Non-stoichiometric curing effect on fracture toughness of nanosilica particulate-reinforced epoxy composites

Non-stoichiometric curing effects on the fracture toughness behaviors of nanosilica particulate-reinforced epoxy composites were experimentally investigated in this study by comparing them with bending strengths to take into consideration the effect of interaction between nanoparticles and network structures in matrix resins. The matrixes were prepared by curing them with an excess mixture of diglycidyl ether of bisphenol A-type epoxy resin as the curing agent for the stoichiometric condition. The volume fractions of the silica particles with a median diameter of 240 nm were constantly 0.2 for all composites. The neat epoxy resins and the composites were cured non-stoichiometrically to change the crosslinking densities of the neat epoxy resins and the matrix resins of the composites within 2740–490 mol/m³. The fracture toughnesses and bending strengths of the composites and the neat epoxy resins strongly depended on the crosslinking densities in the resins. Although the fracture toughness decreased monotonously from that of the stoichiometrically cured resins as the crosslinking density decreased, the fracture toughnesses of composites were largest at a slightly lower crosslinking density of approximately 2490 mol/m³ from the stoichiometric condition of 2740 mol/m³. The fracture toughness and the bending strength were improved for crosslinking densities higher than 2000 mol/m³ by adding particles. At crosslinking density lower than 2000 mol/m³, the particles worked against the mechanical properties as defects in matrix resins.     

The microstructure of rigid polyurethane foams

A study has been made of the microstructure of rigid closed-cell polyurethane foams in the density range 35 to 420 kg m^–3. Existing models for the structure of foams of this type have been evaluated using optical and scanning electron microscope techniques. Foams with a density of the order of 35 kg m^–3 are shown to be best represented as a pentagonal dodecahedron. Medium density foams from 70 to 300 kg m^–3 have a structure described as rounded polyhedra and high-density foams from 300 to 420 kg m^–3 have an isolated spherical structure. A relationship between average cell size and density is given.     

Effects of thermo-mechanical processing parameters on the special boundary configurations of commercially pure nickel

The roles of plastic strain, annealing temperature, and annealing time in affecting the fraction of special boundaries (Fsp), and twin densities of thermo-mechanically processed commercially pure nickel are examined. Different strain levels were achieved by cold rolling the material at different amounts. One-step low strain-recovery processing with strain levels in the range of 3.0–7.5% and annealing temperatures in the range 800–1000 °C were conducted in order to ensure that recrystallization did not occur. From orientation image microscopy analysis it was found that the fraction of special boundaries increased from about 30% for the as-received material to almost 80% for plastically deformed and annealed material. This showed that material strained in the range from 3.0% to 7.5% and annealed at 800 °C for different times all reached Fsp values in the range 75–80%, a considerable increase over the as-received material. Various multi-processing cycle treatments did not increase the fraction of special boundaries to above the value of 80% achieved in single processing cycles.TEM observations indicated dislocation tangles/cells occurred near grain boundaries in material strained at 6%. The density of these dislocation tangles decreased with annealing time at 800 °C and was reduced considerably after 20 min.Experimental results for the variation of twin density with grain size showed that the twin density decreased with increase in grain size. However, there was a tendency for the twin density to decrease more slowly with increase in grain size in the more highly strained samples. The experimental data for twin density were compared with values calculated from the equation suggested by Pande et al. [1]. With appropriate choice of the two variable parameters in the equation, a good “average” fit for all the data was obtained. However, the effect of plastic strain on the twin density could not be accommodated in the model.The experimental twin density–grain size relationship was also compared to values calculated from the formulation developed by Gleiter [2] modified to accommodate the effects of prior plastic train on the twin densities of annealed samples. Calculations from the modified model followed the observed trend that twin densities for the more highly deformed samples decreased more slowly with grain size than those for lesser strained samples. Good quantitative agreement between the experimental values of twin density and those calculated from the modified Gleiter formulation was achieved.     

Knudsen-effect errors with transient line-source measurement in fluids

This paper decribes the Knudsen-effect errors of the transient line-source method used for accurate measurements of the thermal conductivity and thermal diffusivity of fluids. The analysis demonstrates that the instrument can be used with a good accuracy (>0.5%) to lower densities than previously thought. The principal errors are illustrated by measurements on propane in the temperature range 250–300 K at densities less than 9 kg · m^–3.     

