusion reactors. The main drawbacks of tungsten are its brittleness at moderate temperature and the susceptibility to embrittlement during operation. To overcome this limitations tungsten fibre-reinforced tungsten composites (W f /W) have been developed. The interlayer between the tungsten fibre and tungsten matrix enables the activation of extrinsic mechanisms to improve the toughness similar to ceramic fibre-reinforced ceramics. In addition, the evaluation of the interlayer parameter is a necessary for the efforts to model the composite behaviour. Therefore, single fibre model systems with Er 2 O 3 and Y 2 O 3 as interlayer were evaluated by push-out and pull-out tests. The results were used to determine the interfacial shear strength ( τ d e b ), the interfacial frictional shear stress ( τ f r ) and the fracture energy of the interlayer ( Γ i ). In addition, tensile tests on the composite material were performed to evaluate the interfacial frictional shear stress. The evaluation methods were critically discussed as the results show that the influence of the evaluation method is larger than the actual tested interlayer material. • Evaluation of the interlayer properties of W f /W with three different methods. • The interlayer testing method has a larger influence on the evaluated values than the actual tested interlayer material. • For the further material screening pull-out test is preferred over push-out test for W f /W.

Rheological behavior, internal stress and structural changes of ultra-high-filled wood-flour/high-density polyethylene composite in shear flow field

Wood plastic composite (WPC) is a green material with an excellent performance, environmental protection and recyclability, and potential in the field of green construction materials. A rheological study of WPC provides a key theoretical basis for mold design, equipment upgrade, production efficiency and quality improvement. In this study, an innovative approach was proposed to analyze the characters of a wood-flour/high-density polyethylene composite with an ultra-high wood-flour content (70 wt%–90 wt%, UWPC) in a shear flow field, including (1) a redesigned shear rheometer that can seal heat and (2) an analysis of the interactions between components in the UWPC by decomposing and calculating the stress during the rheological process. The stress that was generated by the UWPC melt during shearing originated mainly from wood particles and weak interfacial friction, whereas the stress that was generated by the polymer matrix could be ignored. Therefore, the polymer matrix served only to connect the wood particles and transfer stresses. Maleic-anhydride-grafted polyethylene (MAPE) enhanced the UWPC melt system interface, made the system structure more uniform and increased its fluidity. The enhancement effect of MAPE on the weak interface of UWPC melt was calculated quantitatively by calculating and analyzing the weak interfacial friction stress. The MAPE strengthening rate for the weak interface was 72.3%, 81.1% and 91.9% at 130 °C, 150 °C and 170 °C, respectively. These findings can be applied to improve WPC processing and equipment manufacture.

An improved analytical model for inversely determining multiple interfacial parameters from single fiber micro-Raman and fragmentation tests

Inverse identification of fiber/matrix interfacial properties from single fiber micro-Raman or fragmentation test requires an accurate model prediction of stress transfer near a fiber break. The existing models adopt very simplified interfacial constitutive law, and hence the three key interfacial parameters, i.e. interfacial shear strength, fracture energy and frictional stress, cannot be determined simultaneously from the single fiber composite test. In this work we develop an analytical model for stress transfer analysis near a fiber break, in which the three important interfacial parameters are all introduced. The present model is well validated by finite element simulations through comprehensive comparisons of stress profiles in fiber and fiber/matrix interface. As demonstrated by several examples, our model shows a good ability to inversely identify interfacial shear strength, fracture energy and frictional stress simultaneously for both single fiber micro-Raman and fragmentation tests. This unified model is also expected to minimize the error between the interfacial parameters identified by different test methods.

Acoustic induced antifriction and its effect on thermo-mechanical behavior in ultrasonic assisted friction stir welding

The developed ultrasonic assisted friction stir welding (UaFSW) process can reduce the welding load and improve the joint quality, but the interaction mechanism between the exerted ultrasonic vibration and the thermo-mechanical behavior induced by friction stir welding is not yet elucidated. In this study, the classical Norton friction model is modified with the acoustic stress work, and the effect of ultrasonic vibration on the contact state at the tool/workpiece interface is analyzed. The relative velocity between the tool and its adjacent contacting material is taken as one of the main influencing factors to calculate the interfacial friction shear stress. A non-uniform interfacial friction model is combined with the modified constitution equation. And a fully coupled model of UaFSW process is developed to analyze the ultrasonic field, interfacial contact state, heat generation, plastic material flow and heat transfer phenomena. It is found that ultrasound can effectively reduce the interfacial friction stress, and this antifriction effect is more obvious in the welding direction which coincides with ultrasonic vibration. The ultrasound exerted along the radial direction of tool in UaFSW process results in acoustic softening of base material and friction reduction at the tool/material interface. Both effects change the heat generation, material flow and temperature field in UaFSW. Thus, the torque and traverse force are lowered with the ultrasonic induced reduction of interfacial stress, and the high quality of welds are obtained with the improvement of material flow. The model is experimentally validated by comparison of predicted and measured tool torque, heat input and thermal cycles.

