Viscoelastic behaviour of macro-defect-free cement

The viscoelastic behaviour of macro-defect-free (MDF) cement was studied by dynamic mechanical analysis. MDF specimens with various moisture contents in the range 1.70%–3.20% moisture, were measured at 1 Hz as a function of temperature from 34–96 °C and as a function of frequency from 0.05–5 Hz at 34°C. The viscoelasticity of MDF was found to be affected by moisture content through its plasticizing effect on the PVA binder. Time-moisture and temperature-moisture superposition of the shear moduli were found to be possible for MDF, and linear relationships between log time and linear moisture and between temperature and moisture were found. How the microstructure of MDF affects the viscoelastic response is also discussed through mechanical models. A comparison of the models with known experimental data suggests that the viscoelastic response arises from both direct connections between the inorganic particles and from connections through the polymer binder. Inorganic links are estimated to connect 45% of the inorganic phase.     

Optimization of material characteristics of macro-defect free cement

Macro-defect free (MDF) cement is a high-strength cement-polymer composite produced by mixing cement (commonly calcium aluminate cement) with small amounts of polymer (commonly polyvinyl alcohol acetate) and water, applying high shear, and finally applying relatively low pressure (about 5 MPa) and modest temperature (about 80-100 °C). However, MDF cements lose considerable strength when exposed to water. The objective of this study was to explore the effects of cement and polymer compositions on flexural strength and water sensitivity. Calcium aluminate cements were used with Al₂O₃ contents between 42% and 79%. Production of MDF cement was successful with all cements, but the highest strength (268 MPa) was obtained with 70% Al₂O₃ cement. Secondly, PVAs were used that differed in their degree of hydrolysis between 73% and 99%. Of these, the one with a moderate degree of hydrolysis produced the highest strength (228 MPa). All mixtures had strength loss on exposure to water, but PVAs with moderate degrees of hydrolysis exhibited the lowest strength losses (50-60%).     

Effect of multidirectional forging on microstructures and tensile properties of a particulate reinforced magnesium matrix composite

A particulate reinforced magnesium matrix composite prepared with stir casting was subjected to multidirectional forging (MDF). The results showed that after 1 MDF pass the grain size of matrix in the composites decreased compared with as-cast composite, and increased with increasing the MDF temperature from 370 °C to 450 °C. With increasing the MDF passes at 370 °C, the particle distribution of the composite was improved until 3 MDF passes while the grain size of matrix in the composite reached a minimum after 4 MDF passes. Both the yield strength and the ultimate tensile strength of the composite were enhanced with increasing the MDF passes.     

High temperature behaviour of a Ti-6Al-4V/TiCp composite processed by BE-CIP-HIP method

The high temperature behaviour of a Ti-6Al-4V/TiC_p composite (10% Vol. of TiC) was investigated. A composite produced by Dynamet Technology according to the blended-elemental-cold-hot isostatic pressing (BE-CHIP) method was used. The stress-strain properties of the material were tested at 25, 200, 400, 500, 600 and 800°C. Composite specimens were aged in air at 500 and 700°C or under vacuum at 500, 700 and 1050°C, for periods ranging between 100 and 500 hours. The thermal stability of the matrix/ceramic interfaces was studied by using scanning electron microscope, electron probe microanalysis and x-ray diffraction. Carbon diffusion from the ceramic particles towards the composite matrix occurred (very likely already during the composite fabrication) because the metal matrix of all the composite samples (either in the as received or thermally treated conditions) showed a high content of carbon (more than 1% at.). However, the thermal treatments carried out at both 500 and 700°C under vacuum did not result in a ceramic-metal reaction. In spite of this, the formation of an ordered phase of formula Ti₂C can be inferred. Long period aging under vacuum at 700°C (500 h) did not lower the composite tensile strength. On the other hand, above 500°C in air the titanium matrix rapidly underwent oxidation, which gave rise to the formation of a thick surface reaction layer; this confirms that the composite material cannot be used above this temperature. Furthermore, the thermal treatment performed at 1050°C (under vacuum) resulted in a strong composite microstructure modification: the formation of new mixed carbides of Al and Ti was observed.     