Establishment of an empirical correlation for estimating the thermal conductivity of igneous rocks

A correlation to predict the thermal conductivity of andesitic igneous rocks is developed from measured data on drill cores from wells from the Los Azufres geothermal field, Mexico. The correlation was developed from density, porosity, and thermal conductivity. Seventeen determinations were made on drill cores extracted at varying depths from 12 wells. Thermal conductivity varied from 1.05 to 2.34 W · m^–1 · K^–1, while bulk density varied from 2050 to 2740 kg · m^–3 and grain density varied from 2610 to 2940 kg · m^–3. Total porosity varied from 1.9 to 24.7%. Two polynomial regressions, one linear and one quadratic, were tested on the thermal conductivity-times-bulk density product, with total porosity as the independent variable. The correlation coefficients and residual mean square deviations were 0.83 and 0.00491 for the linear fit and 0.87 and 0.00425 for the quadratic model, respectively. For porosities up to about 18%, both models showed very close predictions, but for larger values, the quadratic model appeared to be better and it is recommended for the porosity range from 0 to 25%. Furthermore, density and porosity may be determined from drill cuttings, which are more readily available than cores.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Thermal conductivity andPVT measurements of pentafluoroethane (refrigerant HFC-125)

By means of the transient and steady-state coaxial cylinder methods, the thermal conductivity of pentafluoroethane was investigated at temperatures from 187 to 419 K and pressures from atmospheric to 6.0 MPa. The estimated uncertainty of the measured results is ±(2–3)%. The operation of the experimental apparatus was validated by measuring the thermal conductivity of R22 and R12. Determinations of the vapor pressure andPVT properties were carried out by a constant-volume apparatus for the temperature range 263 to 443 K, pressures up to 6 MPa, and densities from 36 to 516 kg m^–3. The uncertainties in temperature, pressure, and density are less than ±10 mK, ±0.08%, and ±0.1%, respectively.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Structure and mechanics of cement foams

Lightweight cellular concretes have been available for a number of years. They are made by adding aluminium powder to the cement mix or by introducing a foaming agent to a cement slurry. Materials with densities in the range 320–1600 kgm^–3 are commonly available commercially; they are used for insulated concrete roof-deck systems, masonry blocks, cladding panels and engineered fills for geotechnical applications. Their unique set of properties make them attractive as a foam core material for structural sandwich panels: they have moderate thermal insulation, high heat capacity, high stiffness, excellent fire resistance and low cost relative to polymer foams. The structure and mechanical behaviour of cellular cements ranging in density from 160–1600 kg m^–3 are described.     

Measurements of Saturation Densities in Critical Region and Critical Loci for Binary R-32/125 and R-125/143a Systems

R-32/125 (difluoromethane/pentafluoroethane) and R-125/143a (pentafluoro-ethane/l,l,l-trifluoroethane) binary systems are promising alternative refrigerants to replace conventional refrigerants, i.e., R-22 and R-502. The saturated vapor- and liquid-density data in the critical region of these mixtures were measured using the visual observation of the meniscus disappearance in an optical cell. For the R-32/125 system, 35 saturation density data were measured at three compositions, 10, 35, and 50 mass% R-32. Nineteen saturation density data were also measured for R-125/143a (50/50 mass%). The critical temperatures and densities for these binary refrigerants were determined by taking into consideration the level and location of the meniscus disappearance as well as the intensity of the critical opalescence. Correlations to represent the critical loci of these binary refrigerants for an entire range of compositions have been developed. The experimental uncertainties of the saturation density data are estimated to be within 9 mK in temperature and 0.5 to 5.0 kg · m^–3 in density. The uncertainties of the critical temperature and density are estimated to be within 12 to 14mK and 4 to 8kg · m^–3, respectively.     

