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.     

Wire electrical discharge machining of Al2O3 particle and short fibre reinforced aluminium based composites

Abstract

The wire electrical discharge machining behaviour of alumina particle and short fibre reinforced aluminium based composites was studied. Under the two cutting conditions of this study, namely coarse and fine cutting, the average surface roughness R_a of the particle reinforced composite was found always to be higher than that of the corresponding machined matrix surfaces, however, the opposite was found to be true for the fibre reinforced composite. Comparing the surface quality of the two composites, in terms of roughness measurements, the fibre reinforced composite was superior to the particle reinforced composite. Although cutting conditions have little effect on the overall R_a of the two composites, their corresponding surface topographies were found to be intrinsically different. Surface banding was observed on the fine cut surfaces of the particle reinforced specimens, which was believed to be caused by shifting of the wire. Based on the machined surface morphology and the measured surface profiles, the material removal mechanism for the two reinforced composites is discussed.

MST/3412     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Surface Modification of Transition Metal Dichalcogenide Nanosheets for Intrinsically Self-Healing Hydrogels with Enhanced Mechanical Properties

Nanocomposites with enhanced mechanical properties and efficient self-healing characteristics can change how the artificially engineered materials’ life cycle is perceived. Improved adhesion of nanomaterials with the host matrix can drastically improve the structural properties and confer the material with repeatable bonding/debonding capabilities. In this work, exfoliated 2H-WS₂ nanosheets are modified using an organic thiol to impart hydrogen bonding sites on the otherwise inert nanosheets by surface functionalization. These modified nanosheets are incorporated within the PVA hydrogel matrix and analyzed for their contribution to the composite's intrinsic self-healing and mechanical strength. The resulting hydrogel forms a highly flexible macrostructure with an impressive enhancement in mechanical properties and a very high autonomous healing efficiency of 89.92%. Interesting changes in the surface properties after functionalization show that such modification is highly suitable for water-based polymeric systems. Probing into the healing mechanism using advanced spectroscopic techniques reveals the formation of a stable cyclic structure on the surface of nanosheets, mainly responsible for the improved healing response. This work opens an avenue toward the development of self-healing nanocomposites where chemically inert nanoparticles participate in the healing network rather than just mechanically reinforcing the matrix by slender adhesion.     

Cast metal matrix composites

Abstract

Metal matrix composites have been available in certain forms for at least two decades, e.g. boron fibre reinforced aluminium and various dispersed phase alloys and cermets. Recently, a range of alumina and silicon carbide fibres, whiskers, and particles with diameters <20 μm have become available. The possibilities of incorporating these materials into metals to improve stiffness, wear resistance, and elevated temperature strength without incurring weight penalties have attracted the attention of design engineers in the aerospace and automobile industries. The aim of the present paper is to outline the manufacturing processes for such composites, in particular those based upon liquid metal technology, e.g. squeeze casting and spray forming. Some of the mechanical and physical properties which have been determined for these materials are described. An analysis of how matrix alloy selection may influence tensile and fracture behaviour of short fibre and particle reinforced composites is attempted.

MST/770     

Impact of natural weathering on medium-term self-healing performance of fiber reinforced cementitious composites with intrinsic crack-width control capability

The paper investigates the medium-term self-healing performance of fiber reinforced cementitious composites with intrinsic crack-width control capability under natural weathering. The pre-cracked specimens with different damage levels are exposed to various humidity conditions, namely, water submersion, natural weathering, and a laboratory environment with constant humidity. The medium-term self-healing performance is evaluated using a resonant frequency test, tensile test, SEM, and EDX. It is concluded that the medium-term cracked specimens can moderately recover their mechanical properties within 90 days after being submerged in water or exposed to natural weathering. In particular, they are able to resume the multiple cracking behavior and exhibit a reloading strength larger than the preloading strength. Furthermore, the identified compositions of the medium-term healing products for specimens exposed to water and natural weathering conditions are similarly characterized. The reported results imply that effective medium-term self-healing can be realized in fiber reinforced cementitious composites with intrinsic crack-width control capability under natural weathering.     

