Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour.

The adhesive pads of geckos provide control of normal adhesive force by controlling the applied shear force. This frictional adhesion effect is one of the key principles used for rapid detachment in animals running up vertical surfaces. We developed polypropylene microfibre arrays composed of vertical, 0.3 microm radius fibres with elastic modulus of 1 GPa which show this effect for the first time using a stiff polymer. In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control. A cantilever model for the fibres and the spherical probe indicates a strong dependence on the initial fibre angle. A novel feature of the microfibre arrays is that adhesion improves with use. Repeated shearing of fibres temporarily increases maximum shear and pull-off forces.     

Shear properties of organic films adsorbed on silica fibres

Contact adhesion and sliding friction of octadecylamine and silane molecules adsorbed on silica fibre were measured with orthogonally crossed silica fibres using an electronic microbalance. Interfacial shear strengths, τ, as a function of contact pressure, P, between organic films were deduced from the adhesional model of friction using the measured frictional force and the calculated real area of contact. The pressure dependence of the interfacial shear strength was then interpreted in terms of the molecular interaction of adsorbates with solvent and the surface energetics of molecules adsorbed on the silica fibre. This revised version was published online in November 2006 with corrections to the Cover Date.     

A microfabricated wedge-shaped adhesive array displaying gecko-like dynamic adhesion,directionality and long lifetime

Gecko adhesion has become a paradigmatic example of bio-inspired engineering, yet among the many gecko-like synthetic adhesives (GSAs), truly gecko-like performance remains elusive. Many GSAs have previously demonstrated one or two features of the gecko adhesive. We present a new wedge-shaped GSA that exhibits several gecko-like properties simultaneously: directional features; zero force at detachment; high ratio of detachment force to preload force; non-adhesive default state; and the ability to maintain performance while sliding, even after thousands of cycles. Individual wedges independently detach and reattach during sliding, resulting in high levels of shear and normal adhesion during drag. This behaviour provides a non-catastrophic failure mechanism that is desirable for applications such as climbing robots where sudden contact failure would result in serious falls. The effects of scaling patch sizes up to tens of square centimetres are also presented and discussed. Patches of 1 cm² had an adhesive pressure of 5.1 kPa while simultaneously supporting 17.0 kPa of shear. After 30 000 attachment/detachment cycles, a patch retained 67 per cent of its initial adhesion and 76 per cent of its initial shear without cleaning. Square-based wedges of 20 μm and 50 μm are manufactured in a moulding process where moulds are fabricated using a dual-side, dual-angle lithography process on quartz wafers with SU-8 photoresist as the mould material and polydimethylsiloxane as the cast material.     

Normal and sliding contact experiments on gneiss

The development of reliable discrete element models to simulate the mechanics of granular media requires knowledge of the grain-to-grain contact laws of the material in question. We have conducted a series of normal and sliding contact experiments on material used in laboratory triaxial experiments to obtain such contact laws for DEM simulations of the experiments. The contact experiments employed segments of 14.72 mm-diameter spherical grains from the triaxial specimens and flat specimens of the same material. The spherical grains had a uniform diameter with a smooth surface finish. Monotonic and cyclic loading paths were applied in both the normal and sliding modes, and both sphere-sphere and sphere-flat contact behavior were examined. Force-displacement behavior and frictional loss were measured in all cases. The behavior was generally Hertzian in the normal contact experiments, which involved forces up to approximately 100 N. The normal contact stiffness increased from ≈2 to 15MN m⁻¹ over the range of normal force examined. The sliding experiments employed several normal forces up to approximately 25 N, and produced a value of the coefficient of static friction of 0.28. The shear stiffness of the sliding contact increased with normal force, and ranged from 0.8 to 1.2MN m⁻¹ under normal loads ranging from ≈1 to 7.5 N, respectively, for virgin contacts. The shear stiffness observed for the sphere-flat contact decreased with wear. Surface roughness measurements were obtained on both tested and untested regions of the spheres and flat specimens. The average roughness (Ra) for untested regions of the sphere and flat specimens were 270 and 230 nm, respectively. Repeated testing in the sliding mode reduced these values by 29–45% for the flat surfaces, and by 20% for the spherical contact. Frictional losses were observed in both the normal and sliding modes. In the sliding mode, frictional loss decreased with increasing normal load. We observed stable sliding (associated with significant contact movement under an increasing shear force) at forces that were below the macroscopic frictional limit and resulted in permanent displacement of the contact. There was generally a distinct threshold in shear force for this permanent sliding. The extent of sliding increased significantly with wear for the sphere-flat contact and was accompanied by a substantial drop in shear stiffness.     

