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.     

Photoelectrocatalytic properties and bactericidal activities of silver-treated carbon nanotube/titania composites

Two types of carbon nanotube/TiO₂ and silver-treated carbon nanotube/TiO₂ electrodes were prepared and characterized by X-ray diffraction, transmission and scanning electron microscopy, energy dispersive X-ray analysis, photoelectrocatalytic activity and antibacterial activity. The photoelectrocatalytic degradation of a methylene blue solution was attributed to the combined effects of the photo-degradation of TiO₂, the electron assistance of carbon nanotube network and the enhancement of silver and was a function of the applied potential. The composites treated with silver showed enhanced photo-degradation of methylene blue, and the photoelectrocatalytic activity increased with increasing amount of silver. The silver-treated carbon nanotube/TiO₂ prepared as bactericides have stronger antibacterial activity against Escherichia coli K-12 than standard ampicillin, tetracycline and carbon nanotube/TiO₂ under sunlight or dark conditions. The presence of silver in the silver-treated carbon nanotube/TiO₂ composites enhanced the inactivation of the E. coli K-12.     

Investigations on grinding process of woven ceramic matrix composite based on reinforced fiber orientations

Fiber orientations play the decisive role in grinding process of woven ceramic matrix composites, but the influence of woven fibers in grinding process is not clear. This paper studies the surface quality and grinding force by comparing different woven surfaces. Through a series of experiments in optimized sampling conditions, we analyze characteristics of the material surface topography height, wave distribution and surface support properties in details. And we find some outstanding characteristics of the surface microstructure. We also study the influence of grinding processing parameters on surface microstructure. The results show that machining surface which contains more parallel fibers is rougher and more keenness than gauss surface. Grinding wheel speed and depth of cut have great influence on surface topography and surface support properties. And it is discovered that grinding forces are also highly dependent on fiber orientations. The mechanism of the grinding phenomena is also analyzed in this paper according to knowledge of fracture mechanics and mechanical damage phenomenology. The research obtained will be an important technical support on improving the processing quality of woven ceramic matrix composites.     

Probabilistic multiconstraints optimization of cooling channels in ceramic matrix composites

This paper presents a computational reliable optimization approach for internal cooling channels in Ceramic Matrix Composite (CMC) under thermal and mechanical loadings. The algorithm finds the optimal cooling capacity of all channels (which directly minimizes the amount of coolant needed). In the first step, available uncertainties in the constituent material properties, the applied mechanical load, the heat flux and the heat convection coefficient are considered. Using the Reliability Based Design Optimization (RBDO) approach, the probabilistic constraints ensure the failure due to excessive temperature and deflection will not happen. The deterministic constraints restrict the capacity of any arbitrary cooling channel between two extreme limits. A “series system” reliability concept is adopted as a union of mechanical and thermal failure subsets. Having the results of the first step for CMC with uniformly distributed carbon (C-) fibers, the algorithm presents the optimal layout for distribution of the C-fibers inside the ceramic matrix in order to enhance the target reliability of the component. A sequential approach and B-spline finite elements have overcome the cumbersome computational burden. Numerical results demonstrate that if the mechanical loading dominates the thermal loading, C-fibers distribution can play a considerable role towards increasing the reliability of the design.     

High-temperature dielectric and electromagnetic interference shielding properties of SiCf/SiC composites using Ti3SiC2 as inert filler

Ti₃SiC₂ filler has been introduced into SiC_f/SiC composites by precursor infiltration and pyrolysis (PIP) process to optimize the dielectric properties for electromagnetic interference (EMI) shielding applications in the temperatures of 25–600 °C at 8.2–12.4 GHz. Results indicate that the flexural strength of SiC_f/SiC composites is improved from 217 MPa to 295 MPa after incorporating the filler. Both the complex permittivity and tan δ of the composites show obvious temperature-dependent behavior and increase with the increasing temperatures. The absorption, reflection and total shielding effectiveness of the composites with Ti₃SiC₂ filler are enhanced from 13 dB, 7 dB and 20 dB to 24 dB, 21 dB and 45 dB respectively with the temperatures increase from 25 °C to 600 °C. The mechanisms for the corresponding enhancements are also proposed. The superior absorption shielding effectiveness is the dominant EMI shielding mechanism. The optimized EMI shielding properties suggest their potentials for the future shielding applications at temperatures from 25 °C to 600 °C.     

