Thermal shock resistance of SiC fibrerein-forced borosilicate glass and lithium aluminosilicate matrix composites

Thermal shock resistance of an SiC fibre-(Nicalon®) reinforced borosilicate glass (Pyrex) and lithium aluminosilicate (LAS) matrix composite has been investigated experimentally in the temperature range 0–1000 K. Longitudinal Young's modulus and flexure strength of the composites after thermal shock were obtained as a function of thermal shock temperature. The results are discussed with the observed damage of the composite. The borosilicate glass matrix composite showed multiple cracking of the glass matrix perpendicular to the fibre axis when the thermal shock temperature was above 600 K. Decreases in Young's modulus and flexure strength were also recognized after multiple cracking of the matrix was initiated. On the other hand, the LAS matrix composite showed no damage at thermal shock temperatures below 800 K. However, at 800 K and above, microcracking of the matrix along the fibre axis was observed. After thermal shock, no decrease in the flexure strength was recognized, while the Young's modulus decreased due to microcracking of the matrix when the thermal shock temperatures were 800 K and above. It was found that the major advantage of the composite against thermal shock was to retain non-catastrophic failure properties even after the development of thermally induced damage in the composite.     

Influence of shear properties and fibre imperfections on the compressive behaviour of GFRP laminates

The influence of shear strength properties and fibre misalignment on the compressive behaviour of unidirectional glass fibre-polypropylene laminates has been examined. Tests were conducted between 20°C and 120°C to provide variation in the constitutive behaviour of the polymer matrix and consequently variation in the support provided to the glass fibres. It was found that the laminate loses strength as the operating temperature increases and failure occurs due to fibre microbuckling. At temperatures higher than 50°C the failure mode switches from in-plane to out-of-plane microbuckling. As the test temperature increases the shear strength and stiffness of the resin are considerably reduced; this decreases the amount of side support for the fibres and reduces the strain level at which fibre buckling initiates. Growth of this damage requires little additional load, suggesting that compression strength is controlled by initiation, rather than propagation of microbuckling. Fracture characteristics have been identified using optical and scanning electron microscopy. Recent theoretical models have been employed to predict the compressive stress-strain response and strength.     

Carbon fibre reinforced glass matrix composite tension and flexure properties

Carbon fibre reinforced borosilicate glass matrix composites have been fabricated to determine their mechanical properties in tension and flexure. Composite tensile stress-strain properties, including elastic modulus, proportional limit and ultimate tensile strength, have been measured as a function of fibre content. Composite tensile properties were also obtained at temperatures of up to 625° C through the testing of 0/90 cross-plied specimens. Composite short-beam shear strength was found to depend on specimen orientation and also on the composition of the glass matrix. This compositional dependence was associated with an independent measurement of the fibre-matrix interfacial shear strength and was related to the degree of fibre-matrix reaction taking place during composite fabrication.     

Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply Aramid Fibre (KRP) Tubes Under Hoop Loading

A comprehensive study was undertaken to characterise Kevlar reinforced plastic (KRP) angle ply filament wound tubes at different temperatures. Quasi-static burst tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. An experimental rig and two conditioning tanks were designed and built to test the specimens at three temperatures; –46°C (low temperature) and +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded on suitable digital processing equipment.For a particular batch of tubes tested at three different temperatures, an increase in ultimate hoop strain and a decrease in hoop modulus of the 55° tubes with increasing temperatures was recorded; the temperature effect was less pronounced on the corresponding properties of 25° and 75° tubes. The use of a non-structural thin liner during the tests led to a higher ultimate strength of 55° tubes but had negligible effect on the behaviour of 25° and 75° tubes. The 75° tubes failed in a catastrophic fibre fracture under all test conditions. The mode of failure of 55° changed from weeping at 70°C to fibre fracture at –46°C. The 25° tubes failed by weeping with matrix cracking. The matrix cracking was particularly severe when a liner was used.     

The environmental response of hybrid composites

Hybrid composite specimens containing a total of 60 or 75 vol % of unidirectional fibre were prepared from HT S-carbon fibre and E-glass fibre, HT S-carbon fibre and Kevlar 49 fibre, and E-glass fibre and Kevlar 49 fibre with a standard anhydride cured epoxide resin. The specimens were divided into four groups and subjected to the following environments: (A) room temperature and humidity; (B) soaked in water for 300 h at 95° C and then oven dried at 60° C to a constant weight; (C) thermally cycled 100 times between –196 and 95° C; (D) cycled 35 times between –196 and water at 95° C. The flexural properties of the samples were measured at room temperature after exposure. The modulus of the hybrid materials was not significantly affected by any of the treatments, although thermal cycling with or without water caused a large decrease in the modulus of all Kevlar fibre/resin and to a lesser extent all glass fibre specimens. The flexural strength of the unexposed carbon fibre/glass fibre and glass fibre/Kevlar fibre hybrids showed a positive deviation from the rule of mixtures behaviour at low volume loadings of the lower extension fibre. Wet thermal cycling or soaking in water caused a substantial reduction in the flexural strength of glass fibre/Kevlar fibre specimens. The interlaminar shear strength of all three fibre combinations was not affected by dry thermal cycling, but the effects of soaking in water and especially thermal cycling with water exposure were significant and irreversible.     

