Fabrizio Sarasini

EMC Impact of Advanced Carbon Fiber/Carbon Nanotube Reinforced Composites for Next-Generation Aerospace Applications

This paper presents a comparative analysis of the electromagnetic properties of new composite materials that are of interest to future aircraft/aerospace structures. The fabrication process of single-phase and new multiphase micro/nanocomposites is described. Carbon black, carbon fibers, and multiwall carbon nanotubes are randomly mixed into an epoxy resin matrix at various weight fractions and compositions. The experimental characterization in the frequency range 8-18 GHz shows that the dispersion characteristics of short-carbon-fiber-reinforced composites can be properly controlled by the addition of nanopowders and nanotubes into the mixture. Numerical simulations demonstrate the feasibility of the fabricated materials for the design of new electromagnetic micro/nanostructured shields and radar-absorbing laminates. Thin dielectric Salisbury screens are especially designed to exhibit minimum reflection coefficient at 15 GHz. It shows that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made of three-phase composites.

Electromagnetic Design and Realization of Innovative Fiber-Reinforced Broad-Band Absorbing Screens

This paper presents the design and the realization process for radar absorbing panels made of composite materials. It is demonstrated that the proper selection of the carbon fiber length and volume fraction allows reducing sensibly the overall thickness of the screen compared to a standard absorber, and obtaining at the same time broad-band absorption response in the X- and Ku-bands. To this end, the carbon fiber composite material realizing the lossy sheet of the absorbing screen is designed by simulation in order to have tailored complex effective permittivity. Two prototypes of absorbing screens are realized and tested experimentally. The first one is a three-layer panel, characterized by reflection coefficient lower than -10 dB in the frequency range from 8 to 14 GHz, and total thickness of 4.5 mm. The second one is a five-layer panel with reflection coefficient less than -20 dB in the range 9-18 GHz, and total thickness of 5.5 mm.

Tensile behavior of New Zealand flax (Phormium tenax) fibers

The objective of this study is to characterize the tensile properties of New Zealand flax (Phormium tenax) technical fibers to be used as potential reinforcement in polymer matrix composites. Single fiber tensile tests were performed at three gage lengths to assess the effect of gage length on tensile strength and Young’s modulus. The results were analyzed through a two-parameter Weibull distribution. The morphology, diameter, and fracture modes of P. tenax fibers were also characterized through optical and scanning electron microscopy.

A Global Method for the Identification of Failure Modes in Fiberglass Using Acoustic Emission

The various failure mechanisms in bidirectional glass/epoxy laminates loaded in tension are identified using acoustic emission (AE) analysis. AE data recorded during the tensile testing of a single layer specimen are used to identify matrix cracking and fiber failure, while delamination signals are characterized using a two-layer specimen with a pre-induced defect. Parametric studies using AE count rate and cumulative counts allowed damage discrimination at different levels of loading and Fuzzy C-means clustering associated with principal component analysis were used to discriminate between failure mechanisms. The two above methods led to AE waveform selection: On selected waveforms, Fast Fourier Transform (FFT) enabled calculating the frequency content of each damage mechanism. Continuous wavelet transform allowed identifying frequency range and time history for failure modes, whilst noise content associated with the different failure modes was calculated and removed by discrete wavelet transform. Short Time FFT finally highlighted the possible failure mechanism associated with each signal.

Post-impact mechanical characterisation of glass and basalt woven fabric laminates

Two woven fabric laminates, one based on basalt fibres, the other on E-glass fibres, as a reinforcement for vinylester matrix, were compared in terms of their post-impact performance. With this aim, first the non-impacted specimens were subjected to interlaminar shear stress and flexural tests, then flexural tests were repeated on laminates impacted using a falling weight tower at three impact energies (7.5, 15 and 22.5J). Tests were monitored using acoustic emission analysis of signal distribution with load and with distance from the impact point. The results show that the materials have a similar damage tolerance to impact and also their post-impact residual properties after impact do not differ much, with a slight superiority for basalt fibre reinforced laminates. The principal difference is represented by the presence of a more extended delamination area on E-glass fibre reinforced laminates than on basalt fibre reinforced ones.

