A model of damage accumulation in metallic materials in the complex stressed state

A phenomenological model of damage accumulation in metallic materials under static loading was proposed earlier by Lebedev et al. (Probl. Prochn., No. 7, pp. 31–40, 1995). According to this model, the dependence of the degree of damage to the material on strains has the formε _l ^p = [1 − 2μ(ε)]ε, where μ(ε) is the current value of the coefficient of shear strains. We generalize this model to the case of a complex stressed state by introducing an influence functionf(σ _ij ) which reflects the distinctive features of the evolution of structures depending on the type of the stressed state. Numerical results are in good agrement with experimental data, which proves that the theoretical assumptions are correct. Translated from Problemy Prochnosti, No. 3, pp. 55–63, May–June, 1997.     

Controllable synthesis and morphology-dependent microwave absorption properties of iron nanocrystals

Iron nanospheres, nanoflakes and nanofibers were synthesized via a simple pyrolysis method. When the pyrolysis temperature increased from 523 to 623 K and the flow rate of Ar carrier gas maintained at 100 sccm, the as-prepared iron nanocrystals showed a morphology evolution from isotropic nanospheres to isotropic nanofibers. The phase structures and morphologies of the composite were characterized by X-ray diffraction and scanning electron microscopy. The complex permittivity (ε′ − jε″) and permeability (μ′ − jμ″) of these composites were measured using the transmission/refection coaxial line method in the frequency range of 1–18 GHz by a vector network analyzer. The iron nanofibers exhibited superior microwave absorbing properties compared to iron nanoparticles and nanoflakes. The optimal reflection loss (RL) reached −17.8 dB at 9.9 GHz with a layer thickness of 2.0 mm. The RL below −10 dB can be obtained in the frequency range of 7.3–11.7 GHz. Considering the low cost and high efficiency; the iron nanofibers are favorable for application as microwave absorber.     

Dielectric and impedance spectroscopy studies on sisal fibre-reinforced polyester composite

Frequency and temperature dependence of dielectric constant (ε_r), dielectric loss (tanδ), ac conductivity (σ_ac) and complex impedance spectroscopy studies on cured polyester matrix and sisal fibre-reinforced polyester composites (SFRPC) have been investigated in the frequency range from 180 Hz to 1 MHz and temperature range from room temperature to 200 °C. The experimental results showed that with the incorporation of sisal fibre, the values of ε_r, tanδ and σ_ac are increased. It is also found that the values of ε_r and tanδ for both cured polyester matrix and SFRPC are decreased with increasing frequency, which indicates that the major contribution to the polarization may come from orientation polarization and interfacial polarization. The increasing value of ε_r with increasing temperature at a particular frequency is due to free motion of the dipole molecular chains within the cured polyester matrix and SFRPC at higher temperature.     

Studies on a.c. properties of Ca<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> perovskites

A.c. measurements were preformed on bulk samples of Ca_1−x Sr _x TiO₃ (CST) perovskites with x = 0, 0.1 and 0.5 as a function of temperature range 300–450 K and frequency range 10³–10⁵ Hz . The experimental results indicate that the a.c. conductivity σ_a.c.(ω), dielectric constant ε′ and dielectric loss ε′′ depend on the temperature and frequency. The a.c. conductivity as a function of frequency is well described by a power law Aω ^S with s the frequency exponent. The obtained values of s > 1 decrease with increasing temperature. The present results are compared to the principal theories that describe the universal dielectric response (UDR) behavior.     

Notch effect and brittle-ductile transition

We present the fresults of the investigation of the effect of notches on the brittle-ductile transition in the process of fracture of structural materials. It is shown that the local elastoplastic state of a material in the vicinity of the notch can be described by using the corresponding stress intensity factorsK _I ^σ , strain intensity factorsK _i ^ε , and strain-energy intensity factorsK _I ^ω . We prove that the factorsK _I ^σ andK _I ^ε are linear functions of the notch radius ρ on the coordinates (wherei=σ or ε) and propose to characterize the brittle-ductile transition in the process of fracture of the material by the slopes of these functions. Bay Zoltan Institute of Logistics and Production Systems, Miskolc-Tapolca, Hungary. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 5, pp. 27–33, September–October, 1998.     