Modelling and simulation of earthquake resistant 3D woven textile structural concrete composites

Concrete is a composite material composed of water, sand, coarse granular material called aggregate and cement that fills the space among the aggregate particles and glues them together. Conventional building structures are made up of steel skeleton with concrete impregnation. These are very heavy weight structures with steel vulnerable to corrosion. The conventional concrete structures tend to undergo large deformations in the event of a strong earthquake. Mechanical simulation of various textile structural concretes is carried out successfully for their ductility behaviour. 3D woven reinforced concretes display superior ductile character showing ray of hope to develop seismic resistant building. Simulation of three 3D woven fabrics and their composites was carried to predict ductility and strengths of fabric reinforced concrete structures. Maximum deformation was observed for beam reinforced with orthogonal interlock fabric under the same load and minimum deformation was observed for plain concrete. Maximum equivalent stress was observed to be highest for plain concrete followed by beam reinforced with angle interlock fabric followed by orthogonal fabric and warp interlock fabric under similar loading conditions. From the results it was clear that 3D fabric reinforced structures are more ductile than the traditional steel reinforced structures. Hence 3D fabric reinforced concrete structures are much better in strength and ductility as compared to conventional construction materials. Among the three 3D fabric, orthogonal fabric reinforced composites are most ductile and are also less stiff. They can deform more than the other two fabric composites. Hence, orthogonal fabric reinforced composites can undergo higher deformations without collapsing. These composites can be more elastic under earthquake shaking.     

Consolidation of polymer-derived SiC matrix composites: processing and microstructure

SiC fiber reinforced SiC matrix composite (SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling physical characteristic, polyvinylsilane (PVS) that is liquid thermosetting organo-silicic compound was utilized as the matrix precursor. Based on the pyrolytic behavior of PVS, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites after multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Structural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed.     

Multi-walled carbon nanotube/nanostructured zirconia composites: Outstanding mechanical properties in a wide range of temperature

Multi-walled carbon nanotube (MWCNT)/nanostructured zirconia composites with a homogenous distribution of different MWCNT quantities (ranging within 0.5-5 wt.%) were developed. By using Spark Plasma Sintering we succeeded in preserving the MWCNTs firmly attached to zirconia grains and in obtaining fully dense materials. Moreover, MWCNTs reduce grain growth and keep a nanosize structure. A significant improvement in room temperature fracture toughness and shear modulus as well as an enhanced creep performance at high temperature is reported for the first time in this type of materials. To support these interesting mechanical properties, high-resolution electron microscopy and mechanical loss measurements have been carried out. Toughening and creep hindering mechanisms are proposed. Moreover, an enhancement of the electrical conductivity up to 10 orders of magnitude is obtained with respect to the pure ceramics.     

Multi-walled carbon nanotube-reinforced silicon carbide fibers prepared by polymer-derived ceramic route

Multi-walled carbon nanotube (MWNT)-reinforced silicon carbide (SiC) ceramic fibers were successfully prepared by blending MWNTs (0-0.5 wt.%) with polycarbosilane, followed by melt spinning, curing, and pyrolysis. The MWNTs used in this study were modified with a chemical treatment. It was found that the MWNTs were well-dispersed in the matrix and aligned with the axis of the fibers after ultrasonic dispersion combined with melt spinning. Mechanical measurements revealed that significant improvement in Young’s modulus and tensile strength was achieved by incorporating MWNTs into the ceramic fibers. The addition of 0.5 wt.% MWNTs led to a 93.6% increase in the Young’s modulus and a 38.5% increase in the tensile strength.     

