Effect of thermal cycling on whisker-reinforced dental resin composites

The mechanical properties of dental resin composites need to be improved in order to extend their use to high stress-bearing applications such as crown and bridge restorations. Recent studies used single crystal ceramic whiskers to reinforce dental composites. The aim of this study was to investigate the effects of thermal cycling on whisker-reinforced composites. It was hypothesized that the whisker composites would not show a reduction in mechanical properties or the breakdown of whisker–resin interface after thermal cycling. Silicon carbide whiskers were mixed with silica particles, thermally fused, then silanized and incorporated into resin to make flexural specimens. The filler mass fraction ranged from 0% to 70%. The specimens were thermal cycled in 5 °C and 60 °C water baths, and then fractured in three-point bending to measure strength. Nano-indentation was used to measure modulus and hardness. No significant loss in composite strength, modulus and hardness was found after 10⁵ thermal cycles (family confidence coefficient=0.95; Tukey's multiple comparison test). The strength of whisker composite increased with filler level up to 60%, then plateaued when filler level was further increased to 70%; the modulus and hardness increased monotonically with filler level. The strength and modulus of whisker composite at 70% filler level were significantly higher than the non-whisker controls both before and after thermal cycling. SEM revealed no separation at the whisker–matrix interfaces, and observed resin remnants on the pulled-out whiskers, indicating strong whisker–resin bonding even after 10⁵ thermal cycles. In conclusion, novel dental resin composites containing silica-fused whiskers possessed superior strength and modulus compared to non-whisker composites both before and after thermal cycling. The whisker–resin bonding appeared to be resistant to thermal cycling in water, so that no loss in composite strength or stiffness occurred after prolonged thermal cycling.     

Release of chlorhexidine digluconate and flexural properties of glass fibre reinforced provisional fixed partial denture polymer

The objective of this study was to determine the flexural properties and the release of chlorhexidine digluconate (CHX) of CHX laced unidirectional E-glass fibre reinforced provisional fixed partial denture polymer. Bar shaped test specimens (3.3 × 10.0 × 65.0 mm) were fabricated from provisional fixed partial denture polymer (mixture of poly[ethylmethacrylate] powder and n-poly[butyl methacrylate] monomer liquid) with E-glass fibre reinforcements. Poly(methyl methacrylate) preimpregnated continuous unidirectional glass fibre reinforcement was laced with CHX. The glass fibre reinforcements were incorporated into the polymer and the polymerised to the form of test specimens. In addition test specimens without CHX in glass fibre reinforcement were made for comparison. Control specimens did not contain glass fibres in the test specimens. Flexural strength and modulus of test specimens (n = 6) was tested with three-point bending test after storing the specimens dry or in water (two weeks). Released CHX was determined with high performance liquid chromatography during 180 days water immersion. In dry conditions, the flexural strength and the modulus of the polymer was 43 MPa and 1.7 GPa, and with glass fibre reinforcement 96 MPa and 3.5 GPa. With the reinforcement laced with CHX, the strength was 92 MPa and the modulus was 3.2 GPa. The water storage of test specimens did not weaken the reinforced polymer. The majority of the CHX released from the glass fibre reinforced polymer during the first days of storage in water. Flexural properties of provisional fixed partial denture polymer were increased using glass fibre reinforcement. The fibre reinforcement that was laced with CHX resulted in similar reinforcing effect.     

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.     

The compression strength of composites with kinked,misaligned and poorly adhering fibres

Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction,V _f=0.4 with Kevlar and high strength carbon fibres. The effective fibre strength atV _f<0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10° for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.     

