Method of calculating the endurance limit of specimens with a stress concentrator

A method is proposed for calculating the endurance limits of specimens with geometric stress concentrators. The method is based on the actual stress distribution in the concentration zone during cyclic loading, and it takes into account the size of the propagating crack. The method of equivalent stresses is used to calculate the actual stresses. The results of calculation of endurance limits are compared with experimental data.Kharkov. K. E. Zhukov Kharkov Aviation Institute. Prochnost ONIL. Translated from Problemy Prochnosti, No. 11, pp. 84–86, November, 1989.     

Analysis of the effect of the decomposition of char-forming heat-insulating materials on the free discharge of a gas mixture

The article presents results of a numerical solution of a nonsteady problem on the free discharge of a mixture of gases from a hemispherical volume with allowance for thermal decomposition of heat-insulating materials.Notation V volume - S area - t - P p - T - u v - Q q, dimensional and dimensionless time, pressure, temperature, TIM decomposition rate, and heat flux - adiabatic exponent - R gas constant - density - H specific enthalpy - c specific heat - thermal conductivity - , , _s dimensionless complexes - coefficient expressing the radiative properties of the gas medium and the heat-transfer surface - Stefan-Boltzmann constant Indices 0 initial state and scale factors - s surface - coke - M TIM material - P pyrolysis front - A ablation front - v volatile degradation products - adiabatic conditions - c completion of discharge Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 5, pp. 787–793, May, 1988.     

Creep of ceramics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of 5 and 3, respectively. It is concluded that the behaviour with an exponent of 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.     

Low-cycle fatigue of metal of rolled sheets with initial anisotropy of mechanical properties

The results are presented of examination of the relationships governing low-cycle deformation and failure of the metal of rolled sheets (03Kh16N9M2 steel) with initial anisotropy of the mechanical properties. The results show that the cyclic characteristics of this material depend on the direction in which the specimens are cut out. The given dependence is nonmonotonic. The lowest endurance is recorded for the specimens cut out in the Z-direction normal to the plane of rolled stock. With an increase of the angle , between the Z axis and the direction in which the specimen is cut out, the endurance increases, and at 45° it is almost independent of angle and corresponds to the values obtained for specimens cut out in the rolling direction. The results of evaluation of the cyclic endurance of specimens obtained on the basis of strain approaches and energy criterion are presented.Translated from Problemy Prochnosti, No. 6, pp. 12–16, June, 1991.     

The influence of ageing on the stability of β-γ/γ′ derived microstructures in Ni-Al-Cr-(Co) alloys

Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L1₂), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L1₀ type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L1₀ martensite is examined.     

Effect of Intense Self-Irradiation on the Phase Composition of Metallic Plutonium

The crystal structure of film samples of "high-level" (based on ²³⁸Pu) and low-level (based on ²³⁹Pu) metallic plutonium during their prolonged (up to 343 days) storage (self-irradiation) at room temperature was studied by X-ray diffraction. In the samples of high-level plutonium, the -Pu and -Pu lattices coexist. In the period of 40-60 days, the other known crystal modifications of plutonium (-Pu, -Pu, -Pu, and -Pu) are also present. Low-level plutonium had only the -Pu lattice. A possible origin of this phenomenon is discussed.     

Distinctive features of corrosion cracking of high-strength steels

Experimental data on the corrosion cracking resistance of high-strength steel with a yield limit _0.2 1000 MPa in 3.5 % NaCl solution are presented. It is shown that cracking resistance can be estimated by using the so-called net rated fracture stresses. Corrosion cracking in the surface layers of the metal operating in contact with the medium is explained by the suppression of the processes of plastic deformation.Central Scientific-Research Institute of Structural Materials Prometei, St. Petersburg. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 42–45, January–February, 1994.     

