Softness evaluation of keratin fibers based on single‐fiber bending test

The evaluation of single‐fiber softness by bending is an ingenious and vital approach for the basic investigation of both the fiber bending properties and the textile softness. The bending behavior and bending modulus of wool, alpaca and silk fibers have been measured by an axial‐buckling method developed by the authors, which uses the fiber compression bending analyzer (FICBA). The bending properties of single fibers were quantified by calculating the equivalent bending modulus and the flexural rigidity by measuring the protruding length and diameter of fiber needles and the critical force, P_cr, obtained from the peak point of the force‐displacement curve. The measured data showed that the equivalent bending modulus of the alpaca fiber is higher than that of wool fiber, and even the rigidity is 10 times as high as wool, but its friction coefficient is lower than that of wool, which means that the soft handle of alpaca fabrics is mainly due to the smooth surface and low friction coefficient of alpaca fibers in contrast to that of wool fiber. For the silk fiber, despite high equivalent bending modulus, the smoother handle of silk should be mainly due to the thin fiber diameter in contrast to that of keratin fibers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 701–707, 2006     

Computational and Experimental Studies of Flexible Fiber Flows in a Normal-Stress-Fixed Shear Cell

Flow properties of flexible fibers are poorly understood compared to those of rigid particles. In this study, a Schulze ring shear tester is used to measure the flow properties of fibers made of cut fishing wire and cut rubber cord, which have different levels of flexibility. For a comprehensive study, the discrete element method is employed to simulate flexible fiber flows. The simulations are validated by comparing with the experimental measurements. Studies show that an increase in fiber bending modulus leads to a reduction in the deformation and solid volume fraction, but it has no effect on the shear stress with the same normal stress. An increase in fiber-fiber friction coefficient, below a critical value of 0.8, can augment the angle of internal friction. The contact stiffness, contact damping coefficient, and bond damping coefficient only have limited impact on the shear flow behavior in the ranges considered. © 2018 American Institute of Chemical Engineers AIChE J, 65: 64–74, 2019     

An Investigation on triaxial compression of flexible fiber packings

This article examines the mechanical response of flexible fiber packings subject to triaxial compression. Short fibers yield in a manner similar to typical granular materials in which the deviatoric stress remains nearly constant with increasing strain after reaching a peak value. Interestingly, long fibers exhibit a hardening behavior, where the stress increases rapidly with increasing strain at large strains and the packing density continuously increases. Phase diagrams for classifying the bulk mechanical response as yielding, hardening, or a transition regime are generated as a function of the fiber aspect ratio and fiber–fiber friction coefficient. Large fiber aspect ratio and large fiber–fiber friction coefficient promote hardening behavior. The positions of boundaries between different regimes depend on the confining pressure and fiber flexibility. The hardening packings can support much larger loads than the yielding packings, but larger internal axial forces within fibers and larger fiber–fiber contact forces occur.     

Transverse compression failure of unidirectional composites

The mechanical response of unidirectional composites subject to uniaxial transverse compressive loads was measured and analyzed by finite element simulation. Consistency in failure plane orientation was observed when comparing simulated matrix shear band angle to measured crack angle. A model based on hexagonal packing of fibers was proposed and the shear band angle was shown to depend on the fiber volume fraction. The effects of strong and weak fiber–matrix interfaces were considered using models with randomly distributed fibers for a valid statistical analysis. The results of these models showed that the composite compressive strength increased with the fiber loading for the strong interface case, while the strength was independent of the fiber loading for the weak interface case because of interface debonding. POLYM. COMPOS., 36:756–766, 2015. © 2014 Society of Plastics Engineers     

Estimation of the volume resistivity of electrically conductive composites

The modeling of the electrical conductivity of polymer composites reinforced with conductive fibers is investigated. Existing models generally can be divided into percolation theories and non-percolation theories. The basis of the percolation theory is the fact that the conductivity of the composite increases dramatically at a certain fiber concentration called the percolation threshold. This theory can be used to model the behavior of the composite or to predict the percolation threshold itself. Non-percolation theories include terms, which account for microstructural data such as fiber orientation, length, and packing arrangement. A comparison of experimental data with predictions from the various models reveals that only the percolation theory is able to accurately model the conductive behavior of an actual composite. Two alternative new models, which predict the volume resistivity of a composite using microstructural data, are evaluated. The first model relates resistivity to the concentration and orientation of the fibers, while assuming perfect fiber-fiber contact. The relationship between resistivity and fiber concentration predicted by the model is in qualitative agreement with actual data, and predictions of the anisotropy in volume resistivity compare well with experimental results. The second model accounts for the effect of fiber-fiber contact and fiber length on composite resistivity. Predictions are in excellent agreement with experimental data for polypropylene composites reinforced with nickel-coated graphite fibers.     

