Search

Additive Manufacturing (AM) techniques have recently attracted the attention of scientists for the development of prototypes with complex or particular geometry in a fast and cheap way. Among the different AM processes, the Fused Deposition Modelling process (FDM) offers several advantages in terms of costs, implementation features and design freedom. Recently, it has been adopted to realise auxetic structures, which are characterised by negative Poisson ratio, enhanced mechanical properties, and a higher compression resistance than conventional structures. This review outlines the use of AM processes, in particular FDM, to design and obtain auxetic structures, with the final aim to exploit their applications in different fields. The first part of this work presents a brief classification of auxetic structures and materials. Subsequently, a summary of additive manufacturing processes is presented, focusing on the use of FDM and its limitations. Finally, the studies on the use of additive manufacturing to produce auxetic structures are shown, evidencing the potential of the concurrent combination of a fast prototyping technique such as FDM and the characteristics of polymer- and/or composite-based auxetic structures. Indeed, this new technological field opens the possibility of realising novel structures with integrated smart behaviour, multifunctional properties, compression resistance, and a tailored microstructure and shape.

In this study, the effects of surface treatment of a low-temperature atmospheric oxygen plasma on basalt/epoxy composites were investigated to improve the hydrophobility of the composite surface. After the plasma treatment, the unmodified and surface treated composite laminates have been experimentally characterized by performing contact angle measurements, low-velocity impact tests and indentation depth on the impacted laminates. Results have showed a dependence of such composite properties on the plasma coating deposition and on the treatment parameters outlining the need to optimize both the plasma power and exposition time to plasma in order to assess the efficiency of the plasma treatment and establish the optimal processing conditions.

The low-velocity impact behavior of polyester glass fiber composite laminates has been investigated under different clamping conditions, by wetting the specimens surface with fluids having different densities. Unidirectional composite laminates, based on polymer resin and fibers of naval interest, with different thicknesses have been manufactured by resin transfer molding process in order to have a high control of the final properties and a high fiber volume fraction. Impact tests in air have been, first, performed up to penetration on all manufactured composites having four different thicknesses to get information on the material impact behavior. The effect of the clamping conditions and the interaction of the laminates with two fluids different in density, i.e., water and paraffin oil, have been investigated, then, only on the thickest laminates that showed a lower tendency to impact damage. In particular, the impact tests have been performed by impacting the front surface of the sample and by wetting the back surface of the composite samples under unclamped and clamped conditions, replicating the boundary constraints suggested by ASTM D7136 Standard for the conventional impact tests. Experimental results showed a lower initial rigidity, higher absorbed energy and deflection for specimens exposed to liquids with respect to those exposed to air, confirming the need of taking into account the fluid interaction in the case of marine composite applications.

The strong demand for high performance structural and mechanical systems for aerospace, defense, and advanced manufacturing industries has promoted the evolution of advanced materials. Materials cannot respond to changes of the surrounding environment and are not able to operate under the service conditions for which they are not optimally designed. There is a significant need for the evolution of advanced active materials with enhanced performance whose elastodynamic response can be optimally adapted in real time under a broad range of service conditions. Unlike the common structures, shape-memory materials are stimuli-responsive materials, they have the extraordinary feature to respond to external stimuli adapting their structure to the surrounding environment and compensating for undesired effects. A shape change because of a temperature change is called thermally induced shape-memory effect.

International audience; Investigation on the fatigue life of hybrid composites is critical to extend their structural applications. However, there is a lack of research on fatigue performance of hybrid composites. In this study, the fatigue life of a flax-basalt woven-ply hybrid composite is investigated and compared with the behaviour of 100% flax and 100% basalt composites. After the optimisation of the epoxy resin curing cycle, tension-tension fatigue tests were performed on samples with two orientations, 0°/90° and ± 45°. S-N curves with absolute values and with normalised values are discussed. Results showed that the hybridisation was able to produce a positive effect on the fatigue resistance of basalt laminates: a better normalised fatigue resistance was obtained for the hybrid composite in comparison with the 100% basalt one in both orientations. A two-parameter fatigue life model was then applied and showed its ability to predict the fatigue behaviour of the hybrid composite for the two orientations. An analysis of damage mechanisms by means of infrared thermography and scanning electron microscopy allowed observing the different types of damage occurring during fatigue loading in hybrid composites.

