Fracture resistance of polyblends and polyblend matrix composites: Part II Role of the rubber phase in Nylon 6,6/ABS alloys

A comprehensive study was undertaken on the specific role of rubber on toughening when other rigid polymer or non-polymer phases were present. Nylon 6,6/SAN blends of various SAN concentrations ranging from pure SAN to pure nylon 6,6 were investigated with and without fibre reinforcements. These results could be compared with the toughness values of unreinforced and fibre-reinforced nylon 6,6/ABS alloys from a previous study in order to elucidate the role of rubber. Fracture behaviour was investigated rigorously by characterizing the fracture initiation toughness, JIC, and the steady-state fracture toughness, Jss. These were then related to the microstructure and failure modes determined by microscopy and fractography methods. It was found that rubber increased both fracture initiation and propagation toughness in the presence of the rigid phase, while the rigid phase toughened the alloy only when the rigid phase/matrix interface was strong enough. The role played by glass fibres was found to be critically related to the fibre/matrix interfacial strength. Toughening was generally observed, both in the presence and absence of rubber, when the interface was strong. In all cases toughening could be related to the enhancement of plasticity in the crack tip by the presence of the rubber phase or the reinforcing glass phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Toughening performance of glass fibre composites with core–shell rubber and silica nanoparticle modified matrices

The fracture energies of glass fibre composites with an anhydride-cured epoxy matrix modified using core–shell rubber (CSR) particles and silica nanoparticles were investigated. The quasi-isotropic laminates with a central 0°/0° ply interface were produced using resin infusion. Mode I fracture tests were performed, and scanning electron microscopy of the fracture surfaces was used to identify the toughening mechanisms.The composite toughness at initiation increased approximately linearly with increasing particle concentration, from 328 J/m² for the control to 842 J/m² with 15 wt% of CSR particles. All of the CSR particles cavitated, giving increased toughness by plastic void growth and shear yielding. However, the toughness of the silica-modified epoxies is lower as the literature shows that only 14% of the silica nanoparticles undergo debonding and void growth. The size of CSR particles had no influence on the composite toughness. The propagation toughness was dominated by the fibre toughening mechanisms, but the composites achieved full toughness transfer from the bulk.     

Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Fracture resistance of polyblends and polyblend matrix composites Part III Role of rubber type and location in nylon 6,6/SAN composites

The role of rubber particle type, location and morphology on toughening in blends of nylon 6,6 with styrene acrylonitrile (SAN), with and without fibre reinforcements was examined in this study. The rubber used was ethylene propylene diene monomer (EPDM) rubber and the results were compared to a previous study that used butadiene rubber. The compositions of the blends ranged from pure nylon 6,6 to pure SAN. EPDM rubber was chemically compatibilized with one of the matrix phases rather than grafted, as in the ABS. In order to study the effect of rubber location on fracture behaviour, the approach was to compatibilize EPDM with either the minor phase or the major phase component of the blend. Attention was focused on fracture initiation toughness and fracture propagation toughness, measured through the parameters J _IC and J _SS, respectively. J _SS refers to the steady-state, or plateau value of the material R-curve and was therefore a measure of total toughness which included the additional component derived from crack extension. The results indicated that EPDM rubber was not as effective a toughening agent as was butadiene in the Acrylonitrile Butadiene Styrene (ABS) system, primarily due to the morphology of EPDM and its interface character with the nylon 6,6 or SAN matrix. It was demonstrated that the embrittlement effects of a second rigid polymer phase can be mitigated by selectively adding rubber to that phase in the alloy or blend. With regard to the role of fibre reinforcement, a strong fibre matrix interface was found to be essential for toughening. Further, the extent of rubber toughening was larger when fibres were present than when fibres were absent, provided the fibre matrix interface was strong. Fibres also, like rubber, enhanced local matrix plasticity as well as reduced the embrittlement effects associated with a second polymer phase.     

Toughening of polystyrene by natural rubber-based composite particles: Part III Fracture mechanisms

Impact testing has allowed the toughness of PS blends to be correlated with the morphology of the dispersed rubber phase, which was a natural rubber (NR) in particle form, coated with a shell of polystyrene (PS) or polymethylmethacrylate (PMMA). PS subinclusions were also introduced into the NR core. The impact resistance of the prepared PS blends began to rise steeply at a particle content of about 18 wt %. Transmission electron microscopy (TEM) in combination with osmium tetroxide staining techniques, allowed direct analysis of the crazing and cavitation processes in the composite natural rubber particle-toughened PS blends. Bulk samples were studied at high and slow deformation speeds. Different deformation mechanisms were effective, depending on the location of the observed stress-whitened zone relative to the notch tip. The apparent fracture mechanisms in rubber-toughened PS blends were also studied by scanning electron microscopy. PS blends containing polydisperse natural rubber-based particles or monodisperse poly(n-butylacrylate)-based particles, and commercial high-impact polystyrene, were compared. This revised version was published online in November 2006 with corrections to the Cover Date.     

