Self-Healing Nanofiber-Reinforced Polymer Composites. 1. Tensile Testing and Recovery of Mechanical Properties.

Abstract: The present work aims at development of self-healing materials capable of partially restoring their mechanical properties under the conditions of prolonged periodic loading and unloading, which is characteristic, for example, of aerospace applications. Composite materials used in these and many other applications frequently reveal multiple defects stemming from their original inhomogeneity, which facilitates microcracking and delamination at ply interfaces. Self-healing nanofiber mats may effectively prevent such damage without compromising material integrity. Two types of core-shell nanofibers were simultaneously electrospun onto the same substrate in order to form a mutually entangled mat. The first type of core-shell fibers consisted of resin monomer (dimethylsiloxane) within the core and polyacrylonitrile within the shell. The second type of core-shell nanofibers consisted of cure (dimethyl-methyl hydrogen-siloxane) within the core and polyacrylonitrile within the shell. These mutually entangled nanofiber mats were used for tensile testing, and they were also encased in polydimethylsiloxane to form composites that were also subsequently subjected to tensile testing. During tensile tests, the nanofibers can be damaged in stretching up to the plastic regime of deformation. Then, the resin monomer and cure was released from the cores and the polydimethylsiloxane resin was polymerized, which might be expected to result in the self-healing properties of these materials. To reveal and evaluate the self-healing properties of the polyacrylonitrile-resin-cure nanofiber mats and their composites, the results were compared to the tensile test results of the monolithic polyacrylonitrile nanofiber mats or composites formed by encasing polyacrylonitrile nanofibers in a polydimethylsiloxane matrix. The latter do not possess self-healing properties, and indeed, do not recover their mechanical characteristics, in contrast to the polyacrylonitrile-resin-cure nanofiber mats and the composites reinforced by such mats. This is the first work, to the best of our knowledge, where self-healing nanofibers and composites based on them were developed, tested, and revealed restoration of mechanical properties (stiffness) in a 24 h rest period at room temperature.

Pub.: 19 Aug '15, Pinned: 13 Apr '17

Comparative assessment of the fire structural performance of carbon-epoxy composite and aluminium alloy used In aerospace structures

Abstract: An experimental investigation is presented comparing the fire structural survivability of two important types of aerospace materials: carbon fibre-epoxy composite and aluminium alloy. The thermal response, softening rate, deformation behaviour and structural survivability for the two materials are compared when exposed to a thermal flux. Simulated fire structural tests are performed on quasi-isotropic carbon-epoxy composite and aluminium alloy (AA2024-T3) involving one-sided unsteady-state heating and constant tension loading. When exposed to the same radiant thermal flux, the surface and internal temperatures of the composite and aluminium alloy were different due to differences in their thermal conductivity as well as fire-induced damage (e.g. delamination cracks, fibre-matrix debonding) and decomposition to the composite. Under tensile loading, the aluminium deformed when exposed to fire via elastic, plastic and creep-induced softening processes whereas the composite deformed at a much slower rate due to matrix softening and decomposition. For the experimental test conditions, the tensile load-bearing capacity of the composite was superior to the aluminium alloy when exposed to the thermal flux, and this was due to the capacity of the load-bearing carbon fibres to retain high tensile stiffness and strength to much higher temperatures.

Pub.: 07 Jul '16, Pinned: 13 Apr '17

Thermal degradation analysis of innovative PEKK-based carbon composites for high-temperature aeronautical components