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 this work, the evaluation of damaged areas due to low velocity impact tests at different impact energy values on Carbon Fibre (CF) laminates with 3D fabric, made by infusion technology and reinforced with epoxy resin, were investigated by NDI (Non-destructive Investigation) technique based on speckle interferometry. Composites are characterized by several interacting failure modes such as matrix breakage, fibre failure and delaminations, which can be simultaneously induced by low velocity impacts. It can be very difficult to detect by visual inspections of the structures such phenomena. The aim of this study is to evaluate the damaged area due to an impact event. CF composite laminates were impact damaged with four energy levels (5, 10, 20, 30J) at room temperature, by drop weight impact machine. The results showed that ESPI technique was able to identify barely visible low-velocity impact damages.

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.

Hydrophobic surfaces are highly desired for several applications due to their exceptional properties such as self-cleaning, anti-icing, anti-friction and others. Such surfaces can be prepared via numerous methods including plasma technology, a dry technique with low environmental impact. In this paper, the effect of a one-step sulfur hexafluoride (SF6) plasma treatment upon the low velocity impact behavior of basalt/epoxy composites has been investigated by using several characterization techniques. A capacitive coupled radiofrequency plasma system was used for the plasma surface treatment of basalt/epoxy composites, and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the water droplet contact angle of treated specimens. The contact angle measurements showed that treating with SF6 plasma would increase the hydrophobicity of basalt/epoxy composites, moreover, the impact results obtained on reinforced epoxy basalt fiber showed damage in a confined area and higher impact resistance for plasma-treated basalt systems.

The research reported in this paper was aimed mainly to investigate the different NDE techniques on specimens made by a new process labeled as "pulsed infusion", very crucial for voids content under critical loading conditions. The impact load, in fact, is critical for composite laminates due to their anisotropy, in particular in extreme temperature conditions due to their brittleness. An additional and very relevant aim was to collect a large number of experimental results to supply useful information for the numerical models needed to simulate the dynamic behavior of composite laminates. At the aim to investigate the response under dynamic loads of laminates fabricated by a new vacuum assisted technology labeled as "pulsed infusion", rectangular carbon fiber composite specimens were subjected to low velocity impact tests. Experimental tests up to complete penetration and at different energy levels, were carried out by a modular falling weight tower. All the parameters related to the phenomenon, like penetration energy, maximum force and indentation depths, were used to validate existing semi-empirical and numerical models. The largely used ultra sound technique (US) was adopted to investigate the delamination together with the thermo graphic technique. The results of the measurements were compared with data obtained on the same specimens by holographic analysis (ESPI). One of the scope was to investigate the crucial internal impact damage and assess the ability of an unconventional ND system (ESPI) in giving right information about non-visual damage generated inside composite laminates subjected to dynamic loads. Moreover, some of the specimens were cut to allow the fractographic analysis. The efficiency of the above mentioned new fabrication technology was studied also comparing the results with measurements from literature on impacted autoclave cured laminates. By the comparison between the results, good agreements were found denoting the efficiency and the applicability of the new fabrication and the used NDE methods. © 2015 Elsevier Ltd. All rights reserved.

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.

The aim of this work is the study of the influence of a large notch damage in a stiffened aeronautical panel. In particular, the damage onset and evolution due to a cut-out located in the bay of an omega stiffened composite panel subjected to a compressive load is investigated. Three different cut-outs are considered: parallel, normal, and 45° oriented respect to the load. The effects of such configurations are compared in terms of fibre and matrix failures, in order to better understand which configuration is the most sensitive to these type of damages.

Glass fiber composite materials in epoxy resin with different content of a nitrile rubber modified hybrid resin were manufactured by infusion with two different rubber concentrations. SX 10 epoxy resin was modified to improve the response of the laminates under impact loads. At the aim to test the improvement of the impact resistance over the conventional epoxy resin system, the rectangular specimens 100∗150 mm2 were dynamically loaded in the center by a falling weight machine, Ceast Fractovis, using a cylindrical impactor with a hemispherical nose 19,8 mm in diameter and a total mass of 3.6 kg. Impact tests were carried out at penetration to obtain the complete load-displacement curve useful to measure the penetration energy and to highlight the characteristics points for the increasing energies evaluation, 5J, 10J and 20J, to investigate the damage start and propagation. After the impact tests, the specimens were observed by visual inspection to investigate the delamination extension whereas a confocal microscope was used for the indentation measurements. The obtained results on the modified epoxy system were compared to neat GFRP laminates impact performances. No significant impact performance difference was recorded between samples with different nitrile rubber modifier content. A higher indentation was recorded on the modified samples related with a lower delamination extension. It could represent a good practical results since from a simple external damage measurement it can be possible to obtain information about a low internal delamination. © 2016 The Authors.