Mechanical behaviour of bamboo and bamboo composite

The tensile, flexural and impact strengths of bamboo and bamboo fibre-reinforced plastic (BFRP) composite have been evaluated. The high strengths of bamboo, in the fibre direction, have been explained by its anatomical properties and ultra structure. Bamboo fibres and bamboo orthogonal strip mats (bamboo mat) have been used to reinforce epoxy resin. BFRP composites with unidirectional, bidirectional and multidirectional strengths have been made. In bamboo mat composites, the fibre volume fraction,V _f, achieved was as high as 65%. The tensile, flexural and impact strengths of bamboo along the fibres are 200.5 MN m^–2, 230.09 MN m^–2 and 63.54 kJ m^–2, respectively, whereas those of bamboo fibre composites and bamboo mat composites are 175.27 M N m^–2, 151.83 MN m^–2 and 45.6 kJ m^–2, and 110.5 MN m^–2, 93.6 M N m^–2 and 34.03 kJ m^–2, respectively. These composites possess a close to linear elastic behaviour. Scanning electron microscopy studies of the fractured BFRP composite specimens reveal a perfect bonding between bamboo fibres and the epoxy. Furthermore, high strength, low density, low production cost and ease of manufacturing make BFRP composite a commercially viable material for structural applications.     

Longitudinal shear properties of human compact bone and its constituents,and the associated failure mechanisms

Human compact bone specimens were tested in longitudinal shear at two different strain rates. The maximum stress and energy absorption capacities were 50.40±14.08 MN m^–2 and 20720±9310J m^–2 respectively for 14 embalmed specimens tested at a cross head speed of 2.1×10^–6 m sec^–1. The maximum stress was found to be 75% of the transverse shearing strength. Bone specimens were also tested after selectively dissolving the organic and mineral components. The results showed that the composite strength of bone was much higher than the summation of strengths of its organic and mineral phases. Fractographic examination of the fracture surfaces showed that debonding of the interfaces between the osteons and the surrounding bone matrix and between the osteonal lamellae were the main mechanisms of longitudinal shear failure.     

Fracture of a plasticized epoxide under superposed hydrostatic pressure

The deformation and fracture behaviour of rubber-coated and uncoated epoxy specimens has been studied under superposed hydrostatic pressures extending to 300 MN m^–2. Maximum shear stress at yield for this epoxy were about 25 MN m^–2 at atmospheric pressure and rose to about 48 MN m^–2 at 300 MN m^–2 superposed pressure. Yielding and failure of all specimens tested beyond pressures of 75 MN m^–2 took place when all the (macroscopic) principal stresses, though unequal, were compressive. Fractographic examination revealed three distinct zones of the failure surfaces at atmospheric pressure. The behaviour of all uncoated specimens and those coated and tested below 100 MN m^–2 was similar. A fracture-mechanics interpretation of failure could be applied to these tests assuming the deformation-produced first zone was the fracture initiating site. Coated samples tested beyond 100 MN m^–2 superposed pressure failed with no evidence of Zones II or III of failure; Zone I appeared to spread over the entire failure surface. An interpretation involving fluid penetration of Zone I failure nuclei, along the lines suggested by Duckett, can account for the failure stresses of the uncoated specimens but is not tenable for the coated samples. It appears that crack nucleation and (slow) growth, as opposed, perhaps, to (catastrophic) crack propagation, can take place in this polymer when all the principal stresses are compressive.     