High temperature acidic stress corrosion of glass fibre composites: Part I Effect of fibre type

The stress corrosion characteristics of uniaxial glass fibre reinforced thermosetting resin composites have been examined in hydrochloric acid at 80°C. A simple technique based on linear elastic fracture mechanics (LEFM) is presented for characterizing crack growth in these materials subjected to hostile acidic environments. The environmental stress corrosion cracking is investigated both for different types of resin and different types of glass fibre reinforcements. Two matrices were used: DERAKANE* 411-45 epoxy vinyl ester resin (based on Bisphenol-A epoxy resin) and DERAKANE 470-30 epoxy vinyl ester resin (based on epoxidized novolac resin). Two glass fibre types were employed: standard E-glass fibre and ECRGLAS®, a special type of E-glass with superior acid resistance. Model experiments using a modified double cantilever beam test with static loading have been carried out on unidirectional composite specimens in 1 M hydrochloric acid solution at 80°C. The rate of crack growth in the specimen depends on the applied stress, the temperature and the environment. Consequently, the lifetime of a component or structure made from glass fibre reinforced plastics (GRP) subjected to stress corrosion conditions, could be predicted provided the dependence of crack growth rate on stress intensity at the crack tip is known. Scanning electron microscope studies of the specimen fracture surfaces have identified the characteristic failure mechanisms. The most important finding of this work is that the selection of DERAKANE epoxy vinyl ester resins reinforced with ECRGLAS® fibre exhibited superior resistance to crack growth at 80°C compared to similar E-glass reinforced composites at room temperatures.     

Mechanical properties of thermally cycled nylon bonded Nd-Fe-B permanent magnets

The effect of thermal cycling on the stress-strain behavior of polyamide (nylon) and polyphenylene-sulfide (PPS) based injection molded Nd-Fe-B magnets was investigated after test specimens were cycled between –40 and 150°C for 50, 500, or 5000 repetitions. It was found that PPS based magnets exhibit higher ultimate strengths, higher modulus and lower toughness than nylon based magnets. Furthermore, formulations containing platelet morphology particles exhibited higher strengths and modulus than those containing spherical morphology particles, with increases in particle volume fraction leading to a decrease in strength. Differences in strength, modulus, and toughness were attributed to the degree of bonding between the matrix and the magnet powder in the various formulations, the degree of crosslinking, along with the effects of powder morphology. Additionally, it was found that while the stiffness of these materials increased with thermal cycling, their toughness decreased significantly, by as much as 99%. The extent of these effects was found to be dependent on the polymer matrix, powder morphology, and volume fraction of powder in the magnet. Finally, it was found that the PPS composites showed less relative change due to thermal cycling than the Nylon composites.     

Thermoplastic fibre-reinforced composites for dentistry

The effect of a deterioration of matrix and matrix-fibre interface, caused by moisture, on the flexural properties (modulus, strength and ultimate strain) of unidirectional E-glass fibre-reinforced thermoplastics [polycarbonate, poly(ethyleneterephthalate glycol) and nylon 12] was investigated. Specimens of span/thickness ratio varying from 15 to 30, prepared from pultruded thermoplastic prepregs, were tested in directions parallel to and perpendicular to the fibre orientation as moulded and after immersion in water at 85°C for 100 h. No significant reduction in longitudinal properties, controlled by fibre behaviour, resulted from exposure to water. A significant reduction in transverse properties, controlled by the matrix and interface behaviour, was observed. Polycarbonate/bare E-glass fibre composite annealed at 275°C for 1 h before immersion in water exhibited a superior resistance to moisture attack. This phenomenon is attributed to the transcrystalline interphase near the fibre surface and good wetting of fibres by the matrix. Resistance against moisture favours this material for further clinical application as an orthodontic wire.     

Mechanical performance of hybrid bismaleimide composites reinforced with three-dimensional braided carbon and Kevlar fabrics

Short, unidirectional and laminated hybrid composites have been extensively investigated. However, very limited work has been conducted on three-dimensional (3-D) braided hybrid composites. In this work, 3-D braided carbon and Kevlar fibres were hybridized to reinforce a bismaleimide (BMI) resin. The purpose of this paper was to investigate the effect of carbon to Kevlar ratio on such mechanical properties as load–displacement behaviour, flexural strength and modulus, shear strength, and impact properties. The effect of surface treatment of hybrid fabrics on the flexural properties was also determined. Experimental results showed that the flexural strength and modulus of the 3-D braided carbon/Kevlar/BMI composites increased with relative carbon fibre loading up to a carbon to Kevlar ratio of 3:2 and then dropped. Positive hybrid effects were observed for both flexural strength and modulus. The results presented in this work proved that hybridization with certain amount of ductile Kevlar fibre markedly promoted the shear strength, impact energy absorption characteristics and damage tolerance of the all-carbon composite, which is of importance for the 3-D braided composites to be used in bone fixations. Fracture surfaces and microstructures of various 3-D braided hybrid composites were analyzed to interpret the experimental findings.     

