Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

Requirements to Crack Resistance of Materials of Oil and Gas Equipment Operating in Hydrogen-Sulfide Media

We investigate the crack resistance of low-alloy pipe steels, their welds, and nickel and titanium alloys in air and in a hydrogen-sulfide solution NACE. We determine the stress intensity factors in the course of testing in air K _c and in hydrogen sulfide K _Issc as well as the coefficients of medium influence B _Issc = K _Issc /K _c. We establish minimum admissible values K _Issc ^min and B _Issc ^min that ensure high serviceability of structural materials of oil and gas industry. We obtain that K _Issc ^min 33MPa· for low-alloy steels and welds, K _Issc ^min 70MPa· for nickel alloys, and K _Issc ^min 50MPa· for titanium alloys. The criterion B _Issc ^min 0.6 is the same for various materials. We recommend to include the criteria of crack resistance K _Issc ^min and B _Issc ^min in the corresponding standards, specifications, etc.     

Coherent Brillouin Spectroscopy in Solid Parahydrogen

We applied coherent Brillouin spectroscopy to solid parahydrogen, and measured the Brillouin spectra of longitudinal acoustic modes at 5.6K. It was found that the linewidth of these spectra is 1.5MHz. From the observed Brillouin shift and the crystal orientation, the elastic stiffness was determined as C ₁₁=0.355±0.016GPa and C ₃₃=0.432±0.022GPa.     

Fractal Surfaces of ZrO2 , WO3 , and CeO2 Powders

The surface fractal properties of ZrO₂, WO₃, and CeO₂ powders prepared by the thermal decomposition of ZrO(NO₃)₂, (NH₄)₄W₅O₁₇, and (NH₄)₂Ce(NO₃)₆, respectively, were studied by mercury porosimetry. The results demonstrate that these oxides may, in principle, have fractal surfaces owing to topochemical processes of the type A(s) B(s) + C(g). The surface fractal dimension of individual crystallites and their aggregates are determined.     

Absorption and Circular Dichroism Spectra of Bi12SiO20<Nd> Crystals

The absorption and circular dichroism spectra of Nd-doped Bi₁₂SiO₂₀ crystals were measured in the range 10000 to 20000 cm^–1. The spectra showed electronic transitions related to only one type of Nd-related center: Nd substituting for Bi in position with symmetry C ₁. The bands due to the transitions from the first Stark component of the ground state to Stark components of excites states (⁴ I _9/2 ⁴ G _5/2, ⁴ I _9/2 ² G _7/2, and ⁴ I _9/2 ⁴ G _7/2) were analyzed in detail. The dipole strength D _0k , rotatory power R _0k , and anisotropy factor G _0k of these transitions were calculated. The intensities of transitions to Stark components were shown to vary by more than one order of magnitude within an excited-state multiplet. The anisotropy factors of the ⁴ I _9/2 ² G _7/2 and ⁴ I _9/2 ⁴ G _7/2 transitions, allowed in the magnetic dipole approximation, are, on the average, larger than that of the ⁴ I _9/2 ⁴ G _5/2 transition, which points to a significant contribution of the magnetic moment (<>²) to the total intensity of the transition.     

Optical Properties of Ca3Ga2Ge4O14 Crystals

The optical absorption, induced-absorption, luminescence, and excitation spectra and temperature-dependent luminescence intensity of thermochemically colored Ca₃Ga₂Ge₄O₁₄ crystals were measured. The results indicate that the induced-absorption bands at 34000–28000 and 28000–20000 cm^–1 and the emission band at 14800 cm^–1 are related to F-centers.     

Viscosity of the Grain-Boundary Phase and Sintering of TiB2-Modified Alumina

A model is proposed for the sintering of powders via the sliding of particles facilitated by a grain-boundary phase. Data on the initial-stage sintering kinetics of TiB₂-modified alumina are used to assess the temperature-dependent viscosity of the grain-boundary phase, formed by Al₂O₃, oxidation products of TiB₂, and impurities present on the surface of alumina particles. In the range 1570–1620 K, the temperature variation of viscosity is fitted well by the expression 25.9exp(143308/RT).     

Gc versus crack velocity in single groove double cantilever beam specimens of polycarbonate

The dependence of G _c on crack velocity in single-groove double cantilever beams (SG DCB) is negligible over the range of crack velocities from 0.2 to 1100 in. min^–1. The constancy of G _c appears to reflect a counterbalancing of the rise in yield stress by a decrease in crack opening displacement. The latter occurs through a decrease in the gauge length of material engulfed by yielding rather than by a decrease in the ultimate plastic strain in the crack tip plastic zone. The energy of shear lip formation at any crack velocity appears to be accurately estimated from G _c's for SG DCB specimens fractured at low velocity.     