Temperature and Rate Effects on GRP Tubes Under Tensile Hoop Loading

A comprehensive study was undertaken to characterise glass fibre reinforced plastic (GRP) tubes at different temperatures and strain rates. The tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. Two separate rigs were used, one for the static and the other for the dynamic tests. The tests were carried out at three temperatures; –46°C (low temperature), +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded on suitable digital processing equipment. For a particular batch of tubes tested at three different temperatures, there is in general a decrease in hoop strength with increasing temperature during quasi-static tests. The use of a non-structural liner during such tests led to an increase in ultimate hoop strain of 55° tubes, especially at high temperature. The corresponding increase in ultimate hoop strain was markedly less in the case of 75° and almost negligible in the case of 25° tubes. Testing the tubes at high strain rates resulted in substantial increases in burst strength and ultimate hoop strain as compared with the quasi-static and low strain rate values. The mode of failure of 75° tube is a catastrophic fibre breakage under all test conditions. The mode of failure of 55° tube is a combination of weeping and fibre failure. The 25° tubes are characterised by matrix failure, which is very severe at high strain rates.     

The effects of thermal fatigue on a SiC fibre/aluminosilicate glass composite

The thermal fatigue (TF) of ceramic matrix composites introduces stresses within the composite due to the thermal expansion mismatch of fibre and matrix; this will affect the lifetime and dimensional stability of the composite. A review of various laboratory TF methods is given, and the controlled, photon heating method used in this research is explained. A Nicalon fibre/glass matrix composite was subjected to rapid, controlled TF from 250 to 700 °C and 250 to 800 °C under no load and dead load conditions in order to illustrate a variety of elastic and inelastic cyclic strain conditions. To characterize simple environmental exposure at elevated temperature, ageing experiments were also run. After TF, the surfaces of the composites were characterized using SEM for evidence of thermal damage and microcracking. The composites were then tested for flexural strength. Results show that the tensile modulus after TF testing remains constant, and that dimensional changes are slight, except near any local hot spots. The 700 °C maximum TF specimens showed appreciably greater embrittlement and lower strength than the 800 °C maximum TF specimens. Observations in the SEM of the surfaces of the 700 °C specimens showed little matrix flow of the type which could decrease oxygen infiltration. Greater matrix flow was observed for the 800 °C specimens. Thermally aged specimens gave results similar to those for the TF experiments.     

High-strength silicon carbide fibre-reinforced glass-matrix composites

Silicon carbide fibre-reinforced glass-matrix composites have been fabricated and tested. Two fibre forms, a 140 μm diameter monofilament and a 10 μm diameter filamentary yarn, were incorporated into a matrix of borosilicate glass. The hot-pressing fabrication procedure resulted in fully dense unidirectionally reinforced specimens with excellent flexural strength and fracture toughness over the temperature range 22 to 700° C. In addition, composite thermal expansion was found to be nearly independent of fibre orientation indicating that multiaxially reinforced composites should be readily fabricable without the occurrence of extensive cracking.     

Fabrication, mechanical properties and thermal stability of a novel glass matrix composite material reinforced by short oxycarbide fibres

An aluminosilicate glass matrix composite material reinforced by randomly oriented SiC-based (Tyranno) chopped fibres was fabricated. Slurry dipping and hot-pressing techniques were used to prepare dense composites containing 45 vol% fibres uniformly dispersed in the glass matrix. The mechanical properties and fracture mechanisms of the composite under flexion and compression loading were studied. In flexure, the composite showed higher modulus and strength than the unreinforced glass. However, in compression, the strength of the composite was lower than that of the monolithic glass. Considering the potential application of the material at high temperatures, the thermal aging behaviour of the composite in air at temperatures between 500 and 700°C was investigated. The composite retained its room-temperature compressive strength after exposure for 26 h at 500°C. The variation of compressive strength measured after exposures at higher temperatures was ascribed to mechanisms of fibre/matrix interface oxidation and to the softening of the glass matrix.     

