M. Zarrelli

Investigation of cure induced shrinkage in unreinforced epoxy resin

Abstract Changes in volume and thermal expansion coefficient have been investigated during the cure of a high temperature curing epoxy resin containing a thermoplastic modifier. The measurements were carried out using a combination of standard and novel thermoanalytical techniques. It is shown that the chemical shrinkage of the curing resin is a linear function of the degree of cure, whereas the coefficient of thermal expansion depends on the temperature and on the degree of cure. This experimental information is translated to an incremental model that simulates the volumetric changes occurring as the resin follows a programmed thermal profile. Such a model can serve as a density submodel in simulating heat transfer or residual stress development in composites during the manufacturing process.

A Global–Local Numerical Model for the Prediction of Impact Induced Damage in Composite Laminates

Delamination and other damage mechanisms, such as matrix cracks, fibre-matrix debonding and fiber failure can appear as a consequence of impact events with foreign objects under in service conditions and maintenance operations. These phenomena are seldom analyzed together without discussing how the interferences between the different damage mechanisms can influence their evolution under different loading conditions. The present work is focused on the development of a specific numerical procedure, able to take into account the failure modes interaction in composite laminated structures subject to a low velocity impact. As a matter of fact, a very fine mesh refinement is required to correctly evaluate the stress state where the impact induced damage onsets. Hence, in order to reduce the computational cost without compromising the accuracy of results, a global/local approach, characterized by a very refined mesh in the critical impact region interacting with a coarser mesh in the rest of the geometrical domain, has been implemented in the FE model. In the present work, Multi-Point-Constraints (MPC) has been used to link the refined local domain to the coarse global domain without using transition meshes. The implementation and the analyses have been performed in the ABAQUS® FE environment.

C-4 Gem-Dimethylated Oleanes of Gymnema sylvestre and Their Pharmacological Activities

Gymnema sylvestre R. Br., one of the most important medicinal plants of the Asclepiadaceae family, is a herb distributed throughout the World, predominantly in tropical countries. The plant, widely used for the treatment of diabetes and as a diuretic in Indian proprietary medicines, possesses beneficial digestive, anti-inflammatory, hypoglycemic and anti-helmentic effects. Furthermore, it is believed to be useful in the treatment of dyspepsia, constipation, jaundice, hemorrhoids, cardiopathy, asthma, bronchitis and leucoderma. A literature survey revealed that some other notable pharmacological activities of the plant such as anti-obesity, hypolipidemic, antimicrobial, free radical scavenging and anti-inflammatory properties have been proven too. This paper aims to summarize the chemical and pharmacological reports on a large group of C-4 gem-dimethylated pentacyclic triterpenoids from Gymnema sylvestre.

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.

Enthalpy relaxation of an epoxy matrix/carbon nanotubes

The enthalpy recovery of an Epoxy/Carbon Nanotube systems (E/CNT) manufactured at low nanotube content, have been investigated by using Differential Scanning Calorimetry. Two different nanotube concentration were considered, 0.5 and 0.1 w/w and results compared with the behaviour of the neat epoxy material. Final results reveal that the aging phenomenon is not monotone with the nanofiller content but a significant “depression” of the physical aging seems be highlighted by the lowest nanotube content, i.e. 0.5 w/w. This observation has not been rationalised yet, although it seems reasonable to correlate this later effect with the topological feature of nanotube network within the hosting epoxy. Future work will verify this hypothesis

A Linear Numerical Approach to Simulate the Delamination Growth Initiation in Stiffened Composite Panels

In this article, a simplified linear analysis-based approach to simulate the delamination growth initiation in stiffened composite panels, suitable as preliminary design and optimization tool implemented into a finite element code, is presented. The proposed approach is based on the determination of the delamination buckling and on the evaluation of the energy released during the delamination propagation by means of eigenvalue and linear static analyses. Stiffened composite panels with circular embedded bay delaminations, under compression loads, were adopted as a benchmark to test the simulation capabilities of the method. Obtained results, in terms of delamination growth initiation load and energy release rate distributions along the delamination front, have been compared to nonlinear results obtained by the virtual crack closure technique and experimental data for preliminary validation purposes. Comments and considerations upon the applicability of this methodology are, finally, provided with particular focus on delamination sizes and locations within the considered structural elements.

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.

Three-dimensional modeling of composites fire behavior

The present paper introduces a numerical study on the fire behavior of composites during exposure to a heating source at high-incident power. A three-dimensional novel numerical model is proposed, which is able to simulate the behavior of composite materials in fire environment providing the composites mass loss rate, heat release rate, and total heat released during the heating source application. The ANSYS commercial FEM software has been selected as the platform for the implementation of the proposed numerical model. The use of the ANSYS Parametric Design Language has allowed the ANSYS FEM code to numerically simulate, by a stop–restart incremental procedure, all the most relevant physical phenomena related to fire. As an application, a cone calorimeter experiment over a laminated composite plate has been numerically simulated, and the numerical model has been validated by comparing the ANSYS numerical results to experimental literature data in terms of temperature profile over the panel thickness, mass loss rate, heat release rate, and total heat released. An excellent agreement has been found between the obtained numerical results and the experimental test, confirming the validity of the proposed three-dimensional tool, taking into account the specimen edge effects, which has perspectives of application to the assessment of the fire performance of complex composite structural components.

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

Insight on mendable resin made by combining Diels-Alder epoxy adducts with DGEBA

Formation of micro-cracks is a critical problem in polymers and polymer composites during their service in structural applications. In this context, materials endowed with self-healing features would lead to the next polymers generation. In the present paper, an epoxy system integrating Diels-Alder epoxy adducts is investigated by thermal and spectroscopic analysis. The direct and retro D-A reaction have been studied by FTIR and specific absorption bands have been identified. Finally, mechanical tests have been performed on the system. The polymer is able to heal fracture and micro-cracks recovering its stiffness after a thermal treatment.