Showing papers in "Composite Interfaces in 1996"

Effect of polymer-filler interface interactions on percolation conductivity of thermoplastics filled with carbon black

Abstract: Conductive composites based on PE, PP, PS, PMMA and PA filled with carbon black have been studied. It was found that percolation behavior of the conductivity depended on the degree of polymer-filler interaction. Equations taking into account the polymer-filler interactions were proposed to describe the conductivity of composite above and below the percolation threshold.

The role of chemical bonding and surface topography in adhesion between carbon fibers and epoxy matrices

Abstract: A series of PAN-based IM6 carbon fibers which were surface treated to different levels with a commercial method were pretreated with compounds representing the constituents encountered in epoxy mat...

On the use of energy methods for interpretation of results of single-fiber fragmentation experiments

Abstract: _We consider fragmentation experiments as a set of experimental results for fiber break density as a function of applied strain. This paper explores the potential for using fracture mechanics or energy methods in interpreting fragmentation experiments. We found that energy does not control fiber fracture; instead, fiber fracture releases much more energy than required to fracture the fiber. The excess released energy can lead to other damage mechanisms such as interfacial debonding. By assuming that all the excess released energy causes interfacial debonding and balancing energy using the energy release rate for debonding, we were able to determine interfacial toughness from fragmentation experiments. A reliable determination of interfacial toughness requires prior knowledge of interphase stress-transfer properties, fiber failure properties, actual damage mechanisms, and the coefficient of friction at the interface.

Were McGarry or Shiriajeva right to measure glass fibre adhesion

Abstract: The fibre/matrix interaction is a critical aspect of the composite behaviour for all composite families and has to be optimized for each new fibre/matrix system. In order to help this optimization, dedicated tests on model composites have been developed since fibre adhesion to organic matrices was first measured reproducibly in terms of bonding strength in the early sixties. Such tests provide a lot of information about the interface and related microphenomena and thus help to improve the materials. The progress is especially fast whenever the material is in an early stage of development. However, difficulties in the mechanical analysis of those apparently simple tests stand in the way of the introduction of relevant parameters into models of composite behaviour.

Single-fiber electromechanical pull-out testing and its application to studying the interface between steel fiber and cement

Abstract: The experimental technique of single-fiber electromechanical pull-out testing was introduced and used to study the interface between steel fiber and cement. The technique involves measuring both th...

FTIR spectroscopy applied to woodflour

Abstract: The features of infrared spectra of woodflour obtained by transmission and diffuse reflectance (DRIFT) are discussed. Intrinsic differences between the two techniques, as well as differences in sample preparation for transmission (pressed pellet) or DRIFT (simple mixing with powered KBr), affect the appearance of the spectra recorded. Other important differences arise from the water content in the sample, as well as in the KBr powder utilized as reference and dilution. Some of these differences are discussed since they occur in regions usually utilized to check modifications in the wood. The effect of sample concentration is investigated for both techniques. In transmission, the opacity of the pellet limits the sample concentration to about 2 wt% in KBr in order to obtain useful spectra. While no concentration limitations are present for DRIFT, distortion of the intensities of bands occur, mainly at low wavelengths. Finally, the use of DRIFT subtraction to detect small surface variations in the woodflour ...

Adhesion and composites

Abstract: Adhesion is vital to composites because cracks are stopped at adhesive interfaces, deflected along them, or converted into interfacial dislocations. Failure is therefore dominated by the adhesion between the components of the composite. This paper describes new ideas about adhesion and considers the relevance of these ideas to composite materials. In particular it emphasises the difference between adhesion at the molecular level and adhesion in engineering terms. There is no doubt that molecules of solid materials are attracted to each other by van der Waals and other influences. Also, they can be separated by applying mechanical forces. The problem is explaining the connection, i.e. the mechanism, between molecular attractions and mechanical measurements. Ideas such as 'keying', 'glueing' and 'friction' require critical assessment because they operate differently at the molecular scale. Interesting mechanisms such as 'adhesive hysteresis', 'adhesive stringing', and 'adhesive aggregation' deserve evaluati...

Interfaces in metal matrix composites

Abstract: The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through whic...

