Numerical characterisation of the fibre-matrix interface crack growth in composites under transverse compression

Inter-fibre failure under compression transverse to the fibres is studied at micromechanical level. Interfacial fracture mechanics concepts, associated to both the open model and the contact model, are applied. A numerical study is performed using the boundary element method aimed at explaining the origin and evolution of the damage at micromechanical level, considered as fibre-matrix interface cracks. Assuming that the damage starts as small debonds originated by shear stresses at the position where their maximum values are reached, it has been found that the crack shows different morphologies at both tips: an open one and a closed one with a large contact zone. Then the interface crack grows unstably in mixed mode only on the open tip side until this growth changes to stable, once the crack closes at this tip, with the generation of a contact zone.     

Fracture toughness of alumina/lanthanum titanate laminate composites with a weak interface

Layers of lanthanum titanate (La₂Ti₂O₇) and α-alumina (α-Al₂O₃) were employed to form a layered composite in order to improve the fracture toughness of monolithic alumina. The composites were produced by two different processing methods. In the first, individually presintered pellets of α-Al₂O₃ and La₂Ti₂O₇ were stacked together and hot-forged. In the second, tape cast molten salt La₂Ti₂O₇ and dense α-Al₂O₃ were stacked together and hot-forged. The forged composite samples were investigated by optical microscopy, scanning electron microscopy (SEM), Vickers indentation and three-point bending. During the hot-forging process, an interphase, aluminum titanate (Al₂TiO₅) was found to form as a result of the reaction between α-Al₂O₃ and La₂Ti₂O₇. The flexural strength and the fracture toughness of the resulting laminate composites were found to be 320 MPa and 7.1 MPa m^1 / 2, respectively. Indentation experiments showed that the newly formed Al₂TiO₅ at the interface is sufficiently weak to promote crack deflection and hence increase the fracture energy and mechanical properties of the composite.     

Fracture toughness of weft-knitted fabric composites

The mode I inter-laminar fracture toughness of advanced knitted textile composites was investigated. Two complex weft-knitted glass fabrics were selected for the study: a triple rib knit and a Milano knit were impregnated with a tough epoxy resin and tested using a double cantilever beam geometry. For both knitted composites, the influence of the growth direction was studied by investigating crack propagation in both the wale and course directions. The fracture toughness was quantified by determining the critical strain energy release rate (G_IC) using the modified beam theory. The specimens had to be stiffened with layers of glass woven composites added on top and bottom of the beams. This was necessary in order to avoid plastic deformation of the beams and crack deviation out of the inter-laminar plane. The results clearly showed that knitted fabric composites have exceptional inter-laminar fracture toughness properties, namely, more than 7000 J/m². The origin of the high G_IC values, which are superior to woven or UD laminates, lies in the very complex fabric architecture. The three-dimensional loop structure induces various energy consuming mechanisms, which do not occur in other composites. Toughening mechanisms such as crack branching, friction, yarn bridging and breakage were identified using scanning electron microscopy.     

Fracture toughness and fracture of WC-Co composites

Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 m). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as ¯L _Co,C _WC, ¯L _Co/¯D _WC, ¯L _Co ² /¯D _WC,H _V and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.     

Fracture toughness of metal reinforced glass composites

The effect of hardness and strength of particulate reinforcements on the toughening of a glass matrix composite have been investigated. Spherical particles of two gold-based alloys were blended with a low-fusing glass powder; the mixture was hot-pressed, and disc-shaped specimens prepared for fracture toughness testing using the strength/flaw method. Scanning electron microscopy was used to examine fracture surfaces. It was found that the softer, more ductile alloy was a more effective toughening additive than the harder alloy.     

Fracture toughness of zirconia nanoparticle-filled dental composites

The fracture toughness of dental composites containing zirconia nanoparticles dispersed in a bisphenol A glycol dimethacrylate-based monomer blend (GTE) was studied for several yttria contents. Three-point bend test bars with and without a notch were tested at ambient temperature to determine elastic modulus, flexure strength, and fracture toughness. The ZrO₂ nanoparticles increased the fracture toughness of the nanocomposites compared to previous results for the matrix and Schott glass-filled nanocomposites. X-ray diffraction analyses revealed mostly tetragonal ZrO₂ in the nanocomposites before and after testing, in agreement with a theoretical analysis. The enhancement in fracture toughness in ZrO₂-filled nanocomposites was caused mainly by the higher values of particle toughness and interface toughness in GTE/ZrO₂ compared to those of GTE/Schott glass nanocomposites.     

