Alessandro Pegoretti

Finite element analysis of a glass fibre reinforced composite endodontic post

In this work the mechanical response to external applied loads of a new glass fibre reinforced endodontic post is simulated by finite element (FE) analysis of a bidimensional model. The new post has a cylindrical shape with a smooth conical end in order to adequately fit the root cavity, and to avoid edges that could act as undesired stress concentrators. Mechanical data obtained by three-point bending tests on some prototypes fabricated in the laboratory are presented and used in the FE model. Under various loading conditions, the resulting stress component fields are hence compared with those obtained in the case of two commercial endodontic posts (i.e. a cast metal post and a carbon fibre post) and with the response of a natural tooth. The gold cast post-and-core produces the greatest stress concentration at the post-dentin interface. On the other hand, fibre-reinforced composite posts do present quite high stresses in the cervical region due to their flexibility and also to the presence of a less stiff core material. The glass fibre composite shows the lowest peak stresses inside the root because its stiffness is much similar to dentin. Except for the force concentration at the cervical margin, the glass fibre composite post induces a stress field quite similar to that of the natural tooth.

Biodegradable fibres of poly(L-lactic acid) produced by melt spinning

Starting with poly(l-lactic acid) (PLLA) of molecular weight 330 000, fibres were obtained through a two stage process: (i) melt-extrusion at various collecting rates (ranging from 1.8 to 10 m min−1, and (ii) hot-drawing at various drawing rates. The molecular weight of PLLA fell to about 100 000, as a consequence of the production process. Ninety per cent of the molecular weight loss occurred during extrusion and ten per cent during hot-drawing. At fixed extrusion rate, properties of as-spun fibres strongly depended on their collection rate. The higher the collection rate, the higher the modulus and strength, and the lower the strain at break. While almost amorphous fibres were obtained at lower collection rates (1.8 and 3.1 m min−1), about 30 and 38% crystalline as-spun fibres were produced at rates of 5 and 10 m min−1, respectively. Moreover, the capability of fibres to sustain a further hot-drawing process, was found to be dependent on the collection speed during extrusion. Tensile modulus of 9.2 GPa and tensile strength of 0.87 GPa were obtained for fibres collected at 5 m min−1 and drawn 10 times.

Determining the role of interfacial transcrystallinity in composite materials by dynamic mechanical thermal analysis

The ability of this research group to obtain specimens of isolated transcrystalline layer by microtoming, reported in this paper for the first time, has opened a window for a range of characterization techniques. Here, the interfacial transcrystallinity in aramid fibre-reinforced nylon 66 microcomposites is studied using dynamic mechanical thermal analysis. The results show that the viscoelastic energy damping of the transcrystalline layer, tanδ tc = 0.064, is smaller while the elastic modulus, E ′ tc = 4.6GPa, is higher compared with the crystallized matrix. Moreover, the magnitude of the energy damping by the transcrystalline layer could be used in a rule-of-mixtures expression to calculate the energy damping of an aramid fibrereinforced nylon 66 microcomposite. It is also shown that the activation energy for relaxation, corresponding to the energy barrier for polymer chain movement, increases in the presence of reinforcement and transcrystallinity.

Relaxation processes in polyethylene fibre-reinforced polyethylene composites

Abstract The dynamic mechanical properties of filament-wound composites comprising ultrahigh-molecular-weight polyethylene (UHMWPE) extended-chain fibres in matrices of linear low-density polyethylene (LDPE), high-density polyethylene (HDPE), and thermally treated HDPE have been studied in tensile mode over a wide frequency range. The study focused on the additional effects of the fibres, for three winding angles of 26°, 34° and 45°, on the dynamic properties of the polyethylene-fibre-reinforced polyethylene composites. These effects were expected to result from transcrystallinity, which is induced in the matrix and which may invade a significant proportion of the composites, and from the extra restraint imposed by the reinforcement. The effects of the fibres are expressed by the real and imaginary moduli and the loss tangent and are analysed in terms of frequency dependence and activation energies for the α, β and γ transition processes. The effects of the fibres and of the crystallinity are evident in significantly higher moduli and activation energies of the relaxation processes.

Fatigue crack propagation in polypropylene reinforced with short glass fibres

Fatigue crack propagation (FCP) behaviour of polypropylene composites reinforced with short glass fibres has been investigated as a function of fibre content and frequency of the sinusoidal applied load. The FCP resistance of the composites was found to improve as the fibre weight fraction increased. Results for all composites showed a dramatic decrease in the crack growth rate per cycle as a result of increasing frequency, at any given crack length. A further analysis of the data indicated that crack propagation was governed by viscoelastic creep which produced, at the lower frequencies, a crack speed approximately independent of frequency. However, it was recognized that at the highest frequency hysteretic heating at the crack tip induced a higher crack speed, associated with non-isothermal creep processes.