Properties of glass fibres in cement environment

Fibres produced from a soda-silica-zirconia glass were reacted with Portland cement extracts at 20 and 65° C for various lengths of time and their strength and stiffness determined. The results indicate that these glass fibres resist the attack of cement extracts reasonably well at ambient temperatures. Fibre strengths of the order of 1200 to 1300 N mm^–2 are obtainable after 2 years at 20° C, sufficient to reinforce cement, and there is no change in the Young's modulus of the fibre during this period. At higher temperatures both strength and stiffness are reduced but these temperatures are unlikely to be encountered in practice over extended periods of time. When fibres removed from cement composites containing commercially made alkali-resistant glass fibres are examined, it is found that fibre strengths depend very strongly on the environment in which the composites were kept. For air storage, fibre properties remain relatively unaffected but for composites kept under water continuously, an initial loss in fibre strength is observed. This difference in fibre strength is reflected in the relative strength of the cement composites.     

The thermal behaviour of glass fibre investigated by thermomechanical analysis

A thermomechanical analysis (TMA) procedure has been developed with the capability of probing the thermal behaviour of glass fibre. A single glass fibre was successfully mounted into TMA fibre configuration and several thermomechanical programmes were carried out over a wide temperature range from 20 to 900 °C. It was found that measured coefficient of linear thermal expansion of boron-free E-glass fibre remained constant below 300 °C and the values had an excellent agreement with that found in the literature. At higher temperatures an abrupt length change in glass transition region allowed for the determination of glass transition temperature. The results from isothermal measurement showed significant fibre length shrinkage, which was a function of both temperature and time. It follows that there exist two mechanisms, thermal expansion and structural relaxation, which together account for overall thermomechanical responses of glass fibre. The former is related to the decrease of Young’s modulus at elevated temperatures and the latter is considered responsible for the observed increase of room-temperature Young’s modulus after thermally conditioning glass fibre at various temperatures.     

Thermal cycles effects on interlaminar shear strength (ILSS) and impact behaviour of carbon/PEI composites

In this paper, the effect of thermal cycle on the interlaminate shear strength (ILSS) and impact behaviour of unidirectional carbon fibre reinforced polyetherimide (PEI) matrix composites were studied. Samples were subjected to 100 thermal cycles (by immersing from boiling water (100°C) to ice water (0°C). The effects of thermal cycles were characterized by short beam shear and instrumented impact testers. Also Fractographic investigations were done using a scanning electron microscope (SEM). It is observed that the plastic deformations at the fibre/matrix and interlaminar interface as well as residual stresses lower the ILSS and flexural modulus of the material proportional with the number of thermal cycles. Up to the first 20 thermal cycles the material shows a brittle fracture with lower fracture energy, but after the 20th thermal cycles it is possible to observe that the material fractures with higher fracture energy at longer fracture time. A remarkable difference in the fracture morphology between the thermal cycled and un-treated materials has been observed. It is found that thermal cycles strictly affect the fracture morphology.     

