The effect of matrix composition on fibre/matrix interfacial bond shear strength in fibre-reinforced mortar

The effects of factors associated with the composition of the matrix, i.e. curing conditions and time and mix proportions, on the shear strength of the interfacial bond between steel fibres and a cementitious mortar matrix have been examined experimentally using a single-fibre pull-out test technique. The experimental results indicate that bond shear strength increases significantly with an increase in matrix curing time and, for specimens with the fibre axis perpendicular to the direction of casting and compaction of the matrix, with a decrease in the proportion of water by weight in the matrix mortar. This latter effect is attributed to bleed water gain under the embedded fibre, as it is not observed in specimens with the fibre axis parallel to the direction of casting and compaction of the matrix. Furthermore, the results indicate that there is no correlation between interfacial bond shear strength and matrix mortar compressive strength.     

Experimental techniques for measuring fibre/matrix interfacial bond shear strength

Experimental techniques for directly obtaining some measure of the shear strength of the interfacial bond between fibrous reinforcements and cemetitious matrices are reviewed. These techniques— essentially single- or multiple-fibre pull-out out tests — are classified according to specimen type and configuration and are then evaluated and compared using a set of criteria adapted by the author for this purpose. None of the techniques are found to satisfy completely all of the criteria, but several probably could be modified to do so. The objective of this state-of-the-art review is to provide a basis for discussion of the possible future standardization of a test technique for determining fibre/matrix interfacial bond shear strength in cementitious matrix composite specimens.     

The properties of dry-spun carbon nanotube fibers and their interfacial shear strength in an epoxy composite

Continuous fibers composed of carbon nanotubes have been adopted as reinforcements for polymeric composites. This paper presents several fundamental studies relevant to the mechanical behavior of CNT fibers, including fiber tensile behavior; in situ SEM observation of fiber deformation mechanisms; and fiber modulus, ultimate strength and fracture strain measurements. A modified Weibull strength distribution model that takes into account the flaw density variation with fiber diameter has been adopted for the statistical strength analysis. The interfacial shear strength between the carbon nanotube fiber and the epoxy matrix has been measured using fragmentation tests of single-fiber composites.     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

Effects of environmental exposure on fiber/epoxy interfacial shear strength

The TRI microbond technique for direct determination of fiber/resin interfacial shear strength in composites has been used for investigation the influence of environmental conditions on adhesive bonding in certain systems. The small dimensions involved in the method facilitate uniform exposure and short exposure times. We have observed significant changes in both average shear strength and in shear strength distributions on exposing aramid/epoxy and glass/epoxy microbond assemblies to steam or hot water. Shear strength dropped to a plateau value in both cases, the reduction being more drastic with the glass fiber. Vacuum drying restored shear strength completely in aramid/epoxy microassemblies, even when the surface of the aramid fiber had been chemically modified, but there was only partial regeneration of bond strength with the glass/epoxy system.     

The effect of liquid-induced adhesion changes on the interfacial shear strength between self-assembled monolayers

Friction between chemically-modified tips and surfaces has been studied with chemical force microscopy (CFM) to evaluate the effect of changing solid/liquid free energy on energy dissipation in sliding tip-surface contact. Well-controlled conditions were necessary to attain a single asperity contact in these experiments. We found that in a series of methanol- water mixtures the interfacial shear strength between CH3-terminated surfaces of the siloxane self-assembled monolayers (SAMs) was independent of the adhesion force. The shear strength value of 10.2 ± 1.0 MPa found for this interface under methanol-water media is consistent with the previous studies of similar systems under dry gas conditions. A comparison to available data on interfacial shear strengths demonstrated that siloxane monolayers were much more effective in reducing friction than various carbon coatings.     

Effects of the short‐fiber tip geometry and interphase properties on the interfacial debonding behavior of rubber matrix composites

An axisymmetric finite element model of a single fiber embedded in a rubber matrix was established. A cohesive zone model was used for the fiber–matrix interface because of the interfacial failure. The effect of the fiber tip shape on the interfacial debonding of short‐fiber‐reinforced rubber matrix sealing composites (SFRCs) was investigated; the shapes were flat, semi‐elliptical, hemispherical, and conoid, respectively. The initial strain of the interfacial debonding (ε₀) was obtained. We found that among the researched fiber tips, ε₀ of the SFRC reinforced with the hemispherical tip fiber appeared to be the maximum. The initial locations of interfacial debonding were also determined. The results show that the initial locations of the interfacial debonding moved from the edge to the center of the fiber tip when the ratio of the semimajor axis and semiminor axis of the semi‐elliptical fiber tip increased gradually. Further study on the effect of the interphase properties on ε₀ with the hemispherical fiber tip was conducted. The results indicate that an interphase thickness of 0.2 μm and an interphase elastic modulus of about 752 MPa were optimal for restraining the initiation of the interfacial debonding. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42774.     