Aerobic composting of tobacco industry solid waste—simulation of the process

Solid waste accumulated during the processing of tobacco for cigarette manufacture mostly contains tobacco particles and flavoring agents. Its main characteristics are a high content of nicotine (2,000 mg per kg of total solids), which is a toxic compound, and high value of total organic carbon of the aqueous extract (12,620.0 mg l^–1). Because of this fact tobacco waste cannot be disposed of with urban waste.The aim of this work was to stabilize tobacco solid waste by aerobic composting. The experiments were carried out in closed thermally insulated column reactors (1.0 l and 25 l) under adiabatic conditions and at an airflow rate of 0.9 l min^–1 kg^–1 of volatile solids for 16 days. During the process, temperature changes in the reactor, CO₂ production and the numbers of mesophilic and thermophilic organisms in the mixed microbial culture were closely monitored. Nicotine concentration in the samples was analyzed at the start and at the end of process. It was estimated that at the end of composting the volume and mass of total solids in the tobacco waste were reduced by about 50% and those of nicotine by 80%. A simple empirical model was used to simulate the biodegradation rate of the organic fraction of the solid waste. It was found that the selected model describes aerobic composting fairly well, although only two kinetic parameters (k₀ and n) were estimated.List of symbols c_pS specific heat capacity of the substrate, kJ kg^–1 K^–1 - c_pz specific heat capacity of air, kJ kg^–1 K^–1 - F_Ku and F_Ki molar airflow at the reactor inlet and outlet, mol h^–1 - H_r reaction enthalpy, kJ kg^–1 of dry substrate - k specific rate, Eqs. (5) and (9), h^–1 - k_o constant in Eq. (9), day^–1 - m_o initial mass of the substrate, kg - m_S mass of dry substrate, kg - n order of the reaction in Eq. (5) - n_K molar amount of oxygen, mol - Q_v airflow volume, m³ h^–1 - r_K oxygen depletion rate, mol kg^–1 h^–1 - r_S degradation rate, kg kg^–1 h^–1 - _z air density, kg m^–3 - SD mean square deviation - t time, h - T temperature in reactor, °C - T_o temperature of substrate at the beginning of reaction, °C - T_K temperature of compost at the end of reaction, °C - T_u temperature of air at the reactor inlet - space time, day - w_S mass fraction of compost, m_sm_o^–1, kg kg^–1     

Impact-modified epoxy resin with glassy second component

A resorcinol-based epoxy resin was modified by incorporating a glassy second component. The mixture showed a heterogeneous morphology with two clearly defined phases, one phase rich in oligomer, the other phase composed mainly of resorcinol epoxy resin. The fracture toughness measured asG _1c andK _1c values showed an increase from 174J m^–2 and 0.89 MN m^–1.5 S in pure epoxy resin to 431 J m^–2 and 1.36 MN m^–1.5 in 30% oligomer modified resins. The scanning electron micrographs showed that the oligomer-rich phase exhibited ductile failure behaviour and formed the dispersed phase at low concentrations while it was the continuous matrix when the concentration was 30%. Optical observations on the failure mode of thin films of the oligomer-modified epoxy resin showed the existence of both inter face failure and considerable distortion in both phase.     

Effect of mixing on thermal and mechanical properties of aerogel-PVB composites

Silica aerogels were synthesized via solvent exchange/surface modification of wet gels from waterglass using IPA/TMCS/n-Hexane solution. The densities, porosities, and specific surface areas of the aerogel were in the range of 0.128–0.153 g/cm³, 93–95%, and 598–795 m²/g, respectively. Aerogel-PVB composites were manufactured by hot pressing. Aerogels were crushed and mixed with PVB. Three mixing methods were performed. The aerogel-PVB composites had densities of 0.19–1.09 g/cm³, and thermal conductivities of 0.03–0.12 W/(m · K). Density and thermal conductivity can be controlled by aerogel volume fraction and the mixing method. The modulus of rupture of aerogel-PVB composites was in the range of 0.15–46.5 MPa.     

Measurements of Gaseous PVTx Properties and Saturated Vapor Densities of Refrigerant Mixture R-125+R-143a

The experimental PVTx properties of a binary refrigerant mixture, R-125 (pentafluoroethane)+R-143a (1,1,1-trifluoroethane), have been measured for a composition of 50 mass% R-125 by a constant-mass method coupled with an expansion procedure in a range of temperatures from 305 to 400 K, pressures from 1.5 to 6.1 MPa, and densities from 92 to 300 kg·m^–3. The experimental uncertainties of the present measurements are estimated to be within ±7.2 mK in temperature, ±3.0 kPa in pressure, ±0.12 kg·m^–3 in density, and ±0.040 mass% in composition. The sample purities are 99.953 mass% for R-125 and 99.998% for R-143a. Seven saturated vapor densities and dew point pressures of the R-125+R-143a system were determined, on the basis of rather detailed PVTx properties measured in the vicinity of the saturation boundary as well as the thermodynamic behavior of isochores near saturation. The second and third virial coefficients for temperatures from 330 to 400 K were also determined.     

