Bioinspired self-healing of advanced composite structures using hollow glass fibres

Self-healing is receiving an increasing amount of worldwide interest as a method to autonomously address damage in materials. The incorporation of a self-healing capability within fibre-reinforced polymers has been investigated by a number of workers previously. The use of functional repair components stored inside hollow glass fibres (HGF) is one such bioinspired approach being considered. This paper considers the placement of self-healing HGF plies within both glass fibre/epoxy and carbon fibre/epoxy laminates to mitigate damage occurrence and restore mechanical strength. The study investigates the effect of embedded HGF on the host laminates mechanical properties and also the healing efficiency of the laminates after they were subjected to quasi-static impact damage. The results of flexural testing have shown that a significant fraction of flexural strength can be restored by the self-repairing effect of a healing resin stored within hollow fibres.     

Tensile strength of glass fibres with carbon nanotube–epoxy nanocomposite coating

A study has been made of a concept of ‘healing’ coatings applied onto the brittle fibre surface to reduce the stress concentrations and thus to improve the reinforcing efficiency in a composite. Coatings made from neat epoxy and carbon nanotube (CNT) reinforced epoxy nanocomposite were applied onto the individual glass fibres as well as rovings. It is shown that the 0.3 wt.% CNT–epoxy nanocomposite coating gave rise to a significant increase in tensile strength of the single fibre for all gauge lengths, better than the neat epoxy coating. The results on glass fibre roving also indicated a clear beneficial effect of nanocomposite impregnation on tensile strength. The rovings impregnated with the CNT nanocomposite exhibited a more uniform strength distribution and higher strengths than those impregnated with the neat epoxy. The changes in prevailing failure mechanisms influenced by the epoxy and nanocomposite coatings have been identified.     

Investigation into the behaviour of hollow glass fibre bundles under compressive loading

In this study the behaviour of small epoxy resin-bonded hollow glass fibre (HGF) tows, in a range of fibre hollow fractions and external diameters, was investigated under axial compressive loading. The relative magnitudes of compression strength for HGF tows and nominally identical bundles of solid fibres of the same external diameter (D) were measured. The interactions between fibres in a micro-tow replicated the conditions found in unidirectional composite materials. A significant degree of scatter in compression strengths were observed. However, experimental and calculated compression strengths for the range of specimens tested show a strong dependence on fibre geometry. Large fibre D and fibre hollow fraction (K²) appears to give higher experimental strength values relative to the equivalent solid fibre specimens. Conversely, strength values calculated using actual glass cross-sectional area indicate that smaller D, high K² fibres should offer the best compressive performance. It would appear from these findings that the arrangement of the fibres within the bundle (i.e. alignment, spacing, etc.) rather than the individual fibre property has an overriding effect on subsequent compression performance.     

A new method to normalize the effect of matrix properties on the value of interfacial shear strength obtained from the fragmentation test

A new method, based on tensile yield strength and strain, has been developed to normalize the effect of matrix properties on the critical fibre length and the interfacial shear strength obtained from the fragmentation test. It is argued that the conventional data normalization technique which employs elastic properties of the matrix, is fundamentally flawed because the model employed to calculate interfacial shear strength assumes perfect plasticity. Single embedded fibre fragmentation in a range of epoxy resins with differing mechanical properties has been used to validate the new method. Stoichiometric quantities of the current agent were used to keep the same interfacial chemistry. The proposed method provides more consistent interfacial shear strength data than existing theories. Furthermore, this normalization technique can also be used to predict the interfacial shear strength of glass fibres embedded in a range of support resins, such as vinyl ester or epoxy resins. For these cases, a thin layer of the phenolic resin was used on the glass fibre to keep the interface chemistry the same. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties of a self-healing fibre reinforced epoxy composites

Inspired by biological systems in which damage triggers an autonomic healing response, a polymer composite material that can heal itself when cracked has been developed. In this work, compression and tensile properties of a self-healed fibre reinforced epoxy composites were investigated. Microencapsulated epoxy and mercaptan healing agents were incorporated into a glass fibre reinforced epoxy matrix to produce a polymer composite capable of self-healing. The self-repair microcapsules in the epoxy resin would break as a result of microcrack expansion in the matrix, and letting out the strong repair agent to recover the mechanical strength with a relative healing efficiency of up to 140% which is a ratio of healed property value to initial property value or healing efficiency up to 119% if using the healed strength with the damaged strength.     