Fatigue of VT20 titanium alloy with vacuum-plasma coatings at high temperatures

We study the influence of vacuum–plasma TiN, (TiAl)N, and (TiC)N coatings on the high-cycle fatigue of VT20 titanium alloy in the temperature range 350–640°C for a loading frequency of 10 kHz. It is shown that, in this temperature range, the fatigue limits of VT20 alloy with the indicated coatings 6 μm in thickness become 15–25% higher than for the material without coating. The possibility of replacement of steel blades with titanium blades with vacuum-plasma coatings is demonstrated. Translated from Problemy Prochnosti, No. 4, pp. 101–107, July–August, 2009.     

Impedance-spectroscopy analysis of oriented and mercerized bamboo fiber-reinforced epoxy composite

Bamboo fiber-reinforced epoxy composites were fabricated with untreated and alkali treated bamboo fibers. Dielectric, electric modulus, ac, and dc conductivity studies were carried out to rationalize the dielectric behavior of bamboo/epoxy composites. Composites of two fiber orientation parallel and perpendicular to the electric field were prepared. The dielectric behavior and electric modulus spectra of the composites were characterized using standard impedance analyzer. Dielectric properties were analyzed as a function of frequency (95 Hz–2 MHz) for temperatures in the range from 30 to 180 °C. Real part of dielectric constant (ε′), conductivity, and dielectric dissipation factor (tan δ) of 0° oriented bamboo/epoxy composites were higher than that of 90° oriented composites. Conductivity activation energy, tan δ, ε′, and volume resistivity decreased with increase in frequency at all the temperatures under study. Mercerization reduces the water absorption in bamboo fibers and thus improves corresponding dielectric properties of composites. Relaxation times 39.80 μs and 258.5 μs for 0° and 90° oriented bamboo/epoxy composites were calculated respectively from the relaxation peaks observed in electric modulus spectra at 180 °C.     

Frequency-dependent dielectric and electric modulus properties of Li–Ni–Sm–Fe–O spinel embedded in conducting polymer

Conducting polymeric nanocomposite containing Li–Ni–Sm–Fe–O spinel was synthesized by the chemical oxidizing of aniline in the presence of LiNi_0.5Sm_0.08Fe_1.92O₄ particles. The dielectric and electric modulus properties of the as-prepared samples were investigated over a frequency range from 10⁶ to 10⁹ Hz. The dielectric constant (ε′), dielectric loss (ε″) and dissipation factor (tan δ) for all samples presented relatively high values at low frequency and were found to decrease with the frequency. The values of ε′, ε″ and tan δ of the nanocomposite were lower than that of the pristine PANI. Electric modulus analysis had been carried out to understand the electrical relaxation process. The dielectric relaxation time for the nanocomposite became longer due to the introduction of LiNi_0.5Sm_0.08Fe_1.92O₄ particles lowering the crystallinity of PANI.     

Study of dielectric properties of nanophase silver iodide

Dielectric properties of nanophase AgI are studied over the frequency range from 100 kHz to 3 MHz at different temperatures. The values ofε, tanδ andσ _ac are considerably larger than those reported for crystalline pellets of AgI, but they show a similar trend in variation with frequency and temperature. The increase in these values are attributed to the defect structure of the nano-particles.     

Asymptotic analysis for a crack on interface of damaged materials

An asymptotic analysis for a crack lying on the interface of a damaged plastic material and a linear elastic material is presented in this paper. The present results show that the stress distributions along the crack tip are quite similar to those with HRR singularity field and the crack faces open obviously. Material constants n, μ and m0 are varied to examine their effects on the resulting stress distributions and displacement distributions in the damaged plastic region. It is found that the stress components σrr, σθ θ, σr θ and σe are slightly affected by the changes of material constants n, μ and m0, but the damaged plastic region are greatly disturbed by these material parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the conditional energy σ, of austenitic grain boundaries and analyze changes in boundary energy at elevated temperatures for low-carbon, chrome-manganese and chrome-manganese-molybdcnum steels. It is established that σ, varies within the range 1.0–1.3 J/m² and that, in almost all cases of alloying, the effect caused by a decrease in the surface energy of iron is so strong that even in the presence of chorophobic elements σ, decreases at elevated temperatures. For multicomponent steels, we analyze variations of the temperature coefficients of boundary energy. Philadelphia, United States of America. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 63–68, July–August, 1996     