The effect of interphase curing on interphase properties and formation

Fiber-optic evanescent wave FTIR spectroscopy was combined with phase imaging AFM to examine two thermosetting polymer matrix composite systems. The epoxy/NMA system data from the fiber-optic, evanescent wave FTIR analysis showed incomplete curing (～75% complete) in the region near the fiber, but essentially complete (～95% complete) curing in the bulk. Conversely, the unsaturated polyester system exhibited essentially complete curing (～95% complete) both near the fiber and in the bulk material. For the same samples, phase imaging AFM indicated that the epoxy/NMA system had an ～2.5 micron thick interphase, while the unsaturated polyester system showed no interphase between the fiber and the matrix. Therefore, the presence of the interphase in the epoxy/NMA system can be attributed to the incomplete curing next to the fiber. In addition, the systems chosen allowed the reactivity of adsorbed γ-APS coupling agent to be assessed simultaneously with polymer curing. For the epoxy/NMA system, the amine band decreased about 54% during curing. For the polyester system, the amine band decreased 43% during curing.     

Tensile strength and fiber/matrix interfacial properties of 2D- and 3D-carbon/carbon composites

The tensile and fiber/matrix interfacial properties of 2D and 3D carbon/carbon composites (C/C) were compared. To elucidate the effect of three-dimensional reinforcement, both C/Cs were composed of the same constituents and prepared via. the same process route. The tensile fracture strain of both C/Cs degraded with increasing bulk density, and the fracture strain of the 3D-C/Cs were larger than that of the 2D-C/Cs at the same bulk density. The interfacial bonding strength of the 3D-C/Cs were found to be much lower than that of the 2D-C/Cs. From the comparison of the interfacial and tensile fracture behavior, high tensile fracture strains of 3D-C/Cs were concluded to be attributed to the weak interfacial bonding. This low interfacial strength of the 3D-C/Cs was suggested to be caused by the residual stresses induced during processing in the 3D-C/Cs due to three-dimensional restriction of the fibers.     

The influence of silane coupling agent composition on the surface characterization of fiber and on fiber-matrix interfacial shear strength

It is well known that the fiber-matrix interface in many composites has a profound influence on composite performance. The objective of this study is to understand the influence of composition and concentration of coupling agent on interface strength by coating E-glass fibers with solutions containing a mixture of hydrolyzed propyl trimethoxysilane (PTMS) and n -aminopropyl trimethoxysilane (APS). The failure behavior and strength of the fiber-matrix interface were assessed by the single-fiber fragmentation test (SFFT), while the structure of silane coupling agent was studied in terms of its thickness by ellipsometry, its morphology by atomic force microscopy, its chemical composition by diffuse reflectance infrared Fourier transform (DRIFT), and its wettability by contact angle measurement. Deposition of 4.5 ‐ 10 m 3 mol/L solution of coupling agent in water resulted in a heterogeneous surface with irregular morphology. The SFFT results suggest that the amount of adhesion between the glass fiber and epoxy is dependent not only on the type of coupling agent but also on the composition of the coupling agent mixture. As the concentration of APS in the mixture increased, the extent of interfacial bonding between the fiber and matrix increased and the mode of failure changed. For the APS coated glass epoxy system, matrix cracks were formed perpendicular to the fiber axis in addition to a sheath of debonded interface region along the fiber axis.     

