Interfacial mobility and its effect on interlaminar fracture toughness in glass-fibre-reinforced epoxy laminates

The effects of interfacial treatment of glass fibres in glass/epoxy composites were studied through Mode I delamination fracture toughness tests using a double cantilever beam specimen. The treatment of glass fibres with two similar silane coupling agents has been shown to improve the mechanical properties of the composite as a function of the type of coupling agent, -aminopropyltriethoxysilane (APS) and -aminobutyltriethoxysilane (ABS) have similar chemistry, but differ in mobility (molecular motion) at the coupling agent-epoxy interface. The critical energy release rate, G _1c, for the APS-treated composites (0.59±0.05 kJ m^–2) was shown to be higher than that of the ABS-treated one (0.37±0.01 kJ m^–2) and also the untreated one (0.31±0.02 kJ m^–2). In this case, the bulk structural property appears to be a function of the microscopic interfacial properties including the dynamics of the coupling agent layer. Optical characterization of the fracture surfaces reveal delamination at the epoxy-glass interface for the untreated samples, while the ABS- and APS-treated samples showed less interfacial delamination, respectively.     

Effect of block copolymer dopants and alkylsilane monolayers on the fracture toughness of polystyrene-glass interfaces

The asymmetric double cantilever beam fracture test has been used to study the fracture toughness of polystyrene (PS)-glass interfaces reinforced with poly(deuterostyrene-b-2 vinyl pyridine) (dPS-PVP) as a function of degree of polymerization of the blocks. The effect of modifying the glass substrate with various selfassembled monolayers is also described. For the block copolymer with degrees of polymerization,N _dPS=656 andN _PVP=46 (referred to asN _dPS —N _PVP or 656-46), located at the interface between glass and PS, the interface fails by chain scission at areal chain densities, , of the block copolymer below a critical value, ^*. Above this value, e.g. > ^*, the interface fails by crazing followed by chain scission. For the 656-46 diblock copolymer, the transition is located at ^*=0.03 chains nm^–2, which results in a calculated force to break a C-C bond along the polymer backbone of approximately 2 × 10^–9N. For the 800–870 diblock copolymer at the interface between glass and PS, failure occurs due to chain scission. Fracture of both the 656-46 and the 800–870 block copolymers at the interface between (OTS) octadecyltrichlorosilane monolayer coated glass and PS is due to chain pulloff of the block copolymer from the OTS coated glass. Very little additional stress was transferred across the interface, resulting in fracture toughnesses comparable to that of a PS-glass interface with no block copolymer added.     

Adhesion of epoxy resins to metals

The adhesion of an epoxy resin above its glass transition temperature to aluminium, steel and gold surfaces has been studied using the methods of fracture mechanics. The results are compared with those of a previous study of elastomeric adhesives by Andrews and Kinloch, and the “intrinsic failure energies”,θ ₀, for the epoxy-metal bonds are deduced by similar methods. Correspondence, within a factor of two, is found betweenθ ₀ and the thermodynamic work of adhesion,W _A, for most cases of interfacial failure, indicating both the absence of specific or chemical interactions at the interface and a purging of surface contaminants by the epoxy. An exception occurs when an excess of epoxy groups exist in the uncured resin. Here, for steel and aluminium but not for gold, the interfacial bonding is stronger than the cohesive strength of the resin due probably to the formation of strong bonds with the metal oxide surface layer.     

Investigation on adhesion, interphases, and failure behaviour of cyclic butylene terephthalate (CBT)/glass fiber composites

Interphases exist in hybrid materials and significantly influence their mechanical performance. To find a bridge between the microscopic and macroscopic mechanical properties, this work investigates the microscopic nature of glass fiber surfaces and glass/CBT interphases in terms of topography, fractography, and adhesion properties. The variations in glass fiber surface properties result from the different sizings. Using the single fiber pull-out test, AFM, and ζ potential tests, it is shown that the interfacial bond strengths in CBT resin composites can vary depending on the kind of sizing formulation and properties. The greatest adhesion strength is achieved by aminosilane sizings with epoxy resin film former. The surface roughness of the fibers can be varied by sizings with different content and ζ potential values, which has no significant contribution to interphase adhesion strength from ‘mechanical interlocking’. For the systems with film formers, cohesive failure occurs and similar values of both interfacial adhesion strength, τ_d, and fracture energy release rate, G_ic, are obtained, in which τ_d approaches the shear yield strength of CBT matrix. A further enhancement of interfacial adhesion is limited by the mechanical properties and the non-homogeneous microstructure of CBT resin due to the less-than-perfect CBT polymerization.     

