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The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Hybridization offers intermediate properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials and leading to a composite with more tailored behaviour The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems. In this framework, hybrid reinforced laminates with flax and basalt twill layers alternatively stacked, were manufactured by resin infusion fabrication technology and impacted at low velocity to investigate their dynamic behaviour, in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with only basalt or flax fibres. The experimental results confirmed the positive role played by fibre hybridization in terms of damage. The Electronic Speckle Pattern Interferometry technique was adopted to analyze the internal damage and to provide information on the shape and the extent of the delamination, that was found concentrated under the impactor-material contact point for the basalt and flax/basalt laminates.

This paper aims to investigate the ability of ultrasonic and electronic speckle pattern interferometry to analyse the low-velocity impact internal damage mechanisms on basalt composite laminates and to provide information on the shape and the extent of the delamination in non-destructive way. Basalt/epoxy composites with different thicknesses have been realised and characterised by mechanical tests to investigate both fibre-dominated (tensile and flexural behaviour) and matrix-dominated properties (interlaminar shear strength). Specimens were impacted at penetration and at increasing energy values, to explore the damage onset and propagation. The results showed that the damage was concentrated under the impactor–material contact point and that the composite with intermediate thickness had the best balance between the different kinds of impact damages: delamination and indentation. Further, a good agreement was found between the overall data obtained by the two non-destructive techniques, confirming the capability of both techniques to examine the composite impact damage.

In an attempt to increase the low-velocity impact response of natural fiber composites, a new hybrid intraply woven fabric based on flax and basalt fibers has been used to manufacture laminates with both thermoplastic and thermoset matrices. The matrix type (epoxy or polypropylene (PP) with or without a maleated coupling agent) significantly affected the absorbed energy and the damage mechanisms. The absorbed energy at perforation for PP-based composites was 90% and 50% higher than that of epoxy and compatibilized PP composites, respectively. The hybrid fiber architecture counteracted the influence of low transverse strength of flax fibers on impact response, irrespective of the matrix type. In thermoplastic laminates, the matrix plasticization delayed the onset of major damage during impact and allowed a better balance of quasi-static properties, energy absorption, peak force, and perforation energy compared to epoxy-based composites.

Epoxy based laminates reinforced with woven basalt fibres have been manufactured and characterized by quasi-static mechanical tests and low-velocity impact tests. Samples were prepared by infusion of 16 plies of basalt under pre-optimized conditions. All the interlaminar shear strength, flexural and impact tests were carried out at room temperature, −25 °C and −50 °C to investigate the effect of temperature on the mechanical characteristics and the impact damage onset and propagation. Ultrasonic technique were adopted to provide information on the shape and the extent of the internal damage. The final aim was to be able to predict impact energy and delamination under different temperature conditions and to assess the influence of the resin matrix stiffening at low temperature on the composite behaviour. Results show that at low temperature due to the stiffening of the resin matrix, the composite tested is more resistant and less deformable. It is interesting since, at low temperatures, it is more straightforward the damage propagation. In particular, ILSS and flexural tests show a maximum increase of 39% and 12.78% respectively at T = −50 °C compared to the value at room temperature. A delamination area increase of wholly 40% at T = −50 °C, is detected.

Basalt-epoxy laminates are establishing gradually in engineering sectors due to increasing attention toward environmental matters. Some properties should be improved to extend their applications and the cold spray (CS) metallization provides a potential solution. Unfortunately, the fragile nature of thermosetting polymers makes it difficult the CS coating formation and grow-up. Therefore, this research work aims to study an innovative solution able to enhance the adhesion mechanisms between the cold sprayed metal particles and the thermosetting polymer-based substrates. For this purpose, an original manufacturing method that provides the surface PLA treatment for thermosetting samples was developed. AlSi10Mg particles were cold sprayed on the treated surface, and the low-velocity impact behaviour of the metallised hybrid structures was analysed in details. The outcomes prove the effectiveness of the method proposed on the deposition process and the beneficial effects of metallization on impact damage mechanisms.

