Giovanni Levita

Crosslink density and fracture toughness of epoxy resins

A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated. The same set of resins was also modified with a reactive rubber. The fracture toughness regularly decreased on increasing the crosslink density for all formulations. The addition of the rubber gave rise to a marked increase in toughness, though it magnified the influence of the molecular weight of the prepolymer. Tests performed with blunt notches showed that the fracture toughness was maximum at medium crosslink densities. A dispersion of rubber particles caused a toughness increase through the formation of microcavities ahead of the crack tip. Failure was preceded by a rapid volume increase caused by void coalescence.

Ramie (Boehmeria nivea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties.

The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. The chemical, physical and mechanical properties of these fibres were also examined in order to evaluate the feasibility to use them in composite materials. Results demonstrate that Ramie grown in the temperate environment can be harvested three times a year. The yield of green Ramie plants per annum was almost 10 000 g m−2. The fresh Ramie plant is composed, by weight, of 30% green leaves and 70% green stems. The yield dry fibre is almost 3% of the green stems, giving a total yield of 200 g m−2. Spanish Broom can reach a fresh biomass yield of 4000 g m−2 per annum, represented by 53% of long slender terete green branches which constitutes the economic products. The dry yield per annum was about 1800 g m−2 with a dry branchlets yield of 900 g m−2. Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins, while in Spanish Broom they are bound together by lignin. Both species showed a thick secondary cell wall indicating a high cellulose content. Ramie ultimate fibres are flattened and irregular in shape, while those of Spanish Broom are more regular in shape. The diameter of the ultimates varies from 10 to 25 μm in Ramie, while the Spanish Broom ultimates ranges from 5 to 10 μm; the diameter of the whole bundle is about 50 μm for both species. Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose which, on the base of X-ray analysis, was evaluated to be in excess of 70%. Ramie and Spanish Broom fibres had tensile strength of 950 MPa and 700 MPa, respectively. The elastic moduli were ≈65 and ≈20 GPa, respectively, which well compare with the modulus of E-glass fibres (70–90 GPa). The strength of the fibre-matrix interface was measured using the single filament fragmentation technique and an epoxy resin as the polymer matrix. Values for carbon and glass fibres in the same resin were also measured for comparison. The interface strength for the vegetable fibres was higher than that of carbon and glass, likely due to a mechanical lock mechanism. These values confirm both fibres as potential replacement for man made fibres in composite materials.

Mechanical properties of jute fibers and interface strength with an epoxy resin

Four different forms of jute fibers, namely untreated jute filament (UJF), sliver jute filament (SJF), bleached jute filament (BJF), and mercerized jute filament (MJF), have been subjected to tensile strength analysis following Weibulls theory. The MJF and BJF were obtained by the chemical modification of the UJF. A minimum of 50 fibers of each type, at three different gauge lengths, i.e., 15, 30, and 50 mm, were used to study the strength distribution and the effect of gauge length. The mean fiber strength was found to be the maximum for UJF followed, in the order, by BJF, MJF, and SJF (∼ 700, ∼ 660, ∼ 580, and ∼ 540 MPa, respectively, at 50-mm gauge length). The strength was also found to decrease with an increase in gauge length. In all cases, good agreement was found with Weibulls statistical model. Single fiber composite tests, with an epoxy resin as the matrix, were carried out determine the critical fragment lengths and interfacial strength, following the Kelly–Tyson approach. The BJF was found to have the maximum interfacial adhesion (τ ≈ 140 MPa) followed by UJF, SJF, and MJF having τ values of ∼ 83, ∼ 57, and ∼ 47 MPa, respectively. Scanning electron microscope pictures showed the fiber surface was physically modified by the various treatments.

Early development of properties in a cement paste: A numerical and experimental study

This paper presents the results of an experimental and numerical study of the properties of a high-performance Portland cement paste (w/c ratio 0.37; 5% silica fume) cured at 20 °C in sealed conditions for 5 days. Properties such as electrical conductivity, strength, stiffness, porosity, Vicat penetration, and autogenous deformation were measured and modelled. The kinetics of hydration was studied by means of isothermal calorimetry. The numerical simulations were performed with CEMHYD3D, developed at the National Institute of Standards and Technology, and HYMOSTRUC, developed at the Delft University of Technology. The results of the simulations were compared with experimental data, and the match was good. Clear correlations were found among electrical conductivity, autogenous shrinkage, and connectivity of solids.

