On the tensile behaviour of filled composites

The shape and concentration of reinforcing fillers and the mechanical properties of the matrix influence the tensile behaviour of composites. The Young's modulus, tensile strength and elongational viscosity of composites containing different shaped fillers (glass beads, wollastonite and talcum) are predicted from simple models using a shape factor for each of the fillers.     

Comparative behaviour of hollow columns confined with FRP composites

Confining columns with fibre reinforced polymer (FRP) composites have been investigated in the last few decades to address the problem of upgrading and retrofitting reinforced concrete (RC) columns; however, most studies have concentrated on solid columns. This paper investigates the comparative behaviour of FRP confined hollow RC columns subjected to concentric loading. A total of twelve RC columns made from high strength concrete (HSC) were cast and tested. Six of the columns had a circular cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole) and the remainder columns had a square cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole). Six columns in total, three from each configuration were left unconfined as control specimens, while the others were confined with FRP. It was found that FRP confinement increased hollow RC columns’ axial load and ductility capacities; and hollow columns having circular holes had better performance compared to hollow columns having square holes.     

Fracture behaviour of Al-Zn-Mg/SiCp composites

Al-Zn-Mg alloys reinforced with different volume fractions of SiC particulates were prepared by a liquid-metallurgy technique. The mechanical properties in uniaxial tension and compression were evaluated, and fractographic observations were made on the fracture surfaces. The distribution of SiC was quite uniform in the extruded condition, and the mechanical-property data show that the composite properties were inferior to those of the control alloy; they essentially showed a decreasing trend with increasing volume fractions. These observations can be explained in terms of the particle distribution, the porosity and the interfacial characteristics.     

Fracture behaviour of pressure die-cast aluminium-graphite composites

Fracture toughness values of pressure die-cast Al-7Si-3 Mg-5 graphite composites were measured and found to be in the range 8–10 MPa m^1/2. Detailed microstructure of the composite and the fractured surfaces were examined. Defects such as clusters, agglomerations and segregation of graphite particles, play a dominant role in accelerating the fracture process. In addition, the acicular silicon phase present in the matrix and the casting defects, such as gas and shrinkage porosities, also initiated and accelerated the crack, thus lowering the fracture toughness of the composites.     

Effect of fibre angle on fracture properties of unidirectional flyash filled FRP composites

The fracture properties of unidirectional flyash filled and unfilled glass fibre and carbon fibre reinforced epoxy resin composites are studied in relation to the variation of width ratio (a/W) and fibre angle. The results indicate that the fracture toughness, fracture surface energy and change in elastic strain energy are dependent on the width ratio but the effect of fibre angle between 30 and 60° is not very dependent on fracture properties due to the arrest of the crack path in fibre composites by flyash particles.     

Fracture mechanisms of epoxy-based ternary composites filled with rigid-soft particles

Epoxy composites filled with different amounts of aggregate-free silica nanoparticles and phase-separated submicron rubber particles were fabricated to study the synergistic effect of multi-phase particles on mechanical properties of the composites. Compared with binary composites with single-phase particles, the ternary composites with both rigid and soft particles offer a good balance in stiffness, strength and fracture toughness, showing capacities in tailoring the mechanical properties of modified epoxy resins. It was observed that debonding of silica nanoparticles from matrix in the ternary composites was less pronounced than that in the binary composites. Moreover, the rubber particles became smaller and their shape tends to be irregular, affected by the presence of rigid silica nanoparticles. The toughening mechanisms in the epoxy composites were evaluated, and the enlarged plastic deformation around the crack tip, induced by the combination of rigid and soft particles, seems to be a dominant factor in enhancing fracture toughness of the ternary composites.     

Fracture behaviour of SiC fibre-reinforced nitrogen glass matrix composites

Both Nicalon and Hi-Nicalon SiC fibre-reinforced nitrogen glass composites were prepared by slurry infiltration and hot-pressing, and the interfacial features, fracture behaviour and mechanical properties of these composites were investigated. It was found that the interfacial characteristics were mainly dictated by the thermal expansion properties of the matrix and the type of SiC fibre. Yttrium sialon glass has a higher thermal expansion coefficient than SiC fibres, so a radial compressive stress on the fibre due to thermal mismatch caused a larger interfacial frictional stress between fibre and matrix. As a result, the composite failed in a brittle manner with no effective strengthening and toughening. Strong reaction between the Hi-Nicalon SiC fibre and matrix also resulted in relatively poor performance of these composites. In contrast, lithium sialon glass provided a matrix for these composites with significantly improved mechanical properties.     

Tensile fatigue behaviour of FRP and hybrid FRP sheets

This paper presents the fatigue behaviour of various fibre reinforced polymer (FRP) composites, namely, carbon, glass, polyparaphenylenl benzobisoxazole (PBO), and basalt fibres, including the effect of hybrid applications such as carbon/glass and carbon/basalt composites. A coupon test was conducted to examine the mechanical characteristics of the FRP composites subjected to monotonic and cyclic loads. Test parameters included the applied load range and different types of hybridization. Study results show that (1) the mechanical properties of the emerging PBO and basalt fibres are comparable to those of the conventional carbon and glass fibres; (2) the tensile modulus of the fibres influences the failure mode of the composite coupons; (3) the progressive damage propagation causes fatigue failure of the composites; (4) the hybrid composites of carbon/basalt significantly improves the fatigue resistance in comparison to the homogeneous basalt composite, whereas the resistance of the carbon/glass hybrid composites does not provide such effects.     

