Frictional Shear Stress Research Articles

Interphase behavior of titanium matrix composites at elevated temperature

This paper examines the effect of temperature and thermal exposure on the interphase behavior of continuous fiber reinforced titanium metal matrix composites. The system considered is SCS-6/Timetal-21S. Elevated temperature fiber push-out tests were conducted to determine the effect of test temperature on interphase shear properties. Corresponding variations of debonding shear strength and frictional shear stress with test temperature are presented and discussed. Thermal exposure, both in a vacuum and an air environment, has been conducted on specimens, with temperatures up to 650°C and exposure times of up to 100 h. The resulting size and composition of the interphase have been examined. Fiber push-out tests were carried out at room and elevated temperature on the aged specimens. Results are discussed in terms of the influence of relaxation and oxidation on the debond shear strength. Using the experimentally determined interphase shear properties, the interphase toughness has been calculated and discussed in relation to interface decohesion models.

Elastic instabilities at a sliding interface

I consider a semi-infinite elastic solid sliding on a flat hard substrate. I present a linear instability analysis to determine when the steady sliding motion becomes unstable with respect to infinitesimal perturbations. I consider a general case where the interfacial frictional shear stress depends not only on the sliding velocity but also on a state variable. I show that when the pressure in the contact area between the solids is constant, no linear instability occurs if the kinetic friction coefficient increases monotonically with the sliding velocity, $d{\ensuremath{\mu}}_{\mathrm{k}}{/dv}_{0}>0.$ However, when the pressure at the interface varies spatially, elastic instabilities may also occur when $d{\ensuremath{\mu}}_{\mathrm{k}}{/dv}_{0}>0.$ I discuss the physical origin of this effect, and suggest that these instabilities may be precursors of the Schallamach waves.

Simulation study on interfacial debonding caused by spatially distributed damages in unidirectional fibre composites using 2D model

Influences of spatially distributed damage (broken fibre, matrix and interface) on the progress of interfacial debonding were studied by using a two-dimensional model. It was shown that the debonding progresses intermittently in weakly bonded composite, resulting in serrated stress-strain curves; the overall debonding progresses more rapidly with increasing number of damages due to the enhanced mechanical interactions among damages; and the progress of debonding varies, depending on the geometrical position of the damages, residual stresses and frictional shear stresses at debonded interface. The practically observed feature that the strongly and weakly bonded composites tend to show noncumulative and cumulative fracture modes, respectively, could be realised in the computer by the proposed simulation method.

Effect of Twisting on Tensile Failure of Impregnated Yarns with Broken Filaments

The tensile strength of impregnated strand/unidirectional composite depends on many factors including the strength of the interface. Recent efforts in trying to increase the strength of composites have been directed towards increasing the strength of the interface through fiber surface treatment and better resin systems that give better interfacial shear strength. However, the role of frictional forces and analysis of twisted impregnated yarns have largely been ignored. Twisted geometry gives rise to lateral pressure that contributes to the frictional shear stress; therefore, for a twisted yarn, interfacial shear strength would be higher than that of a impregnated strand/unidirectional composite. Twisted yarns have the obvious disadvantage that their unidirectional strength suffers because of fiber obliquity. This work examines the effect of the lateral pressure on the tensile strength of twisted yarns with broken filaments. Studies have been carried out on both the helical wound impregnated yarns and helical twisted impregnated yarns.

Simulation Study on Influences of Damage-Induced Mechanical Interactions, Residual Stresses and Interfacial Frictional Shear Stress on Interfacial Debonding in Multifilamentary Composites

Simulation Study on Influences of Damage-Induced Mechanical Interactions, Residual Stresses and Interfacial Frictional Shear Stress on Interfacial Debonding in Multifilamentary Composites

Bridging Stress Distributions During Fatigue Crack Growth in Continuously Reinforced [0] Metal Matrix Composites

An optimization procedure was developed to deduce the fiber bridging stresses from crack opening displacements measured in situ during crack growth. This procedure was used to determine the bridging stress distribution during fatigue crack growth in a unidirectionally reinforced metal matrix composite (SCS-6/TIMETAL®21S). The bridging stress is non-zero at the crack tip contrary to predictions from conventionally used shear lag models. The bridging stress at the crack tip is proportional to the applied far-field stress. The deduced bridging law is similar to the new shear lag models with non-zero bridging stresses at the crack tip. Any bridging model can be used to predict the crack growth behavior by choosing appropriate values of the frictional shear stress (T). Consequently, the magnitude of the stresses in the fibers bridging the crack will depend on the fiber bridging model. Hence, the fiber tensile strength required to predict the onset of fiber failure will also depend on the fiber bridging model.