Single fibre pullout tests on auxetic polymeric fibres

A novel route for production of auxetic fibres has been adapted from conventional melt extrusion techniques. These fibres were reproduced, characterised and tested, for the first time, to assess the potential of auxetic fibres as reinforcements in composite materials. Initial experimental work has included the embedding of single fibres in modified epoxy resin. Auxetic fibre specimens were then compared with conventional fibre specimens through a specially designed fibre pullout testing procedure. The auxetic specimens displayed superior anchoring properties. The average maximum force at de-bonding of the auxetic fibres (0.95 N) was observed to be over 100% higher than that for conventional ones (0.44 N) and the average energy required to fully extract the auxetic fibre from the modified resin was 8.3 mJ while the conventional fibre required only 2.5 mJ on average. The results indicate that composites employing auxetic fibres as the reinforcement phase will exhibit enhanced resistance to failure due to fibre pullout.     

Strain-rate sensitive tough fibre-reinforced composites

A new type of composite was designed and tested which has greater fracture toughness under impact loading conditions than conventional fibre-reinforced composites. This composite is strain-rate sensitive and can be more than twice as tough as conventional composites having the same matrix and fibre. The key concept used was to coat the reinforcing fibres with a thin layer of viscous fluid in order to maximize the shear stress acting on the fibres during the fibre pull-out. At a given strain-rate the shear stress can be optimized by changing the fluid viscosity and thickness of the coating. The optimum results are obtained when the frictional force is equal to the fibre strength.Composites were made with uniaxial and randomly oriented E-glass fibres in a polyester resin. Samples with uncoated fibres were used as reference. The viscous fluids used included Dow Corning 200 Fluid with viscosities of 10⁵ cP and 10⁶ cP, Zelec U.N., petrolatum and silicone vacuum grease.Notched uniaxial samples with uncoated fibres (fibre volume fraction of 0.06) showed an energy absorption of 16.8 kJ m^–2 (3.2 ft lb in.^–1) in the lzod test. The uniaxial samples coated with Dow Corning 200 Fluid showed an energy absorption from 6.7 kJ m^– (1.28 ft lb in.^–1) to 41.4 kJ m^–2 (7.87 ft lb in.^–1) depending on the thickness of the coating. The samples with random uncoating fibres (fibre volume fraction of 0.20) had an energy absorption of 14.2 kJ m^–2 (2.71 ft lb in.^–1) while the samples with coated fibres ranged from 13.7 to 31.6 kJ m^–2 (2.60 to 6.02 ft lb in.^–1).     

A fast algorithm for the elastic fields due to interacting fibre breaks in a periodic fibre composite

Monte Carlo simulations of the failure of unidirectional fibre composites typically require numerous evaluations of the stress-state in partially damaged composite patches. In a simulated composite patch comprised of N fibres, of which \(N_b\) fibres are broken in a common cross-sectional plane transverse to the fibre direction, the stress overloads in the intact fibres are given by the weighted superposition of the unit break solutions associated with each of the breaks. Determining the weights involves solving \(N_b\) linear equations, and determining overloads in the intact fibres requires matrix-vector multiplication. These operations require \(O(N_b^3)\), and \(O(N N_b)\) floating point operations, respectively. These costs become prohibitive for large N, and \(N_b\); they limit Monte Carlo failure simulations to composite patches of only a few thousand fibres. In the present work, a fast algorithm to determine the overloads in a partially damaged composite, requiring \(O( N_b^{1/3} N \log N)\) floating point operations, is proposed. This algorithm is based on the discrete Fourier transform. The efficiency of the proposed method derives from the computational simplicity of weighted superposition in Fourier space. Computations of the stress state ahead of large circular clusters of breaks in composite patches comprised of about one million fibres are used to demonstrate the efficiency of the proposed algorithm.     