Thermo-mechanical performance of an ablative/ceramic composite hybrid thermal protection structure for re-entry applications

Hybrid thermal protection systems for aerospace applications based on ablative material (ASTERM™) and ceramic matrix composite (SICARBON™) have been investigated. The ablative material and the ceramic matrix composite were joined using graphite and zirconia–zirconium silicate based commercial high temperature adhesives. The thermo-mechanical performance of the structures was assessed from room temperature up to 900 °C. In all the joints there is a decrease of shear strength with the increase of temperature. Analysis of the fractured surfaces showed that above 150 °C the predominant mode of fracture is cohesive failure in the bonding layer. The joints fabricated with the zirconia–zirconium silicate based adhesive present the best performance and they have the potential to be used as hybrid thermal protection systems for aerospace applications in the temperature range 700–900 °C.     

Damage-based failure theory and its application to 2D-C/SiC composites

Damage evolution is of great importance to determine the final failure of ceramic matrix composites (CMCs). In order to characterize the damage coupling behavior and its effect on failure strength of CMCs, a new methodology for damage coupling analysis was formulated. And, new kind of damage-based failure criteria were established under plane stress state, including maximum damage criterion and quadratic damage criterion. Both the criteria require investigation upon the damage evolution laws. The proposed failure theory was applied to predict the coupled damage behavior and the off-axis strength of a 2D C/SiC composite. The theoretical results agree well with the experimental data.     

Experimental investigation of three-dimensional woven composites

This paper summarizes an extensive experimental study of composites reinforced with three-dimensional woven preforms subjected to tensile, compressive and in-plane shear loading. Three innovative three-dimensional woven architectures were examined that utilize large 12 K and 24 K IM7 carbon tows, including two ply to ply angle interlock architectures and one orthogonal architecture. Additionally, a two-dimensional quasi-isotropic woven material was evaluated for comparison. Loads were applied in both the warp and the weft directions for tensile and compressive loading. Digital image correlation was used to investigate full field strains leading up to quasi-static failure. Experimental results including ultimate strengths and moduli are analyzed alongside representative failure modes. The orthogonal woven material was found to have both greater strength and modulus in tension and compression, though a ply to ply woven architecture was found to outperform the remaining three-dimensional architectures. Recommendations are made for improving the manufacturing processes of certain three-dimensional woven architectures.     

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si₃N₄) composites with various amounts (0-8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc-ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si₃N₄/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube-nanotube contacts and across intrinsic defect clusters within the nanotubes.     

Effects of Cf/C density on microstructure and properties of 3-D Cf/ZrC composites prepared by liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composites, 3-D C_f/ZrC, were fabricated by Liquid metal infiltration process at 1200 °C. Porous carbon/carbon (C_f/C) composites with various densities were used as preforms, and the effects of C_f/C density on microstructure and properties of the 3-D C_f/ZrC composites were investigated. The results show that the composites are composed of carbon, ZrC and residual metal. Both microstructure and properties of the 3-D C_f/ZrC composites are apparently affected by C_f/C density. With increasing density of C_f/C preform, the density of 3-D C_f/ZrC composites decreases while the open porosity increases. The composites obtained from the C_f/C preform with a density of 1.12 g/cm³ have the best mechanical properties, with flexural strength of 286.2 ± 11.4 MPa, elastic modulus of 83.5 ± 6.8 GPa and fracture toughness of 9.2 ± 0.6 MPa m^1/2. The composites exhibit excellent ablation resistance, and the mass rate and the linear ablation rate under an oxyacetylene torch are as low as 5.1 ± 0.4 mg s⁻¹ and 1.1 ± 0.3 μm s⁻¹, respectively.     

