Aromatic hyperbranched polyester/RTM6 epoxy resin for EXTREME dynamic loading aeronautical applications

The effects of the addition of an aromatic hyperbranched polyester (AHBP) on thermal, mechanical, and fracture toughness properties of a thermosetting resin system were investigated. AHBP filler, synthesized by using a bulk poly-condensation reaction, reveals a glassy state at room temperature. Indeed, according to differential scanning calorimetry measurements, the glass transition temperature (Tg) of AHBP is 95 °C. Three different adduct weight percentages were employed to manufacture the AHBP/epoxy samples, respectively, 0.1, 1, and 5 wt%. Dynamical Mechanical Analysis tests revealed that the addition of AHBP induces a negligible variation in terms of conservative modulus, whereas a slight Tg reduction of about 4 °C was observed at 5 wt% of filler content. Fracture toughness results showed an improvement of both critical stress intensity factor (+18%) and critical strain energy release rate (+83%) by adding 5 wt% of AHBP compared to the neat epoxy matrix. Static and dynamic compression tests covering strain rates ranging from 0.0008 to 1000 s−1 revealed a pronounced strain rate sensitivity for all AHBP/epoxy systems. The AHBP composites all showed an increase of the true peak yield compressive strength with the best improvement associated with the sample with 0.1 wt% of AHBP

Fracture Toughening Mechanisms in Epoxy Adhesives

Fracture toughness is generally considered as the main properties of a polymer or a polymer adhesive system for measuring the material resistance to the extension of cracks. Epoxy adhesives are generally brittle in nature; however, the addition of a second dispersed phase could induce a remarkable increase of damage tolerance performance by an enhancement of the material fracture toughness. The fracture behavior of a filled epoxy resin is strongly affected by the dimensions, the shape, and the chemical nature of the considered filler. The chapter describes the different toughening mechanisms for polymer adhesives with special attention toward innovative nanofiller such as graphene nanoplatelets and hyperbranched polymer nanoparticles

Effect of silica nanoparticles on the compressive behavior of RTM6 epoxy resin at different strain rates

RTM6 epoxy resin is considered the main matrix material for high performance aeronautical grade composites which can be subjected to impact loads. Research efforts have been spent to improve the strength and the overall impact properties of these resins. One of the routes followed focuses on the enhancement of the epoxy resin by the addition of rigid nanoparticles with different weight percentages and sizes. The purpose of this work is to study the compressive behavior of RTM6 epoxy resin filled with silica nanoparticles at different strain rates. Two types of epoxy resins, namely neat epoxy resin and epoxy resin filled with silica nanoparticles at 0.1% weight percentage were tested. Quasi-static experiments were performed at strain rates of 0.008, 0.08, and 0.8 /s , while high strain rate experiments were performed at strain rates approx. 1000 /s using the split Hopkinson pressure bar technique. The behavior of both the nanoparticle filled resin and neat resin is compared and discussed. The effect of the silica nanoparticles on the compressive yield strength is also presented

Stiffened panels damage tolerance determination using an optimization procedure based on a linear delamination growth approach

The damage tolerance of delaminated composite panels under compressive load is usually numerically evaluated by means of computationally expensive non-linear approaches. In this study, an alternative numerical linear approach, able to mimic the delamination propagation initiation, is proposed. With the aim to exploit its benefits, in terms of computational costs reduction, the proposed linear methodology has been used in this study in conjunction with an optimization analysis to assess the damage tolerance of stiffened composite panels with an impact induced delamination under compression. Indeed, the optimization was aimed to find the minimum delamination growth initiation load for a delaminated stiffened panel with variable delamination size and position, providing indications on the damage tolerance capability of the stiffened panel with an arbitrary positioned and sized delamination induced (as an example) by a low energy impact

Cyclic moisture sorption and its effects on the thermomechanical properties of epoxy and epoxy/MWCNT nanocomposite

Funding Information: This research was funded by ERDF project No. 1.1.1.2/VIAA/1/16/066 for the support of the post-doctoral research project "Environmental effects on physical properties of smart composites and fiber-reinforced plastics modified by carbonaceous nanofillers for structural applications". Publisher Copyright: © 2019 by the authors.The aim of this work was to reveal the moisture absorption-desorption-resorption characteristics of epoxy and epoxy-based nanocomposites filled with different multiwall carbon nanotubes (MWCNTs) by investigating the reversibility of the moisture effect on their thermomechanical properties. Two types of MWCNTs with average diameters of 9.5 and 140 nm were used. For the neat epoxy and nanocomposite samples, the moisture absorption and resorption tests were performed in atmospheres with 47%, 73%, and 91% relative humidity at room temperature. Dynamic mechanical analysis was employed to evaluate the hygrothermal ageing effect for unconditioned and environmentally "aged" samples. It was found that moisture sorption was not fully reversible, and the extent of the irreversibility on thermomechanical properties was different for the epoxy and the nanocomposite. The addition of both types of MWCNTs to the epoxy resin reduced sorption characteristics for all sorption tests, improved the hygrothermal and reduced the swelling rate after the moisture absorption-desorption.publishersversionPeer reviewe

Mechanical properties of 3-D printed truss-like lattice biopolymer non-stochastic structures for sandwich panels with natural fibre composite skins

A full mechanical characterisation of three types of 3-D printed lattice cores was performed to evaluate the feasibility of using additive manufacturing (AM) of lightweight polymer-based sandwich panels for structural applications. Effects of the shape of three selected lattice structures on the compression, shear and bending strength has been experimentally investigated. The specimens tested were manufactured with an open source fused filament fabrication-based 3-D printer. These sandwich structures considered had skins made of polypropylene (PP)-flax bonded to the polylactic acid (PLA) lattice structure core using bi-component epoxy adhesive. The PP-flax and the PLA core structures were tested separately as well as bonded together to evaluate the structural performance as sandwich panels. The compression tests were carried out to assess the in-plane and out of plane stiffness and strength by selecting a representative number of cells. Shear band and plastic hinges were observed during the in-plane tests. The shear and three-point bending tests were performed according to the standard to ensure repeatability. The work has provided an insight into the failure modes of the different shapes, and the force-displacement history curves were linked to the progressive failure mechanisms experienced by the structures. Overall, the results of the three truss-like lattice biopolymer non-stochastic structures investigated have indicated that they are well suited to be used for potential impact applications because of their high-shear and out of the plane compression strength. These results demonstrate the feasibility of AM technology in manufacturing of lightweight polymer-based sandwich panels for potential structural uses.Peer reviewe

Hydrothermal Aging of an Epoxy Resin Filled with Carbon Nanofillers

The effects of temperature and moisture on flexural and thermomechanical properties of neat and filled epoxy with both multiwall carbon nanotubes (CNT), carbon nanofibers (CNF), and their hybrid components were investigated. Two regimes of environmental aging were applied: Water absorption at 70 °C until equilibrium moisture content and thermal heating at 70 °C for the same time period. Three-point bending and dynamic mechanical tests were carried out for all samples before and after conditioning. The property prediction model (PPM) was successfully applied for the prediction of the modulus of elasticity in bending of manufactured specimens subjected to both water absorption and thermal aging. It was experimentally confirmed that, due to addition of carbon nanofillers to the epoxy resin, the sorption, flexural, and thermomechanical characteristics were slightly improved compared to the neat system. Considering experimental and theoretical results, most of the epoxy composites filled with hybrid carbon nanofiller revealed the lowest effect of temperature and moisture on material properties, along with the lowest sorption characteristics

Assessment of Recycled PLA-Based Filament for 3D Printing

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