Measuring effective interfacial shear strength in carbon fiber bundle polymeric composites

Adhesion in composite materials is often quantified using the single fiber fragmentation (SFF) test. While this method is believed to provide accurate values for the fiber–matrix interfacial shear strength (IFSS), these may not accurately reflect the macroscopic mechanical properties of specimens consisting of tows of thousands of tightly spaced fibers embedded in a resin matrix. In these types of specimens, adhesion may be mitigated by fiber twisting and misalignment, differences in the resin structure in the confined spaces between the fibers and, most importantly, by any incompleteness of the fiber wetting by the resin. The present work implements fiber band fragmentation (FBF) testing to obtain effective interfacial shear strengths, whose values reflect the importance of these factors. The fiber fragmentation in these specimens is tracked through the counting and sorting of acoustic emission (AE) events occurring during the tensile testing of the specimen and yields the average critical fiber fragment length. AE results, in conjunction with stress-strain data, show that fiber breakage events occur at acoustic wavelet amplitudes substantially greater than those generated by fiber/matrix debonding. Kelly–Tyson analysis is applied, using the measured critical fiber fragment length together with known values for the fiber diameter and tensile strength to yield the effective IFSS. FBF tests are performed on carbon fiber/poly(vinyl butyral) (PVB) dog-bone fiber-bundle systems, and effective IFSS values substantially lower than those typically reported for the single fiber fragmentation testing of similar systems are obtained, suggesting the importance of multi-fiber effects and incomplete fiber wetting.     

Plasma surface treatments on carbon fibers. II. Mechanical property and interfacial shear strength

Carbon fiber are surface treated by oxygen, argon, and styrene plasma to study the effects on fiber strength and interfacial shear strength with PPS resin. Interfacial shear strength between carbon fiber and high melting temperature thermoplastic resins is successfully measured with the microbond pull-out test with the help of scanning CO₂ laser beam which solved the difficulties in preparing PPS microspheres. Tensile tests show that etching by oxygen plasma and deposition with plasma–PS increase strength of the fibers in some cases. ESCA spectra deconvolutions demonstrate that the improved interfacial strength is strongly related to the hydroxyl, ether, or aromatic groups on the surface. On the other hand, hydrocarbon segments are detrimental to the interface. Surface area and roughness have little influences on the interfacial strength of carbon fiber/PPS composites.     

Effect of polymer matrix and nanofiller on non-bonding interfacial properties of nanocomposites

To investigate the effect of polymer matrix and nanofiller on interfacial mechanical properties of their resulting nanoreinforced composites, pull-out tests of different nanofillers, such as graphene (GE), graphane (GA) and carbon nanotube (CNT), from various polymer matrix including polyethylene (PE), poly(methyl methacrylate) (PMMA), polytetrafluoroethylene (PTFE) and poly(vinylidene chloride) (PVDC), are simulated using molecular dynamics method (MD). The velocity-load model is applied in MD simulations, and the variation of non-bonding energy (van der Waals interaction), pull force and the average interfacial shear strength (ISS) in the pull-out process are obtained and presented graphically. Under the same mass density, when PE is used as polymer matrix for GE and CNT nanofillers, the resulting nanoreinforced composite possesses the highest non-bonding interfacial energy and the strongest ISS, and the pull force required for pulling out the nanofiller is the largest. For GA nanofiller, the GA-PMMA produces the highest non-bonding interfacial energy and the ISS. With the increase of diameter of CNT, the effect of its reinforcement becomes weak gradually. The chirality of GE does not influence the interfacial mechanical property of GE-reinforced nanocomposite. The (3, 3) CNT nanofiller produces the almost identical interfacial characteristic compared with GE nanofiller. However, when the GA nanofiller is used, the non-bonding energy, pull force and the average ISS of nanocomposite increases by nearly 100%.     

Effects of carbon nanotubes and interphase properties on the interfacial conductivity and electrical conductivity of polymer nanocomposites

L_c is the minimum length of carbon nanotubes (CNTs) required for efficient transfer of filler conductivity to polymer matrix in polymer CNT nanocomposites (PCNTs). In this work, L_c is correlated with the dimensions of the CNTs and the interphase thickness. Subsequently, the interfacial conductivity as well as the effective length and concentration of CNTs are expressed by CNT and interphase properties. Moreover, a simple model for the tunneling conductivity of PCNTs is developed with these effective terms. The impacts of all parameters on L_c, the interfacial conductivity, the fraction of CNTs in the networks and the conductivity of the PCNT are explained and justified. In addition, the predictions of the percolation threshold and conductivity are compared with the experimental results of several samples. The desirable values of interfacial conductivity are achieved by thin, short and super‐conductive CNTs, high waviness and a thick interphase. However, thin and long CNTs, low waviness, a thick interphase, poor tunneling resistivity due to the polymer matrix and a short tunneling distance advantageously affect the conductivity of PCNTs, because they produce large conductive networks. The predictions also show good agreement with the experimental measurements of percolation threshold and conductivity, which confirms the developed equations. © 2020 Society of Chemical Industry     