The potential of using date palm fibres as reinforcement for polymeric composites

Interfacial adhesion of natural fibres as reinforcement for fibre polymeric composites is the key parameter in designing composites. In the current study, interfacial adhesion of date palm fibre with epoxy matrix is experimentally investigated using single fibre pull out technique. The influence of NaOH treatment concentrations (0–9%), fibre embedded length and fibre diameter on the interfacial adhesion property was considered in this study. Scanning Electron Microscopy (SEM) was used to observe the surface morphology and damage feature on the fibre and bonding area before and after conducting the experiments. The results revealed that 6% concentration of NaOH is the optimum solution for treating the date palm fibre to maintain high interfacial adhesion and strength with epoxy matrix. The embedded length of the fibre controlled the interfacial adhesion property, where 10 mm embedded length was the optimum fibre length.     

Mode I interfacial toughening through discontinuous interleaves for damage suppression and control

An investigation is described concerning the interaction of propagating interlaminar cracks with embedded strips of interleaved materials in E-glass fibre reinforced epoxy composites. The approach deploys interlayer strips of a thermoplastic film, thermoplastic particles, chopped fibres, glass/epoxy prepreg, thermoset adhesive film and thermoset adhesive particles ahead of the crack path on mid-plane of Double Cantilever Beam (DCB) specimens. During these mode I tests, the interlayers were observed to confer an apparent increase in the toughness of the host material. The crack arrest performance of individual inclusion types are discussed and the underlying mechanisms for energy absorption and the behaviour of the crack at the interaction point of the interleave edge were analysed using scanning electron microscopy.     

Thermally activated healing in a mendable resin using a non woven EMAA fabric

This paper explores the efficacy of polyethylene-co-ethacrylic acid (EMAA), as a thermally activated thermoplastic healing agent embedded within a carbon fibre reinforced epoxy composite. EMAA fibres have been shown to effectively restore mode I properties in a fibre reinforced composite after thermal activation yet other forms of the healing agent or modes of deformation have so far not been studied at all. This work, uses EMAA in the form of a non-woven mesh, rather than a woven fabric to study the healing mechanism and effectiveness of property restoration for mode I (crack opening) and mode II (shear) failure as well as for high speed impact. Property restoration after mode I damage was found to be over 200% and increased with increasing EMAA concentration. For mode II shear failure, the property restoration was reduced to a little over 100% regardless of EMAA concentration. Mode II analysis also showed that the modulus could be restored to about 80% of its original value when modified with EMAA. Repeated impacting using a falling weight test produced no property restoration after healing, yet the modified laminates appeared protected from further damage compared with an unmodified laminate. This was attributed to the formation of a ductile thermoplastic layer mitigating further damage. Scanning electron microscopy revealed that regardless of the extent of healing, the form of the healing agent or the mode of damage, the unique pressure delivery mechanism previously identified, was always observed to occur.     

Thermal loading of short fibre composites and the induction of residual shear stresses

A study has been undertaken to identify the effect of thermal residual stresses on the stress transfer between a short fibre and resin using the photoelastic method. As expected, it was observed that fibre fracture occurred at a higher applied load for the fibre embedded in the thermally (80 °C) cured epoxy matrix than in the room-temperature-cured epoxy. Under plane polarised light bright birefringent patterns were observed in the hot-cured epoxy matrix around the fibre-ends prior to loading. These were not present in the room-temperature-cured epoxy, indicating that thermal residual stresses had been induced during thermal-curing. On loading, the birefringent patterns in the hot-cured matrix at the fibre-ends were almost extinguished but at a particular stress reappeared as a bright region, and increased in intensity on further loading.Using a phase-stepping polariscope, four images of the fibre-ends were captured simultaneously so that detailed contour maps of fringe order could be created. To examine the micromechanical response in the matrices at the interfaces the profile of interfacial shear stress at the fibre-ends was calculated. Under a given external load the shear stress at the interface in the hot-cured matrix was significantly lower than that in the cold-cured epoxy matrix. The thermal load which is applied to a resin on cooling from manufacture requires a shear stress at the interface to put the fibre into compression. At the fibre-ends a residual shear stress of opposite sign (to that induced mechanically) leads to extension of birefringent patterns on loading.     

Healable thermoset polymer composite embedded with stimuli-responsive fibres

Severe wounds in biological systems such as human skin cannot heal themselves, unless they are first stitched together. Healing of macroscopic damage in thermoset polymer composites faces a similar challenge. Stimuli-responsive shape-changing polymeric fibres with outstanding mechanical properties embedded in polymers may be able to close macro-cracks automatically upon stimulation such as heating. Here, a stimuli-responsive fibre (SRF) with outstanding mechanical properties and supercontraction capability was fabricated for the purpose of healing macroscopic damage. The SRFs and thermoplastic particles (TPs) were incorporated into regular thermosetting epoxy for repeatedly healing macroscopic damages. The system works by mimicking self-healing of biological systems such as human skin, close (stitch) then heal, i.e. close the macroscopic crack through the thermal-induced supercontraction of the SRFs, and bond the closed crack through melting and diffusing of TPs at the crack interface. The healing efficiency determined using tapered double-cantilever beam specimens was 94 per cent. The self-healing process was reasonably repeatable.     