FRACTURE CHARACTERISTICS OF BOROSILICATE GLASSES REINFORCED BY METALLIC PARTICLES

Abstract— The fracture behaviour of borosilicate glass reinforced by molybdenum and/or vanadium particles has been investigated. For the addition of 5 vol% molybdenum particles, two processing procedures have been tested and the influence of volume fraction of vanadium particles (in the range 2 to 30 vol%) on fracture resistance has been assessed. The use of chevron-notched specimens in three-point bending has been shown to be a reliable method for the evaluation of fracture toughness even at toughness levels of order 0.7 to 1.3 MPa √m. The existence of subtle differences in fracture behaviour of glass-composites having comparable volume fractions of molybdenum particles but obtained by two different processing procedures has been established by statistical treatment of the fracture toughness data. An increase in the volume fraction of metallic particles results in an increase of the fracture resistance and the measured fracture toughness level. Toughening mechanisms which have been identified include both the plastic deformation of particles and the bridging of cracks by ductile particles. Some particle cleavage and debonding has been observed, which indicates that a decrease in particle plasticity, probably induced by processing or due to constraints imposed by the rigid matrix, is responsible for a smaller than expected enhancement of the fracture toughness.     

Interlaminar fracture (model II) of commingled yarn-based GF/PP composites

A 5050 wt % mixture of commingled glass/polypropylene fibre system was selected to study the correlations between the morphological details, mode II interlaminar fracture toughness and corresponding failure mechanisms. Mode II interlaminar fracture tests were performed by using the end-notched flexure test procedure. Compared to conventional composite laminates, mode II interlaminar crack extension in these commingled yarn-based composites was very stable, and extensive fibre nesting occurred along the main crack plane. Crack jumping and non-broken matrix links were observed.R-curve behaviour for these materials was identified and the toughness for initiation was much lower than that for propagation. Compared to mode I interlaminar fracture toughness, similar trends in effects of cooling rates and isothermal crystallizations on mode II interlaminar fracture toughness were observed. However, the effects were not as significant as those found for mode I interlaminar fracture toughness.Alexander von Humboldt Fellow.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

The Influence of Fibre Volume Fraction on the Mode I Interlaminar Fracture Toughness of a Glass-Fibre/Vinyl Ester Composite

This paper investigates the influence of fibre volume fraction on the mode I interlaminar fracture toughness G _Ic of a glass-fibre/vinyl ester composite. Two fibre volume fraction parameters are defined; a global value for the composite specimen and a value for the fibre-dense intralaminar regions. The range of global fibre volume fraction studied was 32–52 %. Results show that G _Ic values for crack initiation are independent of fibre volume fraction and similar to matrix resin G _Ic . Variations in the G _Ic for steady-state crack propagation, and the amount of fibre bridging, are not completely explained by changes in global fibre volume fraction. Instead they are consistent with fibre volume fraction in the fibre-dense intralaminar regions, through which the crack preferred to grow. It is concluded that this latter parameter is more relevant for G _Ic characterisation as a function of fibre volume fraction.     

Fracture toughness and failure mechanisms in unreinforced and long-glass-fibre-reinforced PA66/PP blends

In this paper unreinforced and long-glass-fibre-reinforced PA66/PP blends with different glass-fibre sizing were studied with respect to their fracture toughness determined by the typical K_c method. The fracture surfaces of these blends were studied by scanning electron microscopy in order to characterize the failure mechanisms. For the unreinforced blends a decrease in fracture toughness was observed when 25 wt% of polyamide (PA) was added to the polypropylene (PP) matrix, compared with the plain PP and PA matrices. On the other hand an increase in fracture toughness was observed when 25 wt% of PP was added to the PA matrix. This was explained by the differences in thermal expansion of PP and PA. The fracture toughness of the long-glass-fibre (LGF) composites were not affected by the glass-fibre sizing up to a PA/PP ratio of 50/50. After the phase inversion from a continuous PP to a continuous PA phase in the matrix (between PA/PP ratios of 50/50 and 75/25) the PA glass-fibre sized composite showed higher fracture toughness than the PP sized. This was explained by the change of the fibre-related failure mechanism from frequent fibre pull-out to fibre fracture. In addition the matrix affected the fracture toughness of the PA/PP75/25 blend with PA glass-fibre sizing in a positive way, resulting in the highest fracture toughness observed in this study.     

