Vincenza Antonucci

A methodology to reduce thermal gradients due to the exothermic reactions in composites processing

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.

Tuning the insulator to conductor transition in a multiwalled carbon nanotubes/epoxy composite at substatistical percolation threshold

A fine tuning of the electrical conductivity from insulator to conductor behavior has been obtained for a multiwalled carbon nanotubes epoxy composite at a fixed substatistical percolation threshold content by varying the organization of the nanotubes network. A multiscale characterization has been carried out by transmission optical microscopy technique and small angle x-ray analysis that revealed a two level structure characterized by different topological arrangements for the micron sized clusters and nanosized isolated bundles, respectively. A picture of the multidimensional organization of the nanotubes network has been proposed to account for the observed transition modulation.

Impact damage investigation on composite laminates: comparison among different NDT methods and numerical simulation

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.

Thermal decomposition and fire behavior of glass fiber–reinforced polyester resin composites containing phosphate-based fire-retardant additives

The thermal degradation and the fire behavior of a polyester resin containing phosphate-based fire-retardant additives and its corresponding glass fiber composites were investigated. An unsaturated commercial polyester resin was modified by the addition of three phosphate-based fire retardants: ammonium polyphosphate, silane-coated ammonium polyphosphate, and melamine pyrophosphate, at 35% w/w. The effects of the fire retardants on resin thermal decomposition and small-scale fire behavior were studied using dynamic thermogravimetric tests at different heating rates and microcalorimetric measurements according to ASTM D7309-07. Different modes of degradation with different activation energy levels for the neat resin and the phosphate-loaded resins were identified by analyzing the thermogravimetric data through the Kissinger method. Since the ammonium polyphosphate-containing resin showed greater thermal and fire performance than the other systems, it was used to manufacture unidirectional glass fiber composites by a vacuum infusion process. The oxidative pyrolysis and fire behavior of the composites produced were studied using thermogravimetric and cone calorimeter tests that demonstrated improvement of their thermal stability and fire performance.

A new cost-saving vacuum infusion process for fiber-reinforced composites: Pulsed infusion

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.

Aggregates of Chemically Functionalized Multiwalled Carbon Nanotubes as Viscosity Reducers

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.

Fabrication and Thermo-Mechanical Characterization of a Shape Memory Alloy Hybrid Composite

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.

Thermomechanical Modelling and Experimental Testing of a Shape Memory Alloy Hybrid Composite Plate

Shape memory alloys (SMA) exhibit functional properties associated with the shape memory effect, responsible of the SMA shape recovery after a cycle of deforming-heating and of a simultaneous generation of mechanical work. Composite systems incorporating SMA wires have the ability to actively change shape and other structural characteristics. The functional properties of such adaptive composites are related to the martensitic transformation in the SMA elements and to the constraining behaviour that the composite matrix has on the SMA wires. In this work the behaviour of a shape memory alloy hybrid composite (SMAHC) is numerically and experimentally investigated. A plate was fabricated using prestrained SMA wires embedded in an epoxy resin pre preg glass fibres composite system. Upon calorimetric and mechanical material characterization, a finite element model was used in order to predict the structural behaviour of the SMAHC. In the experimental tests, the plate was clamped at one side and actuated via electrical heating. Temperature and displacement data were collected and compared with the prediction of the finite element model. The results show that the model is able to capture the shape change in the actuation region, although a thorough description of the SMAHC behaviour requires further modelling work, including the simulation of the SMA loading history during composite manufacturing.

Investigation of the Effective Reinforcement Modulus of Carbon Nanotubes in an Epoxy Matrix

Polymer nano-composite matrix could be the ideal solution for a new generation of composite materials. The continuous demand for new high performance polymer composite for various applications, in different industrial sectors, has lead many researchers to investigate the potential use of the carbon nanotubes (CNTs) as nano-reinforcements of polymer matrix for the manufacturing of traditional laminate composite. CNTs have attracted considerable attention due to their unique mechanical, surface, multifunctional properties and strong interactions with the hosting matrix mainly associated to their nano-scale features. CNTs’ impressive mechanical properties, with stiffness and strength values falling within the range of 100-1000 GPa and 2.5-3.5 GPa, respectively, make them ideal candidates to develop novel composites characterized by advanced polymer matrices (Treacy, 1996). Despite the enormous amount of experimental data available in literature (Coleman et al. 2006, Thosterston et al., 2003), there are still controversial results concerning elastic property, strength and fracture toughness; moreover, due to inherent difficulties in processing these unconventional nanostructure as nano-fillers in polymer system, a reliable theoretical correlation of the experimental data is still in shadow. Different approaches to build an appropriate theory for predicting reinforcement efficiency of CNTs within an hosting matrix have been presented in the literature.Indeed, the reinforcement capability of carbon nanotubes in a polymeric matrix will depend on their amount, but, undoubtedly, their arrangement within the hosting medium plays a fundamental role in the load transfer mechanism. For this reason, the state and level of dispersion need to be accounted in any attempt for predicting the mechanical behaviour of the final nano-composite system. In literature the enhancing reinforcement of CNT loading for the Young’s modulus is commonly reported. However, at the same time, discrepancy among the different data is highlighted. Therefore, an important issue for modelling purpose is the lack of a reliable database for this property. Characterization and structure-properties of nano-mechanics modelling research have shown that enhancement in mechanical properties of nano-composites are strongly

Effect of the Loading History on Shape Memory Alloy Transformation Temperatures

Due to the complex microstructure and phase transformations taking place in Shape Memory Alloys (SMA), the behaviour and properties of these materials are deeply influenced by their loading history in terms of stress, strain and temperature. Indeed, a thorough understanding of the effects of the loading history on SMA properties is fundamental for a correct modelling and design of SMA applications, in particular in the case of complex loading. An example are shape memory alloys embedded in composite systems, in which stress, strain and temperature vary simultaneously depending upon the properties of the SMA and composite. This work presents the first results of an experimental investigation on the effects of the loading history on SMA transformation temperatures. Nitinol wires of untrained material were considered. Specimens consisting of Martensite and R-phase were subjected to different loading histories, keeping in turn stress, strain or temperature at a constant value. Transformation temperatures at zero stress of these samples were measured via Differential Scanning Calorimetry. Contrarily to most constitutive models assumptions, the DSC results highlight a dependence of the transformation temperatures on the loading history, influencing in particular the Martensite to Austenite phase transformation.