Metal nanoparticles (NPs) were prepared from acrylamide complex of iron and nickel nitrates by controlled thermolysis at constant temperature in a self-generated atmosphere and used to fabricate epoxy resin nanocomposites. X-ray diffraction performed on the nanoparticles revealed a metal core/polymeric shell structure for the nanoparticles thermolysed at higher temperature as also confirmed by cross analysis of thermo-gravimetrical and vibrating magnetometer data. Optical microscopy was used to investigate the achieved dispersion of the nanoparticles within the uncured resin and transmission electronic microscopy was used to analyse the morphology and the average dimensions of the nanoparticles. Thermal stability of manufactured nanocomposites was studied by thermo-gravimetric analysis in inert and oxidative conditions, reporting that negligible variations are induced by the presence of nanofillers. A complete thermo-mechanical and fracture characterization was carried out to assess the effect of nanoparticles on final nanocomposite system. A slight increase of the modulus is recorded at room temperature whereas a significant variation is induced for higher temperature. The presence of nanoparticles induces a global increase in the fracture energy of nanocomposite associated to different failure mechanisms. Magnetic data indicate that iron particles based nanocomposites display room-temperature ferromagnetic behaviour with saturation magnetization and coercivity depending on the diameter of the metal core.

Two types of basalt fibre reinforced epoxy laminates were realized by overlapping flat and twill woven basalt fabrics by resin infusion. Rectangular specimens, cut from the panels were impacted at penetration and at increasing energy values, to investigate the damage onset and propagation. A non-destructive technique, Ultrasound testing (UT), was adopted to investigate the internal damage. Despite the difficulties to obtain information by UT method due to the high amount of signal absorbed, the technique, properly calibrated, proved to be very useful in providing information about the presence, the shape and the extent of the delaminations. The results were compared at the aim to investigate the effect of the fiber architecture (textile). The experimental results indicate a similar impact behavior between basalt flat and twill composites but in the case of the twill a minor delaminated area was detected, even if a higher absorbed energy was recordedTwo types of basalt fibre reinforced epoxy laminates were realized by overlapping flat and twill woven basalt fabrics by resin infusion. Rectangular specimens, cut from the panels were impacted at penetration and at increasing energy values, to investigate the damage onset and propagation. A non-destructive technique, Ultrasound testing (UT), was adopted to investigate the internal damage. Despite the difficulties to obtain information by UT method due to the high amount of signal absorbed, the technique, properly calibrated, proved to be very useful in providing information about the presence, the shape and the extent of the delaminations. The results were compared at the aim to investigate the effect of the fiber architecture (textile). The experimental results indicate a similar impact behavior between basalt flat and twill composites but in the case of the twill a minor delaminated area was detected, even if a higher absorbed energy was recorded

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 flammability behavior of an epoxy resin, modified by the addition of Posidonia Oceanica at three concentration levels (8%, 10%,12%), has been investigated by performing cone calorimeter tests. The plant has been preliminary dried and milled to obtain a powder with average size of 0.08 micron, then has been dispersed within the resin prior the cure. Scanning electron microscopy both on the Posidonia and the hybrid composites has been carried out to verify the dispersion and the action mechanism of plant within the resin. Results from cone calorimeter tests showed that the incorporation of Posidonia Oceanica significantly affect the flammability of the epoxy resin by a strong reduction of PHHR up to 52%.The flammability behavior of an epoxy resin, modified by the addition of Posidonia Oceanica at three concentration levels (8%, 10%,12%), has been investigated by performing cone calorimeter tests. The plant has been preliminary dried and milled to obtain a powder with average size of 0.08 micron, then has been dispersed within the resin prior the cure. Scanning electron microscopy both on the Posidonia and the hybrid composites has been carried out to verify the dispersion and the action mechanism of plant within the resin. Results from cone calorimeter tests showed that the incorporation of Posidonia Oceanica significantly affect the flammability of the epoxy resin by a strong reduction of PHHR up to 52%.