The tensile properties of pultruded GRP tested under superposed hydrostatic pressure

The failure mechanisms in waisted tensile specimens of pultruded 60% volume fraction glass fibre-epoxide were investigated at atmospheric and superposed hydrostatic pressures extending to 350 MN m^–2. The maximum principal stress at fracture decreased from 1.7 GN m^–2 at atmospheric pressure to 1.3 GN m^–2 at 250 MN m^–2 superposed pressure and remained approximately constant at higher pressures, as had been observed with carbon fibre reinforced plastic (CFRP) and a nickel-matrix carbon fibre composite. In the high-pressure region the failure surfaces were fairly flat, consistent with the fracture process being solely controlled by fibre strength. Pre-failure damage, in particular debonding, was initiated at 0.95 GN m^–2 at atmospheric pressure and this stress rose to 1.2 GN m^–2 at 300 MN m^–2 superposed pressure, i.e. by about 9% per 100 MN m^–2. Unlike the pressure dependence in CFRP, this contrasts with the pressure dependence of the resin tensile strength, about 25% per 100 MN m^–2, but can be associated with that of the fibre bundle/resin debonding stress, about 12% per 100 MN m^–2 superposed pressure. Consistent with this interpretation, glass fibres of the failure surfaces were resin-free, again in contrast to CFRP.     

The stability of sapphire whiskers in nickel at elevated temperatures

Composites of 1 to 20 vol % sapphire whiskers contained in a nickel matrix were produced by roll-bonding and by hot-pressing. The composites were examined using transmission electron microscopy, X-ray and electron diffraction, optical microscopy and mass spectrographic analysis. Composites were annealed in vacuum (1.3×10^–3 N m^–2), in low-pressure air (13.3 N m^–2) and in dried hydrogen (101.3 N m^–2) in the temperature range 1100 to 1400° C for times up to 3800 h. Whiskers in situ and the whisker/matrix interface were observed by transmission electron microscopy; matrix dislocations were associated with whiskers and were stable after annealing at 1400° C. Whiskers extracted from annealed composites showed significant morphological changes. These were attributed to: (i) ovulation from the tips of whiskers by interfacial diffusion, (ii) waisting from surface undulations, (iii) Ostwald ripening and (iv) constant-volume shape changes.     

Moisture adsorption and desorption properties of some tropical woods

The sorption of moisture for ten species of tropical wood, in the density range 480–1120 kg m^–3 was investigated by measuring the moisture content (MC) and dimensions (DC) during relative humidity (RH) cycling. In general, it was found that there was an increase in dimensions as well as moisture content on increasing the RH, while there was a decrease in dimensions as well as MC on decreasing the RH. Hysteresis effects were observed both in the DC-RH and the MC-RH curves on RH cycling. The hysteresis properties were observed to be not only anisotropic but also species specific. In addition, an anomalous contraction was observed at moisture contents below 2%. The shell-core model is advanced to explain this anomalous behaviour of wood.     

Transport properties of 1,1-difluoroethane (R152a)

Based on reliable. carefully selected data sets. equations for the thermal conductivity and the viscosity of the refrigerant R 112a are presented. They are valid at temperatures from 240 to 440 K, pressures up to 20 MPa. and densities up to 1050 kg · m^–3. including the critical region.     

Thermal conductivity and viscosity of 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123)

The thermal conductivity and the viscosity data of CFC alternative refrigerant HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane: CHCI₂-CF₃) were critically evaluated and correlated on the basis of a comprehensive literature survey. Using the residual transport-property concept, we have developed the three-dimensional surfaces of the thermal conductivity-temperature-density and the viscosity-temperature-density. A dilute-gas function and an excess function of simple form were established for each property. The critical enhancement contribution was taken no account because reliable crossover equations of state and the thermal conductivity data are still missing in the critical region. The correlation for the thermal conductivity is valid at temperatures from 253 to 373 K, pressures up to 30 MPa, and densities up to 1633 kg m^–3. The correlation for the viscosity is valid at temperatures from 253 to 423 K, pressures up to 20 MPa. and densities up to 1608 kg·m^–3. The uncertainties of the present correlations are estimated to be 50% for both properties, since the experimental data are still scarce and somewhat contradictory in the vapor phase at present.