Axial compressive behaviour of reinforcing fibres and interphase in glass/epoxy composite materials

Axial compressive behaviour of reinforcing fibres and interphase in glass fibre/epoxy resin composites were examined. Axial compressive strengths of glass fibres were evaluated by the tensile recoil method. The effects of silane-based coupling surface treatment agent on the fibre compressive strengths were investigated. The glass fibres showed higher compressive strengths when coated by the surface treatment. Interphase behaviour was also investigated by means of the single-fibre embedded compressive test. The particular stress and strain distributions inside the specimen were examined by a three-dimensional finite element analysis. The parameter interfacial transmissibility instead of the conventional critical fibre length theory was introduced as an index of interfacial properties. This parameter was useful to estimate the interfacial properties at the elastic state apart from the complicated critical state. It was confirmed that the surface treatment improved the glass/epoxy interphase under axial compressive load.     

Fracture mechanism in short fibre reinforced thermoplastic resin composites

The properties of two types of short carbon fibre (CF) reinforced thermoplastic resin composites (CF-PPS and CF-PES-C), such as strength (_y). Young's modulus (E) and fracture toughness (K _1c), have been determined for various volume fractions (V _f) of CF. The results show that the Young's modulus increases linearly with increasingV _f with a Krenchel efficiency factor of 0.05, whereas _y andK _1c increase at first and then peak at a volume fraction of about 0.25. The experimental results are explained using the characteristics of fibre-matrix adhesion deduced from the load-displacement curves and fractography. By using a crack pinning model, the effective crack tensions (T) have been calculated for both composites and they are 57 kJ m^–1 for CF-PPS and 4.2 kJ m^–1 for CF-PES-C. The results indicate that the main contribution to the crack extension originates from localized plastic deformation of the matrix adjacent to the fibre-matrix interface.     

A simplified method for determining K IC in glass with liquid solutions impregnation

In order to determine K _IC in a simplified form, rectangular pieces of glass were scratched with a glazier's diamond, considering this scratch as the crack tip. The sides of the crack were made by sticking two rectangular pieces of proper thickness with epoxy resin as was made in a previous work. The K _IC was determined in glass in which the cracks are impregnated with liquid solutions of kerosene, sodium hydroxide and sodium silicate. For the samples impregnated with kerosene the mean value of K _IC remains constant, whereas for the other two solutions a decrease in the mean value of K _IC is observed, a 10% in samples impregnated with sodium hydroxide and 16% in samples impregnated with sodium silicate, when comparing them with their respective groups of samples of glass without impregnating. The corresponding Weibull diagrams were made and Weibull functions of three parameters were obtained. The respective parameters of the probability distribution functions of K _IC were estimated by means of the graphic method of nomograms.     

Substrate/film interactions in thick films of YBaCuO on alumina

The substrate-film interaction in thick films (>10m) of YBa₂Cu₃O_7–x on alumina processed under normal conditions is investigated using electron-probe microanalysis. The formation of a mixture of barium aluminate and alumina over a thickness of about 2m in the interfacial region is established quantitatively using compositional mapping and point-count analysis across the substrate-film interface. Diffusion of aluminium into the film over severalm beyond the reaction layer is also observed. The variation of oxygen composition across the interface has been mapped. Leaching of oxygen from the 1-2-3 phase in the bulk is suggested as the reason for the observed decrease inT _c(0) and increase in T _c in films of YBa₂Cu₃O_7–x on alumina.     

Analysis of a compressive shear test for adhesion between elastomeric polymers and rigid substrates

A compressive shear test for investigating adhesion between an elastomeric polymer and a rigid substrate has been studied. The test consists of loading a specimen comprising of a 3-ply laminate: substrate/polymer/ substrate, in compression and shear at a specified angle to the loading direction. Under displacement control and when adhesion is sufficiently low, an interfacial crack nucleates at one interface early during loading and propagates stably up to a critical load at which unstable propagation with an associated load drop ensues. The case of an isothennal hyperelastic material has been analyzed by computing the energy release rate for an interfacial crack as a function of crack length. The analysis shows that for a range of initial crack size interfacial crack propagation is stable until crack length reaches a critical size at which unstable propagation ensues. The energy release rate at this instability is relatively insensitive to angle of loading, strain, and hyperelastic parameters, which allows one to extract an interfacial toughness, ₀, from overall measurement of stress and strain. The analysis has been extended to consider combined hyperelasticity and viscoelasticity by using a cohesive zone model for crack propagation implemented as a cohesive finite element. The energy release rate and cohesive zone analyses give identical results for an hyperelastic material. For a viscoelastic-hyperelastic material, the cohesive zone approach allows the viscous losses in the bulk polymer to be estimated separately from the value of interfacial fracture toughness. Both analyses have been applied to experiments on glass/polyvinyl butyral (Butacite®)/glass laminate specimens. The intrinsic interfacial toughness, consisting of contributions from bond rupture and a near-tip process zone, is found to be rate-dependent and lies in the range 50–200 J m^–2.     