The effect of stacking sequence on the impact damage mechanisms and the matrix-fiber dependent properties, such as flexural and interlaminar strength , of epoxy hybrid basalt/flax composites has been experimentally investigated. The basalt and flax fabrics have been appropriately selected with approximately the same areal weight and stacked in the same number in three different symmetrical analysed configurations to manufacture composites with comparable conditions regarding fibre content by weight. No influence of fibre sequence was observed for the flexural modulus , whereas some differences were exhibited by the flexural and the interlaminar shear strength depending on the resin content. The impact tests were carried out at penetration and different energy levels. After impact, indentation measurements have been performed by a confocal microscope to get information on the damage propagation and characteristics that resulted affected by the hybridisation type and by the impact energy. The results evidenced the relevance of the stacking sequence in the composite design to meet specific requirements.

To improve the impact behaviour of composite laminates and to reduce the problem of the complex damages after dynamic loads, a two-component commercial epoxy resin was modified with a commercial nitrile liquid rubber modified DGEBA epoxy prepolymer that, prior the cure, has been mechanically dispersed at two concentration levels (20%w/w; 30%w/w). Experimental characterization of both neat and modified resins was performed to investigate the effect of the rubber addition on the resin properties, i.e. glass transition temperature, modulus and dynamic mechanical characteristics. Further, the rubber domain distribution in the epoxy resin was analyzed by scanning electron microscopy (SEM). The hybrid rubber modified resins did not show high viscosity allowing to manufacture E-glass fiber reinforced composites by vacuum infusion process. The laminates were characterized by tensile mechanical tests that evidenced an overall enhancement of maximum stress and deformation for the hybrid composites. In particular, results demonstrated an improvement of 10% for the shear failure stress and of 110% for the shear failure strain. Further, impact tests were carried out at penetration, to evaluate the improvement of the impact resistance over the conventional epoxy resin system, and at increasing energy levels, 5 J, 10 J and 20 J, to investigate the damage start and propagation. A higher indentation has been observed for the modified composite samples related with a lower delamination extension. © 2017 Elsevier Ltd

The damage resistance of a fibre reinforced composite materials subjected to a concentrated indentation force has been investigated. Composite samples were manufactured by vacuum infusion process and then tested according to the concentrated quasi-static indentation test standard (QSI). The composite coupons were sensorized with embedded Bragg grating fibres in various positions, to monitor the indentation-induced deformations during the loading event. Numerical simulations, performed by using a commercial FEM software, were carried out and predictions results compared with experimental data. Final results showed that Bragg wavelength changes sensitively depending on the applied quasi-static forces and it identifies correctly the non-linear pattern of the force–displacement curve. It was determined that strain transfer to the Bragg fibre is strongly influenced by the embedding conditions and final location of the optoelectronic sensors in term of shifting and in-plane rotation. Comparison between simulated and experimental data showed some discrepancy mainly due to the displacement of the embedded fibre sensors from their original location, caused by the manufacturing. However, by an inverse procedure based on iso-strain curves both shifting and rotation of the FBG sensors respect to their original position could be evaluated. This work confirms that although Bragg fibres are effective and reliable to monitor deformations of composite elements during an impact event but unwanted and unpredictable effects, due to manufacturing, should be carefully taking into account to analyse correctly the sensor signal and quantitatively evaluate the inter-laminar strains.

The electrical percolation behavior of multiwall carbon nanotubes epoxy composites produced using sonication as dispersion process is studied. The electrical properties of the cured composites are related to the different internal carbon nanotube networks induced by the processing parameters. A fine tuning of the electrical conductivity is obtained by dispersing the nanotubes in the matrix and subsequently inducing agglomeration by curing at different temperatures. Optical microscopy and scanning electron microscopy of the formed microstructures reveal a strict correlation between structural and electrical properties that suggests that agglomeration provides a higher electrical conductivity.