Dielectric properties of Be(IO<Subscript>3</Subscript>)<Subscript>2</Subscript>⋅4H<Subscript>2</Subscript>O crystals

The article studies the dielectric properties, dc conductivity and ac conductivity of Be(IO₃)₂⋅4H₂O single crystals. The dielectric constant ε has been defined for the three directions of the vectors a, b and c in the crystals in the temperature interval 280–340 K and frequency range 100 Hz–10⁶ Hz. The crystals show strongly expressed anisotropy, at 20 ^∘C and frequency 100 Hz ε_a = 235, ε_b = 30 and ε_c = 85. The frequency dependence of ε is evidence of the presence of low-frequency relaxation polarization in the crystals. The activation energies of the three directions in the crystals have been derived from the temperature dependence of dc conductivity, and they are 1.03 eV, 0.836 eV and 1.2 eV respectively.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.     

Electrical conductivity and dielectric properties of cadmium thiogallate CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> thin films

Cadmium thiogallate CdGa₂S₄ thin films were prepared using a conventional thermal evaporation technique. The dark electrical resistivity calculations were carried out at different elevated temperatures in the range 303–423 K and in thickness range 235–457 nm. The ac conductivity and dielectric properties of CdGa₂S₄ film with thickness 457 nm has been studied as a function of temperature in the range from 303 to 383 K and in frequency range from 174 Hz to 1.4 MHz. The experimental results indicate that σ_ac(ω) is proportional to ω ^s and s ranges from 0.674 to 0.804. It was found that s increases by increasing temperature. The results obtained are discussed in terms of the non overlapping small polaron tunneling model. The dielectric constant (ε′) and dielectric loss (ε″) were found to be decreased by increasing frequency and increased by increasing temperature. The maximum barrier height (W _m) was estimated from the analysis of the dielectric loss (ε″) according to Giuntini’s equation. Its value for the as-deposited films was found to be 0.294 eV.     

Dielectric properties of Fe(OH)3 thin films formed at solution-gas interface

Dielectric properties of solution-gas interface-formed Fe(OH)₃ thin-film capacitors (Al/Fe(OH)₃/Al) of various thicknesses have been studied in the frequency range 10–10⁶ Hz at various temperatures (300–443 K). Dielectric constant, ε, increases with increasing film thickness (d) and temperature (T) and decreases with increase of frequency (f). The loss factor (tan δ), showing pronounced minimum with frequency, increases with rise of temperature, and tan δ_min shifts to a higher frequency. The large increase in dielectric constant towards low frequency region indicates the possibility of an interfacial polarization mechanism in this region.     

The Influence of the Interaction Potential on the Diffusion of Neutral Particles in Oxyfluoride Glasses

A systematic study of the effects of the interaction potential upon the diffusion coefficients of neutral particles (P0s) and ions in the 60GeO₂–40PbF₂ glass has been performed using molecular dynamics simulations. The P0–P0 and P0–ions pair potentials were described by a Lennard–Jones potential with two parameters: the well depth (ε) and the size (σ). These parameters were varied in the ranges: (σ) = 0.1-1.0 eV and σ=1.0−4.0 ?. The diffusion coefficients were calculated for several combinations at several temperatures (300–1200 K). The estimated glass transition temperature (T _g) presented a slight dependence upon the potential parameters, but it is near the crystallization temperature of a glass of similar composition. Whereas, the diffusion coefficients of the P0s presented a strong dependence with the potential parameters, where increasing ε and σ causes a decrease of their diffusion coefficients. These results suggest that the atoms should be responsible for forming any surface film, since neutral particles larger than atom, such as, nanoparticles, should have negligible diffusion.     

Dielectric constant, magnetic permeability and microwave absorption studies of hot-pressed Ba-CoTi hexaferrite composites in X-band

The microwave absorption, complex permittivity and complex permeability studies of hot-pressed hexaferrite composites prepared with Ba(CoTi)_xFe_12-2xO₁₉ (x = 0.0, 0.2, 0.4, 0.8, and 1.0) were made in the frequency range from 8.0 to 12.4 GHz. The hexaferrite composites with x > 0.0 exhibit significant dispersion in the complex permittivity (ε_r′-jε_r″). However the dispersion in complex permeability (μ_r′-jμ_r″) is not significant and is attributed to the shielding effect of polymer matrix over the ferrite crystallites. The reflection loss has been studied as function of frequency, composition and thickness of absorber. A comparison of reflection loss of hot-pressed ferrite composites with that of normal sintered ferrite composites was made and analyzed. A minimum reflection loss of—24.0 dB is obtained at 9.9 GHz for 2.8 mm thick sample of BaCo_0.4Ti_0.4Fe_11.2O₁₉ hot-pressed hexaferrite composite.     