Continuum contact models for coupled adhesion and friction

We develop two new continuum contact models for coupled adhesion and friction, and discuss them in the context of existing models proposed in the literature. Our new models are able to describe sliding friction even under tensile normal forces, which seems reasonable for certain adhesion mechanisms. In contrast, existing continuum models for combined adhesion and friction typically include sliding friction only if local contact stresses are compressive. Although such models work well for structures with sufficiently strong local compression, they fail to capture sliding friction for soft and compliant systems (like adhesive pads), for which the resistance to bending is low. This can be overcome with our new models. For further motivation, we additionally present experimental results for the onset of sliding of a smooth glass plate on a smooth elastomer cap under low normal loads. As shown, the findings from these experiments agree well with the results from our models. In this paper we focus on the motivation and derivation of our continuum contact models, and provide a corresponding literature survey. Their implementation in a nonlinear finite element framework as well as the algorithmic treatment of adhesion and friction will be discussed in future work.     

Molecular dynamics investigation on the atomic-scale indentation and friction behaviors between diamond tips and copper substrate

Classical molecular dynamic (MD) simulations are used to investigate the atomic-scale indentation and friction behaviors of the spherical diamond(111) or diamond(001) tip in contact with a flat copper(001) substrate. In the indentation simulations, six radii ranging from 5 to 30 nm are adopted for each tip and the contact radius is examined as a function of normal load. The results demonstrate that the contact radii calculated from the MD simulation always deviate from the continuum theory predictions and the deviation varies with the tip surface atomic structure, tip radius, and normal load. Furthermore, the atomic-scale friction behaviors are investigated using 10 nm and 30 nm diamond(111) tips sliding over the copper(001) surface with a variety of loads. Apparent atomic stick–slip behavior is observed on such ordered but incommensurate contact interface; moreover, it does not disappear with increasing tip radius. It is also revealed that the friction versus load relationship is approximately linear, which is not in agreement with the continuum theory predictions and many reported atomic force microscope (AFM) experiments.     

Effect of the fiber geometry on the pullout response of mechanically deformed steel fibers

A simple model to predict the influence of fiber geometry on the pullout of mechanically deformed steel fibers from cementitious matrix is proposed. During the pullout the mechanically deformed fiber is subjected to repetitive bending and unbending which cause an increase of the tension in the fiber. This increase of the tension depends on the amount of plastic work needed to straighten the fiber during pullout. The model input parameters are mechanical and geometrical properties of mechanically deformed fibers. Model predictions were compared to the experimental results on the hooked-end and crimped steel fiber pullout and good agreement was observed.     

Influence of normal contact force model on simulations of spherocylindrical particles

How the choice of elastic normal contact force model affects predictions from discrete element method simulations of spherocylindrical particles is investigated in this article. Three force models were investigated: (1) a Hertzian force model (HFM) which assumes a circular contact area; (2) a linear force model (LFM) with a constant stiffness; and (3) a modified HFM (MFM) that accounts for various contact areas and contact transitions. With the MFM, transitions between contact area types must be accounted for otherwise discontinuities in the contact force can occur. It is found that simple force models (HFM, LFM) can be substituted for more accurate force models if only force data and bulk properties are of interest. However, if more detailed contact information, such as contact area, contact overlap, contact duration, or collision frequency, are needed, for example, in population balance models and transient liquid bridge modeling, then a more accurate force model should be used. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1986–2001, 2018     

An investigation of the comparative behaviour of alternative contact force models during elastic collisions

In this paper the rebound kinematics obtained using different contact force models are compared for the simple problem of an elastic sphere impacting obliquely with a target wall. It is shown that, for an appropriately calibrated normal spring stiffness and a realistic ratio of the tangential to normal spring stiffnesses, excellent results can be obtained by using a simple linear spring model. The paper also demonstrates that for non-linear contact models, integral equations for the tangential force-displacement cannot be used as the spring stiffness varies during the collision. Finally some comments are provided regarding the limitations of the linear spring model and alternatives are discussed.     