SUMMARY The fire behavior and the smoke emission of an unsaturated polyester resin modified by the addition of three phosphorus-based fire-retardant materials (ammonium polyphosphate (APP), silane-coated APP, and melamine pyrophosphate) at two concentration levels (20% w/w, 35% w/w) have been investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy and optical microscopy analysis have been performed to verify the dispersion and the action mechanism of additives within the resin. Results from cone calorimetric tests demonstrated that the incorporation of the fire retardants at 35% w/w has a strong effect on flammability and smoke suppressant properties with respect to both the neat resin and the loaded systems at 20% (w/w). In particular, the smoke formation and smoke parameters are reduced by 50% and 80%, respectively, leading to the conclusion that APP can be used single-handedly without combination with specific smoke suppressors. Copyright © 2011 John Wiley & Sons, Ltd.

The consolidation of a commercial thermoset resin has been completely monitored by using a unique fiber optic sensor integrating a refractometer and a fiber Bragg grating. In particular, the advancement and the evolution of the resin polymerization reaction have been followed by measuring the optical properties changes induced by the thermo-chemical transformations. On the other hand, the fiber Bragg grating has been used to detect on-line the resin gelification onset. The experimental results demonstrated the capability and the efficiency of the developed sensor system to provide complete information on the main phenomena occurring during the cure cycle of a thermoset based system.

A new innovative infusion technology, pulsed infusion, has been developed for the manufacturing of fiber-reinforced thermoset-based composites. Pulsed infusion is a double-bag vacuum infusion process that is based on the use of a proper designed reusable pressure distributor and able to better control the vacuum pressure in pulsed way. Thus, the transverse resin flow through the dry fiber reinforcement is promoted and a better adhesion between the resin and the fibers is achieved. The new process allows to obtain laminates with the same fiber volume fraction and tensile properties of those produced by conventional infusion technologies. An average increase up to 9% for the flexural modulus and up to 24% for flexural strength has been assessed for pulse-manufactured composites compared to traditional vacuum infusion ones. Furthermore, due to a minor consumption of resin and the absence of the distribution net, pulse infusion provides a material cost-saving advantage around 19% and a significant waste reduction.

As-received and pre-treated sepiolite/epoxy systems, characterised by an inorganic content from 2 to 10 wt.%, were investigated in order to assess induced-filler effect on degradation and fire behaviour. Thermogravimetrical results show that the thermal stability of the hosting epoxy, is slightly affected by the presence of sepiolite for both typologies; whereas, changes induced in char morphology by the pre-treated clay will significantly affect the fire behaviour of the final nanocomposite. Modelling of thermo-gravimetrical results in air atmosphere, by means of Kissinger procedure, shows a noteworthy reduction of activation energies associated with each degradation steps, especially at highest sepiolite content either by using as-received and pre-treated inorganic filler. This substantially indicates that the presence of sepiolite shorten the whole degradation process on the temperature scale. On the other hand, the different morphology of the char layer during the burning process can have relevant flame retardant effects acting on both condensate and vapour phase. Analysing the cone calorimetric data, a reduction of about 27% of the peak of heat release rate for the highest sepiolite percentage is measured and the burning total period is increased thus confirming that sepiolite when pre-treated represents a valid fire retardant inorganic filler for such a system.

Epoxy/carbon fiber composites have been manufactured by Pulse Infusion. Pulse Infusion allows to control the pressure of the vacuum bag on the dry fiber reinforcement by using a proper designed pressure distributor that induces a pulsed transverse action and promotes the through thickness resin flow. The adopted one-component commercial epoxy resin has been preliminary modified by adding 0.05% (w/w) of multiwalled carbon nanotubes, in order to take advantage of carbon nanotubes at low concentration. Both neat and hybrid realized composite panels have been mechanically characterized by performing experimental tests to evaluate tensile, interlaminar, and fracture properties in order to investigate the effect of Pulse Infusion and carbon nanotubes on the mechanical and fracture behavior of composites. Results demonstrated an improvement of 36.2% for the interlaminar shear strength, of 35% for the fracture energy at the crack initiation and of 14% for the fracture toughness in mode II. POLYM. COMPOS., 38:2254–2260, 2017. © 2015 Society of Plastics Engineers