The effect of hydrostatic pressure on transverse strength of glass and carbon fibre-epoxy composites

An attempt was made to measure indirectly the transverse tensile strengths of uniaxially aligned fibre pultrusions by the diametral compression of disc-shaped samples using concave loading anvils. Two types of composite were investigated, containing 60%V _f of either type AS carbon fibres (CFRP) or S glass fibres (GRP), both in an epoxy resin matrix. Testing was carried out at atmospheric and under superposed hydrostatic pressures, –H, extending to 300 M Pa. The resultant principal stresses at the disc centre were ₁ = _A +H; ₂ =H; ₃ = –3_A +H, where _A = 2P/dt for a disc of diameter,d, and thickness,t, subjected to a loadP. Deviations from linearity in the load-deflection response were detected throughout the pressure range at ã70% and ã90% of the failure load for CFRP and GRP, respectively, and these were associated with resin yielding. The pressure dependence of _A, approximately –0.1H, was consistent with a two-parameter yielding criterion predicting hypothetical yield stresses in simple tension and compression of ã81 and –109 MPa, respectively, for both matrix materials. Irrespective of pressureeventual fractures took place along the loading diameter, but in the CFRP specimens tested under pressure initial cracks at the disc centres were at ã45 ° to the loading axis, i.e. on the plane of maximum shear stress. Fractographic observations were consistent with transverse failure taking place by fibre-matrix decohesion in GRP and by resin fracture in CFRP. Other than the atmospheric datum point for CFRP, the pressure dependence of _A for failure, _F, was also approximately –0.1H. Of the various stress, strain and strain energy criteria for failure examined, only critical tensile strain was found consistent with this pressure dependence.     

The energy of crack propagation in carbon fibre-reinforced resin systems

The energy expended during controlled crack propagation in unidirectionally reinforced composites of carbon fibre in a brittle resin matrix has been evaluated in terms of the energy dissipated during fibre-snapping, matrix-cracking and fibre pull-out. The work of fracture, _F, is found to depend principally on the frictional shear stress at the fibre/resin interface opposing pulling out of broken fibres. Differences in _F for carbon fibre/resin composites exhibiting a range of interfacial shear strengths and void contents have been explained with reference to variations in fracture surface topography of the fibrous composites. The effect of environment on properties of the interface and work of fracture was also investigated. The energy required to propagate a crack has been compared with the energy for fracture initiation, _I, using a linear elastic fracture mechanics approach. It was found that fibre pull-out energy is the principal contribution to _F, and _I is similar to the elastic strain energy release rate at the initiation of fracture of a brittle, orthotropic solid. For crack propagation parallel to fibres, _F and _I are similar and not unlike the fracture surface energy of the resin alone. The strength of the interface is important only in so far as it affects the value of _I.     

Structural Perfection of Four-Layer Ga x In1 – x As y P1 – y Laser Heterostructures

The structural perfection and coherent growth conditions of epitaxial layers in four-layer Ga _x In_{1 – x} As _y P_{1 – y} heterostructures on InP(001) substrates were studied by x-ray diffraction and topography. The composition of the active layer corresponded to a peak photoluminescence wavelength of 1.34 m. The coherent growth region was delineated, and the critical thickness of epitaxial layers was determined as a function of the interfacial lattice mismatch. The critical thickness determined experimentally exceeds the calculated value. It is shown that heterostructures up to 9 m in layer thickness, free of interfacial dislocations can be grown in a broad range of elastic strains in adjacent layers.     

