Shanti V. Nair

Fracture resistance of polyblends and polyblend matrix composites: Part I Unreinforced and fibre-reinforced nylon 6,6/ABS polyblends

The deformation behaviour and the fracture resistance of a range of nylon 6,6/ABS alloys of varying composition both with and without the presence of glass fibres were investigated. The deformation behaviour was characterized by careful measurements of the volumetric strain during tensile tests in order to understand the relative roles of cavitation and shear yielding in these materials. The fracture resistance was investigated in detail in the fracture mechanics sense by characterizing the J-integral fracture initiation toughness. In materials exhibiting stable crack growth, a new parameter, namely, the plateau value of the J-integral fracture resistance curve, was measured directly and represented the resistance of the material to stable crack growth. The results showed that the relationship between the deformation behaviour and fracture resistance was related to the extent of damage that developed in the crack-tip zone. Substantial additional toughening was developed during the crack extension stage both in the presence and absence of glass fibres. Glass fibres were found to promote shear yielding and, as a result, enhance both the fracture initiation as well as the fracture propagation resistance of the nylon 6,6/ABS alloys.

Deformation mechanism and fibre toughening of nylon 6,6

Abstract The effects of glass-fibre reinforcement on the fracture toughness, KIC, of nylon 6,6 were examined and the deformation mechanisms of unreinforced nylon 6,6 were studied by varying the deformation rate, by dilatational measurements and by i.r. spectroscopy. In the unreinforced nylon 6,6 a flow stress plateau was observed in the stress-strain behaviour prior to the onset of necking. Of the 25–30% inelastic strains stored in this plateau a substantial portion appears to be related to a crystallographic plastic deformation due to the crystalline segments in nylon 6,6. In glass-fibre-reinforced nylon 6,6 a brittle to ductile transition was found to occur when the mean fibre-end spacing was less than a critical value. The observed brittle-ductile transition was found to originate from an observed enhanced matrix plasticity at fibre ends when the glass fibres are sufficiently closely spaced. Such enhanced localized plasticity at fibre ends was suggested to result from interactions of stress fields with nearby fibre ends when the fibre-end spacing is less than the critical value. It is further postulated that the enhanced localized fibre-end plasticity is made possible due to the ability of the matrix to exhibit a large degree of crystallographic plasticity. The toughening behaviour of fibre-reinforced nylon 6,6 was also compared qualitatively to that of rubber-toughened nylon 6,6 and a general principle for microstructural toughening in nylon 6,6 was addressed. Strategies for fibre toughening in fibre-reinforced nylon 6,6 were also discussed.

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.

Effect of glass-fibre reinforcement and annealing on microstructure and mechanical behaviour of nylon 6,6

The effects of glass-fibre reinforcement and annealing on the deformation and fracture behaviour of nylon 6,6 were investigated. The roles of glass fibres were examined by varying the glass fibre content and the fibre length, and by in situ fracture studies in front of crack tips. The effects of microstructural changes were investigated by imposing various annealing conditions on the specimens. The results indicated that the fracture toughness showed a sharp decrease due to stress concentrations at fibre ends when the fibre volume fraction was small. Above a critical fibre volume fraction, it was found that the fracture toughness can be substantially increased by enhanced localized matrix plasticity at fibre ends. The competing roles of glass fibre ends were consistent with microstructure sensitive fracture mechanics models of failure based on the attainment of a critical stress or strain over a critical microstructural distance in the crack-tip region. Upon annealing above a critical annealing time the unreinforced nylon 6,6 showed a drastic decrease in the strength and ductility, corresponding to a loss of the constant-load deformation region prior to necking. However, the fracture toughness of unreinforced nylon 6,6 was only moderately reduced by annealing. On the other hand, the fracture toughness of the composites showed a significant increase upon annealing. The combined effects of glass fibres and annealing on microstructures and overall property optimization of the composites are also discussed.

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.

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 JIC and JSS, respectively. JSS 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.

Strength and Toughness of Interface Between EBC and Ceramic Substrates at Elevated Temperature

In a previous study, the strength of the interface between an environmental barrier coating (EBC) and the substrate was measured at ambient temperature using a unique compression test methodology. In this study, the method was extended for elevated temperature characterization of the strength of this interface. The extension of the method also allowed for the characterization of the elevated temperature toughness ofthe EBC/substrate interface. An essential component of measuring elevated temperature interface strength and toughness was characterization of the elevated temperature elastic modulus of the EBC coating layers. The elastic moduli of EBC layers were characterized at both ambient and at elevated temperature. The results point to a substantial decrease in the moduli of the EBC coating layers at elevated temperature. Preliminary results also indicate a decrease in both the strength and toughness of the EBC/substrate interface at elevated temperatures compared to ambient temperature.Copyright

Characterization of the Cohesive Strengths of Environmental Barrier Coatings to Ceramic Substrates Using Compression Tests

The cohesive strengths of environmental barrier coatings applied to silicon carbide substrates were characterized using a compression test containing a strip of coating along a portion of the gage length. The substrate sample design and test fixture are similar to that described in ASTM D 695-96. The theory needed to extract the cohesive/adhesive strengths from the data is presented. The results of the compression tests are compared to the standard test method for the determination of the cohesive/adhesive strengths by the tension-adhesion test (TAT) (ASTM C 633-79). The preliminary results indicate that the onset of failure in the compression tests can be correlated with TAT test results, allowing for the extraction of the cohesive strength of the coating. For this system, this strength was found to be 15–20 MPa. The compression test has the advantage that it can be conducted at elevated temperature without the use of adhesives and, furthermore, is not limited by the adhesive strengths of polymeric adhesives.© 2002 ASME

High Strength, Porous, Brittle Materials

Contrary to existing models, strengths need not be a strong function of porosity for intermediate density, brittle materials. Flaw sizes can remain small (