In autoclave technology, polymer based composites are manufactured under the application of pressure and heat. The heat transferred between the energy carrying fluid and the bag-composite-tool element activates exothermic curing reactions, leading to composite consolidation. The convective heat transfer mechanism is the most relevant aspect controlling the rate of chemical and physical transformations associated with composite curing. Moreover, the fluidodynamic regime that results from the interactions between the autoclave and the tool geometry, even if totally predictable in theory, is unattainable in practice. In this study, the heat transfer phenomena occurring during the autoclave manufacturing cycle have been analyzed. The assumption of a negligible through-the-thickness thermal gradient led to simplified energy balance equations. In this case, the thermal evolution of the manufacturing elements has been completely determined by two parameters: the global convective heat exchange coefficient, setting the rate of the heat transfer between the autoclave environment and the bag-composite-tool element, and the adiabatic temperature rise, establishing the relevance of the polymerization exotherm. A scaling analysis has been performed in order to identify the dimensionless parameters controlling the autoclave process. The developed semitheoretical methodology has been extensively tested by comparison with experimental data from an industrial autoclave.

In this paper, the electrical and dielectric properties of epoxy composites containing multi walled carbon nanotubes (MWCNTs) are investigated by means of direct and alternate current measurements and correlated with the internal composite microstructure. A systematically study on the role of different nanotube network structures produced by varying the processing parameters: sonication time and curing temperature is performed. The percolation behavior of the produced composites is studied and the correlation between carbon nanotube networks induced by processing and the final properties is analyzed. An equivalent electrical circuit is proposed as a model to describe the dielectric property behavior due to different multi-scale composite microstructure and the interaction between tubes-tubes and tubes-matrix. © 2014 Elsevier Ltd.

Sepiolite/epoxy systems, characterised by an inorganic content from 2% to 10% by weight, were tested by using different experimental techniques in order to assess the effects thermo-mechanical and fracture behaviour. It was found that filler dispersion is independent on the type of sepiolite used, i.e. hydrated and de-hydrated, whereas the addition of hydrated sepiolite strongly increases the viscosity of the final suspension. Mechanical properties have been investigated and discussed. The conservative modulus of the sepiolite/epoxy system increases as much as 21% and 94% respectively, at 35 °C and 200 °C respect to the unfilled polymer, with the incorporation of 10 wt% of sepiolite, both hydrated and dehydrated, with a slightly increasing of the corresponding glass transition temperature. The effect of sepiolite on the coefficient of thermal expansion lead to a variation in both glassy and rubbery region, respectively of about 7–8% and 9%. Fracture toughness analysis revealed that the critical stress intensity factor negligibly changes by increasing the sepiolite content, whereas the critical strain energy release rate (GIC) reaches an noteworthy value around 30% higher than neat epoxy.

A representative volume element (RVE) based model to evaluate the mechanical performance of non-crimp fabric (NCF) composites has been developed and presented hereafter. By means of the stiffness averaging method, the modelling procedure is able to simulate the NCF’s elastic properties taking into account the effects of process-induced defects and final geometrical configuration. Microscopy analysis has been used to quantitatively evaluate the effects of tow waviness, stitching geometrical parameters and matrix porosity; these features have been included as sub-models into the final model. Numerical predictions have been compared to the results of tensile tests performed on composite coupons. A geometrical parameter characterising the undulation of the tows has been introduced and a sensitivity analysis has been performed on the model with different undulations.

The potentiality of carbon nanotubes as reinforcement material is not only due to their exceptional high modulus, but also to their high aspect ratio. Indeed, the nanotubes contribution to the mechanical reinforcement in a polymer is strongly dependent on their distribution within the hosting matrix. In fact, the clustering of carbon nanotubes does limit the theoretical enhancement of the composite mechanical properties by a reduction of their effective aspect ratio. In this work, the reinforcement efficiency of carbon nanotubes having different aspect ratios has been experimentally investigated at low filler contents in an epoxy system. From a theoretical point of view, the classical theory (Cox, 1952 [25] ) concerning the mechanical efficiency of a matrix embedding finite length fibers has been modified by introducing the tube-to-tube Random Contact Model (Philipse, 1996 [33] ) which explicitly accounts for the progressive reduction of the tubes effective aspect ratio as the filler content increases. The validity of the proposed model was assessed by a comparison with available literature data, providing a good agreement.

