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

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

Size limitations on achieving tough and healable fibre reinforced composites through the use of thermoplastic nanofibres

Phase-separated blends of epoxy and poly(ε-caprolactone) (PCL) provide crack repair in composites after a thermal treatment at 150 °C, but decrease the material’s fracture toughness. This article investigates the combination of healing with interlaminar fracture toughness improvement using electrospun PCL nanofibrous veils, interleaved between glass fibre reinforcement layers. Cure temperature close to PCL melting leads to both phase-separated domains and intact nanofibre regions. With the fast cure kinetics of the epoxy resin, phase-separated domains consist of small epoxy particles (1–5 μm diameter) surrounded by a PCL matrix. Interlaminar crack propagation in Mode I demonstrates up to 48% toughness increase when 30 g/m2 of nanofibres are inserted between each layers. Thermal treatment however results in limited healing due to slow flow of PCL in the narrow channels. Further insight is provided regarding the channel width and polymer viscosity requirements to provide a microstructure efficient for both crack healing and interlaminar toughness improvement

Electrospun nanofibrous interleaves for improved low velocity impact resistance of glass fibre reinforced composite laminates

This study analyses the damage tolerance of nanofibre interleaved composites when subjected to low velocity impact. Cross-ply glass/epoxy composite laminates are produced. Drop-weight impact and residual compressive strength measurements are performed on these laminates according to the ASTM D7136 and ASTM D7137 standards for a range of impact energies around the Barely Visible Impact Damage energy limit. Polyamide 6, polyamide 6.9 and polycaprolactone nanofibrous veils with two different veil densities are selected to assess their effect on the damage tolerance. The low velocity impact resistance of nanofibre interleaved laminates increases considerably compared to the virgin material. The (projected) damage area decreases up to 50–60%, especially at higher impact energies where the virgin material shows widespread delamination. As more energy is absorbed in the interleaved laminates by the nanofibres, less damage to reinforcing fibres and matrix resin is produced. Analysis of fracture surfaces shows that the development of nanofibre bridging zones is the main reason for the improved impact damage tolerance