Packing mechanics of fiber reinforcements

Theories of fiber packing, of use in manufacturing composite materials, are developed. The maximum packing fraction of force free fibers is estimated based on a statistical analysis of the distribution of fiber-fiber contact points. The new expressions are more general than previous ones by allowing for a distribution in fiber length and orientation. The forced packing beyond this limit is governed by the bending of fiber segments between contact points. A micromechanical theory is developed for this, again based on the contact point statistics, and equations relating the force response per unit area of a fiber bed to the fiber volume fraction are derived for three basic types of assembly: a general 3D wad, a planar mat of dispersed fibers, and a bundle of almost parallel fibers. Other types of reinforcement structure, such as woven fabrics, and the effect of lubrication are also treated briefly.     

Analysis of the pullout of single fibers from low-density polyethylene

In the present work, a single-fiber pullout test was used to study the interface/interphase between various fibers and low-density polyethylene (LDPE) and between glass fibers and a range of other thermoplastic matrices. For well-bonded fibers, experimental evidence suggests the involvement of plastic deformation and strain-hardening prior to debonding and pullout. The interfacial shear strength was determined to be the ultimate shear strength of the matrix and was found to be insensitive to the fiber surface structure. A new theoretical model was developed to predict the relationship between the debonding force and the embedded length. The contribution of friction to the debonding force was found to be insignificant when compared with the contribution of plastic deformation. © 1994 John Wiley & Sons, Inc.     

Mechanical behavior of closed cell plastic foams used as cushioning materials

Two methods for the prediction of force–deformation curves of closed cell plastic foams at any strain rate from a limited number of experiments are described. These methods are based on the “modified Boltzman integral model” and on the “reference model.” Both methods use constitutive equations and experimentally determined parameters. The modified Botlzman integral model uses data obtained in a limited number of stress–relaxation experiments while the reference model uses a very limited number of stress–relaxation and one force–deformation curve data. Both models predict well the force–deformation curves, the reference model providing somewhat better predictions.     

Sliding Friction Phenomena of a Ball-on-Disc Contact under Different Lubricating Conditions

Experiments were carried out to measure the friction of a ball-on-disc contact in a simple sliding experiment under different lubricating conditions using a Universal Micro Materials Tester (UMT-2). The measurements were also taken for the direct contact between the ball and the disc. The sliding speed was between 5.0 × 10^–6 m/s and 1.0 × 10^–2m/s. The load of the contact ranged between 1 N and 40 N giving the nominal maximum Hertzian contact pressures of 0.59 GPa to 2.03 GPa. It was found that when the sliding speed was below 1.0 × 10^–5m/s, the contact is in boundary lubrication and the measured friction coefficients for all the lubricants could be considered as independent of the sliding speed. It is suggested that the boundary film-contact interfacial slippage in the loaded zone of the asperity contact between the two surfaces is responsible for this friction phenomenon and the friction coefficient is determined by the boundary film-contact interfacial shear strength. Under the same load condition, the boundary film-contact interfacial shear strength for the 68 mechanical oil was found to be the highest; that for the 32 mechanical oil was higher than that for the PB2400 oil; while that for the purified and deionized water was the lowest and significantly lower than that for the other three lubricants. The friction coefficients measured for the direct contact between the ball and the disc were found to be close to those for water lubrication. When the sliding speed was above 1.0 × 10 ^–5m/s, the friction phenomena, taking into account sliding speed variation, was found to be quite different for the different lubricants. For the applied loads, the measured friction coefficients can be considered as independent of the load. From this phenomenon, it is suggested that the asperity contacts formed between the coupled surfaces in the experiment are almost in full plastic deformation mode.     