Structural,Electrical and Dielectric Properties of Co–Mn Spinel Nanoferrites Prepared by Co-precipitation Technique

Cobalt manganese nanoceramics with nominal composition Co_1−x Mn _x Fe₂O₄ (x=0.00, 0.15, 0.30, 0.45, 0.60, 0.75) were synthesized by a chemical coprecipitation technique. The X-ray diffraction patterns revealed the cubic spinel structure. The average crystallite size from the Sherrer formula was in the range of 28 nm–56 nm. The lattice parameter increased with a concentration of manganese. Scanning electron microscopy (SEM) was used for microstructural study. The electrical properties such as electrical resistivity (ρ), drift mobility (μ _d), and activation energy (ΔE) were measured using a two-probe apparatus in the temperature range 323–573 K. The electrical resistivity decreased with temperature and manganese concentration. The dielectric constant (ε′), loss factor or imaginary loss (ε″) and dielectric tangent loss (tan δ) were measured at room temperature using a LCR meter in the frequency range 100 Hz to 5 MHz. All the dielectric parameters decreased with increasing frequency. The AC electrical conductivity (σ _ac) was calculated from the dielectric measurements. It increases with increase of frequency and manganese concentration. The increase in electrical conductivity may be attributed to the formation of Co⁺³ and Mn⁺³ from Co⁺² and Mn⁺³, respectively, at both A and B sites.     

Magnetic and dielectric properties of the M-type barium strontium hexaferrite (Ba x Sr1−x Fe12O19) in the RF and microwave (MW) frequency range

This paper presents results for the dielectric permittivity (ε′), dielectric loss (tg δ _E) dielectric permeability (μ′) and magnetic loss (tg δ _M) in the radio-frequency and microwave frequency range of Ba _x Sr_1−x Fe₁₂O₁₉ hexaferrite (0 ≤ x ≤ 1). The samples were prepared by a new route of the ceramic method. The magnetic permeability (μ′) and magnetic loss (tg δ _M) measurements in the range 100 MHz to 1.5 GHz, reveals that 1.32 ≤ μ′ ≤ 1.68 for the permeability of BFO100 (BaFe₁₂O₁₉) and 1.16 < μ′ ≤ 1.88 for SFO100 (SrFe₁₂O₁₉) in the range of studied frequencies. The BFO100 sample presented lower loss (tg δ _M = 4.10⁻³ at 1.5 GHz). The permittivity of BFO100 and SFO100 in 1.5 GHz are, respectively 8.18 and 8.19, and in the 1 MHz are, respectively 52.04 and 19.09. The samples presented coercive field in the range of 3–5 kOe and remanence magnetization in the range of 33–36 emu/g. The subjects of paper were study the dielectric and magnetic properties of the barium and strontium hexaferrite, in view of applications as a material for permanent magnets, high density magnetic recording and microwave devices.     

Impact of metal filler on the dielectric properties of Nylon 11

Dielectric measurements of pure Nylon 11 in comparison with metal (Zn) filled Nylon 11 have been carried out using an impendence analyzer in the frequency range of 10²–10⁷ Hz and temperature range 20–120 °C. Two different concentrations (1% and 5% (w/w)) of metal (Zn) fillers were used. It was observed that at low frequencies and particularly at high temperature dielectric permittivity (ε′) for 1% Zn filled sample is more than that of pure Nylon 11 whereas ε′ for 5% Zn filler is less as compared to that for pure Nylon 11. But at very high frequencies dielectric permittivity (ε′) for pure Nylon 11 is less than Zn filled samples. Also it is found that for all frequencies and particularly at high temperature ε′′ as well as tan δ are maximum for pure Nylon 11 and decrease for filled Nylon 11 samples. The Cole–Cole arcs have also been plotted for these samples. Using these plots the static and instantaneous values of dielectric permittivity and orientation polarization parameter ‘S’ have been calculated.