Confinement and surface effects provided by nanoparticles have been shown to produce changes in polymer molecules affecting their macroscopic viscosity. Nanoparticles may induce rearrangements in polymer conformation with an increase in free volume significantly lowering the viscosity. This phenomenon is generally attributed to the selective adsorption of the polymer high molar mass fraction onto nanoparticles surface when the polymer radius of gyration is comparable to the nanoparticles characteristic dimensions. Carbon nanotubes seem to be the ideal candidate to induce viscosity reduction of polymer due to both their high surface-to-volume ratio and their nanometric sizes, comparable to the gyration radius of polymer chains. However, the amount of nanotube in a polymer system is limited by the percolation threshold as, above this limit, the formation of a nanotubes network hinders the viscosity reduction effect. Based on these findings, we have used multiwalled carbon nanotubes MWCNT "aggregates" as viscosity reducers. Our results reveal both that the use of nanotube clusters reduce significantly the viscosity of the final system and strongly increase the nanotube limiting concentration for viscosity hindering. By using hydroxyl and carboxyl functionalized nanotubes, this effect has been rather maximized likely due to the hydrogen bridged stabilization of nanotube aggregates.

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.

Shape memory alloy (SMA) elements embedded in a structure allow to control the structural properties, thanks to their coupled thermo-mechanical behavior. In this study, the manufacturing and the shape control abilities of glass fiber/polyester resin SMA hybrid composites (SMAHCs) were experimentally investigated. Mechanical and calorimetric characterizations of SMA were carried out and correlated to the thermo-mechanical behavior of the SMAHC. The SMAHC was manufactured by the Vacuum Infusion Process that uses dry fibers and liquid matrix for the composite production. A polyester resin having a curing temperature lower than the SMA Austenite transition temperature was chosen as matrix, so a blocking system for the SMA wires was not needed during the cure. The thermo-mechanical behavior of the SMAHC was investigated by monitoring the shape change of the plate during heating of the embedded wires. The results give some indications on the appropriate characterization and choice of materials for SMAHC manufacturing.

Smoke is considered to be the main hazard of fires involving epoxy resins but its production depends on many variables, principally the chemical character and the burning rate of the polymer plus the availability of oxygen. The work reported aimed to study the smoke suppressant effect and flammability performance of zinc-based compounds (FR system) in epoxy matrix composites used in the aerospace and aeronautical industry. The flammability performance of neat and FR-loaded systems was screened using microcombustion calorimetry, while smoke generation, in terms of carbon monoxide (CO) and carbon dioxide (CO2) production, was analysed under dynamic conditions using cone calorimetry. Final results indicate that the dispersion of zinc borate and zinc hydroxystannate (ZHS) into epoxy matrices leads to a significant variation in flame retardant properties reducing both total heat release by about 25 and 30%, respectively, and heat release capacity by about 30 and 50%, respectively. The system containing ZHS shows an enhancement in all smoke suppressant properties; both tin compounds (zinc stannate (ZS) and ZHS) give a reduction of CO2/CO ratio from 41 to 25 for ZS and from 41 to 36 for ZHS compared to neat matrix. Copyright © 2010 Society of Chemical Industry

Flame behaviour is a fundamental requirement for advanced aerospace composites. In this work, a commercial, low-viscosity epoxy system, typically used in liquid infusion composite processes, and its mixtures with three different zinc-based flame retardants (ZB, ZS, ZHS) at different weight percentages has been investigated by cone calorimetry and thermogravimetric analysis. Cone calorimetry has been performed to verify the flame retardancy effects induced by each filler composition. Nevertheless manufacturability issues require the evaluation of the rheological changes induced by filler on the unloaded matrix system. Rheological tests have been, therefore, performed to identify the maximum concentration of filler. Based on these results thermogravimetric tests have been performed to investigate thermal degradation kinetics of selected systems. The feasibility of Kissinger and Flynn-Wall-Ozawa method for the determination of characteristic degradation kinetics parameters has been evaluated and results were analysed. A simplified decomposition model was assumed to analyse epoxy degradation behaviour; it was found that this model gives appreciable matching with experimental TGA curve trend for neat epoxy whereas for the filled compounds additional stages were assume to occur.