Effect of polymeric precursors on properties of semiconducting carbon/carbon composites

Microstructure, oxidation behaviour, and electrical and mechanical properties of quasi-carbon fibre-reinforced quasi-carbon matrix (QC/QC) composites were investigated. The composite was prepared by heat treating a QC fibre or OXPAN fibre-reinforced polymer matrix composite at a temperature of 500 °C. Different polymer precursors have resulted in the QC/QC composites with varying thermal behaviour. The phenolic matrix derived QC/QC composites followed a self-acceleration mechanism and had better oxidation resistance than the polyacrylonitrile (PAN) matrix-derived QC/QC composites. Because of fewer chemical reactions involved in the pyrolysis process, the QC/QC composites obtained from QC fibre-reinforced composite precursors exhibited higher flexural modulus and strength and were superior to those derived from oxidized PAN (OXPAN) fibre-reinforced composite precursors. Unique semiconducting and switching characteristics have been observed in the QC/QC composites, which would make them promising for electronic device applications. © 1998 Chapman & Hall     

The effect of physical ageing on the relaxation time spectrum of amorphous polymers: the fractional calculus approach

Empirical models corresponding to a constitutive equation with fractional derivatives are proposed for linear viscoelastic polymers. For these models, the relaxation modulus, the dynamic moduli, the relaxation time spectra, and other material functions can be calculated as a function of a few parameters that characterise the behaviour of a viscoelastic polymer. The fractional calculus approach allows us to calculate the relaxation time spectrum H() via the Stieltjes inversion in the linear viscoelastic zone. Polymethylmethacrylate (PMMA) is chosen as a model amorphous polymer in a temperature range from T_g + 90°C to T_g + 25°C. This polymer is characterised by a non-equilibrium state between at least the and relaxations. The structural recovery of PMMA has been investigated using dynamic mechanical thermal analysis (DMTA) by varying the preparational history. The effect of time and temperature on the model parameters and on the relaxation time spectra are also investigated in the neighbourhood of the glass transition.     

Dynamic mechanical analysis of heat,microwave and visible light cure denture base resins

The clinical durability and performance of a denture are limited by the properties of the denture base resins used in the fabrication of the prosthesis. Among the properties considered important for denture base resins are viscoelastic properties such as storage modulus, damping and glass transition. In this study, dynamic mechanical analysis using a flexural mode of deformation in the temperature range 25–180°C has been used to characterize the viscoelastic properties of three denture base resins with different curing modes including conventional thermal cure, microwave cure and visible light cure. The resins studied were popular commercial systems. The results indicate that the microwave cure and conventional thermal cure resins are significantly different in their viscoelastic properties from the current visible light cure resin system. The latter resin is characterized by higher flexural modulus and loss modulus across the entire range of temperatures investigated and in addition shows higher glass transition temperature relative to the other resins studied. The results indicate that filler loading and crosslinking effects may be responsible for this behaviour of the visible light cure resin and may indicate a potential brittle behaviour not desirable in a permanent denture.     

Tensile behaviour of a niobium/alumina composite laminate

The tensile properties of a ceramic/metal laminate were examined. Alternating layers of niobium and alumina were fabricated into composite material and tensile tested at room temperature and 982 °C (1800 °F). The tensile behaviour of the laminate material was predicted using established models based on classical laminate theory. The estimated and measured properties showed excellent agreement. Relative to monolithic niobium, the composite material was found to have lower density and substantially higher stiffness.     

Curing characteristics of acrylic bone cement

Commercial acrylic bone cements are supplied as two components, a polymer powder and a liquid monomer. Mixing of the two components is followed by a progressive polymerization of the liquid monomer to yield a solid mass, a high level of heat being generated during this exothermic reaction. The exposure of bone to high temperatures has led to incidences of bone necrosis and tissue damage, ultimately resulting in failure of the prosthetic fixation. The aim of this study was to determine the thermal properties of two acrylic bone cements as they progress through their polymerization cycles. It was also felt that there was a need to quantify the variations in the curing characteristics as a function of preparing bone cement by different techniques, hand mixing and vacuum mixing. A number of parameters were calculated using the data gathered from the investigation: peak temperature, cure temperature, cure time, and the cumulative thermal necrosis damage index. The results show the temperature profile recorded during polymerization was lowest when the cement was prepared using the Howmedica Mix-Kit I® system: 36 °C for Palacos R® and 41 °C for CMW³® respectively. When the acrylic cements were prepared in any vacuum mixing system there was evidence of an increase in the cure temperature. The main factor that contributed to this rise in temperature was an imbalance in the polymer powder : liquid monomer ratio, there was a high incidence of unmixed powder visible in the mixing barrel of some contemporary vacuum mixing devices. Observing the thermal characteristics of the polymethyl methacrylate (PMMA) bone cements assessed, it was found that particular formulations of bone cements are suited to certain mixing methodologies. It is vital that a full investigation is conducted on a cement mixing/delivery system prior to its introduction into the orthopaedic market.     