Time-dependent shear velocities in channel routing

The main aim of this study is the experimental investigation of friction velocities and shear stresses in rivers under unsteady flow conditions. Special measurements of mean velocities and other hydraulic parameters were made in two small lowland rivers in central Poland. Four controlled flood waves were released and analysed in the selected reaches. The main hydrometric characteristics and the relationship between water level and discharge were established. Friction velocities were obtained directly from the full St Venant equations of motion, as well as from only the steady momentum equation, and their time-dependent forms were established. Both these approaches provided similar results when the unsteadiness parameter was relatively low. It appeared that real friction velocities were much larger than those obtained from the common uniform flow formula. The passing hydrograph influenced the value of the shear velocity significantly.

Interfacial shear force microscopy

We present an experimental investigation of lossy and reactive shear forces at the nanometer scale using quartz-crystal tuning-fork shear-force microscopy. We show that this technique allows us not only to quantitatively measure viscous friction and elastic shear stress with a combination of high spatial and force resolution (better than 10 nm, and less than 1 pN, respectively), but also to obtain such quantities with the tip positioned at any arbitrary distance away from direct electrical tunnel contact with the sample surface. We are proposing that, even under vacuum conditions, the measured viscous and elastic shear stress (i.e., velocity dependent) are directly attributable to a third body filling the tip-sample gap. A simple model is given that allows us to obtain its local viscosity and shear modulus as a function of the tip-sample distance, showing that tuning-fork shear-force microscopy can be applied to quantitative analysis in nanotribology.

Modeling and Experimental Validation of Superconductor Tape Rolling

Efficient, defect-free manufacturing of high-temperature superconducting (HTS) wires and tapes is critical to a variety of defense and electrical power applications. To contribute to the improvement of these manufacturing operations, an analytical and experimental study of the early stages of the multipass rolling process for transforming HTS wires into tapes was conducted. The rolling process was simulated by a three-dimensional (3D) finite element model that uses the Drucker-Prager Cap plasticity model to represent the powder core and a Von-Mises plasticity model with isotropic hardening to represent the silver sheath. The predicted cross-sectional geometry of the tapes is compared with experiments. The results show that the tape cross-sectional geometry and powder core sizes can be predicted accurately. Further, alternate boundary conditions were found to have minimal effect on the predicted cross-sectional geometry for the range of reductions considered, even though the frictional shear stress distributions were significantly different.

A Study of the Lubrication Behavior of Solid Lubricants in the Upsetting Process

Four kinds of solid lubricants were tested in order to examine the frictional characteristics and the yield shear stress by the friction testing apparatus developed by the authors. The frictional shear stresses τf increased approximate linearly with the punch pressure p in every solid lubricant, and the friction coefficients μ were approximately constant. The yield shear stress k was also increased with the punch pressure p. In order to investigate lubrication behaviors of solid lubricants in upsetting processes, FEM simulations for upsetting of circular plates have been carried out. Some experimental upsetting tests of circular plate were tried using some kinds of solid lubricants. It has been confirmed that the solid lubricants can lubricate successfully without metal-to-metal contact when μD at the interface between tool and solid lubricant is relatively low and μM at the interface between work piece and solid lubricant is relatively high. On the other hand, metal-to-metal contact occurs easily at the peripheral regions of the work piece, when μD is relatively high and μM is relatively low. [S0742-4787(00)00804-3]

Friction under elastic contacts

The friction coefficients and shear stresses for a sliding contact at low loads are calculated on the basis of the elastic contact and adhesion beneath a sliding spherical indenter and compared to experimental data. The contribution of the adhesion appeared to be negligible. The method allows either the critical shear stress to be assessed from experimentally determined friction coefficient and normal load or the friction coefficient to be calculated as a function of normal load for a material. As model materials, different hybrid organic-inorganic coatings as well as hot-pressed alumina and float glass are used.