Ferroelectric Lead Zirconate Titanate Films Prepared by Spray Pyrolysis of Carboxylate Solutions

Ferroelectric PbTi_0.6Zr_0.4O₃films 0.5–1.5 m in thickness were produced on platinum substrates by spray pyrolysis of carboxylate solutions. The optimized compositions of the precursor solutions, containing methacrylic acid and ethylene glycol, are stable under normal conditions, allow the annealing temperature to be reduced, and lead to higher quality film surfaces and large grains. The film exhibit the following electrical properties: T _C= 360–460°C, _max= 1750 at T _C, tan = 0.02–0.1 at 1 kHz and room temperature, P _{s max} = 18 C/cm², P _{r max} = 15 C/cm²at 50 Hz, and E _c= 42–120 kV/cm.     

The separation of the fracture energy in metallic materials

The total plastic strain energy which is consumed during fracture of a plain-sided CT specimen is separated into several components. These are the energies required for deforming the specimen until the point of fracture initiation, for forming the flat-fracture surfaces, for forming the shear-lip fracture surfaces, and for the lateral contraction and the blunting at the side-surfaces, W _lat. Characteristic crack growth resistance terms, R _flat and R _slant, are determined describing the energies dissipated in a unit area of flat-fracture and slant-fracture surface, respectively. R _flat is further subdivided into the term R _surf, to form the micro-ductile fracture surface, and into the subsurface term, R _sub, which produces the global crack opening angle. Two different approaches are used to determine the fracture energy components. The first approach is a single-specimen technique for recording the total crack growth resistance (also called energy dissipation rate). Plain-sided and side-grooved specimens are tested. The second approach rests on the fact that the local plastic deformation energy can be evaluated from the shape of the fracture surfaces. A digital image analysis system is used to generate height models from stereophotograms of corresponding fracture surface regions on the two specimen halves. Two materials are investigated: a solution annealed maraging steel V720 and a nitrogen alloyed ferritic-austenitic duplex steel A905. For the steel V720 the following values are measured: J _i=65kJ/m², R _surf=20kJ/m², R _flat=280kJ/m², R _slant=1000kJ/m², W _lat=30J. For the steel A905 which has no shear lips, the measured values are: J _i=190kJ/m², R _flat=1000kJ/m², and W _lat=45J. Apart from materials characterization, these values could be useful for predicting the influence of specimen geometry and size on the crack growth resistance curves. Key words: Elastic-plastic fracture mechanics, fracture energy, energy dissipation rate, fracture surface analysis.     

Effect of Gamma Irradiation on the Piezoelectric Properties of K2Sr4Nb10O30–K6Li4Nb10O30 Solid Solutions

The effect of gamma irradiation on the dielectric ( ₃₃/₀, tan) and piezoelectric (d ₃₁, g ₃₁, K _p, Q _m) properties of ₂Sr₄Nb₁₀O₃₀–K₆Li₄Nb₁₀O₃₀ solid solutions (tetragonal tungsten bronze structure) was studied as a function of K₆Li₄Nb₁₀O₃₀ content. The results demonstrate that increasing the gamma dose to 9 × 10⁵ Gy reduces ₃₃/₀ and tan. With increasing Li⁺ content (filling of triangular channels), d ₃₁, g ₃₁, K _p, and Q _m increase. The results are interpreted in terms of the generation of stable defects and effective redistribution of the energy of gamma radiation over the ceramic sample.     

The AC electrical characterization of the solid-state reaction derived CaSnO3

The potential of CaSnO₃ (calcium metastannate) for its application as a capacitor component possessing small temperature coefficient of capacitance (TCC) in electrical systems, is examined via the ac small-signal measurements. The ac electrical data were acquired on these samples sintered at various combinations of temperature-time frames (1200°CT1350°C; 24 ht60 h) in the frequency range from 5 Hz to 13 MHz. The measurements were carried out over the temperature range 25–300°C. The electrical response was found to exhibit relaxation processes in more than one complex plane formalism in a simultaneous fashion. The resistance of the sintered samples was dominated by the grain boundaries. The capacitance showed almost linear behavior in this measurement temperature range. The resulting electrical behavior has been discussed with the evolved microstructure in the sintered bodies.     

Investigation of the Electrophysical Characteristics of SF6Gas: Obtained Results

The radiation-induced electrical conductivity of SF₆gas is experimentally investigated at different pressures. The dependences of the coefficient of attachment of electrons to SF₆molecules on the E/Nratio are determined. The obtained results are compared with the results of numerical calculation by the Monte Carlo method and with the data of other authors.     

Corrosion of Iron in a Carbonate-Bicarbonate Solution. Part 1. Crystallographic Analysis of Passive Films

Visible passive films are formed on the surface of 1010 steel at 325K in a solution of 1N NaHCO₃ + 1N Na₂CO₃ at anodic potentials up to –320mV_NHE inclusively. For potentials below –400mV, they are dull and contain FeCO₃. For –(390–320)mV, the films are thin blackish bright. Most probably, the diffraction peaks are caused by Fe₃O₄–Fe₂O₃. For –(430–400)mV, the films are dull blackish. Probably, this color is caused by a composite of magnetite and siderite.     