The aim of this paper is to investigate the ability of two different interferometric NDT techniques to detect and evaluate barely visible impact damage on composite laminates. The interferometric techniques allow to investigate large and complex structures. Electronic Speckle Pattern Interferometry (ESPI) works through real-time surface illumination by visible laser (i.e. 532 nm) and the range and the accuracy are related to the wavelength. While the ESPI works with the "classic" holographic configuration, that is reference beam and object beam, the Shearography uses the object image itself as reference: two object images are overlapped creating a shear image. This makes the method much less sensitive to external vibrations and noise but with one difference, it measures the first derivative of the displacement. In this work, different specimens at different impact energies have been investigated by means of both methods. The delaminated areas have been estimated and compared. © 2016 Author(s).

The autoclave process is the main and the most largely diffused technology for high quality polymer composite manufacturing. In autoclave, an heated and highly pressurized air flow allows the curing and the extremely effective compaction of the composite, achieving excellent mechanical properties of the composite. Nevertheless, the high exothermicity of the polymerization and the low thermal conductivity of the thermo-set matrix may increase the risk of the material failure during the manufacturing process. In this paper, the case of a thick composite cylinder that has shown some failures in the real manufacturing process is analysed. Fiber Bragg Gratings (FBG) optical sensors embedded in the composite cylinder during the manufacturing allowed the measurement of process induced strains for the validation of the numerical FEM model of the process, ad-hoc developed. The aim of the model is the evaluation of the risk of failure during cylinder manufacturing. The mechanical model takes into account geometry, stacking sequence, multistep manufacturing process, material thermo viscoelastic anisotropic behaviour and tool-composite interaction as well as heat transfer, cure kinetic and mechanical modelling of gelation during cure.

The mechanical response of reinforced composite structures under impact loads is particularly challenging owing to the rising of multiple and simultaneous failure phenomena. Indeed, low velocity impacts may produce intra-laminar damages, like fibre breakage and matrix cracking, and inter-laminar damages, such as delaminations and skin-stringer debonding. As already remarked, these failure phenomena often take place simultaneously, leading to a significant reduction in strength and stability of the composite components. In this paper, the behaviour of stiffened composite panels, with omega shaped stringers, under low velocity impacts is numerically investigated by means of non-linear explicit FEM analyses. Different impact energy levels are considered and correlation with experimental data is provided, in terms of impact force, displacement and energy. A sensitivity analysis has been performed to investigate the influence of numerical models’ approximations on the accuracy of the obtained numerical results. Models with an increasing level of damage simulation details have been adopted to study the effects of combined and separated intra-laminar and inter-laminar failures providing an interesting insight on the modelling requirements for an accurate simulation of the investigated phenomena.

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.

The consolidation of a polymer-based composite part is usually accompanied by the build-up of the stresses and deformations that are caused mainly by the polymer matrix shrinkage, the resin rheological and mechanical properties changes associated with the advancement of the cure reaction and by the material properties mismatch of the system constituents, mold, fibers reinforcement, and thermoset resin. These process-induced residual stresses and strains are obviously undesired phenomena that should be avoided leading to low performance, shape distortions, warpage, matrix cracks, and delaminations. Therefore, the optimization of the manufacturing cycle coupled to the capability to predict and measure the processing residual stress and strains represent important issues to be addressed. In this work, after a preliminary introduction on the most relevant mechanisms inducing the formation of stresses and strains in polymer-based composite parts, the most recent advances both in the modeling of the cu...

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.

In this chapter, we examine the techniques used for characterizing alloy properties. This discussion is useful for a better understanding of material behavior as well as for proper exploitation of the material properties for specific engineering applications and practical implementations. Proper determination of transformation temperatures by calorimetric analysis is the preliminary characterization test that has to be performed on the alloys. On the basis of calorimetric results, additional investigations, such as thermomechanical tests, can be set up. These tests are of fundamental importance for the use of alloys in structural or actuation devices, because—for any kind of application—a quantitative knowledge of material properties is required.