Bed material transport in the Virgin River, Utah

Detailed information concerning the rate and particle size distribution of bed material transport by streamflows can be very difficult and expensive to obtain, especially where peak streamflows are brief and bed material is poorly sorted, including some very large boulders. Such streams, however, are common in steep, arid watersheds. Any computational approach must consider that (1) only the smaller particle sizes present on the streambed move even during large floods and (2) the largest bed particles exert a significant form drag on the flow. Conventional methods that rely on a single particle size to estimate the skin friction shear stress acting on the mobile fraction of the bed material perform poorly. Instead, for this study, the skin friction shear stress was calculated for the observed range of streamflows by calculating the form drag exerted on the reach‐averaged flow field by all particle sizes. Suspended and bed load transported rates computed from reach‐averaged skin friction shear stress are in excellent agreement with measured transport rates. The computed mean annual bed material load, including both bed load and suspended load, of the East Fork Virgin River for the water years 1992‐1996 was approximately 1.3×10 5 t. A large portion of the bed material load consists of sand‐sized particles, 0.062–1.0 mm in diameter, that are transported in suspension. Such particles, however, constituted only 10% of the surface bed material and less than 25% of the subsurface bed material. The mean annual quantity of bed load transported was 1060 t/yr with a median size of 15 mm.

Growth of interfacial debonding in notched two-dimensional unidirectional composite under stress and displacement controls

A simplified calculation method for study of the growth of interfacial debonding between elastic fiber and elastic matrix ahead of the notch-tip in composites under displacement and stress controlled conditions was presented based on the shear lag approach in which the influences of residual stress and frictional shear stress at the debonded interface were incorporated. The calculation method was applied to a model two-dimensional composite. An outline is given of the difference and similarity in the growing behavior of the debonding between the displacement and stress controls, and of the influences of the residual stresses, frictional shear stress, the nature of the final cut component (fiber or matrix) and sample length on the debonding behavior.

A technique for the determination of interfacial properties from debond length measurement

Fiber-matrix interfacial debonding is observed and the debond length is directly measured during flexure tests performed on transparent SiC fiber-reinforced borosilicate glass composites. The relationship among the debond length, applied stress, and interfacial properties is investigated both experimentally and theoretically. A new technique based on debond length measurement is introduced for measuring fiber-matrix interfacial properties such as interfacial shear strength, frictional shear stress, and interfacial debond energy. Analytical models are employed for the new technique to interpret the experimental data. Fiber pushout technique is also employed to measure the interfacial properties independently. It is shown that these two different techniques of debond length measurement and fiber pushout test for measuring the interfacial properties can provide comparable results.

Coarse-grained debris-flow deposits in the Miocene fan deltas, SE Korea: a scaling analysis

Abstract The viscoplastic and inertial grain-flow models have been widely used as tools for description and interpretation of ancient debris-flow deposits, providing a basis to estimate yield strength, viscosity, cohesion, and internal friction angle. Recent studies suggest, however, that a debris flow is an intimate mixture of solid and fluid, in which a number of momentum-transfer mechanisms operate. It is therefore necessary to evaluate the relative importance of different momentum-transport processes to properly describe a debris flow. Scaling analysis may be useful to this end, using the Bagnold number (the ratio of inertial grain stress to viscous shear stress), Savage number (the ratio of inertial grain stress to shear stress borne by sustained grain contacts), friction number (the ratio of frictional shear stress to viscous shear stress), and Darcy number (the ratio of grain–fluid interaction stress to inertial grain stress). Scaling analyses of Miocene gravelly debris-flow deposits in Korea suggest that different types of debris flows can be better distinguished by the analyses, providing some implications for debris-flow processes.

Tribology of Shearing Polymer Surfaces. 1. Mica Sliding on Polymer (PnBMA)

The energy-dissipating processes associated with shearing polymer junctions were investigated at the molecular, microscopic, and macroscopic levels using the surface forces apparatus-FECO optics technique. For a solid surface of mica sliding across a polymer surface of poly-n-butyl methacrylate (PnBMA) at low sliding velocities and at temperatures close to the glass transition temperature, T{sub g} = 25 C, the friction was mainly of the stick-slip variety; the static friction force F{sub s} was always high and remained relatively constant, attaining its equilibrium steady-state value immediately on commencement of sliding, but the kinetic friction force F{sub k} decreased from an initially high value to a very low value as sliding progressed. The friction forces exhibit complex time, temperature, load, and velocity dependencies and cannot be properly described in terms of a single parameter such as a friction coefficient or shear stress. These, and other tribological characteristics of this type of system, appear to be very different from those of hard surfaces or simple liquid-lubricated surfaces. Where comparison with literature data is possible, the tribological results on these molecularly smooth model surfaces are similar to those for engineering surfaces sliding on bulk polymer. The molecular mechanisms and relaxation processes responsible for the observedmore » tribological behavior and adhesion hysteresis of this type of system (solid surface sliding on polymer) are discussed.« less