Analysis of single-fiber pull-out from composites by using stress birefringence

Abstract: During a fiber pull-out test, it is desirable to analyze the stress profiles along the embedded fiber directly within the same time scale as the normal pull-out tests. In the present study, the axial tensile stress profiles of the fiber in a model composite are measured during the single-fiber pull-out tests by using stress birefringence of the fiber. It is concluded from the analysis of the measured stress profiles that an effective radius of matrix, i.e. a radius defining the region of the matrix where the major deformation takes place, is not constant but is an increasing function of the interfacial shear stress. By incorporating the variable values of the effective radius of matrix into the shear-lag model, the axial tensile and the interfacial shear stress profiles are calculated. To accurately estimate the interfacial shear strength, the stress distribution along the embedded fiber and the variability of the effective radius of matrix should be taken into account instead of calculating the interfaci...

A refined method for estimating fiber and interfacial shear strength by using a single-fiber composite

Abstract: In estimating interfacial shear strength from the fragmentation process of fibers in single-fiber composites, a problem arises as to the value of the fiber strength if the fiber strengths distribute widely and strongly depend on the fiber length. To overcome this problem, a refined analysis method for simultaneously estimating the fiber and the interfacial shear strength from the fragmentation process has been shown. Agreements between the values estimated with the proposed method and the results of the single-fiber tensile and the direct shear tests have been obtained. It has been shown that the estimation of the interfacial shear strength using the proposed method is insensitive to the matrix properties if the interfacial shear strength is unaltered by the matrix properties, and that the variations of the distribution parameters of the fiber strength is significantly smaller for the proposed method as compared with the single-fiber tensile tests. The results obtained by applying the proposed method to v...

Interphase characterization in epoxy resin/carbon fibre composites

Abstract: In this paper, interphase properties of carbon fibre/epoxy resin single-fibre model and unidirectional (UD) composites are reported. To study the contribution of the carbon fibre surface chemistry and morphology and of the resin itself to the overall properties of the composites, untreated, oxidized and sized fibres are used with bi- and tetrafunctional, diglycidylether of Bisphenol A, DGEBA and tetraglycidyl 4,4'-diaminodiphenylmethane, TGDDM-based resins, cured with amine and anhydride hardeners. Adsorption measurements and single fibre contact angle experiments, as well as the pull-out test were applied to characterize the surface of carbon fibre and the interfacial shear strength with different matrices. It was shown that the presence of the size on the surface can drastically affect the wettability as well as the starting rate of the cure reaction of epoxide in the vicinity of the fibre surface, as revealed by FTIR microscopy. Different elastic-plastic behavior of model composites before debonding is...

The effect of GF/PP matrix interfacial properties on the weldline strength in short glass fiber reinforced polypropylene

Abstract: The strength of GF/PP injection molded parts is reduced by the existence of a weldline. In this paper, the effect of molecular weight of matrix PP, GF content, GF surface treatment, GF shape (diameter and chopped strand length), MAH-PP content and interfacial properties on the weldline strength in GF/PP has been studied. Since the fibers orient vertically to the flow direction, effective reinforcement cannot be obtained at the weldline. Such fiber direction at the weldline cannot be controlled by the molecular weight of matrix, GF content and GF shapes. On the other hand, strength of the weld specimens increases when the amount of silane coupling agents and the MAH-PP content increase. Therefore, it is suggested that the weldline strength is affected by the strength of interfacial adhesion between GF and matrix.

Modified null field method for P- and SV-wave scattering from a partially debonded fiber

Abstract: The modified null field approach to elastic P- and SV-wave scattering (in plane strain) from a partially debonded fiber has been developed. The debonded region on the fiber surface is subjected to traction free boundary conditions, whereas the fiber is assumed to be in welded contact with the host medium elsewhere. Additional null field equations for an elliptical extension of the actual surface of the scatterer are introduced which require additional expansions for the field in the region between the extended mathematical boundary and the actual boundary of the scatterer. The numerical accuracy of the modified null field method has been tested for the case of a perfectly bonded fiber for P- and SV-wave incidence. Scattering cross-section plots are presented for different degrees of debonding.