Effect of different lignocellulosic fibres on poly(ε-caprolactone)-based composites for potential applications in orthotics

This work compares the mechanical and thermal behaviour of fully biodegradable biocomposites based on polycaprolactone reinforced with three different natural fibres, namely hemp, sisal and coir, for potential applications in the field of orthoses. The same properties were further compared to those of two commercially available materials commonly used in the same prospective field. The results confirmed that the addition of natural fibres, irrespective of the origin of the fibres (leaf, bast or fruit) to a biodegradable matrix allows for significant improvement of the mechanical behaviour of the ensuing composites compared to traditional thermoplastic materials used in orthotics.

Optical and Mechanical Excitation Thermography for Impact Response in Basalt-Carbon Hybrid Fiber-Reinforced Composite Laminates

In this paper, optical and mechanical excitation thermography was used to investigate basalt-fiber-reinforced polymer, carbon-fiber-reinforced polymer, and basalt-carbon fiber hybrid specimens subjected to impact loading. Interestingly, two different hybrid structures including sandwich-like and intercalated stacking sequence were used. Pulsed phase thermography, principal component thermography, and partial least-squares thermography (PLST) were used to process the thermographic data. X-ray computed tomography was used for validation. In addition, signal-to-noise ratio analysis was used as a means of quantitatively comparing the thermographic results. Of particular interest, the depth information linked to Loadings in PLST was estimated for the first time. Finally, a reference was provided for taking advantage of different hybrids in view of special industrial applications.

Effect of alkali and silane treatments on mechanical and thermal behavior of Phormium tenax fibers

The effect of different treatments on the mechanical (tensile), thermal behavior (TGA), FTIR, and morphology of Phormium tenax fibers has been studied with the aim to investigate methods to improve their compatibility with polymer matrices. Applied treatments included sodium hydroxide (NaOH), silane (APTES, 3-aminopropyltriethoxysilane), and the combined application of silane treatment after NaOH. The effectiveness of the treatments in the removal of non-structural matter from the fibers was confirmed by FTIR investigation and TGA measurements, suggesting also that the alkali treatment has a strong effect on their thermal behavior. The study of tensile properties of the fibers performed using Weibull statistics indicates that the tensile properties are somewhat reduced by chemical treatment. The morphological investigation of treated fibers through scanning electron microscopy indicates that silane treatments, both on raw fibers and on alkalized ones, result in limited fiber degradation.

Synthesis and Characterization of Magnetic Nanocomposites for Environmental Remediation

In the present study, the effect of nano-magnetite (Fe3O4) content on mechanical and magnetic properties of polypropylene matrix is investigated. Magnetite nanoparticles were successfully synthesized by co- precipitation while the composites were prepared by an ex situ processing method involving solvent casting followed by compression molding. The nanoparticles and resulting nanocomposites were characterized by X- ray diffraction, thermogravimetric analysis, scanning electron microscopy, tensile testing and vibrating sample magnetometry. It was found that composites have tailorable mechanical and magnetic properties dependent on the content of magnetic filler. Increase of concentration of magnetite particles provides a significant increase of Young’s modulus without affecting the yield strength and the ductility. As regards the magnetic properties, nanocomposites having 10 wt% of nanoparticles exhibited a superparamagnetic behaviour that can be exploited in environmental applications.

Thermal and mechanical characterisation of Phormium tenax-reinforced polypropylene composites:

Composites including short and randomly arranged Phormium tenax fibres in a polypropylene (PP) matrix (fibre content of 20, 30 and 40 wt%) were produced by twin-screw compounding and injection moulding. They have been characterised by tensile testing, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results indicated that tensile modulus has been increased by reinforcing the matrix with growing amounts of fibres, whilst the effect on tensile strength had lower evidence. Fracture surface characterisation by SEM indicated that debonding and pull-out are prevalent, which suggests the need to optimise the interfacial bonding. Thermal characterisation results have shown that the main degradation peak for PP was slightly shifted to higher temperatures with the increasing fibre contents, thus improving the thermal stability of the composites. The introduction of fibres did not result in a significant variation in the position of the peaks for calorimetric analysis, except for the melting peak, which appeared lower for the composites with respect to the neat matrix. A slight increase in crystallinity was also measured.