A fracture-mechanics model of the microbond test with interface friction

The microbond test is commonly used to measure the toughness of fiber/matrix interfaces. However, interfacial friction tends to stabilize the crack and increase the peak load as compared to a friction-free crack with the same interface toughness. Nevertheless, for a constant friction shear stress both the interface toughness and friction stress can be approximately determined from load vs. crack length or load vs. displacement hysteresis curves (including energy dissipation, residual displacement or compliance measurements). A friction-free crack is used as a reference solution, and friction is taken into account in the reduced crack-tip fiber load and debonded fiber elongation. When expressed in terms of these two quantities, the energy release rate and fiber end displacement are almost independent of the amount of friction. The approximation involved is verified by a finite-element analysis.

Experimental study on gas–liquid two-phase flow regimes in rectangular channels with mini gaps

Experimental investigations were conducted for adiabatic air/water two-phase flow in vertical rectangular channels. Three test sections with the narrow gaps of 0.3, 0.6 and 1.0 mm respectively were studied. The flow regimes were observed by using a high-speed video-camera and were identified by examining the video images. The flow regimes in channels with the gaps of 1.0 and 0.6 mm are similar to those found by previous studies, and can be classified into bubbly, slug, churn and annular flow. With decreasing the channel gaps, the transition lines for bubbly-slug, slug-churn and churn-annular flow shift to left in the flow regime maps with the superficial gas velocity as the horizontal coordinate and the superficial liquid velocity as the vertical coordinate. However, the flow regimes in the channel with the gap of 0.3 mm are dramatically different from the previous studies. Bubbly flow was never observed even at very low gas flow velocity. Cap-bubbly flow can exist in the channels stably. Due to the increased influences of the surface tension and friction shear stress in the mini channels, the liquid droplets adhere on the wall surface and are pushed by the flowing gas. The flow regimes can be classified into cap-bubbly, slug-droplet, churn and annular-droplet flow.

Crack-bridging Processes and Fracture Resistance of a Discontinuous Fiber-reinforced Brittle Matrix Composite

A simple bridging model is proposed for the toughening of a discontinuous fiber-reinforced brittle matrix composite, in which the frictional bridging of fibers during, as well as after, the interfacial debonding is considered. The R-curve behavior and the work-of-fracture of the composite can be theoretically predicted by the computation of the bridging model applying material parameters, such as fiber volume fraction, size and shape of fibers, fiber tensile strength, elastic moduli of fibers and matrix, fracture toughness and work-of-fracture of matrix, and frictional shear stress at interface. The experimental result obtained from a SiC-whisker-reinforced Al2O3 composite confirms the theoretical predictions of the present bridging model. Through the model calculation, the R-curve, crack profile, and bridging stresses of the composite can be estimated correspondingly to the bridging processes.

Gas–liquid two-phase flow regimes in rectangular channels with mini/micro gaps

An adiabatic concurrent vertical two-phase flow of air and water in vertical rectangular channels (12×260 mm) with narrow gaps of 0.3, 0.6–1.0 mm was investigated experimentally. Flow regimes were observed by using a CCD camera and were identified by examining the video images. The flow regimes for gaps of 1.0 and 0.6 mm were found to be similar to those in the existing literature which can be classified into bubbly flow, slug flow, churn–turbulent flow and annular flow. With the decrease of the channel gap, the transition from one flow regime to another occurs at smaller gas flow rates. However, flow regimes for micro-gaps of 0.3 mm or less are quite different from the previous studies: bubbly flow was never observed even at very low gas flow rates. Due to the increased influence of the surface tension force and the frictional shear stress in channels with a micro-gap, the liquid droplets adhered on the wall surface and were pushed by the gas phase. Flow regimes in these micro-gaps can be classified into cap–bubbly flow, slug–droplet flow, churn flow and annular–droplet flow. A previous model has been extended to predict the flow regime transitions from bubbly flow to slug flow, slug flow to churn flow using the bubble rising velocity and the increased frictional coefficient for rectangular channels. A new criterion has been developed to predict the transition of the annular flow. Comparisons of our prediction results with experimental data are discussed for gaps larger than 0.6 mm. With micro-gaps of 0.3 mm or smaller, a new theory needs to be developed.