Impact behavior of sandwich structures made of flax/epoxy face sheets and agglomerated cork

The unremitting quest of natural and renewable materials able to replace their synthetic counterparts in high-performance applications has involved also sandwich structures. In this regard, the aim of this work is to characterize the impact response, in both high- and low-velocity conditions, of green sandwich structures made of agglomerated cork as core and flax/epoxy laminates as face sheets. Both bare cork, flax skins, and complete sandwich structures were subjected to impacts at three different energy levels representing the 25%, 50%, and 75% of the respective perforation thresholds. A gas gun was instead used to assess the high-velocity impact behavior of these green sandwich structures and evaluate their ballistic limit. This study shows that the buckling of cell walls of agglomerated cork enables to tailor the damage extension through-the-thickness in low-velocity impacts compared to traditional synthetic foams coupled with a considerable amount of energy absorption

Analysis of damage in composite laminates with embedded piezoelectric patches subjected to bending action

Smart materials offer the possibility of the integration of active functions into a composite primary structure. This work aim to investigate the impact of the incorporation of smart materials in a conventional composite laminate. Fiberglass specimens with and without PZT piezo elements embedded in the lamination stack are manufactured and then subjected to a four-point bending test. Together with the classical mechanical parameters, the operational behaviour of the piezo as sensor is monitored in terms of electrical capacity during test. Benefits and disadvantages of the embedment of a piezo element in a laminate are evidenced from a comparison of the experimental results. In parallel, numerical models of the experimental setup and test, with a finite element approach, allows to explore the mechanisms of damage of such kind of sensorized specimens

Evaluation of a new green composite solution for wind turbine blades

The wind energy market requires reliable wind turbines with a long and efficient working life, able to generate energy without interruption, at the lowest investment and operating cost. The current material systems used for making wind turbine blades are in majority based on glass fibres and epoxy resins. These thermoset polymer composites with synthetic fibres have proved to be technologically mature and easy to work with during the manufacturing steps, with highly fluid resin that adheres well to the composites reinforcement fibres. However, glass fibre reinforced plastics show shortcomings, such as relatively high fibre density (approximately 40–50% higher than natural fibres), difficulty to be machined and limited recycling options, not to mention the potential health hazards posed by glass fibre particulates. Among its objectives, the SoftWind project should evaluate new green composite solutions that will lead to the design of recyclable and repairable blades with higher mechanical strength and lower weight than blades made of standard materials. Initially, to familiarise with natural fibres, specimens made of hemp fibres embedded in a vinylester matrix were tested in tension, flexure and impact, and compared with traditional glass fibres with different resins to validate their performance characteristics. Hemp fibres are good candidates for this use, since they offer a sufficient compatibility with technical matrices used in impact-resistant applications. This work validates on hemp/vinylester composite plates an analytical model previously introduced in the literature for synthetic composites when subjected to low-velocity/large-mass impacts. The validation is performed by comparison between the derived analytical load curves and the experimental ones. Moreover, in view of the final scope of modelling the behaviour of a full-scale blade under workloads, the obtained mechanical properties are also used to reproduce numerically, through a finite element code, the damage mechanisms of such bio-based composites

Quasi-static and dynamic response of oriented strand boards based on balsa wood waste

This work presents an evaluation of the performance of Oriented Strand Boards (OSB) panels based on balsa wood (Ochroma Pyramidale) waste agglomerated with castor oil polyurethane resin. In this study, were evaluated OSB panels with different densities (300 kg/m3, 400 kg/m3 and 650 kg/m3), with 10mm thickness and castor oil polyurethane resin in different contents (11% and 15%). The OSB panels were preliminary characterized by physical and quasi-static mechanical tests to identify the class of application of this material according to the recommendations of standard EN 300: 2002. Subsequently, the OSB panels were characterized by low velocity impact tests. Panels with the highest density outperformed those with the lowest one in terms of peak force and perforation energy (Ep=22.88 J). Both properties are clearly influenced by the better compaction of the particles, as confirmed by the higher value of internal adhesion (0.46 MPa), which resulted also in better residual flexural properties after impact, with a reduction in strength of 36% for the samples with 650 kg/m3 compared to about 70% for the samples with the lowest density at an impact energy level equal to 50% of the respective perforation energy

High temperature oxidation resistance of modified MCrAlY coatings for thermal barrier systems

