Functional composites with damage control and repair

Fibre Reinforced Polymer (FRP) composite structures are subjected during service to low energy impacts or unexpected loads, leading to damage. Microcracks are generally first formed in the matrix and can reach up to several hundred microns in thickness. In many living systems, an initial self-sealing phase followed by self-healing of the original tissue leads to highly effective repair of minor damage events. A bio-inspired strategy for the efficient healing of microcracks in FRPs is hence to: (i) autonomously close cracks whose thickness is above a certain threshold to ensure better crack faces registry; (ii) use a healing matrix with structural properties close to that of a conventional FRP matrix, that is able to expand and fill the cracked regions repeatedly after multiple damage events. This strategy was investigated in the present work through (i) the use of Shape Memory Alloy wires (SMAs) as stitches in FRPs and (ii) a healing matrix based on thermoset-thermoplastic phase-separated blends. Blends composed of epoxy resin and different thermoplastics (PCL, PLA and PMMA) were evaluated for their potential as healing matrix, based on their room temperature toughness, stiffness and their capacity to heal when subjected to a moderate heating cycle. Three types of blend morphology resulted from polymerisation-induced phase separation during cure, depending on the thermoplastic content, including an interconnected particulate epoxy phase and a co-continuous thermoplastic phase at high thermoplastic contents. An optimal composition was found for epoxy-25vol%PCL blends. The PCL phase expands by 14% in volume upon melting at 150 °C, therefore enabling filling of small cracks. When further integrated to FRPs (with an adapted vacuum infusion moulding process), this healing matrix led to composites with similar stiffness and strength to that of pure epoxy composites, but also to full recovery of compression after impact strength for low damage extent (impacts of 8.5 J). To provide healing for larger damage extent, NiTiCu SMA wires, with a diameter of 150µm, were stitched in the dry fabric before processing. After damage and upon heating at 150 °C, SMA stitches, that have been stretched and partly debonded upon crack propagation, efficiently closed the cracks. This procedure demonstrated the capacity of the wires to close 200 µm thick cracks in the FRPs and led to fully heal impact damage events up to 17 J, corresponding to the main concern of maintenance activities in the composite industry (e.g. tools dropped from 1-2 m height). Finally, as an alternative to blends, PCL electrospun nanofibrous veils were interleaved into fabrics, infused with epoxy and cured to reach a microstructure combining both phase-separated domains as well as intact nanofibre regions. Interlaminar crack propagation demonstrated up to 48% toughness increase as compared to reference specimens. However, healing was prevented due to reduced flow of PCL in the fine channels resulting from phase separation, showing the limitation of this approach as compared to the use of blends and stitches. Phase-separated epoxy-PCL composites (with or without stitched SMA wires), considering their manufacturing feasibility through conventional industrial processes, their acceptable mechanical properties and their ability to fully heal low-velocity impact damage, demonstrated their relevance for composite structures that are subjected to moderate loads and not easily accessible to repair

Tensile elongation of unidirectional or laminated composites combining a brittle reinforcement with a ductile strain and strain-rate hardening matrix

We use the long-wavelength model of Hutchinson and Neale (1977) and Ghosh (1977) to estimate the uniform tensile elongation of two-phase composites deforming quasistatically according to the equistrain rule of mixtures, in which one phase is ductile while the other fractures progressively according to two-parameter Weibull statistics. We use shear-lag models in the literature to quantify load transfer from the ductile phase to the fractured brittle phase, and to estimate the influence of matrix strain and strain-rate hardening, of brittle phase fracture characteristics, and of phase volume and strength ratios, on the composite strain to failure as dictated by the onset of unstable necking. Calculations show that strain and strain-rate hardening of the ductile phase do relatively little to increase the ductility of the composite. Two parameters play a dominant role, namely the brittle-phase Weibull modulus and a dimensionless parameter describing load transfer across the two phases. The main practical implication of this analysis is that, to produce reasonably ductile two-phase composites, the best strategy is to aim for small layer thicknesses. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Designing laminated metal composites for tensile ductility

This contribution draws practical implications of a recently published estimation of the tensile ductility in laminated composites made of two ductile materials, typically metals or alloys, which harden as both the strain and the strain-rate increase. To this end, the literature is surveyed to collect values for the strain hardening exponent, the strain-rate sensitivity and the strength constant for a wide range of engineering metals and alloys. Material combinations that might produce ductile laminated metal composites are then examined in light of the data and theory. A simple graph is proposed, which gives a direct reading of the predicted elongation to failure of composites containing equal volume fractions of any two materials among those surveyed. The resulting plots show material combinations in which a more ductile material can significantly increase, within a laminated metal composite (LMC), the tensile elongation of a less ductile material. In this role, 304 stainless steel and commercial purity iron emerge as sensible possibilities. (C) 2014 Elsevier Ltd. All rights reserved

