Search

In this work, it is presented the application of the Electronic Speckle Pattern Interferometry (ESPI) application in the analysis of composites, used in the aeronautic field. Through realtime surface illumination by visible laser (ie 532 nm), the ESPI technique allows the noncontact, non-destructive detection of micro-deformations, micro-cracks, residual stress and delamination. The method is based on recording the surface field differential displacement, due to thermal or mechanical strains. A CCD camera records the whole field deformation into image to be processed. By this technique, it is possible to reveal hidden defects and to evaluate the effective delamination area due, for example, to impact damages. in this study, ESPI technique has been used to evaluate the effective delamination area of damaged Epoxy-Carbon Fibers Composites that have been manufactured by pulsed infusion.

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.

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.

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 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.

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.

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.

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.

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

Carbon fibre composites were subjected to low velocity impact tests. The scope was to investigate the response of laminates fabricated by a new vacuum assisted technology, labelled as “pulse infusion”, under dynamic loads. At this aim, experimental tests up to complete penetration and at a different energy levels, were carried out. Some of the specimens were destined to CAI tests and the residual strength was evaluated. All the parameters involved in the phenomenon, like penetration energy and indentation depths, were studied to validate existing semi empirical models. By the comparison with results from literature, good agreements were found denoting the efficiency of the new fabrication method.

Ultra sound technique (US), thermography and holography were adopted to investigate the delamination in carbon fibre laminates after low velocity impact loads. The scope was to investigate the ability of new tools in giving information about internal damage on composites. The carbon laminates were fabricated by a new vacuum assisted technology, labelled as “pulsed infusion”. The efficiency of this innovative fabrication technology was studied too, comparing the delamination extensions with the measurements from literature on impacted autoclave cured laminates.

Epoxy/Kevlar composites have been manufactured by conventional Vacuum Infusion process and the Pulse Infusion technique. 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 realized composite panel have been mechanically characterized by performing tensile and short beam shear tests according with the ASTM D3039 and ASTM D2344/D 2344M standard respectively in order to investigate the effect of Pulse Infusion on the tensile strength and ILSS. © 2014 AIP Publishing LLC.

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.

Composites of carbon nanotubes (CNT) in polymeric matrices have attracted considerable attention in the research communities due to their good electrical conductivity, high stiffness and high strength at relatively low CNT contents. Effective utilization of CNT in composites depends primarily on the ability to disperse them homogeneously throughout the polymer matrix, avoiding the formation of bundles due to van der Waals interactions existing between the nanotubes. In this work composites of polystyrene at various percentages of SWNT were fabricated using Latex Technology technique, a polymer type-independent method based on using a surfactant as a dispersing agent. An electrical characterization of SWNT composites was performed both in DC and AC modes. From the analysis of DC data a percolative behavior was found for the conductivity as function of SWNT content. The innovative contribution of this work consists in the modeling of the composite material upon its electrical properties. AC measurements and the analysis of impedance as function of angular frequency lead to the formulation of an equivalent circuit able to model the composite material in correspondence of the percolative threshold. Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

A simulation of the non-isothermal resin transfer molding manufacturing process accounting for both the filling and the consolidation stage has been developed. The flow of an exothermally reactive resin through a porous medium has been analyzed with reference to the Darcy law, allowing for the chemorheological properties of the reacting resin. Thermal profile calculations have been extended to a three phase domain, namely the mold, the dry preform and the filled preform. The mold has been included in order to evaluate the thermal inertial effects. The energy balance equation includes the reaction term together with the conductive and convective terms, and particular attention has been devoted to setting the thermal boundary condition at the flow front surface. The moving boundary condition has been derived by a jump equation. The simulation performance has been tested by comparing the predicted temperature profiles with experimental data from literature. Further numerical analysis assessed the relevance of using the jump equation at the flow front position for both filling time and thermal profile determination.