Micromechanical interpretation of fracture toughness of particulate-filled thermoplastics

The toughness behaviour of particulate-filled thermoplastics is determined by different failure mechanisms in the plastic zone and fracture process zone in front of the macrocrack such as particle-matrix debonding, shear processes or crazing and fracture of matrix fibrils. Theoretical expressions describing the critical strain causing microcrack initiation as well as the critical crack opening and the criticalJ integral value for unstable crack initiation are derived on the basis of a micromechanical analysis. Matrix properties, particle diameter, filler content and phase adhesion are taken into account. Critical particle contents and diameters caused by matrix morphology are discussed. Model calculations are compared with experimental results from acoustic emission analysis and dynamic fracture mechanics tests on PS, PVC and HDPE filled with CaCO₃ or SiO₂ particles.     

Toughening and strengthening of polypropylene using the rigid-rigid polymer toughening concept Part II Toughening mechanisms investigation

Toughening mechanisms in blends of isotactic polypropylene and Noryl polyphenylene oxide/polystyrene (iPP/Noryl) are studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. Large Noryl particles (10–15 m) are formed in iPP/Noryl blend and crazing is found to be the dominant toughening mechanism. A detailed investigation of fracture mechanisms reveals that Noryl particles help trigger and stabilize massive crazes in the iPP matrix. Incorporation of a small amount of styrene-ethylene-propylene (SEP) compatibilizer helps reduce Noryl particle size and improve interfacial adhesion between iPP and Noryl particles. Crazing and shear banding mechanisms are found to operate sequentially in iPP/Noryl/SEP blends. As a result, significantly improved toughness is obtained.     

Low-temperature behaviour of epoxy-rubber particulate composites

Toughness and mechanical property data are presented for a carboxyl-terminated acrylonitrile butadiene (CTBN) rubber-modified epoxy resin in the temperature range 20 to – 110° C. A toughening model based on ultimate strain capability and tear energy dissipation of the rubber, present as dispersed microscopic particles in an epoxy matrix, is used to explain the suppression of composite toughness (G _Ic ) below – 20° C. The toughness loss is attributed to a glass transition in the rubber particles, and to a secondary transition in the epoxy resin, both occurring in the range – 40 to – 80° C. Strain-tofailure and modulus measurements on bulk rubber-epoxy compounds, formulated to simulate rubber particle compositions, confirm a decrease in rubber ductility coincident with the onset of composite toughness loss. An increase in rubber tear energy associated with its transition to a rigid state can explain the observation that even at low temperatures composite toughness generally remains significantly higher than that of pure epoxy. Although the low-temperature epoxy transition reduces molecular mobility in the matrix phase, residual ductility in, and energy dissipation by, the rubber particles determine the extent of composite toughness suppression. The low-temperature data bear out the particle stretching-tearing model for toughening.     

Mode I and mode II delamination properties of glass/vinyl-ester composite toughened by particulate modified interlayers

Various vinyl-ester (VE)/poly(acrylonitrile-butadiene-styrene) (ABS) blends were used for interlayer-toughening of a glass/VE composite to increase delamination resistance of the base material under mode I and mode II loading. Dry ABS powder was mixed with the liquid resin in four weight ratios: 3.5, 7, 11 and 15 phr (parts per hundred parts of resin) while the layer thickness was varied within the range of 150–500 μm. Firstly, mode I fracture toughness and tensile properties of the VE/ABS blends were assessed. By using the Raman Spectroscopy technique a chemical reaction was discovered which occurred during ABS–VE mixing: i.e. butadiene transition from the ABS particles to the VE. A butadiene saturation was discovered to occur in the VE beyond 7% ABS particle content. Both mode I and mode II fracture toughness were significantly improved with application of the interlayers. Mode I fracture toughness was found to be a function of layer thickness and particle content variations. The latter dominated G_Ic after the saturation point. On the other hand mode II fracture toughness was found to be independent of the layer thickness (within the used layer thickness range) and only moderately influenced by the particle content. Important Toughening mechanisms were plastic deformation and micro-cracking of the layer materials. Evidence of both mechanisms has been found using optical and scanning electron microscopy (SEM).     