Abstract: Nowadays, composite materials find a large application in several engineering fields, spanning from automotive to aerospace sectors. In the latter, especially in aircraft civil transportation, severe fireproof requirements must be accomplished, taking into account that the second most frequent cause of fatal accidents involving airplanes, was the post-impact fire/smoke, as reported by the European Aviation Safety Agency (EASA) in 2014. In the light of this, experimental research is of crucial importance in the understanding thermal behavior of composites for aircraft components, when exposed to high-temperature and fire conditions. In this context, a thermal degradation study is carried out for two carbon-reinforced resins: the well known thermosetting phenolic and a thermoplastic polyether–ketone–ketone (PEKK), recently developed specifically for this kind of application. The aim is to evaluate the PEKK behavior and to understand the impact of composite nature in terms of structural strength under fire. To this end, thermogravimetric analyses were performed for three different non-isothermal heating programs, between 30 and 1000 °C. Under inert atmosphere one single global reaction is observed for carbon-PEKK between 500–700 °C, while two for carbon-phenolic, whose pyrolysis begins around 200 °C. This better PEKK strengthening is attributed to the ether and ketone bonds between the three aromatic groups of the monomer. As expected, under oxidative atmosphere, the kinetic process becomes more complex, making more difficult the detecting of single-step reactions, especially for carbon-phenolic. Nevertheless, the oxidative process of carbon-PEKK seems to be driven by three consecutive global reactions. The activation energy is estimated by means of both integral (Starink) and differential (Friedman) isoconversional methods, as a function of the extent of conversion, corresponding to the identified reaction intervals. For carbon-PEKK in inert conditions, with Starink a mean value of 207.71 ± 6.57 kJ/mol was estimated, while 213.88 ± 20.04 kJ/mol with Friedman. This expected slight difference depends on the nature of the considered mathematical approaches. The difficulty in activation energy estimation for polymeric materials prefers the use of at least two different methods, allowing for the identification of an activation energy range, for a resin of which no data are available in the literature. For the decomposition model evaluation, the so-called compensation effect method was implemented, as well as the single-step-based approach proposed by Friedman. The evaluation of a possible decomposition expression has been achieved only for carbon-PEKK under inert conditions, since the considered methods are valid and applicable only for well defined single-step reactions. In fact, the three reactions of the oxidative case cannot be considered as single-step processes. Moreover, the higher difference in the estimated activation energy between Starink and Friedman suggests to check the achieved results by implementing further isoconversional methods, to understand the most reliable for polymer-based carbon composites degradation analysis under oxidative atmosphere. However, the observed higher thermal performance of PEKK resin, attributed to its chemical structure, increases the interest toward its use as matrix for aerospace composite materials, that can be subjected to hazardous environments.

Pub.: 22 Feb '17, Pinned: 13 Apr '17

Cost-weight trades for modular composite structures

Abstract: A design approach for airframe structures is formulated based on the concept of modularity allowing trade-offs and optimization between cost and weight. A modular structure can be created by replacing a collection of parts which all have a unique design by a collection of parts where the same design repeats multiple times. Structures with high levels of modularity have higher weight since it is harder to design a weight-efficient structure when the amount of design options is limited, but this weight increase might be worth the associated decrease in manufacturing cost. In modular design, cost reductions are achieved through learning curve effects and through reduction of the non-recurring cost, for example, due to lower tooling costs. Based on dynamic programming, an approach to determine the optimum number of repeating designs was determined and applied to a composite fuselage structure. Two examples are given where the cost-weight efficiency at different modularity levels is assessed for a composite airframe: the stringers and the frames in a fuselage. The corresponding cost-weight diagrams indicated that the modularity concept provides a useful methodology for designing more cost- weight efficient structures. In both cases it was possible to replace a large amount of designs and increase the level of modularity of the structure, yielding significant reductions in recurring and non-recurring manufacturing costs while keeping the associated weight increase of the structure to a minimum.

Pub.: 29 Nov '13, Pinned: 12 Apr '17

A critical review of nanotechnologies for composite aerospace structures

Abstract: The past decade extensive efforts have been invested in understanding the nano-scale and revealing the capabilities offered by nanotechnology products to structural materials. Integration of nano-particles into fiber composites concludes to multi-scale reinforced composites and has opened a new wide range of multi-functional materials in industry. In this direction, a variety of carbon based nano-fillers has been proposed and employed, individually or in combination in hybrid forms, to approach the desired performance. Nevertheless, a major issue faced lately more seriously due to the interest of industry is on how to incorporate these nano-species into the final composite structure through existing manufacturing processes and infrastructure. This interest originates from several industrial applications needs that request the development of new multi-functional materials which combine enhanced mechanical, electrical and thermal properties. In this work, an attempt is performed to review the most representative processes and related performances reported in literature and the experience obtained on nano-enabling technologies of fiber composite materials. This review focuses on the two main composite manufacturing technologies used by the aerospace industry; Prepreg/Autoclave and Resin Transfer technologies. It addresses several approaches for nano-enabling of composites for these two routes and reports latest achieved results focusing on performance of nano-enabled fiber reinforced composites extracted from literature. Finally, this review work identifies the gap between available nano-technology integration routes and the established industrial composite manufacturing techniques and the challenges to increase the Technology Readiness Level to reach the demands for aerospace industry applications.

Pub.: 08 Jul '16, Pinned: 12 Apr '17