Fracture behaviour of composites based on Al2O3-TiC

The critical stress intensity factor and other related fracture parameters have been measured in three-point bending for pure Al₂O₃-TiC composites containing 4 to 35 volume fractions of TiC. An increase has been observed for all the parameters with increasing volume fraction of TiC. Following a study of the mode of fracture, the results are explained in terms of a linear variation of the fracture energy with the volume fraction of TiC.     

Fracture behaviour of hybrid glass matrix composites: thermal ageing effects

The thermal aging of a glass matrix composite reinforced by short carbon fibres as well as by ZrO₂ particles (hybrid composite) was investigated at temperatures in the range 500–700 °C for exposure durations of 24 h in air. The mechanical properties of as-received and aged samples were evaluated at room temperature by using the three-point flexure chevron notch technique. The fracture toughness values of as-received specimens were in the range 2.6–6.4 MPa m^1/2. Fracture toughness was affected by the thermal aging conditions. For thermal aging at temperatures <700 °C, degradation of fibre–matrix interfaces occurred and therefore the apparent fracture toughness and flaw tolerant resistance decreased. For the most severe ageing conditions tested (700 °C/24 h), fracture toughness values dropped to 0.4 MPa m^1/2. Significant degradation of the material was detected for this aging condition, mainly characterised by porosity formation in the matrix as a result of softening of the glass and oxidation of the carbon fibres.     

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) or Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10^–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al₂O₃/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.     

Fracture toughness and electrical conductivity of epoxy composites filled with carbon nanotubes and spherical particles

The attainment of both high toughness and superior electrical conductivity of epoxy composites is a crucial requirement in some engineering applications. Herein, we developed a strategy to improve these performances of epoxy by combining the multi-wall carbon nanotubes (MWCNTs) and spherical particles. Two different types of spherical particles i.e. soft submicron-rubber and rigid nano-silica particles were chosen to modify the epoxy/MWCNT composites. Compared with the binary composites with single-phase particles, the ternary composites with MWCNTs and spherical particles offer a good balance in glass transition temperature, electrical conductivity, stiffness and strength, as well as fracture toughness, exhibiting capacities in tailoring the electrical and mechanical properties of epoxy composites. Based on the fracture surface analysis, the complicated interactions between multiscale particles and the relative toughening mechanisms were evaluated to explain the enhancement in fracture toughness of the ternary composites.     

Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan δ and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tanδ and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan δ and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan δ) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan δ peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2–91 wt% graphite leads to an increased relaxation time τ of dipoles in polysulphide epoxy from 1.44 × 10⁻⁵− 3.92 × 10⁻⁵ (s) at 90°C by creating the hindrance to the rotation of dipoles.     

Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

The fracture behaviour of two toughened epoxy composite systems was investigated using various microscopy techniques. The Mode I delamination fracture toughness,G _IC, Mode II delamination fracture toughness;G _IIC, and compression after impact (CAI) strength of these model composite systems were also measured. Under Mode I fracture, it was found that these composites exhibit nearly identical toughening mechanisms to those of the rubber-modified neat resins. The composites differ primarily in having smaller damage zones than the neat resin equivalents. Under Mode II fracture, the typical hackles were found to initiate from inside the resin-rich interlaminar region due to the presence of the toughener particles. The CAI strength, based on the present study as well as the work conducted by others, appeared to be related to, but not necessarily strongly dependent on, the interlaminarG _IC andG _IIC, the thickness of the interlaminar resin-rich region, and the type of the interlaminar toughener particles. Approaches for improving theG _IC,G _IIC, and CAI strength of high-performance toughened composites are discussed.     

Probabilistic failure prediction for FRP composites

Results are presented from a theoretical study on failure-locus prediction for unidirectional FRP laminae under complex in-plane loading, taking into account statistical aspects of the basic strengths of the material. The purpose of the analysis is to establish simple rules and methodologies for failure prediction under specific reliability requirements. Therefore, the problem one is faced with is the definition of the cumulative distribution function (CDF) of the failure condition if the respective CDFs of the basic material strengths are known by experiment. To this end, two analytical approaches, namely a functional expansion technique and the introduction of Pearson’s semi-empirical distribution function, were developed and implemented in software. Both methods were shown to predict satisfactory results compared with Monte Carlo simulated ones and experimental data, wherever available. Finally, a semi-deterministic approach was examined according to which the failure criterion is used in the usual deterministic way but with basic strength values of a certain reliability level. Results from this simple and fast method were found in good agreement with those derived by expensive pure statistical methods or numerically simulated ones and experimental data.     

Fracture behaviour at elevated temperatures of alumina matrix composites reinforced with silicon carbide whiskers

Creep tests were performed on alumina matrix composites containing 9.3,18 and 30 vol % of SiC whiskers. Careful examination after failure showed that the fracture characteristics were dependent upon the microstructure of the material. Whisker pullouts were visible at the fracture surfaces of the composites with 30 and 18 vol % of SiC whiskers due to preferential debonding at the whisker-matrix interfaces, and these composites also developed crack networks due to the propagation of cracks along whisker-free channels between whisker agglomerates. No whisker pullouts or crack networks were visible in the composite with 9.3 vol % of SiC whiskers where the whisker distribution was reasonably uniform.