On the Existence of Ternary Magnetic Phases in the In–Cr–Te System

Phase relations along the In₂Te₃–Cr₃Te₄ join of the In–Cr–Te system were investigated by differential thermal analysis, x-ray diffraction, and microstructural analysis, and the 600°C section of the InTe–CrTe–Te phase diagram was mapped out. Under the conditions of this study, the compounds InCr₂Te₄, In₂CrTe₄, and InCrTe₃ were not obtained. The only ternary compound identified is In₂Cr₆Te₁₁.     

Influence of pore volume on laser performance of Nd : YAG ceramics

For present study, 1.1 at% Nd-doped YAG ceramics with a controlled pore volume (150–930 vol ppm) were fabricated by a solid-state reaction method using high-purity powders. The scattering coefficients of Nd:YAG ceramics, obtained from Fresnel' equation, increased simply with increases in the pore volume. The cw laser output power of Nd:YAG ceramics was clearly related to the scattering coefficients of the specimens examined in the present works, which in turn were affected on the pore volume. The effective scattering coefficients of Nd:YAG ceramics with a pore volume of 150 vol ppm were nearly equivalent to those of a 0.9 at%Nd:YAG single crystal by Czochralski method. As the exciting power was increased under excitation by an 808-nm diode laser, however, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal because of the fairly large amount of Nd additives. The lasing performance of the Nd:YAG ceramics changed drastically with change in pore volume. On the other hand, lasing performance was not affected by the existence of grain boundaries in the polycrystalline Nd:YAG ceramics.     

Phase Transitions of Cs3Sb2I9 , Cs3Bi2I9 , and Cs3Bi2Br9 Crystals

The phase transitions of Cs₃Sb₂I₉, Cs₃Bi₂I₉, and Cs₃Bi₂Br₉crystals were investigated by ¹³³Cs nuclear magnetic resonance. Cs₃Bi₂Br₉was also studied by ⁸⁷Br nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs₃Sb₂I₉, the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs₃Sb₂I₉has a trigonal structure. The observed temperature variation in the shape of the ¹³³Cs NMR and ¹²⁷I NQR signals suggests that Cs₃Bi₂I₉may undergo a transition to an incommensurate phase.     

Size effects in concrete fracture – Part II: Analysis of test results

This paper presents an analysis of the extensive experimental program aimed at assessing the influence of maximum aggregate size and specimen size on the fracture properties of concrete. Concrete specimens used were prepared with varying aggregate sizes of 4.75, 9.5, 19, 38, and 76mm. Approximately 250 specimens varying in dimension and maximum aggregate size were tested to accomplish the objectives of the study. Every specimen was subjected to the quasi-static cyclic loading at a rate of 0.125mm/min (0.005in./min) leading to a controlled crack growth. The test results were presented in the form of load-crack mouth opening displacement curves, compliance data, surface measured crack length and crack trajectories as well as calculated crack length, critical energy release rate, and fracture toughness (G ₁). There is a well pronounced general trend observed: G ₁ increases with crack length (R-curve behavior). For geometrically similar specimens, where the shape and all dimensionless parameters are the same, the R-curve for the larger specimens is noticeably higher than that for the smaller ones. For a fixed specimen size, G ₁ increases with an increase in the aggregate size (fracture surface roughness). For the same maximum aggregate size specimens, the apparent toughness increases with specimen size. It was clear that the rate of increase in G ₁, with respect to an increase of the dimensionless crack length (the crack length normalized by the specimen width), increases with both specimen size and maximum aggregate size increase. The crack trajectory deviates from the rectilinear path more in the specimens with larger aggregate sizes. Fracture surfaces in concrete with larger aggregate size exhibit higher roughness than that for smaller aggregate sizes. For completely similar specimens, the crack tortuosity is greater for the larger size specimens. The crack path is random, i.e., there are no two identical specimens that exhibit the same fracture path, however, there are distinct and well reproducible statistical features of crack trajectories in similar specimens. Bridging and other forms of crack face interactions that are the most probable causes of high toughness, were more pronounced in the specimens with larger maximum size aggregates.     

Line Radiation of Multiply Charged Ions with a Fermi–Dirac Level Distribution of Electrons at High Temperatures

The line radiation of multiply charged ions with a Fermi–Dirac level distribution of electrons is investigated in the plasma temperature range where the motion of electrons may be treated as quasi-classical, and the potential in which they move, as Coulomb, which is valid under the conditions ZE _H max(T, _I ) Z ² E _H, where E _H = 13.6 eV is the ionization energy of hydrogen, Z is the atomic number of the ion being treated, and _I is the frequency of electron reversal in the Coulomb field at an energy equal to the ionization energy. The spectrum and intensity of radiation of ions are calculated. In the limit of high plasma densities, the intensity of radiation per ion turns out to be independent of density and proportional to T ².     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.