SWNT composite coatings as a strain sensor on glass fibres in model epoxy composites

The preparation of a model glass-fibre/epoxy composite with single-walled carbon nanotubes (SWNTs) incorporated as a strain sensor on the fibre surface is described. A micromechanical study of stress transfer at the fibre–matrix interface followed using Raman spectroscopy properties is reported. The SWNTs were distributed along the fibre surface either by dispersing them in an amino-silane coupling agent or coating with an epoxy resin solution containing the SWNTs. The point-by-point mapping of the fibre strain in single fibre fragmentation tests has been undertaken for the first time using SWNTs on the fibres and the interfacial shear stress distribution along the fibre length was determined using the embedded SWNTs. The behaviour was found to be consistent with the classical shear-lag model. The effects of SWNT type and preparation procedure on the sensitivity of the technique were evaluated and optimized from single fibre deformation tests.     

Fracture response of fibre-reinforced materials with macro/microcrack damage using the Boundary Element Technique

The Boundary Element Method (BEM) incorporating the Embedded Cell Approach (ECA) has been used to analyse the effects of constituent material properties, fibre spatial distribution and microcrack damage on the localised behaviour of transversely fractured, unidirectional fibre-reinforced composites. Three specific composites, i.e., glass fibre reinforced polyester, carbon fibre reinforced epoxy and a glass-carbon hybrid, are considered. The geometrical structures examined were perfectly periodic, uniformly spaced fibre arrangements in square and hexagonal embedded cells. In addition, numerical simulations were also conducted using embedded cells containing randomly distributed fibres. The models involve both elastic fibres and matrix, with the interfaces between the different phases being fully bonded. The results indicate that the constituent material properties (two phase composite) and spatial distribution have a significant effect on the localised stress distributions around the primary crack tip. However, the strain energy release rate associated with crack propagation is predominantly influenced by the material composition. The three-phase hybrid composite exhibited an apparent intermediate fracture toughness value, compared to the all-glass and all-carbon models. Furthermore, the strain energy release rate for the macrocrack lowers as it enters a zone of localised damage (microcracking). The presence of microcracks relaxes the stress field, which can result in a significant reduction in the energetics of the primary crack.     

Morphology, dynamic mechanical and thermal studies on poly(styrene-co-acrylonitrile) modified epoxy resin/glass fibre composites

Poly(styrene-co-acrylonitrile) (SAN) was used to modify diglycidyl ether of bisphenol-A (DGEBA) type epoxy resin cured with diamino diphenyl sulfone (DDS) and the modified epoxy resin was used as the matrix for fibre reinforced composites (FRPs) in order to get improved mechanical and thermal properties. E-glass fibre was used as the fibre reinforcement. The morphology, dynamic mechanical and thermal characteristics of the systems were analyzed. Morphological analysis revealed heterogeneous dispersed morphology. There was good adhesion between the matrix polymer and the glass fibre. The dynamic moduli, mechanical loss and damping behaviour as a function of temperature of the systems were studied using dynamic mechanical analysis (DMA). DMA studies showed that DDS cured epoxy resin/SAN/glass fibre composite systems have two T_gs corresponding to epoxy rich and SAN rich phases. The effect of thermoplastic modification and fibre loading on the dynamic mechanical properties of the composites were also analyzed. Thermogravimetric analysis (TGA) revealed the superior thermal stability of composite system.     

X-ray damage characterisation in self-healing fibre reinforced polymers

Realising autonomous healing in advanced composite structures requires a detailed understanding of the damage profile to be repaired. Quantifying the damage volume and mapping its through-thickness location is key to ensuring that the delivery infrastructure can supply sufficient healing to critical locations whilst maximising coverage and minimising structural cost. In this study micro-X-ray computer tomography (μCT) was used to determine the damage volume in quasi-isotropic carbon fibre reinforced plastic (CFRP) laminates subjected to low velocity impacts. The laminates incorporated a layer of hollow glass fibres (HGFs) at either the 3rd or 13th interface for the purpose of delivering a self-healing agent. Analysis of the μCT data indicated that HGF inserted at interface 3 (near back face) altered the through-thickness damage map whilst visualisation of the HGF at both interfaces indicated low levels of HGF fracture.     