Impact performance of glass cloth/epoxy composite tubes with different surface treatment

Abstract: The primary objective of this work is to identify the effects of testing speed and the fiber surface treatment on the crushing performances of glass cloth/epoxy composite tubes. Two kinds of composite tube were investigated; the first tubes were reinforced with glass cloth treated with aminosilane coupling agent, and the others were reinforced with glass cloth treated with acrylsilane coupling agent. Composite tubes were axial compression tested at an impact speed of 8.5 m/s. The impact test results were compared with the results of tubes tested at a quasi-static crushing speed of 1 mm/min. Independent of the testing speed the aminosilane-treated tubes displayed higher energy absorption capability than the acrylsilane-treated tubes. The progressive crushing mode and the energy absorption capability of aminosilane-treated tubes was not influenced by the testing speed. By changing the testing speed from quasi-static to impact, the energy absorption capability of acrylsilane-treated tubes increased by 15%. T...

Interfacial enhancement of flexible PVC-silica composites by silane coupling agents

Abstract: Enhancement of a flexible poly(vinyl chloride) (PVC)-silica composite interface was studied by the application of γ-aminopropyltrimethoxysilane on silica. Composites containing silica and silanized silica up to 25.6 phr (per hundred resin) and prepared by sol-gel technology were subjected to water and water vapor attacks similar to that in their daily use. Silane application resulted in diminishing liquid water and water vapor sorption by about 24.0% and 11.9%, respectively. Equilibrium weight gain values of the composites having different amounts of silica correlated well with a peak at 3400 cm-1 in the IR spectra which was attributed to the stretching vibration of the O-H group of water. Liquid water and water vapor diffusivities in composites determined by the evaluation of weight gain against time data were about 0.4 x 10-13 and 0.4 x 10-12 m2 s-1, respectively. Inhibition of dioctyl phthalate (DOP) migration from composites by silane application was also determined as 24% using UV measurements. The m...

Maxwell- Wagner- Sillars relaxations and crystallinity in PEEK

Abstract: Dielectric studies have been carried out on semi-crystalline poly(ether-ether-ketone) at temperatures from 100 to 300°C and at frequencies between 20 Hz and 10 kHz. A relaxation is seen in the isochronal curves and in the Cole-Cole plots, which is attributed to a Maxwell-Wagner-Sillars polarization taking place at the interface between the crystalline inclusions and the amorphous matrix. The intensity of this relaxation is presented in relation to the morphology of the crystalline part which is known to depend on annealing conditions.

Role of weak interface to retain high strength of fiber in monofilamentary SiC fiber/titanium-aluminide composite

Abstract: The tensile strength of monofilamentary weakly bonded SiC fiber/γ-TiAl intermetallic compound matrix composite, prepared by the sputtering method, was measured and analysed using a fracture mechanical technique. The main results are summarized as follows: (1) The fracture of TiAl occurred prior to that of fiber, resulting in formation of circumferential cracks on the fiber. Interfacial debonding occurred during tensile test, resulting in long pull-out of the fiber. (2) The strength of the fiber in the TiAl matrix was nearly the same as that of the bare fiber. (3) The fracture mechanical analysis showed that (i) the interfacial debonding grows unstably upon initiation and (ii) the stress distribution in the fiber in the cross-section, where the matrix is fractured, approaches to that of bare fiber with increasing debonded length. The reason why the fiber strength was maintained in spite of the formation of cracks on the fiber surface due to the premature fracture of the matrix was accounted for by the full...

The effect of interface on fracture mechanism of GF/PP composites using O2 plasma treatment

Abstract: Polypropylene sheets are treated with oxygen plasma for the interfacial control of GF/PP composites. The interfacial strength between glass fabric and PP resin is estimated by the T-peel test method. The evaluation of T-peel test data is done by both the T-peel strength method and the T-peel amplitude method. The T-peel strength value and T-peel amplitude value were respectively increased to about 50% and 120% compared with each value of non-treated specimens. The T-peel strength relates to the surface energy on the PP-sheet and the T-peel amplitude relates to the fracture pattern of the delamination surface. From SEM observations on the delamination surface, many voids in the space enclosed with fiber bundles are observed in the case of non-treated specimen and no void and fiber bridging are observed on the plasma treated specimens. It is found that interfacial properties between fiber and resin are improved by this plasma process.