Effect of temperature and strain rate on interfacial shear stress transfer in carbon/epoxy model composites

Abstract The amount of stress transferred in a composite from the matrix to the fibre has been determined in an epoxy/carbon system by using a fragmentation test on single-fibre model composites (microcomposites), as a function of temperature and strain rate. Experiments, performed on microcomposites containing commercial carbon fibres, with and without sizing agent, and an epoxy resin of low glass transition temperature (T g ), indicate that the interfacial shear strength, 〈τ〉, decreases sharply as the test temperature approaches the T g of the constituent surrounding the fibre. Moreover, while in desized fibre composites the interfacial shear strengths compare well with the matrix shear strength, in sized fibre composites the measured 〈τ〉 values appear to be strongly influenced by the presence of an interphase. 〈τ〉 values measured for sized fiber microcomposites are higher than the matrix shear strength. The contribution to adhesion of frictional stresses, exerted by the matrix on the fibre as a result of thermal and Poisson contractions, appears to be negligible with respect to the 〈τ〉 values obtained from the fragmentation test.

Evaluation of the statistical parameters of a Weibull distribution

A simple iterative procedure for determination of the statistical parameters of a Weibull distribution is proposed. All experimental results on specimens of different size are considered together as a statistically representative population. The procedure can be used for a population in which each specimen has a unique size. The statistical reliability of the iterative procedure is illustrated by comparison with a minimization analysis and confirmation with existing methods. Experimental confirmation of the analysis is developed using six types of glass and carbon fibres at four gauge lengths each. It is shown that Weibull parameters, obtained separately for populations of fixed length, vary with the fibre length.

Effect of nanoclay addition on the fiber/matrix adhesion in epoxy/glass composites

Various kinds of organo-modified clays were dispersed at different amounts in an epoxy matrix. After clay addition, the viscosity of the epoxy resin resulted still acceptable for a possible usage as matrices for fiber-reinforced composites. The formation of intercalated microstructures led to substantial improvements of the thermal (glass transition temperature) and mechanical (fracture toughness) properties of the epoxy matrix. E-glass fiber/matrix interfacial shear strength was evaluated by the single-fiber microdebonding method. The introduction of organo-modified clays led to the formation of a stronger fiber-matrix interface, with an increase of the interfacial shear strength of about 30%. Concurrently, the evaluation of the fiber/matrix contact angle revealed an improved wettability when organo-modified clays were added.

Fatigue resistance of basalt fibers-reinforced laminates

Fabrics of basalt (BFs), E-glass (GFs), and carbon (CFs) fibers with the same areal density were used to prepare epoxy-based laminates. The BF laminates presented elastic moduli and strength values higher than those of the corresponding GF laminates, with tensile strength values near to that of CF laminates. Investigation of the behavior under fatigue conditions indicated superior performances of BF laminates with respect to the corresponding GF composites, with an improved capability of sustaining progressive damaging and slightly higher damping properties. As far as the fatigue behavior is concerned, BFs may therefore represent a valid substitute of GFs in structural composites.

Thermal stability of high density polyethylene-fumed silica nanocomposites

High-density polyethylene-based nanocomposites were prepared through a melt compounding process by using surface functionalized fumed silica nanoparticles in various amounts, in order to investigate their capability to improve both mechanical properties and resistance to thermal degradation. The fine dispersion of silica aggregates led to noticeable improvements of both the elastic modulus and of the stress at yield proportionally to the filler content, while the tensile properties at break were not impaired even at elevated filler content. Thermogravimetric analysis showed that the selected nanoparticles were extremely effective both in increasing the decomposition temperature and in decreasing the mass loss rate, even at relatively low filler loadings. The formation of a char enriched layer, limiting the diffusion of the oxygen through the nanofilled samples, was responsible of noticeable improvements of the limiting oxygen index, especially at elevated silica loadings. In contrast with commonly reported literature results, cone calorimeter tests also revealed the efficacy of functionalized nanoparticles in delaying the time to ignition and in decreasing the heat release rate values. Therefore, the addition of functionalized fumed silica nanoparticles could represent an effective way to enhance the flammability properties of polyolefin matrices even at low filler concentrations.