Multiscale carbon fibre–reinforced polymer (CFRP) composites containing carbon nanotubes with tailored interfaces

Pristine and functionalized multiwalled carbon nanotubes (MWCNTs) with tailored interfaces were efficiently dispersed in an epoxy matrix using a three‐roll mill and further reinforced with carbon fibres. 1.3‐Dipolar cycloaddition of azomethine ylides was used for the chemical modification of MWCNTs by a solvent‐free approach. The influence of different loadings and types of MWCNTs on the final properties of the epoxy matrix was studied. Moreover, the most promising formulations were selected for manufacturing of prepreg sheets. The transversal tensile properties and the interlaminar fracture toughness under mode I loading (G_IC) of multiscale carbon fibre–reinforced polymer (CFRP) composites were characterized. The results point out that it is not straightforward to transfer the remarkable intrinsic properties of MWCNTs to the composite level, although an overall positive trend was found. Double cantilever beam experiments showed that G_IC of CFRP composites was improved 44% at ultralow content of functionalized MWCNTs (0.043 wt%).     

THE EFFECT OF INTERFACIAL PROPERTIES ON THE FLOW STRENGTH OF DISCONTINUOUS REINFORCED METAL-MATRIX COMPOSITES

Abstract

The effect of interfacial properties on the strength of discontinuous reinforced metal-matrix composites is systematically studied by theoretical modelling. The calculations were carried out within the framework of continuum plasticity theory using cell models and the finite element method. A wide range of inclusion aspect ratios, volume fractions, and interfacial strengths were investigated for perfectly plastic and hardening matrices. Interfaces were modelled either as strongly bonded, or as shearable but strong normal to the inclusions, or as debonding at the reinforcement ends but strong on the sides. Additionally, the effects of reinforcement arrangement and extensive damage to continuous fibre composites were addressed. Debonding at the ends of the inclusions was found to have the most deleterious effect on the strength of the composite. When debonding does not occur but interface sliding takes place freely, an amount of strengthening is seen which is a function of the inclusion volume fraction but is primarily independent of the inclusion aspect ratio. For extensively damaged continuous fibre composites, a weak interface yields a steady-state composite flow strength slightly higher than the volume fraction of the matrix times the yield strength of the matrix. This increases linearly with the interfacial shear strength up to the level for strongly bonded composites and can be estimated from the intact fibre aspect ratio, the matrix yield stress, the volume fraction, and the interfacial strength     

Thermomechanical fatigue of short fibre reinforced aluminium matrix piston alloy

Abstract

The technical potential of short fibre reinforced aluminium matrix composites lies in their higher stiffness and higher strength at elevated temperatures compared with unreinforced matrix alloys. In the present investigation, thermal cycling creep tests were conducted on the piston alloy AlSi12CuMgNi reinforced with 20% Saffil (Al₂O₃) short fibres, to simulate the cold start conditions of combustion engines. After processing of the metal matrix composite (MMC) by direct squeeze casting, four heat treatment conditions were produced. Specimens under constant load were thermally cycled between 50 and 300°C, whereby a heating and cooling speed of 12.5 K s1 was achieved. Series of up to 5000 cycles at tensile stresses between 20 and 80 MPa were executed, comparing reinforced specimens and unreinforced matrix material. The results of these experiments showed that the creep properties of the alloy, especially minimum creep rate and lifetime to fracture, were improved by the reinforcement. Furthermore, the creep rate of the MMC was essentially independent of the heat treatment condition, whereas the minimum creep rate was increased significantly for the matrix material by overaging. It can be concluded that precipitation strengthening influenced the creep properties of unreinforced specimens only, which is in good agreement with theoretical considerations. An analysis of fibre length revealed that the majority of the fibres broke at between 50 and 75% of the lifetime, just before the beginning of tertiary creep. Metallographic investigations using a scanning electron microscope did not show fibre pullout, but multiple fracture of fibres along the whole specimen. Micromechanical models for isothermal creep in short fibre reinforced aluminium alloys confirm the above results, since tertiary creep is assumed to be a consequence of fibre fracture.     

Surface oxidation of carbon fibre on tribological properties of PEEK composites

Abstract

In this work, ozone modification method and air oxidation were used for the surface treatment of polyacrylonitrile (PAN) based carbon fibre. The surface characteristics of carbon fibres were characterised by X-ray photoelectron spectroscopy. The interfacial properties of carbon fibre reinforced PEEK (CF/PEEK) composites were investigated by means of the single fibre pull-out tests. As a result, it was found that IFSS values of the composites with ozone treated carbon fibre are increased by 60% compared with that without treatment. X-ray photoelectron spectroscopy results show that ozone treatment increases the amount of carboxyl groups on carbon fibre surface, thus the interfacial adhesion between carbon fibre and PEEK matrix is effectively promoted. The effect of surface treatment of carbon fibres on the tribological properties of CF/PEEK composites was comparatively investigated. Experimental results revealed that surface treatment can effectively improve the interfacial adhesion between carbon fibre and PEEK matrix. Thus the wear resistance was significantly improved.     