The aim of this paper is to investigate the ability of different NDT techniques to detect and evaluate barely visible and non-visible impact damage on composite laminates. Firstly, a conventional ultrasound technique was adopted to investigate the delamination in carbon fibre laminates after low velocity impact s. Then the results were compared with a thermographic and holographic analysis, as well as a theoretical simulation of the expected delamination. The results were compared and discussed. Overall a good agreement was found between the data obtained by the different techniques. Furthermore, the true values of the damage parameters were confirmed by DT performed on the samples. © 2015 IOP Publishing Ltd.

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

Polymer based composite materials have significant interest and find several engineering applications in many industrial sectors, such as automotive, aeronautic, civil, naval. In particular, composite materials with carbon fibres and polymeric thermoset matrices are good candidates to achieve the required weight savings for aerospace applications. To deliver product competitiveness, composite materials properties, the respective design and manufacturing process must be optimized in an integrated manner. The challenge for material and processing development is to provide the full benefits of composites at economically viable prices. Focus of current technical development is to find cheaper manufacturing methods with less emphasis on improving properties. Liquid molding dry fabric processes have the potential for being the most economic route for many applications. In addition, these processing techniques are flexible and enable the production of proper composite panels replicating delamination phenomena characteristics allowing a better understanding of composites damage mechanisms.

The cure kinetics of an epoxy–amine commercial thermoset system have been investigated with the isothermal differential scanning calorimetry technique. In particular, a kinetic study has been performed in the glass–transition zone, in which diffusion phenomena compete with the chemical transformations and the overall reaction rate is partially slowed by the reduced segmental chain mobility. A generalized form of the Vogel equation has been formulated to account for the effect of the increasing glass–transition temperature on the chain mobility and, therefore, on the overall reaction rate. The kinetic model has been expressed with two factors representing the chemical reaction rate and the segmental mobility reduction. As the main result, the activation energy relative to the diffusion phenomena has been found to be very low, having a value of 42.5 K ≈ 0.356 kJ/mol, which is compatible only with the small-angle rotation of the reactive unit. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3757–3770, 2002

In resin transfer molding (RTM) process, a polymer composite part is fabricated by injecting a thermoset resin into a fiber preform placed in a closed mold cavity. After the infiltration of the resin into the empty spaces in the mold, the manufacturing process is characterized by a curing reaction, which is an exothermic resin polymerization phenomenon that cross-links the resin and results in a solid structure. In most cases, the resin cure is initiated by heating the mold. The heat released during the reaction can cause temperature gradients in the composite, which leads to residual stresses in the part. Residual stresses are undesirable as they can cause shrinkage and warpage. By controlling the temperature of the mold walls, one can control the cure reaction and reduce the thermal gradients through the composite part. In this paper, we present a methodology based on scaling analysis of the energy balance equation to manage the heat generated by the cure reaction that minimizes the temperature gradients before the resin solidifies. The method capability is demonstrated with a highly reactive polyester resin infiltrated into different types of glass fiber preforms in a rectangular mold.

In this work, the application of Electronic Speckle Pattern Interferometry (ESPI) is employed for inspecting composite materials subject to thermal load. The composite samples are of interest in the aerospace industry. Through realtime surface illumination by visible laser (i.e. 532 nm), the ESPI technique allows the non-contact, non-destructive, full frame and real time detection of micro-deformations, micro-cracks, residual stress and delamination. The evaluation of the effective delaminated area of damaged polymer/Carbon Nanotubes fiber reinforced composites after flexural tests is presented. © 2014 IEEE.

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.

Summary An high resolution optical technique based on the film fiber optic refractometer has been used to investigate the effect of the thickness on the glass transition dynamic of Atactic Polystyrene films in the range 20–120 nm. The thermo-optic behavior of a set of layers dip coated onto the fiber has been analyzed upon cooling from long term equilibrated melt state. The glassy transition temperature is observed to be thickness-independent as did both the width and the lower and upper bounds of the transition itself. At lowering the thickness of the film the relevant effect is the reduction of the “contrast” between the melt equilibrium phase and the glassy state. A double layers model where a surface soft layer (3.9 nm at 418 K) do not contribute to the glass dynamic retaining liquid features even at temperature well below the glass transition, while the core layer shows a bulk glass transition, is able to quantitatively reproduce reflectance experimental data.