Local Interfacial Shear Strength Research Articles

Micromechanical Tests on Natural Fibre Composites with Enzymatically Enhanced Fibre–Matrix Adhesion

Natural fibre–reinforced composites are more sustainable than other composites with respect to the raw materials. Their properties are attractive due to high specific properties, and especially so wherever high damping is valued. As the interphase between fibre and matrix is the region of highest stresses, a strong bond between fibre and matrix is essential for any composites’ properties. The present study compares two methods of determining the interfacial shear stress in natural fibre–reinforced composites: the single fibre fragmentation test and the single fibre pullout test. The studied composites are flax fibre reinforced epoxy. For a variety of fibre–matrix interaction, the fibres are treated with a laccase enzyme and dopamine, which is known to improve the fibre–matrix shear strength. In the observed samples, single fibre fragmentation test data, i.e. of fracture mode and fragment length, scatter when compared to pullout data. In single fibre pullout tests, the local interfacial shear strength showed a 30% increase in the laccase-treated samples, compared to the control samples. The method also permitted an evaluation of the frictional stress occurring after surface failure.

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 Interfacial Shear Strength and Critical Energy Release Rate in Single Glass Fiber-Crosslinked Polypropylene Model Microcomposites.

Continuous glass fiber-reinforced polypropylene composites produced by using hybrid yarns show reduced fiber-to-matrix adhesion in comparison to their thermosetting counterparts. Their consolidation involves no curing, and the chemical reactions are limited to the glass fiber surface, the silane coupling agent, and the maleic anhydride-grafted polypropylene. This paper investigates the impact of electron beam crosslinkable toughened polypropylene, alkylene-functionalized single glass fibers, and electron-induced grafting and crosslinking on the local interfacial shear strength and critical energy release rate in single glass fiber polypropylene model microcomposites. A systematic comparison of non-, amino-, alkyl-, and alkylene-functionalized single fibers in virgin, crosslinkable toughened and electron beam crosslinked toughened polypropylene was done in order to study their influence on the local interfacial strength parameters. In comparison to amino-functionalized single glass fibers in polypropylene/maleic anhydride-grafted polypropylene, an enhanced local interfacial shear strength (+20%) and critical energy release rate (+80%) were observed for alkylene-functionalized single glass fibers in electron beam crosslinked toughened polypropylene.

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.

Alkali resistant glass fiber reinforced concrete: Pull-out investigation of interphase behavior under quasi-static and high rate loading

Single fiber model composites of alkali resistant (AR-) glass fibers and a cementitious matrix were used to investigate the pull-out behavior under quasi-static and high speed loading. For fundamental understanding of the effect of the fiber/matrix interphase on the pull-out behavior under impact, differently sized AR-glass fibers were spun. As a first approach, the fiber surface was modified in oppositional ways using the following sizings: one based on a polypropylene (PP, weak) film former and another one based on a styrene-butadiene (strong) film former. Additionally, some of the fibers were kept unsized for comparison. A new ‘alternative’ approach was employed to determine the local interfacial shear strength, τd, and the critical energy release rate, Gic, from the reliable force values of the force–displacement curves. For all fiber surface states, the τd and Gic values for high loading rates appeared to be considerably greater than the corresponding parameters for a quasi-static pull-out test. This can be explained using a model based on Zhurkov's kinetic (thermal fluctuation) theory of the strength of solids, which also enabled to estimate the apparent activation energy for interfacial debonding. Both quasi-static and high-rate pull-out tests on this fiber/matrix pair can be considered as ‘normal’ (slip-dependent interfacial friction was not observed) and their results can be evaluated using the described approaches. The interfacial frictional stress reduced at high-rate pull-out tests for all systems. One of the possible mechanisms responsible for this behavior may be the smoothing of surface asperities upon debonding. As revealed by AFM of fracture surfaces, in the case of unsized fibers or those with the ‘strong’ styrene–butadiene sizing, the interfacial crack occurs through surface layers of the matrix material adjacent to the fiber, but it may propagate through the weak interface when the fiber is sized with ‘weak’ PP film former.