Fatigue behaviour characterization of the fibre-matrix interface

The fracture behaviour of FRP composite materials is significantly influenced by the behaviour of the fibre-matrix interfacial bond. Thus far interfacial bond mechanical characterization has been based upon the critical strength and critical fracture energy of debonding. Characterization of the fatigue behaviour of the interfacial debonding process, however, may be more valuable for composite design and fibre-matrix selection. A fracture mechanics model of interfacial bond fatigue based on the mode II strain energy release rate (G _II) is presented. An expression forG _II is derived for a single fibre in matrix cylinder model. By fitting the model to single fibre pull-out fatigue test data, fatigue crack propagation plots for specific fibre-matrix combinations can be drawn. These should prove useful for the development of fatigue resistant FRP composite materials.     

Some hygrothermal effects on the mechanical behaviour and fractography of glass-epoxy composites with modified interface

The mechanical behaviour of glass fibre-reinforced epoxy composites with introduced layers of materials and with fibre coating is studied. The role of aerosil powder as a filler material is investigated, and the fracture mode is analysed by scanning electron microscopy. The investigation shows a considerable drop in interlaminar shear stress for the higher fibre volume fraction, whereas the introduction of filler materials to the composite causes no change. Surface mat-reinforced samples show a marginal increase in shear stress. Exposure to moisture reduces the interlaminar shear stress value faster for the higher fibre volume fraction, thereby highlighting the role of the interfacial area. Impact values for coated and uncoated fibres show an identical trend with exposure to dry heat, the former always recording lower values. The impact value decreases faster with moisture absorption for the composite with the higher fibre content. Fractography reveals poor adhesion in the coated laminates.     

Fracture toughness of composites reinforced with discontinuous fibres

Measurements of the work of fracture of composites of polyester resin reinforced with chopped steel wires of various lengths are compared with the theory developed by Cooper. For composites containing aligned wires the results agree well with the model except where there is excessive resin cracking. The toughness of composites containing wires which are randomly distributed in the resin can be significantly greater than that of aligned composites with wires of similar length. This is probably due to the plastic shearing of wires not lying parallel or normal to the specimen axis.     

Fracture toughness of unidirectional fibre reinforced composites in MODE II

Experimental investigations have been performed on unidirectional glass fibre reinforced/epoxy composites in Mode II (Forward shear) with the presence of crack parallel to the fibres direction through the use of end-cracked beam. A concentrated load at the Centre of the beam produced bending-induced shear deformation at the crack tip. Calibration factors for Mode II have been obtained. The stress-intensity factor at instability $\text{K}$_IIR(INR) is obtained by experiments on a small end cracked beam through a compliance matching procedure. The crack growth resistance at instability and the corresponding critical strain energy release rate are independent of initial crack in the range of crack length investigated. In composite materials, fibre-matrix interfacial shear stress play an important role in load transfer mechanism: hence Mode II study may be very useful to analyse the interfacial mechanisms and to understand the fracture behaviour of unidirectional fibre reinforced composites in Mode I when load is applied in the direction of the fibres.     

Fracture toughness of discontinuously reinforced aluminium 6061 matrix composites

The fracture toughness of two aluminium 6061 matrix composites reinforced with 20 vol% discontinuous reinforcement has been determined over a range of heat treatments. The materials investigated were Comral-85, reinforced with 20 vol% alumina-based microspheres, and Duralcan reinforced with 20 vol% angular alumina particulates. These were produced in an identical manner. Although the fracture toughness of both materials was relatively insensitive to ageing time, the Duralcan composite was significantly tougher than Comral-85 for all heat treatments examined. The matrix composition of both alloys was determined and it was found that Comral-85 contained higher additions of silicon and iron, resulting in the formation of an increased density of secondary particles. This was found to be the primary cause of the difference in fracture toughness between these two materials.     