Investigation of the effect of surface modifications on the mechanical properties of basalt fibre reinforced polymer composites

Unsaturated polyester-based polymer composites were developed by reinforcing basalt fabric into an unsaturated polyester matrix using the hand layup technique at room temperature. This study describes basalt fibre reinforced unsaturated polyester composites both with and without acid and alkali treatments of the fabrics. The objective of this investigation was to study the effect of surface modifications (NaOH & H₂SO₄) on mechanical properties, including tensile, shear and impact strengths. Variations in mechanical properties such as the tensile strength, the inter-laminar shear strength and the impact strength of various specimens were calculated using a computer-assisted universal testing machine and an Izod Impact testing machine. Scanning Electron Microscope (SEM) observations of the fracture surface of the composites showed surface modifications to the fibre and improved fibre–matrix adhesion. The result of the investigation shows that the mechanical properties of basalt fibre reinforced composites are superior to glass fibre reinforced composites. This work confirms the applicability of basalt fibre as a reinforcing agent in polymer composites.     

Development of a powder coated fibre pre-processing route for production of fibre reinforced composites

Abstract

As a critical material for next generation aeroengines, fibre reinforced composites such as silicon carbide reinforced titanium continue to attract strong attention from both industrial and academic sectors. Reducing the processing costs and increasing component processing flexibility remain the priorities of current research. This paper presents a novel powder coated fibre pre-processing technique to meet such industrial requirements. The proposed technique is based on slurry powder metallurgy and presents itself as a cost effective alternative to current processing methods. It involves firstly, mixing matrix powder particles with an appropriate organic binder and solvent to form a slurry, drawing a continuous silicon carbide fibre through the slurry, drying the coated fibre and finally laying up the fibres into a composite preform for subsequent consolidation. The organic component is removed from the preform matrix via a binder burnout phase prior to composite consolidation.     

Mechanical properties of natural fibre reinforced polymer composites

During the last few years, natural fibres have received much more attention than ever before from the research community all over the world. These natural fibres offer a number of advantages over traditional synthetic fibres. In the present communication, a study on the synthesis and mechanical properties of new series of green composites involving Hibiscus sabdariffa fibre as a reinforcing material in urea-formaldehyde (UF) resin based polymer matrix has been reported. Static mechanical properties of randomly oriented intimately mixed Hibiscus sabdariffa fibre reinforced polymer composites such as tensile, compressive and wear properties were investigated as a function of fibre loading. Initially urea-formaldehyde resin prepared was subjected to evaluation of its optimum mechanical properties. Then reinforcing of the resin with Hibiscus sabdariffa fibre was accomplished in three different forms: particle size, short fibre and long fibre by employing optimized resin. Present work reveals that mechanical properties such as tensile strength, compressive strength and wear resistance etc of the urea-formaldehyde resin increases to considerable extent when reinforced with the fibre. Thermal (TGA/DTA/DTG) and morphological studies (SEM) of the resin and biocomposites have also been carried out.     

Influence of microstructure on axial strength of unidirectional continuous fibre reinforced aluminium matrix composites

Abstract

Systematic empirical investigations on the relationship between microstructural features and mechanical performance of unidirectionally reinforced continuous fibre Al matrix composites (CFAMCs) carried out by the present authors in recent years are summarised. The employment of a high strength matrix alloy and the development of a strong fibre/matrix interface are beneficial to maximise the strengthening effect of the fibre reinforcement. Processing defects, such as second brittle phases in the matrix, non-infiltration defects, matrix solidification shrinkage voids, excessive interfacial reactions, the presence of reaction products on the interface, weak interfacial binding, and excessively high fibre volume fraction reduce composite strength to different extents via a number of different mechanisms. Criteria for the microstructure design of CFAMCs for optimum fibre strengthening efficiency are proposed.     

Healing of low-velocity impact damage in vascularised composites

This research has sought to characterise damage formation and self-healing efficiency within vascularised carbon fibre reinforced polymer (CFRP) laminates over a range of low velocity impact energies. Using ultrasonic C-scanning and compression after impact (CAI) analysis, vascularised laminates were shown to conform to the same damage size to residual compression strength relationship established for conventional laminates. The damage tolerance level of the host laminate was carefully determined, an important consideration in selection of the most appropriate vascule spacing for a reliable self-healing system. The healing functionality imparted full recovery of post impact compression strength over the range of impact energies tested (2.5–20 J), via healing of matrix cracking and delamination damage. The successful implementation of this technology could substantially enhance the integrity, reliability and robustness of composite structures, whilst offering benefits through reduced operational costs and extended lifetimes. However, establishing the benefits of such novel systems to existing design criteria is challenging, suggesting that bespoke design tools will be required to fully attain the potential benefits of self-healing technologies.     