The toughness of epoxy polymers and fibre composites modified with rubber microparticles and silica nanoparticles

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m² by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m² was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.     

Damage tolerance assessment by bend and shear tests of two multilayer composites: Glass fibre reinforced metal laminate and aluminium roll-bonded laminate

The damage tolerance of an aluminium roll-bonded laminate (ALH19) and a glass fibre reinforced laminate (GLARE) (both based on Al 2024-T3) has been studied. The composite laminates have been tested under 3-point bend and shear tests on the interfaces to analyze their fracture behaviour. During the bend tests different fracture mechanisms were activated for both laminates, which depend on the constituent materials and their interfaces. The high intrinsic toughness of the pure Al 1050 layers present in the aluminium roll-bonded laminate (ALH19), together with extrinsic toughening mechanisms such as crack bridging and interface delamination were responsible for the enhanced toughness of this composite laminate. On the other hand, crack deflection by debonding between the glass fibres and the plastic resin in GLARE was the main extrinsic toughening mechanism present in this composite laminate.     

Mechanism of fracture of short glass fibre-reinforced polyamide thermoplastic

The mechanism of fracture of short glass fibre-reinforced polyamide 6.6 thermoplastic was studied by means of optical and electron microscopy and acoustic emission methods. It was found that there were three stages in the failure, i.e. initiation of the interfacial cracks at fibre ends, propagation of the interfacial cracks along fibre sides, and propagation of the crack into the matrix leading to the failure of the composite. On the fracture surface, fibres were almost pulled-out from the matrix, not broken. The close correspondence between the crack initiation and propagation and the amplitude of AE signals was observed. The AE signals of lower amplitude occurring under a relatively low stress were considered to be made in association with the initiation and propagation of the interfacial cracks. The AE signals of higher amplitude observed prior to the failure of the composite were considered to be made in association with the occurrence of the matrix cracks. Furthermore, in order to analyse the effect of the stress state in the composite on crack occurrence and propagation, the stress levels in matrix, fibre and interface were estimated for the composite stressed to the failure stress. The calculation was based on the equivalent inclusion method proposed by Eshelby and on an assumption of a perfect bond between the matrix and the fibres. The result was found to be consistent with the mechanism of the fracture, the occurrence of the interfacial cracking in the initial stage and the matrix cracking in the final one.     

Toughness of glass fibres reinforced glass-ionomer cements

Two fracture toughness parameters, the critical stress intensity factor, K _c and the work of fracture, W _f have been used to characterise the toughness of conventional and resin-modified glass-ionomer cements reinforced with glass fibres. The critical stress intensity factor was determined from the peak load, and the work of fracture was determined as the energy required to extend an introduced crack through the respective glass ionomers. For both materials, crack propagation became more stable as the weight fraction of glass fibres was increased. Additionally, when the weight percent of glass fibres was increased the work of fracture increased. Fibre bridging at the crack tip resulted in the increase in the work of fracture. As the percentage weight of fibres was increased, the critical stress intensity factor decreased proportionally to the increase in porosity.     

Fatigue fracture of nylon polymers

The influence of glass fibres on the fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true fatigue process is suggested for the fatigue fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the fatigue fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the fatigue fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.     

Improving interlaminar fracture toughness of carbon fibre/epoxy laminates by incorporation of nano-particles

Double-cantilever-beam tests were applied to investigate the mode I interlaminar fracture toughness of carbon fibre/epoxy laminates, in which the epoxy matrices were incorporated with rubber and silica nano-particles, either singly or jointly. It is shown that the toughness is improved owing to the presence of these nano-particles although nano-rubber is more effective than nano-silica. Further, by keeping the total particle weight percentage constant in epoxies (e.g., at 8 and 12 wt.%) filled with equal amount of nano-silica and nano-rubber, the interlaminar toughness values of the hybrid laminates are always higher than those with nano-silica filled epoxies but lower than those with nano-rubber filled matrices. Scanning electron microscopy examination of the delaminated surfaces of composite laminates filled with nano-particles revealed that cavitation of nano-rubber particles/void growth and debonding of nano-silica from epoxy matrix are responsible for the improved interlaminar toughness observed. It is also shown that the bulk toughness of nano-particle filled epoxies cannot be fully transferred to the interlaminar toughness of composite laminates, being limited by the constraint effect imposed by the carbon fibres. Finally, the role of fibre-bridging on the delaminated crack and hence delamination toughness is discussed.