Mechanical and structural studies of low density polyethylene

This paper is one of a series concerned with the complete characterisation of the creep behaviour of oriented polymers, the correlation of creep behaviour with other mechanical properties and the interpretation of such data in the light of present structural knowledge. Sheets of oriented low-density polyethylene were prepared from initially isotropic sheets by cold-drawing, cold-drawing followed by heat-treatment at 55° C, drawing at a temperature of 55° C and hot-drawing at temperatures in the range 90 to 100° C. At each draw ratio, specimens were cut at angles of 0°, 45° and 90° to the draw direction. For each specimen, the variation of longitudinal and lateral strain with time, during uniaxial tensile creep at 20° C, was measured simultaneously by direct extensometer methods, for a wide range of applied stresses. All the materials exhibited complex anisotropic non-linear viscoelastic behaviour. The methods of presenting such data are discussed and the results are presented in some detail. Many similarities in the creep behaviour of the cold- and hot-drawn materials are noted. However, marked differences are apparent in the non-linearity and creep rate of the 45° specimens from these two materials at high draw ratio. These, and other effects found at high draw ratio, are discussed with reference to the structural studies reported in part 1. At low draw ratio, it is shown that the anomalous behaviour of the modulus in the draw direction, reported previously for cold-drawn material, may also be found in the hot-drawn material, although at a different creep time. On the basis of obvious differences in wide-angle X-ray patterns other workers had previously predicted that the anomalous mechanical behaviour of cold-drawn LDPE was probably unique. The anomalous behaviour of the hot-drawn material is also explained in terms of the structures discussed in part 1.     

Thermal stress development in fibrous composites

In this work, the thermal stress development in anisotropic fiber-reinforced polymer composites is investigated for temperatures below the glass transition temperature of the resin. By applying two independent experimental methodologies, it was found that the initial thermal (residual) strain in the reinforcing fibers is compressive of about − 0.04% at ambient temperatures. This is due to the mismatch of the thermal expansion coefficient between the polymer matrix and fiber, as the material is cooled down from the processing temperature. However, on reheating the composites the compressive stress in the fiber gradually diminishes and becomes zero at 50 °C. Further heating to 100 °C introduces tensile strains in the fiber of maximum of 0.13%. The conformity of these results to analytical models that relate the composite thermal strain to the thermal expansion coefficients of fiber and resin, as well as, the fiber volume fraction, is examined. Finally, the possibility of tailoring the sign (positive, negative or, even, zero) of the composite thermal expansion coefficient of certain advanced composites by simply varying the thermal expansion of the polymer matrix, is discussed.     

The effects of temperature,pressure and water on the preparation of glass-mica composite material

A systematic investigation on the fabrication of glass-mica composite materials from recycled colourless soda-lime glass powders and phlogopite-type mica powders has been conducted. Mixtures of two specific compositions of the glass-mica system were used and the investigation was based on several chosen processing parameters. When compacted powder samples were sintered at temperatures in the range 780 to 900° C, samples of one composition formed a composite material having a cellular structure; whereas samples of the other composition formed a composite material having a highly-dense ceramic structure. Sample evaluations showed that both the sintering temperature and the quantity of water which is added to the glass-mica mixtures as wetting agent in the powder compaction process are sensitive processing parameters. They can control the physical, mechanical and thermal properties of the glass-mica composite material. It was found that when glass-mica dry mixtures were prepared with the addition of a quantity of water equivalent to about 10% of the sample weight and sintered at the temperature of 850° C, the resultant composite material exhibited optimum physical, mechanical and thermal properties. The compressive strength and thermal insulating value of the glass-mica composite material with the densified structure are found to be superior to those of several conventional building materials, such as masonary products, lightweight concrete and soda-lime glass components. The experimental findings suggest that the glass-mica composite material is a potential structural element for building construction applications as it may contribute to energy conservation.     