Role of matrix modification on interlaminar shear strength of glass fibre/epoxy composites

Interlaminar shear properties of fibre reinforced polymer composites are important in many structural applications. Matrix modification is an effective way to improve the composite interlaminar shear properties. In this paper, diglycidyl ether of bisphenol-F/diethyl toluene diamine system is used as the starting epoxy matrix. Multi-walled carbon nanotubes (MWCNTs) and reactive aliphatic diluent named n-butyl glycidyl ether (BGE) are employed to modify the epoxy matrix. Unmodified and modified epoxy resins are used for fabricating glass fibre reinforced composites by a hot-press process. The interlaminar shear strength (ILSS) of the glass fibre reinforced composites is investigated and the results indicate that introduction of MWCNT and BGE obviously enhances the ILSS. In particular, the simultaneous addition of 0.5 wt.% MWCNTs and 10 phr BGE leads to the 25.4% increase in the ILSS for the glass fibre reinforced composite. The fracture surfaces of the fibre reinforced composites are examined by scanning electron microscopy and the micrographs are employed to explain the ILSS results.     

Effects of coating treatment on mechanical properties of carbon fibres and reinforced silicon carbide composites

Abstract

Three-dimensionally braided carbon fibre reinforced SiC matrix composites have been fabricated and the effects of coating treatment on the mechanical properties have been investigated. It has been found that pyrocarbon coating can improve the strength of the heat treated carbon fibres. When the coating thickness was 0.5 m, the composites had better mechanical properties: a flexural strength of 643 MPa and a fracture toughness of 17.9 MPa m12. The composites also exhibited a toughening fracture mode.     

Matrix chemistry optimisation of Al/Altex composites preserving both bres and matrix alloy integrity

Abstract

The mechanical properties of continuous fibre reinforced metals (CFRMs) are known to be very sensitive to the selection and composition of the metallic matrix; for aluminium based CFRMs, commercial alloys are not suitable for this purpose. In the present study, the chemical composition of the matrix of unidirectionally Altex fibre reinforced aluminium composites was adjusted to meet the requirements previously established for a maximum exploitation of the fibre strengthening potential in CFRMs, particularly accounting for the peculiarities of the aluminosilicate Altex fibre and of the squeeze casting process. It was found that a matrix made from high purity elements based on the ternary Al–Zn–Mg system confers the best tensile strength properties to the CFRM, provided the Zn and Mg additions are tightly controlled to prevent the formation of interfacial intermetallic compounds during processing. The optimum composition was shown to be Al–6Zn–(0.3–0.6)Mg (wt-%); in this case Zn traps the Mg which leads to the formation of fine matrix precipitates indentified as Zn₂Mg (γ' phase), thus preventing a deleterious reaction of Mg at the fibre/matrix interface. The resulting composite material (fibre volume fraction ~ 54%) does not require additional heat treatment after casting and yields a tensile strength up to 1116 and 257 MPa in the axial and transverse directions respectively.     

Self-repair of structural and functional composites with intrinsically self-healing polymer matrices: A review

Self-healing is a smart and promising way to make materials more reliable and longer lasting. In the case of structural or functional composites based on a polymer matrix, very often mechanical damage in the polymer matrix or debonding at the matrix–filler interface is responsible for the decrease in intended properties. This review describes the healing behavior in structural and functional polymer composites with a so-called intrinsically self-healing polymer as the continuous matrix. A clear similarity in the healing of structural and functional properties is demonstrated which can ultimately lead to the design of polymer composites that autonomously restore multiple properties using the same self-healing mechanism.     

Comparative viability of processing routes for intermetallic based materials

Abstract

Processing routes for intermetallic based materials are briefly surveyed and compared. For monolithic intermetallics the two main factors determining process routes both derive from the low room temperature ductility of most intermetallics. They are the need to maintain material cleanness, thus reducing fracture initiation sites, and the desire to achieve fine grain size to seek to improve ductility. For the titanium based aluminides there is also a need to minimise interstitial, particularly oxygen, pick-up during processing. For intermetallic based composites, a broad range of processes is already being investigated. In many of these, issues of cleanness may be more difficult to control than for the monolithic composites. With continuous fibre reinforced composites a further process impetus is the need to control interfacial interactions between the fibre and the matrix.

MST/1560