Influence of plastic predeformation on the fatigue strength of compressor blades with defects

We study the influence of plastic predeformation by bending to create deep residual compressive stresses on the fatigue strength of 13Kh11N2V2MF steel specimens and compressor blades with cut and stamped V-shaped and semicircular defects, and also the use of design-based redistribution of the stresses in the blade (offsetting toward the edges) for the retardation of cracks in the stress concentrators. It is found that plastic predeformation increases the fatigue strength by about 20%, while offsetting with the creation of a compressive mean stress _m = –300 MPa in the defect region increases the fatigue strength by 2–2.5 times. We examine schemes for calculating the fatigue strength with account for the residual and mean stresses, and also examine the defect creation technique. Recommendations are made for increasing the strength of compressor blades with defects.Translated from Problemy Prochnosti, No. 10, pp. 76–88, October, 1994.     

Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.     

Inviscid fluid in high frequency excitation field

Summary The influence of high frequency excitations (HFE) on a fluid is investigated. The response to these excitations is decomposed in two parts: slow motion, which practically remains unchanged during the vanishingly small period , and fast motion whose value during this period is negligible in terms of displacements, but is essential in terms of the kinetic energy. After such a decomposition the slow and fast motions become nonlinearly coupled by the corresponding governing equations. This coupling leads to an effective potential energy which imparts some elastic properties to the fluid and stabilizes laminar flows.     

Morphology of nitrated layers in titanium alloys of different structural classes

This paper generalizes the results of investigations of the effect of nitrogen on titanium alloys at temperatures of isothermal exposure of 800 – 1100°C and gas pressures of 10⁵ – 10^–1 Pa. As a result of nitration, a gas-saturated area appears on the surfaces of the alloys (in addition to the nitride area). Its qualitative and quantitative characteristics depend on the phase composition of the alloys. The gas-saturated area consists of two layers. For all the investigated alloys, the first layer (adjacent to the nitride area) can be described as the -titanium structure stabilized with nitrogen. If the saturation temperature is lower than the temperature of the polymorphic transformation, then the structure of the second layer is identical to the alloy matrix. As a result of nitration in the (+)--region, this structure undergoes the transformation and, for - and pseudo--alloys, turns into an -grain of smaller size but with higher etchability compared to the -structure of the first layer. For (+)-alloys, this is mainly the -phase (-plates) in the -transformed structure. For -alloys, the morphology of the gas-saturated area does not depend on the saturation temperature and is identical to other structural types nitrated in the -region.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 35–41, September–October, 1993.     

Microstructure and mechanical properties of rapidly quenched L20 and L20+L12 alloys in Ni-Al-Fe and Ni-Al-Co systems

Ductile L2₀-type wires and+L1₂-type duplex wires with high strengths and large elongation in the Ni-Al-Fe and Ni-Al-Co ternary systems have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter was in the range 80 to 180m and the average grain size was 2 to 4m for the wires and 0.2 to 1.0m for the+ wires. _y, _f and _p of the wires were found to be about 360 to 760 MPa, 560 to 960 MPa, and 0.2 to 5.5%, respectively, for the Ni-Al-Fe system, those of the+ wires were about 395 to 660 MPa, 670 to 1285 MPa, and 3.5 to 17%, respectively, for the Ni-Al-Fe system, and about 260 to 365 MPa, 600 to 870 MPa, and 4.0 to 7.0%, respectively, for the Ni-Al-Co system. Cold-drawing caused a significant increase in _y and _f and the values attained were about 1850 and 2500 MPa, respectively, for Ni-20Al-30Fe and Ni-25Al-30Co wires drawn to about 90% reduction in area. The high strengths, large elongation and good cold-workability of the melt-quenched and+ compound wires have been inferred to be due to the structural change into a low-degree ordered state containing a high density of phase boundaries, suppression of grain-boundary segregation and refinement of grain size.     