An improved integral non-linear model for the contact of particles in distinct element simulations

In this paper we consider a non-linear model for the elastic-frictional contact of spherical particles based on a modification of the classical Hertz-Mindlin no-slip solution. The characteristics of the original model are described and discussed in terms of the capabilities to simulate collisions using the distinct element method (DEM). Perfectly elastic collisions in normal direction and elastic-frictional mechanisms in tangential direction are considered for impacts of a sphere with a flat wall at various angles. On this basis, we suggest a mathematical modification of Mindlin's tangential solution and demonstrate formally its advantages with respect to the commonly used model. We illustrate a comparison of the proposed model with other commonly used models and a validation of the models against experimental data, available under similar conditions (Kharaz et al., Powder Technology 120 (2001) 281). It is shown that an improved realism and consistency is obtained with our modification, especially regarding the tangential displacement and force-displacement relation, at the cost of a very simple modification of the model algorithm.     

Determining the collision properties of semi-crystalline and amorphous thermoplastics for DEM simulations of solids transport in an extruder

To improve application of the distinct element method (DEM) to polymer processing applications it was necessary to evaluate the contact mechanics of a selection of commonly used polymers. The contact behaviors of high-density polyethylene (HDPE), polystyrene (PS) and polycarbonate (PC) were revealed in this paper from a series of impact studies where spherical polymeric particles struck a steel anvil at various angles of incidence and impact velocity. The coefficients of restitution and friction were calculated from high-speed video analysis of individual impacts. The collected data was used to evaluate the relevance of several popular normal contact force-displacement models to determine their suitability for the tested semi-crystalline and glassy polymers. The models considered were those proposed by Walton and Braun [1986. Viscosity, granular-temperature, and stress calculations for shearing assemblies of inelastic, frictional disks. Journal of Rheology 30, 949-980], with constant (WBCE) and variable (WBVE) normal restitution coefficient, and Thornton [1997. Coefficient of restitution for collinear collisions of elastic-perfectly plastic spheres. Journal of Applied Mechanics 64, 383-386], based on elastic-plastic collisions. Comparison of the impact data with DEM simulations using the various contact force-displacement models revealed that the WBVE model provides the best overall agreement with the viscoelastic-fully plastic behavior of HDPE, while the almost purely elastic nature of PC and PS agreed well with all three models studied in this paper. The influence of the normal force-displacement models on the solids transport zone of an extruder was subsequently discussed based on the findings from the impact study.     

Friction of polystyrene: Consequence on nano-wear

Friction is able to induce major consequences on surface polymer properties (wear, scratch, etc.). These problems are crucial in the case of organic coatings (paints, varnish). The aim of this work is to analyse the influence of friction on the nano-wear behaviour of polystyrene. Studies will be focused on the analysis of the transfer layer induced by the friction of a polystyrene cylinder in contact with a flat and smooth substrate. The model substrate is a hydrophilic silicon wafer (hydroxylated by a piranha treatment). Friction experiments are performed with a translation tribometer which measures the tangential force between the polymer and the substrate for controlled normal force and friction speed. The transfer layer is analysed using atomic force microscopy (AFM). Tentative correlations between transfer layer characteristics and polymer properties are proposed.     

Analysis of a pull-out test with real specimen geometry. Part I: matrix droplet in the shape of a spherical segment

We compared two models of the pull-out specimen – the ‘equivalent cylinder’ and the platelet models in which the matrix droplet is represented as a set of thin parallel disks with the diameters varying along the embedded fiber to approximate the real droplet shape. Analytical expressions for the profiles of the fiber tensile stress and the interfacial shear stress have been derived for the matrix droplet in the shape of a spherical segment, including the effects of residual thermal stresses and interfacial friction. Using these expressions, we analyzed the process of crack initiation and propagation in the platelet model and investigated the effect of the specimen shape on the force–displacement curves. The interfacial stress near the loaded fiber end in the platelet model is higher than in the equivalent cylinder model, which gives rise to earlier crack initiation and smoother shape of the force–displacement curve. As a result, the calculated interfacial shear strength values may be underestimated by 10–20%, if the equivalent cylinder is used instead of the real droplet shape. A method of correction to the equivalent cylinder model in order to avoid this underestimation is proposed.     

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

The effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003