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ABSTRACT
The fibre–matrix interface represents a vital element in the development and characterisation of fibre-reinforced polymers (FRPs). Extensive ranges of interfacial properties exist for different composite systems, measured with various interface characterisation techniques. However, the discrepancies in interfacial properties of similar fibre–matrix systems have not been fully addressed or explained. In this review, first, the interface-forming mechanisms of FRPs are established. Following a discourse on three primary factors that affect the fibre–matrix interface, the four main interface characterisation methods (single-fibre fragmentation, single-fibre pull-out, microbond and fibre push-in/-out tests) are described and critically reviewed. These sections review various detailed data reduction schemes, numerical approaches, accompanying challenges and sources of reported scatter. Finally, following the assessment of several infrequent test methods, comprehensive conclusions, prospective directions and intriguing extensions to the field are provided.
Disclosure statement
No potential conflict of interest was reported by the author(s).
| List of abbreviations and symbols | ||
| 45FBT | = | 45° fibre bundle tensile test |
| AE | = | Acoustic emission |
| AFM | = | Atomic force microscopy |
| ANN | = | Artificial neural network |
| BAM | = | Federal institute for materials research and testing |
| BEM | = | Boundary element method |
| CF | = | Carbon fibre |
| CFRP | = | Carbon fibre-reinforced polymer |
| CKT | = | Cottrell–Kelly–Tyson model |
| CMC | = | Ceramic matrix composites |
| CNT | = | Carbon nanotubes |
| CT | = | Computed tomography |
| CTE | = | Coefficient of thermal expansion |
| CZM | = | Cohesive zone model |
| DEM | = | Discrete element method |
| EPZ | = | Embedded process zone model |
| FBG | = | Fibre Bragg grating |
| FE(M) | = | Finite element (method) |
| FRP | = | Fibre-reinforced polymer |
| GF | = | Glass fibre |
| GFRP | = | Glass fibre-reinforced polymer |
| HM | = | High modulus carbon fibre |
| IFFT | = | Interfacial fracture toughness |
| IFNS | = | Interfacial normal (radial) strength |
| IFSS | = | Interfacial shear strength |
| IFSSapp | = | Apparent interfacial shear strength |
| ILSS | = | Interlaminar shear strength |
| IMD | = | Intermediate modulus |
| LRS | = | Laser Raman spectroscopy |
| MB (MBT) | = | Microbond test |
| MFFT | = | Multi-fibre fragmentation test |
| MRS | = | Micro-Raman spectroscopy |
| PA | = | Polyamide |
| PC | = | Polycarbonate |
| PEEK | = | Polyether ether ketone |
| PEI | = | Polyetherimide |
| PP | = | Polypropylene |
| PPS | = | Polyphenylene sulphide |
| SCF | = | Stress (or strain) concentration factor |
| SEM | = | Scanning electron microscopy |
| SERR | = | Strain energy release rate |
| SFFT | = | Single-fibre fragmentation test |
| SLM | = | Shear-lag model |
| TFBT | = | Transverse fibre bundle tensile test |
| TP | = | Thermoplastic |
| = | Embedded area | |
| = | Crack length | |
| = | Interface effective thickness | |
| = | Change in crack length | |
| = | Change in compliance | |
| = | Fibre diameter | |
| = | Energy summation proposed by Marshall and Oliver | |
| = | Change of the elastic energy inside the fibre | |
| = | Work of friction in the interface | |
| = | Debonding energy associated with the new debonded area | |
| = | Potential energy of the loading system | |
| = | Change in matrix elastic energy | |
| = | Longitudinal Young's modulus of the model composite | |
| = | Axial Young's modulus of the fibre | |
| = | Matrix Young's modulus | |
| = | Transverse Young's modulus of the fibre | |
| = | Force–displacement | |
| = | Initial post-debonding force | |
| = | Catastrophic failure load | |
| = | Debonding force | |
| = | Maximum frictional force | |
| = | Maximum load | |
| = | Shear force | |
| = | Strain energy release rate (fracture toughness) | |
| = | Interfacial fracture toughness | |
| = | Shear modulus of the interface | |
| = | Matrix shear modulus | |
| = | Strain energy release rate for debond propagation | |
| = | Interfacial mode II fracture toughness | |
| = | Height in contact angle | |