Prediction of first matrix cracking in micro/nanohybrid brittle matrix composites

The classical Aveston–Cooper–Kelly shear-lag model for predicting the first matrix cracking strength in a brittle matrix composite is extended to the case of a hybrid brittle matrix composite containing both micro-scale and nano-scale fibers. First, closed-form solutions for the stresses in the two types of fibers and the matrix are derived. These are then used along with an energy analysis to predict the matrix cracking stress as a function of relevant material parameters. The analysis is applied to a typical Nicalon-SiC/CVI-SiC ceramic matrix composite containing additional nanofibers, for a wide range of nanofiber properties. A few volume percent of small diameter, moderate-stiffness nanofibers is predicted to provide significant strengthening and reduced crack opening while maintaining acceptable post-cracking fiber stresses. Various issues in the design of such micro/nanohybrid composites are then discussed.     

Enchanced high-temperature performances of SiC/SiC composites by high densification and crystalline structure

We report the enchanced in situ performances of tensile strength and thermal conductivity at elevated temperatures of the PCS-free SiC/SiC composite with a high fiber volume fraction above 50% fabricated by NITE process for nuclear applications. The composite was fabricated by the optimized combination of the fiber coating, the matrix slurry and the pressure-sintering conditions, based on our previous composites’ study history. The composite showed the excellent tensile strength up to 1500 °C, that it retained approximately 88% of the room-temperature strength. Also, the thermal conductivity of the composites represented over 20 W/m K up to 1500 °C, which was enough high to take the advantage of the assumed design value for nuclear applications. Microstructural observation indicated that the excellent high-temperature performances regarding tensile strength and thermal conductivity up to 1500 °C were the contribution to the high densification and crystalline structure in matrix.     

Effects of weave architecture on mechanical response of 2D ceramic composites

A meso-scale finite element model is developed to investigate effects of weave architecture on strain and stress evolution in an eight harness-satin SiC/SiCN composite. Fiber tows are modeled explicitly using elastic rebar layers embedded within elastic/plastic effective medium elements. Effects of through-thickness constraint are investigated using several idealized test geometries, ranging from a single (unconstrained) ply to a fully-constrained two-ply lay-up with periodic boundary conditions in the through-thickness direction. A parallel experimental study of surface strain evolution in a representative SiC/SiCN composite is used to assess the model predictions. The results indicate that, because of bending and straightening of wavy tow segments at the locations of tow cross-overs, strain and stress concentrations arise. The effects are exacerbated by reductions in the constraints on bending and straightening caused by matrix damage, especially in surface plies. The implications of the results in the fracture process and on potential mitigation strategies are discussed.     

Infrared and microwave absorbing properties of BaTiO3/polyaniline and BaFe12O19/polyaniline composites

BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites were synthesized by in situ polymerization and introduced into epoxy resin and polyethylene to be microwave and infrared absorber. The spectroscopic characterizations of the formation processes of BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites were examined using Fourier transform infrared, ultraviolet–visible spectrophotometer, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron spin resonance. Microwave absorbing properties were investigated by measuring reflection loss in the 2–18 and 18–40 GHz microwave frequency range using the free space method. The thermal extinction measurements in the 3–5 and 8–12 μm were done to evaluate the shielding affectivity of infrared. The results showed that the BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites have good compatible dielectric and magnetic properties and hence the microwave absorbency show broad frequencies absorbing properties. Moreover, the infrared thermal image testing that the detecting ability of infrared thermal imaging was decreased when the BaFe₁₂O₁₉ and BaTiO₃ was coating with polyaniline.     

The fracture energy of metal fibre reinforced ceramic composites (MFCs)

A model is presented for prediction of the fracture energy of ceramic-matrix composites containing dispersed metallic fibres. It is assumed that the work of fracture comes entirely from pull-out and/or plastic deformation of fibres bridging the crack plane. Comparisons are presented between these predictions and experimental measurements made on a commercially-available composite material of this type, containing stainless steel (304) fibres in a matrix predominantly comprising alumina and alumino-silicate phases. Good agreement is observed, and it’s noted that there is scope for the fracture energy levels to be high (∼20 kJ m⁻²). Higher toughness levels are both predicted and observed for coarser fibres, up to a practical limit for the fibre diameter of the order of 0.5 mm. Other deductions are also made concerning strategies for optimisation of the toughness of this type of material.