A hybrid configuration at the micromechanical level is presented and described as a suitable approach to enhance the damping features of advanced polymer composites. A micro-level hybridization was achieved on dry preform reinforcements by embedding visco-elastic fibres within standard carbon tows. Unidirectional composites with two viscoelastic volume fractions (2.5% and 5% vol/vol) were manufactured by a vacuum infusion process and later tested by dynamic mechanical analysis along the principal directions. Final results reveal a significant enhancement (+80% and +56%) of the damping properties, respectively, for the longitudinal and the transverse directions in the case of the highest viscoelastic fibre content. In turn, the elastic properties of the final composite were greatly reduced (−37% and −35%) with respect to the standard composite. Final results support further work in the direction of micromechanical hybridization looking at the potential exploitation of standard textile configurations with different viscoelastic fibre content to enhance damping properties.

The formation and the build-up of process-induced stresses and strains have been investigated both experimentally and numerically, analyzing the non-isothermal cure of a thermoset polymer resin placed into a rectangular mold. In particular, a stress model has been formulated accounting for the evolution of the resin mechanical properties, the mold constrains, thermal expansion effects and shrinkage phenomena induced by the cure reaction. The mechanical equilibrium equations, based on the Cauchy–Navier approach, have been solved by implementing a finite element code based on an incremental transient approach that reduce the difficulty of defining the stress free conditions. The effect of the mold material on the process induced strain field has been investigated performing numerical simulations both with an aluminum and a steel mold. In addition, the model results have been validated by comparing with the experimental strain measurements which were obtained by embedding a single fiber optic Bragg grating sensor within the resin and activating the polymerization reaction in an oven. Residual stresses and strains have been computed numerically performing two-dimensional simulations on the top and side sections of the experimental mold/resin system. In both cases, a good agreement between the numerical strain results and the experimental fiber optic Bragg measurements was found.

The resin film infusion is a promising and low cost technique for the manufacturing of high performance polymer-based composite materials. The processing cycle is usually performed in the autoclave where, under temperature and pressure application, a bagged dry fiber mat is infiltrated by a thermoset polymer resin which is pre-shaped in film form under the fibers. After complete infusion, the consolidation step behaves as in the conventional autoclave process. The infusion process is strongly affected by the permeability of the fiber mat that determines processing issues, such as the cycle time and the complete fiber impregnation and, as a consequence, the performance of the final part. Therefore, the permeability measurement represents one of the most crucial requirements for the design and the optimization of the composite manufacturing by RFI. In this study, a non-conventional technique, based on a fiber optic sensor, has been used to guess the permeability of braided carbon fiber fabrics infiltrated with an epoxy resin by reproducing experimental conditions similar to the RFI industrial manufacturing. The fiber optic sensor, placed through the reinforcement, is able to detect the resin advancement during the infiltration by recording the light intensity drop which takes place as the resin reaches the optical fiber location. This technique represents a valid alternative to the common permeability measurement methods based on flow visualization and in addition, it can be easily used for the real time monitoring of the resin flow advancement during the real processing.

Autoclave manufacturing of thermoset composites is determined mainly by heat transfer phenomena. As a matter of fact, the consolidation of composite laminates takes place by the progress of the polymerization, which is activated thermally. The design and control of the autoclave process relies on the capability to manage the relationship between the temperature-pressure cycle of the heat carrier fluid and the temperature distribution through the manufacturing part. In particular, in industrial cases, the main limitations reside in the correct evaluation of the local convective heat transfer conditions through the autoclave and in the evaluation of the local thermal inertia arising from the bagging-tooling system. In this study, the autoclave manufacturing of thick laminates has been addressed by modeling the heat transport phenomena occurring through the composite, the bagging and the tooling system. A new methodology for the evaluation of the energy transfer regimes has been proposed accounting for the heat fluxes from the bag and the tool side, the temperature through-the-thickness gradients and the heat generated by the resin polymerization reaction. The proposed approach enables the prediction of the temperature history of the autoclave assembly without knowledge of the effective thermal inertia of the two external layers, which could be difficult to evaluate owing to possible deformations of the bag during the manufacturing cycle and nonuniform shape of the metallic tool along the part. Experimental data from industrial autoclave runs have been collected and analyzed to validate the method.