On the effect of different polymer matrix and fibre treatment on single fibre pullout test using betelnut fibres

This study is aimed at exploring the possibility of improving the interfacial adhesion strength of betelnut fibres using different chemical treatments namely 4% and 6% of HCl and NaOH respectively. The fibre specimens were partially embedded into different thermosetting polymer matrix (polyester and epoxy) as reinforcement blocks. Single fibre pullout tests were carried out for both the untreated (Ut) and treated betelnut fibres with different resins and tested under dry conditions. Scanning electron microscopy was used to examine the material failure morphology. The studies revealed the differences of interfacial adhesion strengths for the various test specimens of betelnut fibres treated with the polyester and epoxy matrix which followed in the order of: N6 ? N4 > H4 > Ut > H6. It was proven that fibres treated with 6% of NaOH exhibits excellent interfacial adhesion properties. The interfacial adhesion shear strength of these fibres using polyester and epoxy has improved by 141% and 115% correspondingly compared to untreated fibre under the same treatment.     

Analysis of the single-fibre pull-out test by means of Raman spectroscopy: Part II. Micromechanics of deformation for an aramid/epoxy system

The single-fibre pull-out test has been analysed for Kevlar-49 fibres in a cold-cured epoxy resin by using both a conventional pull-out experiment and Raman spectroscopy. The interfacial shear strength (ISS) has been estimated from the pull-out force for fibres with a range of embedded lengths. Raman spectroscopy has been used to analyse the distribution of fibre strain in the pull-out test by mapping the variation of strain along an aramid fibre undergoing pull-out from the epoxy resin matrix. At low strains the behaviour follows elastic shear-lag analysis but, as the fibre strain is increased, debonding takes place at the fibre/matrix interface. It is found that this debond propagates along the interface until the entire fibre is debonded. The fibre is then pulled out of the resin matrix by a frictional pull-out process. It is shown that the conventional pull-out experiment produces only an apparent value of ISS and that through a partial-debonding model it is possible to use the interfacial parameters obtained from the Raman analysis to predict the data from the conventional test.     

Effects of surface and sizing treatments on axial compressive strength of carbon fibres

Attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibres, and the effects of surface- and size-treatment on compressive strength was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compression force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compression force in a temperature range from room temperature to 80 °C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compression force is zero, increases with increasing shear yield strength at the fibre-matrix interphase. In order to obtain reinforcing fibres with a higher compressive strength, it will be necessary to surface- and size-treat the fibres.     

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part II: Parallel and anti-parallel fibre orientations

Finite element (FE) micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Two cases were studied, i.e. a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results relating to the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a micro-environment as part of a macro-environment, the displacement coupling technique was introduced. The contact analysis operates on both the macro- and the micro-level, applying node-to-node contact elements. The contact results, especially the contact pressure distribution, can characterize the real fibre/matrix micro-system. Displacement and strain results lead to explanations of fibre related phenomena, matrix shear effects, and fibre/matrix debonding events. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composite. For parallel fibre orientation, fibre/matrix debonding as a result of shear stresses at the interface, matrix shear type failure and fibre thinning are the dominant sliding wear mechanisms. If an anti-parallel fibre orientation is considered, matrix shear, tension/compression type fibre/matrix debonding and fibre thinning, associated with fibre cracking events, are the most dominant wear mechanisms. To study the wear mechanisms experimentally, diamond tip scratch tests were carried out, showing that the predicted failure events occur also in reality.     