Crushing behaviour of composite hemispherical shells subjected to quasi-static axial compressive load

This paper examines the effects of composite constituents and geometry on the energy absorption capability of composite hemispherical shells. To examine the effects of matrix types on their energy absorption capability, glass fibre/epoxy and glass fibre/polyester hemispherical shells were fabricated. While glass fibre/epoxy and carbon fibre/epoxy hemispherical shells were fabricated to investigate the effect of fibre reinforcements. Effect of aspect ratio (R/t) was also examined and the results were presented. The results obtained showed that the energy absorption capability of the hemispherical shells significantly affected by the composite constituents as well as R/t ratio.     

Analysis of interfacial micromechanics of model composites using synchrotron microfocus X-ray diffraction

The deformation micromechanics of single-fibre embedded model composites of poly(p-phenylene benzobisoxazole) (PBO) and poly(p-phenylene terephthalamide) (PPTA) fibres, embedded in an epoxy resin have been examined using synchrotron microfocus X-ray diffraction. Single fibres (in air) were deformed and the c-spacing monitored to establish a calibration of crystal strain against applied stress. Subsequently, the variation in crystal strain along fibres, embedded in the resin matrix was mapped using synchrotron microfocus X-ray diffraction. Raman spectroscopy was then used to map molecular deformation on the same samples (recorded as shifts in the Raman band wavenumber) in order to provide a complementary stress data. A shear-lag analysis was conducted on the axial fibre stress data in order to calculate interfacial shear stress and identify different stress-transfer modes at fibre/resin interfaces. The results establish that the axial fibre stress distributions measured by synchrotron microfocus X-ray diffraction correlate well with those obtained using Raman spectroscopy. The interfacial shear stress data derived from the stress-transfer profiles also show a good degree of correlation.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.     

Monitoring the micromechanics of reinforcement in carbon fibre/epoxy resin systems

The technique of laser Raman spectroscopy (LRS) was employed to obtain the interfacial shear stress (ISS) distribution along a short high-modulus carbon fibre embedded in epoxy resin at different levels of applied stress. Up to 0.6% applied strain, the ISS reached a maximum at the bonded fibre ends and decayed to zero at the middle of the fibre. At higher applied strains, the maximum value of the ISS distribution shifted away from the fibre ends towards the middle of the fibre. At the point of fibre fracture, fibre/matrix debonding was found to initiate at the fibre breaks. Further increase of applied strain resulted also in debonding initiation at the fibre ends. Current analytical stress-transfer models are reviewed in the light of the experimental data.     

Microcapsule induced toughening in a self-healing polymer composite

Microencapsulated dicyclopentadiene (DCPD) healing agent and Grubbs' Ru catalyst are incorporated into an epoxy matrix to produce a polymer composite capable of self-healing. The fracture toughness and healing efficiency of this composite are measured using a tapered double-cantilever beam (TDCB) specimen. Both the virgin and healed fracture toughness depend strongly on the size and concentration of microcapsules added to the epoxy. Fracture of the neat epoxy is brittle, exhibiting a mirror fracture surface. Addition of DCPD-filled urea-formaldehyde (UF) microcapsules yields up to 127% increase in fracture toughness and induces a change in the fracture plane morphology to hackle markings. The fracture toughness of epoxy with embedded microcapsules is much greater than epoxy samples with similar concentrations of silica microspheres or solid UF polymer particles. The increased toughening associated with fluid-filled microcapsules is attributed to increased hackle markings as well as subsurface microcracking not observed for solid particle fillers. Overall the embedded microcapsules provide two independent effects: the increase in virgin fracture toughness from general toughening and the ability to self-heal the virgin fracture event.     

Embedded single carbon fibre to sense the thermomechanical behavior of an epoxy during the cure process

A new approach that uses a single carbon fibre for sensing the thermomechanical behavior of an epoxy during the cure cycle is presented. By recording and analyzing the electrical resistance and temperature history of a carbon fibre embedded inside an epoxy specimen during the cure cycle, the interaction between the carbon fibre and the surrounding polymer can be revealed. Compared with reported TMA and DMA results, this embedded carbon fibre sensor approach successfully detects the glass transition zone covering the final transition temperature, the main transition temperature, and the starting transition temperature that respectively have similar values as: (i) T_g corresponding to the abrupt change in CTE, (ii) T_g by storage modulus (E′) onset, and (iii) the upper temperature limit for the linear relationship between E′ and the temperature. The future applications for this sensor method are also discussed.