An alternative method of determining the local interfacial shear strength from force–displacement curves in the pull-out and microbond tests

The traditional way of determining the local interfacial shear strength (IFSS), τd, between the fiber and the matrix in microbond and pull-out test includes the measurement of the debond force, Fd, at which interfacial crack initiates, and subsequent τd calculation using the shear-lag analysis. However, for some fiber–matrix pairs the debond force, which manifests itself as a ‘kink’ in the force–displacement curve, cannot be reliably measured. We propose a new approach, based on measuring the maximum pull-out force, Fmax, and the initial post-debonding force, Fb, which does not require the knowledge of the Fd value. The comparison of the τd values calculated for several fiber–matrix pairs using three different methods of data reduction demonstrated that the new method yielded results very similar to the traditional one. Thus, it can be recommended as an alternative technique if the debond force is indeterminable.

Poly (vinyl alcohol) fiber reinforced concrete: investigation of strain rate dependent interphase behavior with single fiber pullout test under quasi-static and high rate loading

A new single fiber pullout device was used to investigate interfacial debonding in the poly (vinyl alcohol) (PVA) fiber–cementitious matrix system under impact loading. For comparison, PVA fibers with two different surface states were used – as-received and after fiber finish removal by extraction in solvents. The indirect method was employed to determine the interfacial strength parameters, namely, the local interfacial shear strength, , and the critical energy release rate, G ic, from the peak load values reached in the pullout test. For both fiber surface states, the and G ic values for high loading rates appeared to be considerably greater than the corresponding parameters determined by means of a quasi-static pullout test. This can be explained using a model based on Zhurkov’s kinetic (thermal fluctuation) theory of the strength of solids, which also enabled to estimate the apparent activation energy for interfacial debonding. Its values (1.3 eV for both as-received and treated fibers) evidenced formation of strong bonds between PVA fiber and the concrete matrix and the identity of debonding mechanisms for both fiber types. Finish removal decreased interfacial adhesion due to smoothing the fiber surface and reducing interfacial mechanical bonding.

A Fluoropolymer Coating on Carbon Fibers Improves Their Adhesive Interaction with PTFE Matrix

Carbon fibers were treated in a HF glow discharge in tetrafluoroethylene and octafluorocyclobutane in order to improve their adhesion to poly(tetrafluoroethylene) matrix. As the result of the plasma treatment, a thin (20–140 nm) fluoropolymer coating was deposited onto the fiber surface. The structure of this coating was studied by means of IR spectroscopy, XPS, AFM and SEM techniques. The coating material appeared to be similar to PTFE in its chemical composition but distinguished by branched, partially crosslinked, amorphous structure and included unsaturated chemical bonds. The coating thickness of 70 nm was sufficient to effectively screen the field of molecular forces of the initial substrate, thus, decreasing the surface energy of the fibers and improving their compatibility with the PTFE matrix. The adhesive strength in the PTFE–carbon fiber systems, measured by means of the microbond test, more than doubled upon the plasma treatment (the local interfacial shear strength increased from 10.7 to 29.7 MPa, apparent IFSS from 4.3 to 7.8 MPa), and the interfacial frictional stress increased by 70%. The new composite material consisting of 20% short coated carbon fibers in the PTFE matrix showed better mechanical, thermal and tribological characteristics as compared with the composite reinforced with untreated fibers.

Is there any contradiction between the stress and energy failure criteria in micromechanical tests? Part III. Experimental observation of crack propagation in the microbond test

A direct observation of crack propagation in the microbond test was carried out for five different fiber/polymer matrix systems. This technique appeared to be a very effective tool for interface characterization. Experimental plots of the force required for further crack propagation as a function of debond length were analyzed using both energy-based and stress-based models of debonding. The fracture mechanics analysis was used to construct families of crack resistance or R-curves which showed the variation of energy release rate, G, with the debond length, and included the effect of interfacial friction in debonded regions. For the first time, analogs of the R-curves were created within the scope of the stress-based model to present the local shear stress near the crack tip, τ, as a function of crack length. In both models, the behavior of the interfacial parameter (G or τ) strongly depends on the assumed value of the interfacial frictional stress (τf). However, for each matrix/fiber system there exists such a τf value for which the investigated parameter is nearly constant over the whole region of stable crack propagation (70–90% of the embedded length). Moreover, these best-fit τf values for each specimen appeared to be practically the same for both energy-based and stress-based approaches. Thus, both interfacial toughness, G ic, and local interfacial shear strength, τd, adequately characterize the strength of a fiber/matrix interface. Extrapolation of R-curves and their analogs to zero crack length allows measurement of the interfacial parameters with good accuracy.