Tensile behaviour and fracture toughness of EPDM filled with untreated and silane-treated glass beads

The Essential Work of Fracture (EWF) theory has been applied to study the fracture behaviour of untreated and silane-treated glass bead-filled EPDM composites. The experimental values of both Young's modulus and tensile strength have been compared with those predicted by the main theoretical and semiempirical models, and the influence of the composite processing temperature on the tensile properties has been studied, noticing a marked drop of stiffness and strength from a processing temperature of 200 °C. A good adhesion between EPDM matrix and glass beads was achieved with the silane Z-6032, resulting in higher tensile strength, and it has been observed that glass bead presence induces plasticity in the EPDM matrix. No differences of the specific essential work of fracture were found in the three filled samples, although results show that the higher adhesion degree between matrix and particles, the higher value of the specific plastic work of fracture, and also the higher final instability in crack propagation.     

Mechanical properties of a lithium glass–ceramic matrix (LZSA) reinforced with TiC or (W,Ti)C particles: A preliminary study

Despite their generally low strength and hardness values, glass–ceramics show good potential to be used in structural applications at room temperature instead of other costlier ceramic materials. This work investigates the effect of dispersed hard carbide particles on the sintering behaviour and the mechanical properties of a lithium glass–ceramic. The glass was mixed with 30 wt.% TiC or (W,Ti)C and hot-pressed at 650 °C (30 MPa, 30 min, Ar). The results obtained compare the properties of the composites with those of the parent glass and demonstrate that the addition of hard particles significantly improves the mechanical strength of the glass–ceramic matrix.     

Fatigue and stress rupture of silicon carbide fibre-reinforced glass-ceramics

Fatigue and stress-rupture testing of unidirectional Nicalon-type silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composites is described. Tensile fatigue testing was performed at 22°C on two different composite systems to contrast the behaviour under applied stresses above and below the levels necessary to cause matrix cracking. The higher strength of the two composites was then also tested in flexural fatigue and constant-load stress rupture at 22, 600 and 900° C in air. It is shown that the level of tensile stress at which composite inelastic stress-strain behaviour begins is an important factor in the control of overall composite performance, and that properties at elevated temperature are significantly different to those at room temperature.     

Time-dependent behaviour of steel/CFRP double strap joints subjected to combined thermal and mechanical loading

Degradation of structural adhesives at elevated temperatures makes the time-dependent behaviour of adhesively-bonded steel/CFRP joints a critical issue for safety considerations of CFRP strengthened steel structures. This paper reports the examination of specimens at different load levels (i.e. 80%, 50%, and 20% of their ultimate load measured at room temperature) and constant temperatures from 35 °C to 50 °C (i.e. temperatures below and above the glass transition temperature T_g, 42 °C of the adhesive). Furthermore, a scenario of cyclic thermal loading between 20 °C and 50 °C was included to represent more realistic exposure. Joint time-dependent behaviour was demonstrated by the stiffness and strength degradation as a function of not only temperature but also time. At the same temperature level close to or above T_g, a higher load level corresponded to a shorter time-to-failure. In addition, up to 47% of strength recovery was found for the specimens subjected to cyclic temperatures compared with those under constant 50 °C which failed at the same load level. Based on the proposed temperature and time-dependent material property models, the time-dependent failure time of steel/CFRP double strap joints was well described and validated by the experimental results.     

Fracture and flexural characterization of SiCw/SiC composites at room and elevated temperatures

The fracture and flexural behaviour of monolithic SiC and SiC-whisker reinforced SiC composites (SiC_w/SiC) has been investigated at room and elevated temperatures. Flexure and fracture tests were conducted in a four-point beam configuration at 23 °C, 800 °C and 1200 °C to study the effects of whisker reinforcements especially in respect of mechanical and thermal stability at high energy environments. Flexural strengths and fracture toughness data within the test temperature range are presented in graphical as well as in Weibull form, and experimental observations are analysed and discussed. Increase in flexural strength as well as in fracture toughness has been observed with the whisker reinforcement. However, it was found that the trend discontinues after a certain range of temperature. Post-failure analyses have been performed with the scanning electron microscope (SEM). Formation of glass phase has been observed at the whisker/matrix interface and the crack growth was found to be shifting from intergranular to transgranular with the rise in temperature. Effects of whisker reinforcement and the degradation of flexural and fracture properties at elevated temperature are investigated. Ultrasonic velocity measurements have been performed through the thickness of the untested as well as fractured specimen, and the variation in the sonic wave velocity is discussed in this paper.     