Research on the selection principle of upper sheet in double-layer sheets hydroforming

By combining the advantages of sheet hydroforming and multi-layer sheet deep drawing in wrinkling suppression, double-layer sheets hydroforming process was proposed. Different from the multi-layer sheet deep drawing, the two sheets are not bonded together, the upper sheet is introduced to eliminate the wrinkling defects occurred in the formed lower sheet. Therefore, how to determine the appropriate upper sheet is the key point in this process. Based on this, some investigations in selecting the appropriate upper sheet have been down, but the selection principle for the appropriate upper sheet is still ambiguous. To make clear this, in this paper, a semi-ellipsoid curved sheet part made of 2198 Al-Li alloy with an axial length ratio of 0.9 was chosen as the target part, by changing the thickness of the upper sheet (5A06-O aluminum alloy); the primary and secondary order of the anti-wrinkle ability and interfacial friction stress in suppressing the wrinkle defects were investigated and the selection principle for appropriate upper sheet is obtained.

Monotonic axial compressive behaviour and confinement mechanism of square CFRP-steel tube confined concrete

Steel tube confined concrete (STCC) is widely used in the vertical members of high-rise buildings such as columns. The axial load is not directly resisted by the steel tube in STCC, but is resisted via the interfacial frictional stress between steel tube and concrete core, which is different with that of concrete filled steel tube (CFT) members and would effectively suppress the outward local buckling of steel tube at early stage. Recently, fibre-reinforced polymer (FRP) confined STCC presents a potential to enhance the ductility and durability of such vertical elements. This paper presents an experimental study on monotonic axial compressive behaviour of carbon FRP (CFRP) confined STCC (CFRP-STCC) stub column and an analytical study on the confinement mechanism of and the ultimate axial bearing capacity of the elements. A three-stage confinement mechanism involving the different contributions of the steel tube and the CFRP wrap in CFRP-STCC elements was proposed based on the test results. A prediction model of the ultimate axial bearing capacity of CFRP-STCC stub columns was developed subsequently. Results show that the presence of CFRP wrap enhances effectively the load-bearing capacity and the ductility of steel tube confined plain concrete and reinforced concrete elements, and significantly prevents the local buckling of the steel tubes in the elements. The proposed prediction model of ultimate axial bearing capacity assesses test results with a great agreement.

Interfacial crack propagation between a rigid fiber and a hyperelastic elastomer: Experiments and modeling

The interfacial adhesion between a metal fiber and an elastomer plays a key role in maintaining the structural integrity of stretchable interconnect composites under large strains, but their interfacial adhesion mechanisms have remained elusive. To this end, the interfacial debonding behavior and physical mechanisms between a rigid metal fiber and hyperelastic elastomer matrix are investigated by means of fiber pullout tests and numerical modeling. An improved cylindrical specimen is adopted for a copper fiber pulling out from polydimethylsiloxane (PDMS). A numerical model is developed to simulate the interfacial cracking using the Yeoh hyperelastic model and cohesive damage model. We report a first-of-its-kind experimental observation on the pullout process of a rigid fiber from elastomers. The results demonstrate that there exist three stages in the fiber pullout process: elastic deformation, interfacial debonding and interfacial friction. The interfacial shear strength between the rigid fiber and the PDMS is proportional to the reciprocal of the mixing ratio of the PDMS, and the interfacial friction stress varies inversely with the 1.25 power of the mixing ratio. Enhancing the mixing ratio of the PDMS decreases the strength of both the PDMS and the interface between the PDMS and the metallic materials while increasing their ultimate stretch due to a decrease of chemical crosslinking degree within the PDMS and at the interface between the PDMS and the metallic materials. The results obtained from the numerical model agree well with the experimental data. This work can aid in the design of advanced stretchable interconnect composites.

Three Specimen Geometries and Three Methods of Data Evaluation in Single-Fiber Pullout Tests

The shapes of force–displacement curves recorded in single-fiber pullout and microbond tests are analyzed within the framework of a stress-based model of interfacial debonding. Three characteristic points allowing one to evaluate the local interfacial strength parameters, the local interfacial shear strength (IFSS), and the interfacial frictional stress, using several different methods, can be marked out in each curve. The “alternative” method based on the measured peak force and initial postdebonding force appeared to be more reliable than the “traditional” one using the debond force for calculating the local IFSS in fiber–matrix systems. The effect of specimen geometry on force–displacement curves and on calculated local interfacial strength parameters was investigated. Though the “equivalent cylinder” approximation often yields a good estimate of these parameters, there is a need for a method which would explicitly include the actual specimen geometry.