Wide-shallow beams with and without steel fibres: A peculiar behaviour in shear and flexure

This paper reports an experimental campaign on Reinforced Concrete (RC) Wide-Shallow Beams (WSBs) with or without fibres, tested under shear and flexure. A wide-shallow beam is a rather frequent structure in residential buildings in Southern Europe (as in Italy and in Spain). In order to study both the shear and flexural behaviour of WSBs and evaluate the possibility of substituting the minimum conventional transverse reinforcement required by Eurocode 2 with steel fibres, full-scale beams have been tested. Specimens, all 250 mm deep, had two different widths, fibre contents and also, minimum amount of classical shear reinforcement. Results evidenced that a relatively low volume fraction of fibres can significantly increase shear bearing capacity and beam ductility. Moreover, WSBs did not show the typical brittle failure in shear, even without any shear reinforcement, as the effect of fibres was more prominent than in deep beams. Peculiarities of WSBs were evidenced in terms of enhancements both in shear and in flexure. Experimental results have been evaluated in terms of strength, ductility, post-cracking stiffness, shear and flexural cracking, collapse mechanism and fibre effect.     

Surface,interphase and composite property relations in fibre-reinforced polymers

The aim of this study is to demonstrate the relations between surface, interphase and different composite properties in composites of carbon and glass fibres and thermosets (epoxy resin) or thermoplastics (polyamide, polypropylene). The surface characteristics of variously treated carbon and glass fibres have been determined by contact angle measurements, using a capillary penetration technique, and zeta potential measurements. Micromechanical tests (single-fibre pull-out) have been applied to model composites to establish the interphase properties. Bulk composites have been manufactured by impregnation with resins and curing by hot pressing or pressing of commingled yarn samples (continuous reinforcing fibres) or twin-screw extrusion and injection moulding (short reinforcing fibres). The composite properties have been tested by tensile and shear tests. Contact angle and electrokinetic measurements permit detection of differences in the surface treatment of glass and carbon fibres. Measurements of shear strength by pull-out, fibre fragmentation and yarn tensile shear tests show comparable results and correlate to the macromechanical properties.     

Investigation into stress transfer characteristics in alumina-fibre/epoxy model composites through the use of fluorescence spectroscopy

In composite materials, fibre/fibre interaction phenomena due to fibre failure are crucial in determining the composite fracture behaviour. Indeed, the redistribution of stress from a failed fibre to its intact neighbours, and stress concentration induced in the neighbouring fibres, determine the extent to which a break in one fibre will cause more breaks in others. In this paper, we have used fluorescence spectroscopy to study the stress transfer and redistribution induced by fibre fracture in two-dimensional Nextel-610 fibres/epoxy-resin micro-composites. The stress along the fibres was mapped at different load levels, and specimens with different inter-fibre distance were used to study the fibre content effect. The interfacial shear stress distribution along broken and intact fibres was derived by means of a balance of shear-to-axial forces argument. The experimental stress concentration factors (SCF) were smaller than values predicted from our model based on the cell assembly approach. As expected the 2D configuration allows access to the upper bound of the SCF in real composites. For the several specimens tested, a region of matrix yielding was observed behind the fibre fracture and no-debonding at the interface was detected. The measured SCF values agree well with those reported in recent study for carbon-fibre/epoxy model composites.     

A liquid droplet measurement technique as a means of assessing the interlaminar shear strength of fibre-reinforced composites

A liquid droplet measurement technique was used to measure the contact angle for low modulus, medium modulus and high modulus pitch-based carbon fibres. A phase transfer catalytic oxidation treatment and an electrochemical oxidation treatment both resulted in falls in the observed glycerol contact angle. Increases in surface energy values were estimated using an equation-of-state approach. The electrochemical oxidation treatment increased the concentration of oxygen- and nitrogen-bearing groups on the fibre surface (as shown by Auger electron spectroscopy), resulting in a fall in the surface energy values estimated using the droplet measurement technique, and corresponding improvements in composite interlaminar shear strength (ilss) were observed. Thus it was shown that the surface energy values determined using this method could be used to assess changes in ilss due to surface treatment of the fibre, provided only chemical changes resulted from the fibre treatment.     