Peak force as function of the embedded length in pull-out and microbond tests: effect of specimen geometry

We have derived the equations which explicitly express the peak force, F max, and the apparent interfacial shear strength, τ app, measured in the pull-out and microbond tests, as functions of the embedded length. Three types of test geometries were considered: (1) a fiber embedded in a cylindrical block of the matrix material; (2) microbond test with spherical matrix droplets; and (3) pull-out test in which the matrix droplet had the shape of a hemisphere. Our equations include the local interfacial shear strength (IFSS), τ d, and the frictional interfacial stress, τ f, as parameters; the effect of specimen geometry appeared in the form of dependency of the effective fiber volume fraction on the embedded length. The values of τ d and τ f were determined by fitting our theoretical curves to experimental F max (l e) plots by using the least squares method. Our analysis showed how the local IFSS and the frictional interfacial stress affected the measured F max and τ app values. In particular, it was revealed that intervals of embedded lengths could exist in which frictional interfacial stress had no effect on F max and τ app, even if the τ f value was high. We also derived an equation relating the scatter in the interfacial strength parameters (τ d and τ f) to the scatter in τ app, which is experimentally measurable, and proposed a procedure to determine the standard deviations of τ d and τ f from experimental pull-out and/or microbond test data.

Characterization of fiber/matrix interface strength: applicability of different tests, approaches and parameters

Different approaches to interface strength characterization in fibrous composite materials by means of micromechanical tests are analyzed. Special attention is paid to single fiber pull-out and microbond techniques. For these tests, advantages and disadvantages of stress-based and energy-based models of interfacial debonding and corresponding adhesional parameters (local interfacial shear strength, critical energy release rate, adhesional pressure) are discussed. The following analytical models developed to measure these parameters as well as to determine the interfacial frictional stress are considered in detail: (1) the use of the debond or “kink” force from the force–displacement curves; (2) two-parameter fit of experimentally measured maximum pull-out force as a function of embedded length; and (3) models assuming two-stage debonding with imperfect interface at the second stage. The analysis is illustrated by our own experimental data on adhesion of polymer and cement matrices to different reinforcement fibers.

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τapp, and the local interfacial shear strength (IFSS), τd. However, it has been demonstrated that the F max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relationship between the maximum force and the embedded length ('scale factor') to separately measure fiber-matrix interfacial adhesion and friction. Using the equation for the current crack length as a function of the applied load, based on a stress criterion of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force value versus the embedded length. By varying τd and interfacial friction, τf, to fit experimental plots, both interfacial parameters were estimated. The micromechanical tests were modeled for three types of specimen geometries (cylindrical specimens, spherical droplets, and matrix hemispheres in the pull-out test) with different levels of residual thermal stresses and interfacial friction. The effect of all these factors on the experimental results is discussed, and the importance of specimen geometry is demonstrated. One of the most interesting results is that the 'ultimate' IFSS (the limiting τapp as the embedded length tends to zero) is not always equal to the 'local' bond strength.

Characterisation of the Interfacial bond Strength between Glass Fibre and Epoxy Resin using the Pull-Out and Push-Out Techniques

Interfacial bond strength between epoxy resin and glass fibre was studied using the pull-out and push-out techniques. For untreated fibres, these micromechanical tests gave similar values of the local interfacial shear strength and critical energy release rate. In the case of fibres treated by γ-APS, both tests showed considerable increase in the bond strength. However, for the modified fibres, the pull-out test gave greater values of both interfacial parameters than the push-out test, a result attributed to the different modes of interfacial loading. The different loading patterns also cause different failure mechanisms in these two tests.