Fracture toughness and mechanical properties of glass fibre-epoxy composites

The present study investigated fracture properties and various mechanical properties of a set of unidirectional glass fibre-epoxy resin composites. This set was comprised of samples with volume fraction of fibres in the range 0.29 to 0.75. An identical set of composites was water-boil treated for 7 days, and the effect of this treatment on the above properties was examined. The work of fracture (γ _F) and the fracture surface energy of initiation (γ ₁) results were compared with existing theoretical models for the prediction of fracture toughness. It was discovered that theγ _F results agreed with the pull-out model [6], suggesting that this was the major contribution to the fracture energy of the complete process. Theγ ₁ values corresponded generally with the surfaces formation model [9], proposing that the creation of new fibre, matrix and fibre-matrix surfaces controls the stage of fracture initiation.     

Fracture toughness of Kevlar-epoxy composites with controlled interfacial bonding

The fracture toughness of Kevlar-epoxy resin composites with intermittent fibre bonding of a silicone vacuum fluid (SVF-200) and a polyurethane varnish (Estapol 7008) have been studied over the temperature range –60 to 40° C and strain rates 0.03 to 5000 min^–1. Whilst both coating materials give similar tensile strengths their effects on toughness are very different. As far as toughening is concerned Estapol 7008 is more effective than SVF-200. The toughening effect increases with increasing intermittent lengths of the Estapol-7008 coating, i.e. coating parameterC, increasing temperature and decreasing strain rate. At low strain rates and high temperatures, forC=1, the toughness increase is some 200 to 300% compared to the uncoated composites. Some initial work has also been conducted for hygrothermally aged uncoated and coated fibre composites. The SVF-200 coated composites do not show any toughness degradation compared to the dry control samples. However, both the uncoated and Estapol-7008 coated composites suffer some toughness loss. Even so, the toughness of the fully coated aged specimens is as good as the uncoated dry controls. A fracture analysis is presented which gives reasonable agreement between predicted fracture toughness values and experimental measurements. It is shown that fibre pull-out toughness and fibre fracture work are the main contributors to the total fracture toughness of these fibre composites; their relative significance being dependent on the type of coating material, the temperature and strain rate of testing.     

Fracture toughness evaluation for short glass fibre reinforced composites

Valid plane-stress fracture toughness evaluation of short fibre reinforced composites relies essentially on the successful separation of the energy absorbed in the localized crack-tip region out of the total energy absorbed by the cracked material body at large. Three different experimental techniques, all stemming from the energetic interpretation of theJ integral, are utilized and their relative merits in the characterization of fracture initiation in short glass fibre reinforced injection-moulded nylon 6.6 examined. Various theoretical aspects concerning these experimental methods are outlined. The rationale behind using a single-edge-notched tension type specimen for theJ _c test is presented. TheJ _c value obtained from the compliance calibration method and the quasistatic energy method agree closely and can be considered to be independent of pre-crack length and specimen geometry when the pre-crack length to specimen width ratio (a/w) is larger than 0.45. The extrapolation method fails nevertheless to yield a physically consistentJ _c value, possibly due to its questionable theoretical representation. As no constraint on boundary conditions is necessitated during the course of crack extension, the quasistatic energy is physically more appealing.     

Angular dependence of hygroelasticity in unidirectional glass-epoxy composites

Swelling and hygroelasticity of unidirectional glass fibre-reinforced epoxy composites in boiling water is studied with respect to the angle between the direction of orientation and the direction of measured dimensional change. The results are expressed in terms of the dependence of of a hygroelasticity coefficient. A very good agreement is observed between the experimental results and the theoretical equation, already verified in the case of the angular dependence of the coefficient of thermal expansion. This is taken as additional support for the analogy between thermoelastic and hygroelastic behaviour in composites. The experimental longitudinal and transverse hygroelasticity coefficients are found to be different from predictions based on Schapery's equation. However, it is shown that these equations can produce possible valid estimates of _L and _T provided the mechanical properties of the swollen constituent materials are used.