Accuracy of temperature measurements in determining the thermal conductivity of substances by stationary methods in the range of moderate temperatures

An empirical comparison is made of the accuracy of platinum-rhodium-platinum and Chromel-Alumel thermocouples in determining the thermal conductivity of substances.Notation T, t temperature - temperature difference - y thermocouple readings - A_i parameters of approximating equation - sensitivity of thermocouple - sensitivity found from the generalized function - _st standard values of sensitivity - S₀ standard deviation of sensitivity for a given series - maximum deviation of from in different series - I, II indices indicating that the values pertain to platinum-rhodium —platinum and Chromel —Alumel thermocouples, respectively Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 306–310, August, 1980.     

A note on the similarity solutions for free convection on a vertical plate

Summary The similarity solutions for free convection on a vertical plate when the (non-dimensional) plate temperature is x and when the (non-dimensional) surface heat flux is –x are considered. Solutions valid for 1 and 1 are obtained. Further, for the first problem it is shown that there is a value ₀, dependent on the Prandtl number, such that solutions of the similarity equations are possible only for >₀, and for the second problem that solutions are possible only for >–1 (for all Prandtl numbers). In both cases the solutions becomes singular as ₀ and as –1, and the natures of these singularities are discussed.     

Deformation and recrystallization textures of copper single crystals and bicrystals

Recrystallization textures in rolled copper specimens have been investigated so as to elucidate the mechanisms of cube texture formation. The specimens are single crystals with the orientations corresponding to the main components of the rolled textures, such as {112}111,{110}112, etc. and bicrystals consisting of such oriented crystals. The cube texture was not observed in any single crystal specimens, but observed in only two bicrystal specimens with {112}/111{100}001 and {110}112/{100}001 orientations. The formation of cube texture seems to require the existence of a cube oriented region in the deformed state, and the favourable oriented matrix to allow the growth of such nuclei.     

Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Initial permeability studies on A13+ and Cr3+ substituted Ni–Zn ferrites

The chemical formula of the samples investigated is Ni0.7Zn0.3Alx/CrxFe2-xO4, where x=0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. The samples were obtained by the usual ceramic technology from high-purity oxides. The initial permeability was calculated from the inductance measurements with a torroidal core of 100 turns, using the formula L=0.0046 iN2h log d2/d1. The initial permeability i decreases in Ni0.7Zn0.3Alx/CrxFe2-xO4 with increase in Al3+/Cr3+. The decrease in i is attributed to a decrease of grain size D from 4.9 m to 4.4 m with Al3+ and to 1.9 m with Cr3+ and to variations in the anisotropy constant K1. The main contribution to the variation of permeability with content of Al3+/Cr3+ in the system is the effect each of them has on domain wall motion. The trivalent substituents (Al3+/Cr3+) cause impedance to the domain wall motion, which increases as the content of these ions increases. Al3+ has a stronger effect than Cr3+. The initial permeability components and do not exhibit much variation with temperature, except near Tc, where they fall sharply. The maximum of near Tc has been attributed to a damping effect of domain wall motion. © 1998 Chapman & Hall     

Experimental and theoretical investigations on morphological changes of γ′ precipitates in Ni -Al single crystals during uniaxial stress-annealing

Influences of the external tension and compression on the morphological changes of precipitates during coarsening in an Ni-15 at. % Al single crystal were experimentally investigated, and theoretical evalutions on the energetically favourable shape and its orientation were also derived based upon the anisotropic elasticity theory. The shape transformation with growth in size is experimentally found to follow the sequence: cuboidrod plate, under all annealing conditions, i.e. compression-annealing, tension-annealing and external stress-free annealing. However, the external stresses give preferential orientation to such microstructures. The tension of the [001] direction aligns the rods and plates along the tension axis, while the compression aligns them on a plane perpendicular to the compression axis. Theoretical evaluations based upon the anisotropic elasticity for the energetically favourable shapes and orientations of are consistent with experimental results except in the appearance of rods after compression-annealing. This discrepancy is understood by a conception that the shape transformation is influenced not only by energetics in the elastic strain and interfacial energies, but also by elastic interaction energy.Formerly at the Graduate School of Nagoya Institute of Technology.