Partition of solute elements during solidification of iron-carbon-chromium alloys

The partition coefficients of chromium between austenite and liquid iron,k _Cr ^A/L , were determined from the experiment of rapid cooling of iron-carbon hypo-eutectic alloys containing a small amount of chromium from coexisting solid—liquid states; the partition coefficients between eutectic and its liquid,k _0,Cr, andk _0,Cr, for the stable and metastable eutectic solidifications were obtained from the zone-melting experiment of iron—carbon eutectic alloys containing a small amount of chromium. Chromium was rejected to liquid iron on the crystallization of primary austenite,k _Cr ^A/L <1. On the eutectic solidification, chromium was enriched in eutectic liquid for the stable system,k _0,Cr<1, and was conversely diluted in the liquid for the metastable system,k _0,Cr>1. The relationship between effective and equilibrium partition coefficients given by Burtonet al. was observed for the results of the zone melting experiment and, from the relationship, the thickness of boundary layer in the liquid ahead of the solid—liquid interface was found to be 0.17 mm for the stable system and 0.11 mm for the metastable system. Thermodynamic calculation of the partition coefficients of chromium and carbon proved to represent the observed partition coefficients well.     

Giant dielectrics from modified boundary layers in n-BaTiO<Subscript>3</Subscript> ceramics involving selective melting reactions of silver/glass composites at the grain boundaries

High permittivity ceramics with _eff > 10⁵ can be realized from semiconducting BaTiO₃ by the two-step processing namely, sintering the donor doped samples in static air followed by electroding with the fired-on silver/glass composites. Doping with Sb⁵⁺ and Bi³⁺ not only enhances the grain conductivity but also increases the grain size (10–60 m), when sintered at 1370 C in static air. The ceramic samples are electroded with the paste containing nanometer particles of silver dispersed in varied amounts of low melting (600–900 C) glass compositions PbO + Bi₂O₃ + B₂O₃ ± SiO₂ ± CuO. High permittivities are obtained for these capacitors stable over a wider range of temperature and over a broad frequency range. The grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding, related to ceramic microstructure is proposed to be responsible for the unusual high permittivity in semiconducting BaTiO₃. The energy dispersive X-ray analyses indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting oxides at the grain boundaries near to the electrodes. Impedance spectroscopy on BaTiO₃ ceramics demonstrates that they are electrically heterogeneous with insulating grain boundaries together with the ceramic/electrode interface acting as barrier layers. On the basis of the symmetrical Schottky-barrier model of the grain boundary region, the barrier height and donor concentration N_d of the grains were obtained by the modified 1/C²–V plot. These modified boundary layer capacitors having high field strength withstandability can be used in a wide range of frequencies.     

The effect of a silane coupling agent on water adsorption at a metal/polymer interface studied by neutron reflectivity and angle-resolved X-ray photoelectron spectroscopy

Neutron reflectivity and angle-resolved X-ray photoelectron spectroscopy (XPS) have been used to examine the adsorption of water to a molybdenum/polyurethane interface. The initial quasi-steady state interfacial D₂O profile (obtained after 1 h exposure to D₂O saturated air at ambient temperature) was determined on a nanometre scale by neutron reflectivity. The profile was obtained as a function of the concentration of a silane coupling agent mixed into the bulk of the polymer. A large concentration of D₂O (> 80 vol %) was observed at the interface when no silane was present. This is interpreted as a partial delamination. Roughly 2 wt% silane led to a sharp reduction in the amount of D₂O at the interface. This is attributed to the silane diffusing to the interface and promoting adhesion. Samples conditioned over longer periods of time and at elevated temperature ( 16 h, 100% humidity at 80 °C) were examined by angle-resolved XPS. For all the conditioned samples the appearance of an additional molybdenum oxidation state, and significant dissolution of the molybdenum oxide into the polymer layer were observed. These effects were readily apparent even for samples with 2 wt% silane, although the effects were slightly more pronounced with decreasing concentration of silane. Therefore, we conclude that the silane promotes adhesion of the polyurethane to the molybdenum oxide surface, but does not provide an impenetrable barrier to water or restrict its degradative effects.     

Apparent interfacial failure in mixed-mode adhesive fracture

Aluminium-epoxy adhesive specimens constructed with the bond at 45? to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by¹⁴C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (G _{I,II) C} ^45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m^?2 compared to 136 J m^?2 for the same polymer in simple opening-modeG _{I C} adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.     