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.     

Oxidation behaviour and strength of B4C-30 wt% SiC composite materials

The oxidation behaviour and the effect of oxidation on the room-temperature flexural strength of B₄C-30 wt% SiC composite material were investigated. The weight changes of the samples exposed to air at temperatures between 500 and 1000 °C were continuously monitored with a microbalance. At temperatures below 800 °C, the weight change of the specimen was negligible. As the temperature was increased to 800 °C, parabolic weight gain was observed. The rate of the weight gain increased with exposure temperature. The oxidation product formed on the surface was found to be a crystalline boric oxide (B₂O₃) by X-ray analyses. The oxide layer was severely cracked due to the thermal expansion mismatch between the oxide layer and the substrate. However, the room-temperature flexural strength was increased when the samples were exposed at temperature between 700 and 900 °C, apparently due to the blunting of strength-limiting defects at the surface. When the temperature was higher than 1000 °C, a severe reduction in strength was observed. The reliability of the composite material was also improved significantly by such exposures.     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

Transverse compressive testing of T300/914

The transverse compressive behaviour of a structurally complex carbon-epoxy unidirectional composite (T300/914) has been investigated over various temperatures and test rates. The mechanical behaviour was found to mirror the transverse tensile behaviour of the material, with the viscoelastic response being different between micro- and macroproperties. Composite macrofracture was found to occur at ±34 ° to the loading axis and was regular, whereas fracture of the bulk resin was irregular and occurred by Poisson forces, rather than compressive forces. The performance of the test fixture was assessed and found to be adequate for composites testing. An optimum specimen gauge length was determined as the ASTM recommendation was found to be unsatisfactory.     

Fabrication of cellular structure composite material from recycled soda-lime glass and phlogopite mica powders

A homogeneous composite material with different physical structures has been fabricated from recycled colourless soda-lime glass powders and phlogopite-type mica powders by mixing the two powder components and sintering the mixture at a temperature above 850° C for a period of 30 min or longer. The physical structure of the composite material can be fabricated into either a cellular structure consisting of both closed and open cells or a highly densified ceramic body. The cellular structure composite material is found to have a compressive strength of about 0.877 MN m^–2 and thermal conductivity values in the range of 0.290 to 0.306 W m^–1 °C^–1 when measured at temperatures in the range of 25 to 100° C. The highly densified composite material, on the other hand, is found to have a compressive strength of about 53.0 MN m^–2 and thermal conductivity values in the range of 0.198 to 0.250 W m^–1 °C^–1. The composite material, when compared with other common building materials, is found to be potential material for construction applications because of its superior mechanical and thermal properties.     

Thermo-viscoelastic modelling of organic matrix composite behaviour – Application to T700GC/M21

The purpose of this study is to propose a thermo-viscoelastic model able to describe the strain rate dependency, from creep to dynamic tests, and temperature dependency, from −100 °C to room temperature, of organic matrix composite materials. A bi-spectral viscoelastic model, proved to be representative of the T700GC/M21 behaviour on a large range of strain rates, is used and improved in this study for temperature dependency. First, thermal residual stresses are introduced and proved not to be sufficient to describe the temperature dependency of the laminate. Subsequently, further to the analysis of the time–temperature superposition principle on DMA tests, the viscoelastic model is improved with the introduction of an Arrhenius like shift factor. The proposed model is proved to be representative and predictive of the strain rate and temperature dependencies of the T700GC/M21 composite material.     

Thermophysical properties of asbestos-reinforced rubbers

Factors influencing the sealing characteristics of gaskets made from asbestos-reinforced rubbers include mechanical properties, thermal properties and moisture absorption characteristics. Existing knowledge of the temperature dependence of the mechanical behaviour has been augmented by investigations of the specific heat capacity, thermal expansion and thermal conductivity at temperatures up to 250° C, as well as moisture absorption characteristics at ambient temperature of as-prepared and thermally cycled material. The measurements have provided an insight into thermally induced structural changes within the matrix, which are relevant to component operation.