INFLUENCE OF THE AGGREGATE ON THE ELECTRICAL CONDUCTIVITY OF PORTLAND CEMENT CONCRETES

This study seeks to prove the capability of electrical methods to monitor microstructure changes in concretes and to give an insight into microscopic features such as the properties of the interfacial transition zone (ITZ). The electrical conductivity and compressive strength of several high-performance Portland concretes with different amounts of crushed aggregate and sand have been measured at early age in isothermal conditions (20 deg C). The total aggregate volume fraction varied from 0 (plain paste) to 0.75 and a constant weight ratio (1.2) between crushed aggregate and sand was used. The water-cement ratio was 0.37 and microsilica (in slurry form) and a superplasticizer in water solution were used. The time taken before the electrical conductivity began to drop correlated very well with the induction period. The drop of conductivity was slightly delayed by the aggregate. The analysis of the electrical data, by means of different numerical and analytical models, allowed an estimate of the properties of the ITZ. In particular, an ITZ thickness of about 9 micrometers and an ITZ to bulk conductivity ratio of about 2.5 were found. The existence of a percolating pathway through the interfacial regions was found by both electrical measurement and modeling when the aggregate volume fractions exceeded 60%. A new relationship among electrical conductivity, compressive strength, and aggregate amount was derived. The results also confirm electrical methods as a viable technique to monitor the properties of high-performance concretes.

Electrical properties of fluidified portland cement mixes in the early stage of hydration

Abstract The electrical conductivity and the dielectric constant of cement mixes have been studied in the initial stages of setting to correlate the time evolution of the electrical parameters with the chemical and microstructural modifications. The formulations, based on a mix of Portland cement and microsilica, were fluidified with various amounts of an acrylic polyelectrolite and had a w/c ratio of 0.24. Measurements were carried out at ambient temperature for times of up to 40 h, in the frequency interval 30 Hz–200 kHz. The conductivity showed a marked decrease correlated with the loss of connectivity of the macroscopic porosity (depercolation). The time to the drop of conductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenomenon was observed which vanished after the porosity depercolated. The temperature profiles of mixtures with different concentrations of the superplasticizer were drawn and a simple model was developed to compute the specific thermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat tests were also carried out.

Enhancing the Electro-Mechanical Response of Maxwell Stress Actuators

The need for high electric fields to drive dielectric elastomers is still retaining their diffusion as actuators in some areas of potential application, as in the case of biomedical disciplines. The development of new materials offering superior electromechanical properties is thus an essential requirement in order to effectively reduce the driving fields. In this light, the present work is aimed to enhance the electromechanical properties of two silicone and polyurethane based dielectric elastomers, both by making particulate composites with high-permittivity ceramic fillers, and by blending with a highly polarisable polymeric phase. Due to a consequent worsening of the mechanical properties, pure composite architectures yielded only limited results on the overall electromechanical response. With the blend approach, instead, both an increase of the dielectric permittivity and an unexpected reduction of the tensile elastic modulus were observed, leading to an overall increase of the electromechanical response. In any case, a key role appears to be played by the nature and intensity of polarisation phenomena arising at interfaces between different phases.

Broad band dielectric analysis of bituminous concrete

Abstract Bituminous concrete were irradiated with intense electromagnetic fields at 2.45GHz in a laboratory microwave oven The observed heating rate was similar to that of an equal volume of water put in the same conditions. Information useful for the comprehension of this behaviour were obtained by exposing individual constituents (bitumen and aggregates) to microwaves and by means of broad band dielectric spectroscopy performed on both concrete and single constituents. Collected data show that bitumen is the less dissipative component of the concrete, while the nature of aggregates and the effects of interfacial polarisation seem to be at the basis of both the finite dissipation observed in the radiofrequencies region of the spectra and the efficient heating obtained with microwaves.

Soft elastomeric electrets for electro-active polymers

The results reported show that electret elastomers may represent a new promising way for obtaining dielectric elastomers with improved dielectric constant to be used as actuators. To this purpose two main issues have to be faced: first a way to keep charges over time (see TABLE II); second, a study on how the pore dimension/dispersion influences the dielectric and electromechanical response.

Synthesis of Bioactive Hydroxyapatite-Zirconia Toughened Composites for Bone Replacement

Hydroxyapatite (HAp) is a major inorganic component of human hard tissues, such as bones and teeth, and its content determines their microstructures and physical properties. Artificial HAp shows strong biocompatibility and bioactivity and thus it has found broad applications in tissue engineering for replacing damaged hard tissues. The artificial HAp, however, suffers from its intrinsic low mechanical properties, so to meet mechanical requirements, HAp can be incorporated with stiff mineral phases (mullite, zirconia, alumina). The performance and long-term survival of these biomedical devices are also dependent on the presence of bacteria surrounding the implants. In order to reduce the incidence of implant-associated infections, several treatments have been proposed, e.g. introduction of silver or fluoride in the HAp. The objective of this research is the sintering of composites based on calcium phosphate, mainly HAp supported on zirconia, for bone replacement with better microstructural features. In fact the use of zirconia can enhance the mechanical properties of bioceramics. Moreover the introduction of small amounts of silver, which should improve the antibacterial properties, will be taken into consideration since it is expected also to further toughen the whole structure.