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 2. Grafting and crosslinking of interphase materials

Abstract: —This paper presents an interphase engineering technique suitable for grafting silane-modified polymers onto glass fibres to be used in composites with enhanced impact tolerance. The silane-modified polymers include ethylene polymers grafted with γ-methacryloxypropyltrimethoxysilane (MPS) and a copolymer of butyl acrylate (BuA) and MPS. The grafting of functionalized interphase materials onto glass fibres is performed in solution. By changing the concentrations of the solutions, different amounts of polymer can be deposited on the fibres. Water crosslinking of the polymer gives the possibility of producing stabilised interfacial polymer coatings over a range of thicknesses. It is concluded that acidic conditions (1) promote the grafting of silane-modified polymers on glass fibres and (2) for a given reaction time, increase the amount of crosslinked polymer in the interphase, i.e. yield more stable interphases. It is also likely that preserving acidic conditions at the fibre/polymer interface is important ...

Surface modification of calcium carbonate: radical graft polymerization of vinyl monomers onto calcium carbonate surface initiated by azo groups introduced onto the surface

Abstract: —The preparation of calcium carbonate modified by 12-hydroxystearate groups and the grafting of polymers onto the surface by the polymerization of vinyl monomers initiated by azo groups introduced onto the surface were investigated. The preparation of calcium carbonate modified by 12-hydroxystearate was achieved by the reaction of calcium chloride with sodium carbonate containing a small amount of sodium 12-hydroxystearate. The introduction of azo groups onto calcium carbonate was successfully achieved by the direct condensation of the carboxyl group of 4,4'-azobis(4-cyanopentanoic acid) with 12-hydroxystearate groups on the modified calcium carbonate using N,N'-dicyclohexylcarbodiimide as a condensing agent. It was found that the radical polymerization of vinyl monomers, such as methyl methacrylate (MMA), styrene, and N-vinylcarbazole (NVC), was initiated by azo groups introduced onto the surface, and the corresponding polymers were grafted onto the surface based on the propagation of polymer from the su...

Autohesive strength characterization of semi-crystalline polymer matrix composites

Abstract: The processing of polymer matrix composites requires the formation of an autohesive bond between the surfaces of adjacent polymer entities as well as with the fiber surface This autohesive bond strength development in amorphous polymers has been extensively studied and modeled This paper describes a study undertaken to describe the development of the autohesive bond between polymer surfaces in semi-crystalline matrix composites The results show that the development of the autohesive bond competes with the formation of crystals and actually results in the decrease of fracture strength at long processing times

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 1. Synthesis and characterisation of alkoxy-silane functionalized polymers

Abstract: This paper presents techniques for the functionalization of polymer materials with alkoxy-silane groups Alkoxy-silane groups are key intermediates in bonding organic materials onto glass-fibre surfaces Two reaction routes are described: (1) Chain copolymerization of butyl acrylate (BuA) and γ-methacryloxypropyltrimethoxy-silane (MPS) and (2) grafting of MPS onto polyethylene (PE) and poly(styrene-block-ethylene-co-1-butene-block-styrene) (SEBS) It is concluded that chain copolymerization is the superior route since it gives almost unlimited possibilities of adjusting the degree of alkoxy-silane functionality The paper also demonstrates the advantage of performing graft copolymerization in the melt instead of in solution Solution grafting is of low efficiency and is probably directed to chains having reactive sites, eg branch points It is proposed that the primary cause is the lower temperatures used in solution grafting in comparison to the melt reaction

Radical graft polymerization initiated by azo groups introduced onto the surface of carbon whisker

Abstract: The radical graft polymerization of vinyl monomers onto carbon whisker, i.e. vapor grown carbon fiber, initiated by azo groups introduced onto the surface has been investigated. The introduction of azo groups onto the surface is achieved by (1) the reaction of 4,4'-azobis(4-cyanopentanoic acid) with isocyanate groups on the surface, which are introduced by the treatment of surface carboxyl and phenolic hydroxyl groups with tolylene-2,4-diisocyanate and (2) the reaction of 2,2'-azobis(2-cyano-n-propanol) with acyl chloride groups on the surface, which are introduced by the treatment of surface phenolic hydroxyl groups with terephthaloyl dichloride. The carbon whiskers having azo groups prepared from the above two methods are abbreviated as CW-Azo 1 and 2, respectively. The azo group content of CW-Azo 1 and 2 is determined to be 0.04 and 0.07 mmol/g, respectively. It is found that the radical polymerization of methyl methacrylate and styrene is initiated by the surface azo groups to give the corresponding p...