Progress in Self-Healing Fiber-Reinforced Polymer Composites

This paper sets out to review the current state of the art in applying self-healing/self-repair to high-performing advanced fiber-reinforced polymer composite materials (FRPs). A significant proportion of self-healing studies have focused so far on developing and assessing healing efficiency of bulk polymer systems, applied to particulate composites or low-volume fraction fiber-reinforced materials. Only limited research is undertaken on self-healing in advanced structural FRP composite materials. This review focuses on what is achieved to date, the ongoing challenges which have arisen in implementing self-healing into FRPs, how considerations for industrialization and large-scale manufacture must be considered from the outset, where self-healing may provide most benefits, and how a functionality like self-healing can be validated for application in real structures. Systems are compared in terms of process parameters, resulting mechanical properties, methods of healing assessment, as well as values of healing quantification. Guidelines are further given for a concerted effort to drive toward standardization of tests and the use of specific reinforcement architectures in order to allow reliable comparison between the available healing systems in structural composites

Composite material

In a composite material (6), comprising a fibrous reinforcement (3), and a polymer matrix (14), wherein the polymer matrix (14) comprises two interpenetrating phases, namely a thermoset phase and a continuous thermoplastic phase, wherein the thermoset phase and the thermoplastic phase form a matrix microstructure (7), wherein the matrix microstructure (7) comprises a thermoplastic matrix formed by the thermoplastic phase, and wherein the matrix microstructure (7) comprises a multitude of thermoset particles (13) formed by the thermoset phase, the thermoset particles (13) have dimensions in a range between 0.1 µm and 10 µm

Shape memory alloys in fibre-reinforced polymer composites

This article reviews the current state-of-the-art in the applications of shape memory alloy (SMA) wires into high performance fibre reinforced polymer composite materials (FRPs). SMAs have been investigated to date to address four main areas of properties improvement: (i) damping and vibrational response, where SMAs are integrated into composites either in the plane of the neutral axis, or as transverse stitches; (ii) impact, where SMAs are integrated in the neutral axis or as stitches; (iii) crack closure, where SMAs are integrated transverse to the crack, as stitches and (iv) shape morphing, where SMAs are integrated in plane into the composite, but in the non-neutral axis. The critical parameters for successful integration of SMAs to FRPs are highlighted, mostly from a composite processing angle. Finally, this review evaluates some hurdles remaining in the implementation of SMAs to FRPs to create smart and efficient composite structures without compromising their processing route, structural properties, weight and cost. Keywords: Shape memory alloys, Fibre reinforced polymer composites, Processing, Smart material

Damage recovery after impact in E-glass reinforced poly(epsilon-caprolactone)/epoxy blends

Damage recovery after low-velocity impact has been assessed in woven E-glass fibre-reinforced polymer composites with an epoxy matrix and a blend of epoxy and 25 vol% of poly(e-caprolactone) (PCL). Impact was carried out at three energy levels (8.5, 17, 34 J) and composites with epoxy-PCL blends demonstrated similar energy absorption capacity as compared to pure epoxy composites even though the extent of damage (quantified by C-scans and optical microscopy) was higher. Ultimate compressive residual strength of the modified composites was, for the different impact energy levels, 23-33% lower as compared to unmodified composites. Healing efficiency after a thermal mending cycle at 150 degrees C for 30 min has been quantified using three complementary characterization methods; impact damage could be recovered from 20% to 100% depending on the impact energy level. These modified matrix composites are thus able to fully recover low-velocity impact damage at energy levels often met in real structures. (C) 2017 Elsevier Ltd. All rights reserved

Thermal mending in E-glass reinforced poly(epsilon-caprolactone)/epoxy blends

Blends of difunctional epoxy monomer with a 4,4'-diaminodiphenylsulfone hardener and poly(epsilon-caprolactone) (PCL) were used as a self-healing matrix in woven glass fibre-reinforced polymer composites (FRPs). FRPs with these blends (containing 0, 25 and 37 vol% of PCL in the blend) were manufactured through Vacuum Assisted Resin Infusion Moulding at high temperature and the matrix, resulting from polymerization induced phase separation, consisted of interconnected epoxy particles embedded in PCL. With 25 vol% PCL in the matrix, similar storage modulus and interfacial shear strength as compared to unmodified systems have been observed, however toughness was decreased by 40%. Up to 45% toughness recovery and over 100% stiffness recovery were observed over several cycles when the blend matrix composite samples were re-tested after a thermal cycle at 150 degrees C for 30 min. These composites can thus provide efficient crack healing, but remain more sensitive to initial crack propagation due to confinement of the thermoplastic phase. (C) 2017 Elsevier Ltd. All rights reserved

Statistical Fatigue Investigation and Failure Prediction of a Healable Composite System

Fiber reinforced polymers are massively used as an alternative to metals in structural applications. The brittle nature of their matrix, however, makes them more susceptible to crack formation and propagation resulting in costly repair operations and increased environmental impact. Intrinsic healable composites provide a good alternative to these conventional composite materials, whereas their mechanical properties in static solicitation or impact testing are well documented, only few studies address fatigue testing. This research focuses on 3-point bending fatigue tests of polymer-blend based healable E-glass composite materials. The S-N curve was first built to compare the fatigue behavior of the healable system to a conventional epoxy composite. A statistical approach based on Weibull statistics was developed to predict the failure probability as a function of the applied stress amplitude, to compare both systems at equivalent probability of failure. The healable system showed a higher fatigue resistance at high cycle fatigue. Furthermore, a full stiffness recovery was obtained and a life extension of at least five times compared to the reference system when healed after reaching a 90% chance of survival. The healable system thus opens new perspectives for more sustainable load-bearing composites