C. Santulli

Binary PVA bio-nanocomposites containing cellulose nanocrystals extracted from different natural sources: part I.

PVA bio-nanocomposites reinforced with cellulose nanocrystals (CNC) extracted from commercial microcrystalline cellulose (MCC) and from two types of natural fibres, Phormium tenax and Flax of the Belinka variety, were produced by solvent casting in water. Morphological, thermal, mechanical and transparency properties were studied while the respective efficiency of the extraction process of CNC from the three sources was evaluated. The effect of CNC types and content on PVA properties and water absorption capacity were also evaluated. Natural fibres offered higher levels of extraction efficiency when compared with MCC hydrolysis yield. Thermal analysis proved that CNC promotes the crystallization of the PVA matrix, while improving its plastic response. It was also clarified that all PVA/CNC systems remain transparent due to CNC dispersion at the nanoscale, while being all saturated after the first 18-24h of water absorption.

Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission

The present study aims to characterise damage due to low velocity impact on jute fibre reinforced polyester composites. To attain this goal, a number of post-impact mechanical tests have been carried out, including tensile tests, three-point bending and indentation, using either a staircase or a continuous loading programme. On all these tests acoustic emission activity (AE) was monitored. The results, compared with damage observed under an optical microscope, show that AE is able to perform a reliable measurement of the level of damage also, on a natural fibre reinforced laminate. The main limitations of this study are owing to the rather low ultimate stress of the material and to the need to apply a loading to evaluate the damage produced by the impact event.

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.

Ternary PVA nanocomposites containing cellulose nanocrystals from different sources and silver particles: part II.

Cellulose nanocrystals (CNC) extracted from three different sources, namely flax, phormium, and commercial microcrystalline cellulose (MCC) have been used in a polyvinyl alcohol (PVA) matrix to produce anti-bacterial films using two different amounts of silver nanoparticles (0.1 wt% and 0.5 wt%). In general, CNC confer an effect of reinforcement to PVA film, the best values of stiffness being offered by composites produced using phormium fibres, whilst for strength those produced using flax are slightly superior. This was obtained without inducing any particular modification in transition temperatures and in the thermal degradation patterns. As regards antibacterial properties, systems with CNC from flax proved slightly better than those with CNC from phormium and substantially better than those including commercial MCC. Dynamic mechanical thermal analysis (DMTA) has only been performed on the ternary composite containing 0.1 wt% Ag, which yielded higher values of Youngs modulus, and as a whole confirmed the above results.

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.

Impact damage evaluation on advanced stitched composites by means of acoustic emission and image analysis

Abstract Polymeric composites that are stitched reinforced show dissimilar behaviours with respect to regular ones. In this paper the mechanical characteristics and different behaviours of stitched reinforced polymeric composites after impact tests are investigated using an alternative approach. Several factors such as grid size, stitching scheme and voids content modify the composite performances and the interlaminar delaminating processes. If, on the one hand, the stitching technique is used to prevent delaminating damage phenomena, then, on the other hand, the process itself introduces a sort of damage into the layers during the assembled preform, that has to be taken into account. Delaminating processes due to impact loads are considered and the post-impact behaviours and damage tolerance are investigated. Impact tests were performed at several energy levels known to have an appreciable damage on the specimens. Damage evaluation analysis was performed on post-impacted specimens using different techniques: as a first step, using a digitalised image technique, the damaged area was measured and different zone shape was evaluated, further compression tests were done to evaluate the material residual properties after impact. The results coming from compression tests do not reveal the better awaited behaviour of the stitched composites with respect to regular ones. On the other hand, the acoustic emission technique is capable of evidencing the different aspects of the stitching wire. The image acquisition technique represents a new experimental method able to fully characterise damage zones and gives more information on the interlaminar behaviour of composite material. This new technique joined to acoustic emission measurements allows important results on the residual life prediction of materials mechanical properties to be obtained.

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.

Development of smart variable stiffness actuators using polymer hydrogels

In this work, compliant actuators are developed by coupling braided structures and polymer gels, able to produce work by controlled gel swelling in the presence of water. A number of aspects related to the engineering of gel actuators were studied, including gel selection, modelling and experimentation of constant force and constant displacement behaviour, and response time. The actuator was intended for use as vibration neutralizer: with this aim, generation of a force of 10 N in a time not exceeding a second was needed. Results were promising in terms of force generation, although response time was still longer than required. In addition, the easiest way to obtain the reversibility of the effect is still under discussion: possible routes for improvement are suggested and will be the object of future work.

The fracture properties of environmental-friendly fiber metal laminates

The tensile and impact properties of environmental-friendly composites and FMLs have been investigated. Of the four composites investigated here, a SRPP composite offered superior properties to basalt-, flax-, and hemp fiber-reinforced PP composites. Adding aluminum layers to the outer surfaces of the composites resulted in a significant enhancement in the tensile and impact properties of the laminates. The tensile strength and modulus properties of the FMLs obey a rule of mixtures approach, suggesting that simple procedures can be used to design these hybrid systems. Under low-velocity impact loading, the SRPP, and its associated FML, offered the highest resistance to perforation, as a result of gross plastic deformation in the composite and metal plies. A semi-empirical model, previously employed to characterize metal plates, was used to characterize the low-velocity impact response of the laminates investigated here. The model was capable of predicting the trends in the experimental data with reasonable success. This evidence suggests that environmental-friendly fiber-based FMLs offer significant potential for use in engineering applications.

Failure modes characterization of impacted carbon fibre reinforced plastics laminates under compression loading using acoustic emission

Composite laminates have low resistance under dynamic loading, particularly impact loading. A low-velocity impact on laminated composites causes various types of damage, such as delamination, fibre breakage, matrix cracking and fibre matrix interfacial debonding. Post-impact compressive strength is one of the greatest weaknesses in carbon fibre reinforced plastics laminates. After impact, due to the delaminations present in the laminates, local instability is triggered, which ultimately reduces considerably their residual strength. In this work, symmetric cross ply carbon fibre reinforced plastics laminates [(0°/90°)2]12 were subjected to falling weight impact at two different velocities, 2.5 and 3.5 m/s. Compression after impact studies showed substantial differences in failure mode between the two cases, passing from end crushing to crack propagation with higher impact energy. Acoustic emission technique was able to confirm this result and characterize the different types of failure modes during compression after impact test, in particular by frequency distribution.