The model analysis of tensile stress distribution in the short-fiber embedded in thermoplastic composites

Abstract: The possibility of experimental model analysis in which stress birefringence is utilized for the simple and accurate evaluation of the tensile stress distribution in short-fibers embedded in composites has been investigated. For this purpose, a birefringence-free polymer blend is used as the matrix resin to avoid any influence of stress birefringence. When the stress applied to the composite is low, the experimental results agree with the theoretical estimates; however, as the stress increases, a slight deviation is detected.

Surface layer of polymer fibers

Abstract: Sorption and dynamic mechanical analysis have been used to study the surface structure of high strength polymeric fibers including aramide and high modulus polyethylene fibers. The method used probes surface layers of the order of micrometers. The change in diffusion coefficient as a function of the radial distance of the fiber indicates structural inhomogeneity, in particular skin-core morphology in these fibers. The skin layer possesses lower density and less order than the fiber bulk.

Properties and interfacial microstructures of K2O* 6TiO2 whisker-reinforced aluminum composites

Abstract: K2O 6TiO2 whiskers have been used to prepare 6061 alloy and pure A1 matrix composites by a squeeze casting process. The flexural strength and microhardness test have been performed for as-cast composites and in the T6 treated state. The interfacial microstructures of composites and the effect of interface reaction on composite properties have been studied with high resolution transmission electron microscope (HRTEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The results showed that the flexural strength of 6061A1 composites was 518 MPa in the as-cast state and decreased to 490 MPa in the T6 state. The tensile strength of pure A1 matrix composites was 238 MPa (as-cast) and 212 MPa ('T6'). In K2O. 6TiO2/A1 composites, the whisker was found to react with matrix easily, thus forming a continuous interlayer. In the case of 6061A1 matrix, the interlayer consisted of TiO, MgTi2O4, MgAl2O4, some 10-20 nm in thickness, which increased a few nanometers after T6 treatment. In the case of pure A1,...

Reactive blending for improving interfacial behavior

Abstract: Multicomponent polymer blends, for improving both process and product related properties, often require the optimization of interfaces/ interphases. This is particularly true in the case of blends ...

Measurement of interfacial shear strength of ultra-high modulus carbon fibers and its effects on macromechanical properties of CFRP

Abstract: Ultra-high tensile modulus (700 GPa) pitch-based carbon fiber was systematically surface treated, and interfacial shear strength (IFSS) between the fiber and epoxy resin was measured by micro-droplet method; then, relations between the IFSS and several mechanical properties of the corresponding CFRP were investigated. First, the method of measuring IFSS of very thin and fragile fiber by micro-droplet technique was developed by modifying the shape of specimen. IFSS was related to fiber surface oxygen, which seems to be related to functional groups on the fiber surface. Next, 0° tensile, 0° flexural and 0° compressive properties, 90° tensile strength, interlaminar shear strength (ILSS), and 0° Izod resistance were measured and were related to the IFSS. On the 0° compressive test, delamination was not the dominant failure mode and hence the compressive strength did not change with IFSS. ILSS was measured by double notch shear method, and was proportional to the IFSS. 90° tensile strength increased with incre...

Surface and core modification of carbon fibers

Abstract: Thermal and thermochemical treatments with amines have been shown to improve both the mechanical strengths of carbon fibers and their interfacial bonding forces with polymer matrix. In this report, the wettabilities of the surface-treated carbon fibers were studied by dynamic contact angles with epoxy resin. Two mechanisms occur during the fiber modification processes, namely, the fiber core restructuring and surface etching. The former occurs at elevated temperature and increases the fiber strength significantly. When surface reaction occurs, the number of surface oxides increases to improve the surface wettabilities, which may be observed by the decreased dynamic contact angles. Accompanying the etch-cleaning effects, the fiber strength would also increase initially. Deeper penetration of amine would destroy the graphite crystallinity on fiber surfaces and lower the fiber strength.