| = | Slope of the force-displacement curve | |
| = | Fibre free length stiffness | |
| = | Cohesive stiffness | |
| = | Droplet length | |
| = | Fibre length, axial location of the crack front | |
| = | Arithmetic mean of the fragment lengths at saturation | |
| = | Critical fibre length | |
| = | Fibre embedded length shorter than | |
| = | Debond length | |
| = | Embedded fibre length | |
| = | Critical embedded length | |
| = | Fibre free-length | |
| = | The point where the results of FEM, variational mechanics and SLM converge | |
| = | Maximum fragment length | |
| = | Maximum fibre length beyond which catastrophic debonding does not occur | |
| = | Maximum fibre length to surpass the frictional dissipation of energy | |
| = | A parameter acquired from the slope of the | |
| = | Applied load | |
| = | Critical load at the debond initiation | |
| = | Debonding load | |
| = | Normal pressure exerted on the fibre due to the matrix shrinkage during cure | |
| = | Axial distance at which | |
| = | Equivalent cylinder radius | |
| = | Indentation position to the fibre centre | |
| = | Slope of the linear region in a push-in load-displacement curve | |
| = | Tensile force on fibre | |
| = | Glass transition temperature | |
| = | Tensile force on matrix | |
| = | Deformation in | |
| = | Total recorded displacement throughout the push-in test | |
| = | Elastoplastic indentation of the fibre surface | |
| = | Fibre surface displacement due to the fibre compression | |
| = | Droplet volume | |
| = | Fibre volume fraction | |
| = | Matrix volume fraction | |
| = | Deformation in | |
| = | Work required to separate the two neighbouring molecular layers of the fibre and the matrix, Work of adhesion | |
| = | Thickness of a push-out specimen (equal to the fibre length) | |
| = | Cross section area of a square specimen | |
| = | Fibre axial axis | |
| = | The | |
| = | Axial thermal expansion coefficients of the fibre | |
| = | Transverse thermal expansion coefficients of the fibre | |
| = | Thermal expansion coefficient of the matrix | |
| = | Shear-lag parameter | |
| = | Cox shear-lag parameter | |
| = | Geometrical correction factor | |
| = | Nayfeh shear-lag parameter | |
| = | Elastic deformation energy of the fibre, matrix and bending of the sample | |
| = | Work of friction | |
| = | Plastic deformation energy of fibre, matrix, and interface | |
| = | Temperature difference | |
| = | Separation in traction-separation | |
| = | Applied strain, Fibre axial strain distributions | |
| = | Fibre strain | |
| = | Matrix strain | |
| = | Contact angle | |
| = | Frictional stress transfer rate | |
| = | Effective normal displacement between the contacting surfaces required for their separation | |
| = | Interfacial friction coefficient | |
| = | Fibre Poisson's ratio | |
| = | Axial Poisson's ratios of the fibre | |
| = | Transverse Poisson's ratios of the fibre | |
| = | Poisson's ratio of the Matrix | |
| = | Net axial stress, Axial stress at the minimum cross-section of the specimen | |
| = | Longitudinal stress in a model composite | |
| = | Debonding initiation stress, Adhesion pressure | |
| = | Fibre failure strength | |
| = | Interfacial tensile stress | |
| = | Normal stress | |
| = | Radial stress in variational mechanics | |
| = | Critical radial stress | |
| = | Critical radial stress value at the onset of the debond initiation | |
| = | Cross-sectional average axial stress of fibre | |
| = | Matrix shear yield strength | |
| = | Apparent interfacial shear strength | |
| = | Local interfacial shear strength | |
| = | Interfacial frictional sliding stress (post-debond frictional shear stress) | |
| = | Interfacial shear stress | |
| = | Interfacial shear strength | |
| = | Shear stress of the matrix | |
| = | Maximum interfacial shear stress | |
| = | Actual interfacial shear strength | |
| = | Maximum interfacial shear stress obtained from laser Raman spectroscopy | |
| = | Interfacial shear strength obtained with the shear-lag model | |
| = | Maximum residual shear stress | |
| = | Interfacial shear stress in variational mechanics | |
| = | Residual thermal stresses | |
| = | Ultimate interfacial shear strength | |