Surface,structural and electrical properties of BaTiO3 films grown on p-Si substrates by low pressure metal organic chemical vapour deposition

Metal organic chemical vapour deposition of BaTiO₃ using Ba(tmhd)₂, Ti(OC₃H₇)₄ and N₂O, where tmhd equals 2,2,6,6-tetramethyl-3,5-heptanedionate, via pyrolysis at relatively low temperatures (370C) was performed in order to produce BaTiO₃ insulator gates. Scanning electron microscopy showed that the surfaces of the BaTiO₃ films had very smooth morphologies. Atomic force microscopy showed that the BaTiO₃ thin film was polycrystalline. X-ray diffraction results indicated that BaTiO₃ crystalline films grew on Si(100) with [110] orientation. High resolution transmission electron microscopy measurements showed that the BaTiO₃ films were polycrystalline, and an interfacial layer in the BaTiO₃/Si interface was formed. The stoichiometry and atomic structure of the BaTiO₃ films were investigated by Auger electron spectroscopy and transmission measurements, respectively. Room temperature capacitance-voltage measurements clearly revealed metal-insulator-semiconductor behaviour for samples with BaTiO₃ insulator gates, and interface state densities at the BaTiO₃/p-Si interface were approximately high, 10¹¹ eV^–1 cm^–2, at the middle of the Si energy gap.     

Adherence-fracture energy of a glass-bonded thick-film conductor: effect of firing conditions

The effect of firing conditions on the adherence of a glass-bonded Pt-Au printed thick film conductor to a 96 wt % Al₂O₃ substrate was determined by a fracture mechanics measurement of the critical fracture energy for catastrophic thick film-substrate separation. The technique also demonstrated that separation by slow crack growth (delayed failure) occurred in this system. Analysis of the thick film microstructure and fracture surfaces showed that optimum adherence was primarily a result of a mechanically interlocked interface formed between the conductor metal and the glass bonding layer which, in turn, was strongly bonded to the alumina substrate. The two step decrease observed in _IC (from 3.7 to 0.2 J m^–2) with firing temperatures over 860° C resulted from the removal of this interlocking metal-glass interface brought on by metal sintering and glass migration to the substrate. Thus, at 860° C firing temperatures, adherence is controlled by cohesive fracture in the glass bonding phase while above 1000° C it is controlled by adhesive failure of the weak chemical-physical bond at the metal-glass interface.     

Relationship between Interfacial Energy and Corrosion of Steel in Heterocyclic Compounds

We have theoretically studied mechanical and electric phenomena at the interface St3 steel–electrolyte solution containing H₂SO₄ (0.1%) and also heterocyclic compounds with a concentration of (0.001–0.01) M. These compounds were N pyridine oxide, 2-aminopyridine, 4-picoline, pyridine, 2-butylpyridine, 3-picoline, 2.6-lutidine, pyridine zaldehyde, 3.5-lutidine, 3.5-dibromopyridine, quinoline, and lepidine. We took into account interfacial energy, interfacial tension, and also the variation of electric potential in the interphase region at T = T _a – T _b. We have established the relation between the interfacial energy and the corrosion coefficient for St3 steel in organic (corrosive) solutions. The results of computations were compared with analogous numerical data for corrosive solutions obtained by the Born–Mayer potential method, and these results agree well with each other. Deviations of the values of _m and also of their variations do not exceed 5%.     

Theory of helium under heat flow near the λ point. II. Dynamics of phase change

The He I-He II interface is a crucial aspect in the transformation processes between the superfluid and normal fluid phases. Its motion is investigated when temperatures and heat flows at boundaries deviate from those of a stationary coexistence state. As a unique feature, the heat flow to the interface from the He I side can be mostly transmitted to the He II side by thermal counterflow, and the latent heat generation (or absorption) at the interface becomes negligibly small. In any case the interfacial motion is so slow that the temperature on the He II sideT is still given by the stationary relationT –T Q ^3/4, whereT is the critical temperature and Q is the heat flow. The temperature profile and the interfacial position are calculated in some nonstationary cases. To this end a simple approximation scheme is developed. First, the interface can propagate with a constant velocity and the superfluid region can expand as a shock wave. Second, if the heat flow at the warmer boundaryQ _w and that in the He II regionQ _– are fixed at different values, the length of the He I region y_i changes in time as (d/dt)y _i ^1+p = const Q _w – Q_–, wherep=1/(1–x), and x is the critical exponent of the thermal conductivity. In particular, ify _i=0 att=0 andQ _w>Q_–, the normal fluid region emerges asy _it ^1/(1+p) at the warmer boundary. Third, ifQ _– and the temperature at the warmer boundary are fixed, the interfacial position approaches an equilibrium position exponentially in time. The uniqueness of the problem arises from the superfluidity on the He II side and the strong critical singularity of the thermal conductivity on the He I side.