MCrAlY coatings obtained by thermal spray techniques are widely used as bond coats in thermal barrier systems to protect the substrate from corrosion and high temperature oxidation and to guarantee the coupling between ceramic top coat and substrate. In this work, the high temperature oxidation resistance of MCrAlY coatings with modified compositions was evaluated; in particular the effect of reactive and refractory elements (Ta, Re, Hf) addition was studied. The investigated MCrAlY coatings were obtained by HVOF and plasma spray (VPS) techniques; the samples were exposed in air at 1150°C up to 100 hours and the oxidation kinetics were evaluated by measuring the thickness of TGO (Thermally Grown Oxide) scale at the several exposure times. Microstructural evolution induced by the heat treatments was studied by means of SEM, EDS and XRD analyses: kinetics data verified that the oxidation resistance of MCrAlY coatings is strictly related to the amount of the reactive and refractory elements in the starting powders and that a thorough understanding of microstructure modifications is essential in controlling the TGO growing. Moreover the effects of an innovative surface treatment were evaluated: selected MCrAlY coatings were protected with a sputtered ceramic film deposited by PVD technique; the system oxidation resistance was assessed and compared to that of unprotected coatings

Application of DIC to Static and Dynamic Testing of Agglomerated Cork Material

In this work, experimental compression tests have been performed on parallelepiped specimens cut from an agglomerated cork slab. The tests have been performed both using a quasi-static testing machine and a polymeric Split Hopkinson Bar, in order to assess the sensitivity of the material to the strain rate. A standard and a high-speed digital camera have been used to collect frames of the samples during the tests. 2D DIC analyses have been conducted on the pictures of lateral faces of the specimens in order to evaluate the actual strain distributions, which showed a significant heterogeneity within each sample. Moreover, the DIC analyses on the dynamic tests have been used for evaluating the local accelerations and to compute the inertia stresses. The latter may affect the global response that can be measured by following the standard Hopkinson bar procedures, and are responsible for the fluctuations in the force histories observed in the tests at highest strain rates

Feasibility of the use of agglomerated cork as the core of sandwich structures subject to impact

Proceedings of: XIII Congreso Nacional de Materiales Compuestos (MATCOMP'19), 3-5 July 2019, Vigo, España.En la fabricación de estructuras sándwich de material compuesto, unas de las opciones más utilizadas como núcleo es el uso de espumas poliméricas. Debido al cada vez mayor esfuerzo en reducir el impacto medioambiental de los procesos industriales, existe un gran interés en incorporar materiales de origen natural que permitan disponer de fuentes renovables y que faciliten los procesos de reutilización y de reciclado. Una de las posibilidades para sustituir a las espumas poliméricas es el corcho aglomerado que presenta unas buenas propiedades mecánicas. No obstante, para que sea posible su uso como núcleo en estructuras sandwich es necesario conocer su comportamiento frente a cargas de impacto, tanto de baja como de alta velocidad. En este trabajo se compara el comportamiento de una espuma de PVC comercial con corchos aglomerados de diferentes densidades. Se ha analizado la fuerza, desplazamiento, energía absorbida y velocidad de perforación. Se ha observado que el comportamiento del corcho aglomerado puede ser similar al de una espuma polimérica convencional, aunque con un incremento de peso. Adicionalmente se ha puesto de manifiesto que el corcho aglomerado puede ser una alternativa mejor a las espumas poliméricas en aquellas aplicaciones que puedan verse sometidas a varios impactos sucesivos.Los autores agradecen al Ministerio de Economía y Finanzas de España por la financiación del proyecto DPI2017-86324-R

Carbon-phenolic ablative materials for re-entry space vehicles: manufacturing, properties and plasma wind tunnel test

Thermal Protection Systems (TPS) are designed to protect re-entry space vehicles from the severe heating encountered during hypersonic flight through a planet’s or the earth’s atmosphere. A carbon-phenolic ablative TPS was developed, manufactured and tested with the aim of fulfilling the thermal and mechanical requirements corresponding to the actual loads experienced by a vehicle during a moon-earth re-entry. Experimental activities were carried out on two different composite systems (a resole resin coupled with a graphitic felt and a graphitic foam), and were aimed to the optimization of the manufacturing procedure and to the characterization of the mechanical behaviour and of the insulation performance of the fabricated composites. On the basis of the preliminary results, the selected ablators were tested in a Plasma Wind Tunnel (Scirocco, CIRA, Italy)