Probabilistic aspects of strength of unidirectional fibre-reinforced composites with matrix failure

In the previous paper [1], the stress distribution and the expected number of successive fibre breakages around broken fibres were calculated. It showed the following results. The fracture process that the crack originates from one isolated broken fibre and propagates due to the stress redistribution following the fibre breakage is unlikely to occur in the real unidirectional fibre-reinforced composite material. The matrix-failure is considered to play an important role in the fracture process of real composite materials. In the present paper, the stress (or strain) distribution and the expected number of successive fibre breakages around broken fibres are calculated when the matrix-damaged regions exist. The stress (or strain) distribution is obtained based on the three-dimensional hexagonalarray shear-lag model. Uniform shear force is assumed to occur in the matrix-damaged region. The expected number of the successive fibre breakages is calculated on the assumption that the flaws in the fibre follow a Poisson process.     

Multiscale tool–fabric contact observation and analysis for composite fabric forming

This paper provides measurements and analysis at the meso and microscopic scales of the real contact area between twill carbon fabric and a flat glass counterface. The mesoscopic contact area associated with tow contacts is about 55–75% of the nominal area. However, the total real contact length within the tow contacts, associated with microscopic contact at the fibre level, is only 4–8% of the idealised contact conditions with parallel touching fibres, for a nominal contact pressure of around 2 kPa. The dependence of real contact area on fabric shear angle is also investigated. The estimated real contact pressure is 15,000 times higher than the nominal contact pressure. Models or experiments of friction in composites forming which do not take into account the real contact situation, which is very far from an idealised packing arrangement, may fail to capture the essential tribological mechanisms.     

Friction and adhesion of silica fibres in liquid media

Contact adhesion and sliding friction between orthgonally configured silica fibres were measured in water, hexadecane, and cyclohexane. In the adhesion model of friction, the friction is interpreted in terms of shearing the junction and the shear strength () can simply be represented by =F/A whereFis frictional force andA is the real area of contact. While this model works well for the frictional behaviour of silica fibre in air, this was not the case in liquid media. The influence of liquid on the force required to break the adhesive junction cannot simply be interpreted in terms of the reduction in adhesional forces between two fibres; but the interpretations have to also include the nature of the liquids.     

Carbon nanotube grafted carbon fibres: A study of wetting and fibre fragmentation

Carbon nanotubes (CNTs) were grafted on IM7 carbon fibres using a chemical vapour deposition method. The overall grafting process resulted in a threefold increase of the BET surface area compared to the original primary carbon fibres (0.57 m²/g). At the same time, there was a degradation of fibre tensile strength by around 15% (depending on gauge length), due to the dissolution of iron catalyst into the carbon; the modulus was not significantly affected. The wetting behaviour between fibres and poly(methyl methacrylate) (PMMA) was directly quantified using contact angle measurements for drop-on-fibre systems and indicated good wettability. Single fibre fragmentation tests were conducted on hierarchical fibre/PMMA model composites, demonstrating a significant (26%) improvement of the apparent interfacial shear strength (IFSS) over the baseline composites. The result is associated with improved stress transfer between the carbon fibres and surrounding matrix, through the grafted CNT layer. The improved IFSS was found to correlate directly with a reduced contact angle between fibre and matrix.     

Study of friction and wear mechanisms at high sliding speed

The aim of this work is to propose an analysis of mechanisms inducing surface interaction by friction during high sliding speed. Specific devices including a ballistic setup were used to reproduce extreme sliding conditions combining high speed and high pressure. The titanium alloy/tantalum tribo-pair is chosen to investigate the frictional and material transfer mechanisms. The tangential force measurement is used to follow the evolution of the friction coefficient at a macroscopic scale. The evolution of the sliding surface was analyzed by confocal 3D microscope to evaluate material transfer and real contact surface area. Numerical modeling of micro-contact at the asperities scale is presented to illustrate the scenarii involved during friction. The energy needed to shear a junction is estimated and analyzed for several types of interaction. Different behaviors have been taken into account in order to investigate the global forces generated by the contact including strong and weak contacts. The analysis of energy is available to predict the global friction force in a large range of velocities. Correlations between experimental measurements and numerical predictions are used to validate the proposed approach. The results can be interpreted as following: (1) at lower velocity the main mechanism dominating the interaction between asperities becomes ploughing with large volume of plastic deformation (2) at higher velocity the main mechanism is shear localization requiring less energy and force for shearing the junctions.