Joining of SiC fibre reinforced borosilicate glass matrix composites to molybdenum by metal and silicate brazing

The brazing of SiC fibre reinforced borosilicate glass matrix composites with Mo plates has been investigated. Molybdenum was chosen as the metallic partner under consideration of system requirements, e.g. thermomechanical stability at temperatures of interest (500–750^∘C), and physical properties, e.g. coefficient of thermal expansion close to that of the glass matrix composite. Two brazing filler materials were investigated: a glass braze (Schott G018-174) and an active filler metal (Incusil ABA, brazing temperature = 740^∘C). When using the glass braze the surface of the metal had to be roughened to ensure a bond of significant strength. Vacuum brazing with the active filler metal resulted in joints with high strength, which allows to fully utilise the mechanical competence of the glass matrix composite when the joint configuration is adapted to the relevant loading conditions. A novel design of a tool for hot glassware handling, made of glass matrix composite/Mo joints, is presented.     

Alternative Uses of Waste Glasses: Issues on the Fabrication of Metal Fibre Reinforced Glass Matrix Composites

In order to investigate a potential use for recycled speciality glasses (specifically those containing hazardous elements), a processing route has been developed for the fabrication of metallic fibre mat reinforced glass matrix composites. Commercially available 3-dimensional stainless steel 316L fibre mats were used as the metal reinforcement, and a borosilicate glass which had been used previously in radiation experiments was used as the glass matrix. The fibre mats were infiltrated with a commercially available silica sol using electrophoretic deposition (EPD), and the glass matrix was laid in between infiltrated fibre mats prior to consolidation using uniaxial cold pressing. It was found that composites with sufficient integrity could be obtained from this recycled waste glass after sintering in air at 850°C for 1 h. The deposited silica remained amorphous at the processing temperature, providing a porous interface between the metallic reinforcement and the waste glass matrix. The processing issues involved in composite fabrication, namely, the EPD infiltration parameters for the silica sol, the quantity and subsequent effect of the impurities present in the waste glass, and the densification of the composite material on sintering, have been discussed.     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

Effect of Temperature on the Tensile Strength and Failure Modes of Angle Ply CFRP Tubes under Hoop Loading

A comprehensive study was undertaken to characterise carbon fibre reinforced plastic (CFRP) tubes at different temperatures. Quasi-static burst tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. An experimental rig and two conditioning tanks were designed and built to test the specimens at three temperatures; -46°C (low temperature), +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded using a digital processing equipment.For a particular batch of tubes, tested at three different temperatures, a decrease in hoop strength and modulus of the 55° tubes with increasing temperature was recorded; the effect was less pronounced on the properties of 25° and 75° tubes. The use of a non-structural liner during the tests led to higher ultimate strength and strain of 55° tubes but had negligible effects on the behaviour of 75° tubes. The use of a liner in 25° tubes altered the mode of failure, resulting in a very large tube deformation with no noticeable increase in burst pressure. Micrographic analysis was also undertaken to study the failure mechanisms during pressurisation of lined and unlined tubes.     

Tensile creep behavior of 3-D woven Si-Ti-C-O fiber/SiC-based matrix composite with glass sealant

The present work investigates the tensile creep behavior (deformation and rupture) at 1100–1300°C in air of a 3-D woven Si-Ti-C-O (Tyranno) fiber/SiC-based matrix composite with and without glass sealant. The composite contained Si-Ti-C-O fibers with an additional surface modification in order to improve interface properties. Although a significant decrease in tensile strength was observed in the unsealed composite beyond 1000°C in air (and attributed to oxidation of the fiber/matrix interface), the composite with glass sealant possessed excellent mechanical properties for short-term (<1 hr.) exposure in air. In this study, tensile creep testing was conducted at 1100–1300°C in air and the effect of glass sealant on medium- and long-term strength was investigated. In addition, chemical stability of the glass sealant was evaluated by X-ray diffraction analysis (XRD) and energy dispersive X-ray spectrometer (EDS). The creep rupture behavior of the composite with glass sealant under long-term exposure is suggested to depend on several factors including decomposition, evaporation, and crystallization of the glass sealant material, in addition to the applied stress.     

Sol–gel route to carbon nanotube borosilicate glass composites

Dense borosilicate glass matrix composites containing up to 3 wt% of multiwalled carbon nanotubes were produced by a sol–gel process. The three different silicate precursors employed (tetramethylsilane (TMOS), methyltriethoxysilane (MTES) and methyltrimethoxysilane (MTMS)) yielded transparent xerogels which were subsequently crushed and densified by hot pressing at 800 °C. The dispersion of the carbon nanotubes was aided by using an organic–inorganic binder (3-aminopropyl triethoxysilane) which limited flocculation of the CNTs in the silica sol. After densification, the borosilicate glass composites containing up to 2 wt% CNTs showed significant improvements in hardness and compression strength, as well as thermal conductivity, whilst percolation effects lead to a dramatic increase in electrical conductivity above 1 wt%. This simple approach to disperse CNTs into a technical silicate glass matrix via the sol–gel process focusses specifically on the borosilicate system, but the procedure can be applied to produce other inorganic matrix composites containing CNTs.