Improvement of Mechanical Properties of Oligomer-modified Acrylic Bone Cement with Glass-fibers

Some mechanical properties of oligomer-modified acrylic bone cement with glass-fibers were studied. Under wet environments, oligomer-filler forms a porous structure in the acrylic bone cement. Test specimens were manufactured using commercial bone cement (Palacos^® R) with different quantities of an experimental oligomer-filler (0–20 wt%), and included continuous unidirectional E-glass fibers (l=65 mm) or chopped E-glass fibers (l=2 mm). The specimens were either tested dry, or after being immersed under wet environments for one week. The three-point bending test was used to measure the flexural strength and modulus of the acrylic bone cement composites (analysis with ANOVA). A scanning electron microscope (SEM) was used to examine the surface structure of the acrylic bone cement composites. Using continuous glass-fiber reinforcement, the dry flexural strength was 145 MPa and modulus was 4.6 GPa for the plain bone cement. For the test specimens with 20 wt% of oligomer-filler and continuous unidirectional glass-fibers, the dry flexural strength was 118 MPa and modulus was 4.2 GPa, whereas the wet flexural strength was 66 MPa and modulus was 3.0 GPa. The results suggest that the reduced flexural properties caused by the porosity of oligomer-modified bone cement can be compensated with glass-fiber reinforcement.     

Influence of glass addition and sintering temperature on the structure,mechanical properties and dielectric strength of high-voltage insulators

Soda-lime glass as a substituent for the feldspar was used to prepare high-tension electrical porcelain by standard chemical solid reaction technique. The effect of glass substitution and sintering temperature on the physical properties, microstructure, hardness, modulus of rupture, flexural strength and Dielectric breakdown strength were examined. Zero water absorption (WA %) and apparent porosity (AP %) were achieved for the samples with glass content >15 wt.% sintered at 1100 °C. The apparent density was found to increase with sintering temperature. The Vicker’s micro-hardness increased with both glass addition and sintering temperature. Both of the modulus of rupture (MOR) and flexural strength (σ_f) had maxima values at 15 wt.% glass addition. The structure and morphology were characterized by X-ray diffraction and scanning electron microscope (SEM). It showed the formation of mullite needles at sintering temperature of 1100 °C, which enhanced the mechanical and electrical properties of the porcelain. The dielectric breakdown strength increased with sintering temperature and glass addition. The highest dielectric strength was found at 10 wt.% of glass addition depending on the Na₂O and Fe₂O₃ content.     

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 hot-pressing time and post-heat treatment on the microstructure and mechanical properties of SiC-fibre-reinforced glass-ceramic composites

Nicalon-SiC-fibre-reinforced (35 vol %) lithium-aluminosilicate (LAS) glass-ceramic composites were fabricated by a slurry-infiltration process followed by hot pressing at 1400°C and 10 MPa for varying soaking times. The ultimate strength and elastic modulus of the as-fabricated composites, as determined by four-point flexural tests, increased rapidly with the densification time, saturating after 30 min at 550 MPa and 130 GPa, respectively. Longer hotpressing times caused a decrease in the elastic modulus via fibre degradation. A carbon-rich interfacial layer formed between the fibres and the matrix, the thickness of which reached a maximum of 400 nm after 30 min soaking time. The flexural strength of post-heat-treated composites in air decreased by a factor of approximately four, due to oxidation and removal of the carbon content of the interfacial layer. The silica-rich bridges left behind between the fibres and the matrix contributed to brittle fracture of the composite.     

Failure mechanisms in mission-cycled carbon/carbon composites under flexural load at room and elevated temperatures

The response of quasi-isotropic laminates of SiC coated carbon/carbon (C/C) composites under flexural load was studied. Mission-cycled as well as virgin specimens were tested to compare the thermal- and pressure-cycling effects. Variation of flexural strength and stiffness with temperature was investigated to study the load-deflection behaviour and the thermal stability of C/C composites up to 1371 °C. Increase in flexural strength and stiffness were observed with the rise in temperature. A distinct shift in failure modes from compressive to tensile was found with the mission-cycled specimens with the increase in test temperatures, while the failure mode for virgin material was found always on the tensile side. Change in the load-deflection behaviour was examined and increase in non-linearity of the stress-strain behaviour with the mission cycling was observed. Although the number of test specimens was few, Weibull characterization on the flexure data was performed to study the variation of Weibull modulii and the characteristic lives. Failed, as well as untested, specimens were C-scanned to identify the location and the extent of the damaged zone. Post-failure analyses through optical microscopy and scanning electron microscopy were performed to study the damage growth and failure mechanisms. Degradation and separation of the porous matrix structure, localized damage of the reinforcing fibres in the transverse direction, complete fibre bundle failure in the mission-cycled specimens, and delamination near the loading zone were observed.     