Modeling of tensile strength in polymer particulate nanocomposites based on material and interphase properties

In this work, a simple model is presented to determine tensile/yield strength in polymer nanocomposites containing spherical nanofillers based on material and interphase properties. The accuracy of the proposed model is estimated by comparing with the experimental strength of several samples from the literature. In addition, the effects of thickness (t) and tensile strength (σ_i) of the interphase as well as the radius (R) and volume fraction ( ) of the nanoparticles on the tensile strength are explained according to the proposed model. The high level of nanoparticle strength (more than 100 GPa) commonly leads to overestimates of the tensile strength of nanocomposites, whereas the assumption of correct interphase properties produces accurate calculations. The tensile strength of nanocomposites does not change at σ_i < 38 MPa, while it increases by 140% at t = 20 nm and σ_i = 90 MPa. However, a maximum 14% growth in tensile strength is obtained with the optimum values of = 0.04 and R = 10 nm. Therefore, the concentration and size of the nanoparticles have minor effects on the tensile strength of nanocomposites, but the major influences of interphase thickness and strength are pronounced. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44869.     

Nanoscale characterisation of thickness and properties of interphase in polymer matrix composites

An overview is presented of the properties and effective thickness of the interphase formed between fibres and polymer matrices. Chemical and physical characterization of the interphase is discussed to portray molecular interactions comprising the interphase layers in silane-treated glass-fibre composites. The gap between physico-chemical investigation on one side and bulk material testing on the other side is bridged by implementation of novel techniques, such as nanoindentation, nanoscratch tests, and atomic force microscopy (AFM), which have been successfully used for nanoscopic characterization of the interphase in the past few years. Salient differences are identified between the major findings of these studies in terms of hardness/modulus of the interphase relative to the bulk matrix material. While there is a significant "fibre stiffening" effect that may cause misinterpretation of the interphase hardness very close to the fibre, the formation of both a softer and a harder interphase is possible, depending on the combination of reinforcement, matrix, and coupling agent applied. This is explained by different interdiffusion behaviour, chemical reactions, and molecular conformation taking place at the interphase region in different composite systems. The effective interphase thickness is found to vary from as small as a few hundred nanometers to as large as 10 µm, depending on the constituents, coupling agent, and ageing conditions.     

Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase

The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (δ) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz–van der Waals work of adhesion (W _a ^LW) and the acid–base interaction parameter (I _a?b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid–base interaction (I _a?b) or by reducing the chemical incompatibility (Δδ) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.     

Effects of crystallinity and supermolecular formations on the interfacial shear strength and adhesion in GF/PP composites

Aim of this study was to screen the morphological effects on the interfacial shear strength (i) in glass fibre (GF) reinforced isotactic polypropylene (iPP) model composites. i was determined by a modified single fibre pull-out technique. It was established that the relation between i (5–6 MPa) and the yield stress of the iPP (y30 MPa) is at about 1:6 and that the i values were not influenced by the mophological superstructure set under isothermal crystallization conditions. Increased i was only observed when specimens were produced non-isothermally, by quenching (i9 MPa). This improvement could not be related to thermal shrinkage stresses. The enhancement in i was attributed to better wetting and improved adhesion due to the enlarged amorphous PP (aPP)-phase. A schematic adhesion model considering the wetting behaviour of aPP and iPP was proposed.     

Dependence of interfacial strength on the anisotropic fiber properties of jute reinforced composites

The upsurge in research on natural fiber composites over the past decade has not yet delivered any major progress in large scale replacement of glass fiber in volume engineering applications. This article presents data on injection‐molded jute reinforced polypropylene and gives a balanced comparison with equivalent glass reinforced materials. The poor performance of natural fibers as reinforcements is discussed and both chemical modification of the matrix and mercerization and silane treatment of the fibers are shown to have little significant effect on their level of reinforcement of polypropylene in comparison to glass fibers. A hypothesis is proposed to explain the poor performance of natural fibers relating their low level of interfacial strength to the anisotropic internal fiber structure. POLYM. COMPOS., 31:1525–1534, 2010. © 2009 Society of Plastics Engineers