Enhanced load transfer by designing mechanical interfacial bonding in carbon nanotube reinforced aluminum composites

The interfacial native oxide layer always acts as barriers preventing effective interfacial bonding and load transfer in carbon nanotube reinforced aluminum (CNT/Al) composites. In this work, annealing-controlled reaction between the interfacial native oxide layer and Mg element was attempted to improve interfacial bonding in CNT/AlMg composite, and mechanical interfacial bonding featured with direct CNT-Al contact and well-reserved CNTs was achieved in the 1 h annealed composite. For the 2 h annealed composite, chemical interfacial bonding featured with interfacial aluminum carbide (Al4C3) was established due to progressive interfacial reaction. Further tensile tests and numerical analysis revealed that, owing to barrier-free load path and increased interfacial friction stress, the mechanical interfacial bonding significantly enhanced CNT load transfer effect from 30.9 MPa to 59.4 MPa, which was close to shear-lag model prediction of 59.9 MPa. While, the chemical interfacial bonding led to a combined strengthening effect of 58.8 MPa from CNT-Al4C3 hybrid as a comprised outcome of Al4C3 strengthening and CNT damage. Thus, the design of mechanical interfacial bonding should be a superior strategy for improving interfacial bonding and enhancing mechanical performance in CNT/Al composites, considering its effective protection of CNT structure integrity and suppressed formation of hydrolysable Al4C3 phase.

Influence of Strain Induced Martensite Formation of Austenitic Stainless Steel on Wear Properties

In this study, the wear characteristics and behaviors of both AISI304 and AISI316 austenitic stainless steels were evaluated under the different rotation speeds, using block-on-ring wear testing machine. From the microstructure analysis, at the first, the transition of strain induced-martensite was observed in wear-affected region of both AISI304 and AISI316. It is considered that the interfacial friction stress between test block and ring may trigger the phase transformation of meta-stable γ-fcc to e/α′ martensite. Moreover, it is also observed that the amount of martensite increases with increasing rotation speed for both AISI304 and AISI316. The thickness of transformed martensite was deeper in AISI304, compared to AISI316 at all rotation speed. It can be explained by that the existence of molybdenum in AISI316 increases stacking fault energy, which can retard nucleation and growth of strain induced-martensite. Furthermore, reverse transformation was also detected at subsurface layer of both AISI304 and AISI316.

Why Should the "Alternative" Method of Estimating Local Interfacial Shear Strength in a Pull-Out Test Be Preferred to Other Methods?

One of the most popular micromechanical techniques of determining the local interfacial shear strength (local IFSS, τd) between a fiber and a matrix is the single fiber pull-out test. The τd values are calculated from the characteristic forces determined from the experimental force–displacement curves using a model which relates their values to local interfacial strength parameters. Traditionally, the local IFSS is estimated from the debond force, Fd, which corresponds to the crack initiation and manifests itself by a “kink” in the force–displacement curve. However, for some specimens the kink point is hardly discernible, and the “alternative” method based on the post-debonding force, Fb, and the maximum force reached in the test, Fmax, has been proposed. Since the experimental force–displacement curve includes three characteristic points in which the relationship between the current values of the applied load and the crack length is reliably established, and, at the same time, it is fully determined by only two interfacial parameters, τd and the interfacial frictional stress, τf, several methods for the determination of τd and τf can be proposed. In this paper, we analyzed several theoretical and experimental force–displacement curves for different fiber-reinforced materials (thermoset, thermoplastic and concrete) and compared all seven possible methods of τd and τf calculation. It was shown that the “alternative” method was the most accurate and reliable one, while the traditional approach often yielded the worst results. Therefore, we proposed that the “alternative” method should be preferred for the experimental force–displacement curves analysis.

Effects of short fiber and pre-tension on the tensile behavior of basalt textile reinforced concrete

Tensile behaviors of basalt textile-reinforced concrete (BTRC) with five layers of textile and different volume contents of short carbon, glass, and steel fibers (0.5 vol%, 1.0 vol%, and 1.5 vol% for carbon and steel fibers and 0.5 vol% for glass fiber) are investigated under uniaxial tensile loading by using an MTS load frame. Pre-tensioned BTRC with 0.5 vol% of short carbon and steel fibers is also tested. The reinforcing mechanisms of different short fibers and pre-tension conditions are illustrated, and digital image correlation analysis is performed to obtain full-field strain and crack distribution. First crack stress, ultimate load, and toughness are significantly improved by short fibers and pre-tension. The improving trend differs with the increase in volume fractions of various short fibers. Short glass, steel fibers, and pre-tension can increase crack numbers with reduced crack spacing and width, whereas short carbon fiber exerts no obvious effect. In addition, the shape of cracks changes from linear pattern to curved form due to the addition of short fibers. The critical crack widens quickly, whereas other non-critical cracks diminish when the strain level goes beyond the ultimate strain. On the basis of the Aveton–Cooper–Kelly model, the relationship of the normalized effective factor and interfacial frictional shear stress is derived.

The surface displacements of an elastic half-space subjected to uniform tangential tractions applied on a circular area

In this paper we derive the complete surface displacement field of an elastic half-space uniformly loaded with unidirectional tangential tractions over a circular area, as those resulting, for instance, from frictional interactions in soft single-asperity contacts. The elastic problem is solved in the framework of linear elasticity, by opportunely integrating over the circular loaded area the fundamental Cerruti solution for the displacements induced by a unit tangential force. We discuss the displacement field peculiarity, in terms of symmetry and slope. The solution here presented allows to treat analytically different contact mechanics problems involving spheres and soft substrates, where interfacial frictional shear stresses can be assumed uniform.