Investigation of cyclic thermal shock behaviour of fibre reinforced glass matrix composites using non-destructive forced resonance technique

Abstract

A non-destructive forced resonance technique was used to assess the damage development in SiC fibre reinforced glass matrix composite materials subjected to cyclic thermal shock. Both elastic modulus and internal friction measurements were conducted. The thermal shock tests involved quenching the specimens from high temperatures (590–710°C) to room temperature in a water bath. Damage in theform of matrix microcracks was induced by quenchingfrom 620 and 660°C, and the extent of damage increased with the number of thermal shock cycles. After a certain number of shocks, this damage was detected by a decrease in the Youngs modulus and a simultaneous increase in the internal friction. The non-destructive dynamic forced mechanical resonance technique employed was shown to be more sensitive than a destructive three point flexural technique for detecting crack development in the early stages of thermal shock damage. The technique was also used to confirm the occurrence of a crack healing process in the thermally shocked specimens: after an annealing heat treatment for 12 h at 550°C, the initial values of Young's modulus and internal friction were recovered. This was attributed to crack closure due to viscous flow of the glass matrix.     

Properties of high-alumina cement reinforced with alkali resistant glass fibres

The changes in the mechanical properties of cement composites made from high-alumina cement and Cem-FIL AR-glass fibres kept in three different environments up to 10 years are described. While the flexural and impact properties of the composite remained largely unaffected with time in a relatively dry atmosphere, in wet conditions a reduction in strength takes place. In natural weather the 10 year modulus of rupture and impact strength values are 22.8 MIN m^–2 and 6.7 KJ m^–2, respectively, corresponding to the 28 day values of 41.2 MN m^–2 and 22.8 KJ m^–2. These values are significantly better than the corresponding results obtained with Portland cement composites made from Cem-FIL fibres. High-alumina cement composites reinforced by E-glass fibre lose a very large proportion of their flexural and impact strength under wet conditions. The strength reduction with time observed for glass fibre reinforced high-alumina cement composites can be related to two sources: (a) the reduction in the strength of the glass fibre due to chemical corrosion and (b) conversion of the matrix. The latter has greater influence on those composite properties that are matrix controlled such as the Young's modulus whereas any significant reduction in fibre tensile strength is reflected in a corresponding loss in composite tensile and bending strength. Matrix conversion may also influence the fibre-matrix bond.     

Thermal cycling of tungsten-fibre-reinforced superalloy composites

The thermal cycling of a tungsten-fibre-reinforced superalloy (TFRS) composite is typical of its application in high-temperature engine environments. The mismatch in thermal expansion coefficients between fibre and matrix causes substantial longitudinal (0°) stresses in the composite, which can produce inelastic damage-producing matrix strains. The case of thermal fatigue is explored as a worst case of the possible matrix damage in comparison with specimens which are also mechanically loaded in tension. The thermally generated cyclic stresses and the attendant matrix plasticity may be estimated using a non-linear finite-element program by proposing a physical analogue to the micro-mechanics equations. A damage metric for the matrix is proposed using the Coffin-Manson criterion, which metric can facilitate comparisons of damage among different candidate materials, and also comparisons for a given material subjected to different temperature cycles. An experimental programme was carried out for thermal cycling of a 37 vol % TFRS composite to different maximum temperatures. The results confirm the prediction that thermal cycling produces matrix degradation and composite strength reduction, which become more pronounced with increasing maximum cyclic temperature. The strength of the fibre is shown to be identical for the as-fabricated and thermally cycled specimens, suggesting that the reduction in composite strength is due to the loss of matrix contribution and also to notching effects of the matrix voids on the fibre.     

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.