Dynamic and fractal properties of SP-28 steel under high-speed loading conditions

Using an interferometric method to record the velocity of the free surface of a target subjected to two-dimensional shock loading, it is shown experimentally that the decrease in the compression pulse amplitude is due to the nonstationary nature of mesoscale processes — the amplitude decrease is progressively larger for higher rates of change of the variance of the mesoparticle velocity. It is shown theoretically that the loading rate influences the spallation strength of a material in a planar collision only if the variance of the particle velocity is nonzero. A fractal analysis of the spallation surfaces of steel samples is performed by quantitative fractography methods. An expression relating the fractal dimension of the spallation fracture surface and the variance of the mesoparticle velocity is derived. For typical values of the load pulse parameters for which back-side spallation occurs the fractal dimension agrees satisfactorily with the fractal dimensions for triadic Koch islands. Zh. Tekh. Fiz. 68, 43–49 (October 1998)     

Theoretical analysis and numerical simulation of development length of straight steel fiber in cementitious materials

In fiber-reinforced concrete, it is important to choose an appropriate length in each fiber to develop its full yield strength without a failure in the bond strength between the fiber and the concrete. This length is called the fiber development length, L_df. The bond capacity is evaluated between the fiber and the concrete using the pull-out tests. This test evaluates the bond capacity of various types of steel fiber surfaces relative to a specific embedded length. If the steel fiber is smooth and straight, the distribution of tensile stresses will be uniform around the fiber at a specific section and varies along the anchorage length of the fiber and at a radial distance from the surface of the fiber. Pull-out tests can be performed on an embedded straight steel fiber in concrete matrix, in this case, the tensile force, P, is increased gradually and the number of cracks and their spacings and widths is recorded. The bond stresses vary along the fiber length between the cracks. The strain in the steel fiber is maximum at the cracked section and decreases toward the middle section between cracks. If the embedded length of the straight steel fiber is greater than the development length, the steel fiber may yield, leaving some length of the fiber in the concrete. The linear elastic behavior of the fiber-matrix system is interrupted by interface debonding which occurs due to overall weak bonding between the concrete matrix and the surface of the steel fiber. This paper introduces new developed shear lag model and explains simplified method to find the development length of straight steel fiber in concrete matrix using finite element model and analysis of shear lag stresses, where the maximum tension force which is applied on the steel fiber is resisted by another internal force related with the ultimate average bond stress, steel fiber diameter and its yield strength.     

The structure of the distortion free-energy density in nematics: Second-order elasticity and surface terms

Summary By means of a phenomenological approach, we demonstrate that the mixed splay-bend elastic constantK ₁₃ in the free energy density of nematic liquid crystals must be considered zero, unless the bulk contributions of the squares of the distortion second-order derivatives are taken into account, together with the squares of the first-order derivatives times the second-order derivatives, and with the fourth powers of the first-order derivatives. Such contributions just reduce to one in the presence of—and close to—a threshold. Furthermore, the saddle-splayK ₂₄-term instead is shown always to play an essential role, as the bulk first-order elasticity, in determining the distortion free energy of nematics with weak anchoring subjected to spatial deformations. Finally, the new surfacelike elastic constants are shown to have a nilpotent character: thus they behave as well asK ₂₄ from the point of view of the variational calculus. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Influence of mesoscale three-phase parameters to the tensile behavior of concrete

In the mesoscopic level, concrete is regarded as three-phase composite material with cement matrix, aggregate, and the interfacial transition zone (ITZ) between them. The mechanical properties of ITZ are regarded weaker than those of the cement matrix and aggregate. In this study, a mesoscale mechanical model based on the interface specimen with a single aggregate is established to study the influence of three-phase parameters on the interface specimen under quasi-static and dynamic direct tensile loading. Besides, the loading rate effect is also considered in this study to further analyze the dynamic performance of ITZ and the whole interface specimen. According to the numerical results, it is indicated that the ITZ properties (elastic modulus and strength) play significant roles in the performance of the interface specimen under quasi-static direct tensile loading. However, the cement matrix is dominant to the mechanical properties of interface specimen under dynamic tensile loading. Moreover, the properties of ITZ (elastic modulus, strength, and DIF values) and the ITZ thickness have some influence on the dynamic performance of ITZ and the whole interface specimen under dynamic tensile loading. In contrast, the Poisson’s ratio and density of ITZ have little influence on the dynamic behavior of the whole interface specimen. Additionally, the aggregate diameter is influential to the time reaching peak stress of ITZ and the whole interface specimen, and the loading rate only influences the time to reach the peak stress of ITZ under dynamic tensile loading.     

Predicting Interfacial Strengthening Behaviour of Particulate-Reinforced MMC — A Micro-mechanistic Approach

The fracture properties of particulate-reinforced metal matrix composites (MMCs) are influenced by several factors, such as particle size, inter-particle spacing and volume fraction of the reinforcement. In addition, complex microstructural mechanisms, such as precipitation hardening induced by heat treatment processing, affect the fracture toughness of MMCs. Precipitates that are formed at the particle/matrix interface region, lead to improvement of the interfacial strength, and hence enhancement of the macroscopic strength properties of the composite material. In this paper, a micro-mechanics model, based on thermodynamics principles, is proposed to determine the fracture strength of the interface at a segregated state in MMCs. This model uses energy considerations to express the fracture toughness of the interface in terms of interfacial critical strain energy release rate and elastic modulus. The interfacial fracture toughness is further expressed as a function of the macroscopic fracture toughness and mechanical properties of the composite, using a toughening mechanism model based on crack deflection and interface cracking. Mechanical testing is also performed to obtain macroscopic data, such as the fracture strength, elastic modulus and fracture toughness of the composite, which are used as input to the model. Based on the experimental data and the analysis, the interfacial strength is determined for SiC particle-reinforced aluminium matrix composites subjected to different heat treatment processing conditions.     

A Generalized Plasma and Interpolation Between Classical Random Matrix Ensembles

The eigenvalue probability density functions of the classical random matrix ensembles have a well known analogy with the one component log-gas at the special couplings β=1,2 and 4. It has been known for some time that there is an exactly solvable two-component log-potential plasma which interpolates between the β=1 and 4 circular ensemble, and an exactly solvable two-component generalized plasma which interpolates between β=2 and 4 circular ensemble. We extend known exact results relating to the latter—for the free energy and one and two-point correlations—by giving the general (k ₁+k ₂)-point correlation function in a Pfaffian form. Crucial to our working is an identity which expresses the Vandermonde determinant in terms of a Pfaffian. The exact evaluation of the general correlation is used to exhibit a perfect screening sum rule.     

Effect of electropulsing treatment on microstructure and tensile fracture behavior of aged Mg–9Al–1Zn alloy strip

The effect of electropulsing treatment (EPT) on the microstructure, mechanical properties, and tensile fracture behavior of aged Mg–9Al–1Zn alloy strip at room temperature was investigated. The results indicated that EPT accelerated the spheroidizing and dissolution of β phase tremendously in the aged Mg–9Al–1Zn alloy strip. The EPT-induced microstructural change resulted in remarkably increasing elongation to failure, remained tensile strength unchanged. A mechanism for rapid spheroidizing and dissolution process of β phase during EPT was proposed based on the reduction of nucleation thermodynamic barrier and enhancement of atomic diffusion. Fracture analysis showed that with increase in frequency of EPT transgranular dimple fracture becomes predominant instead of the quasicleavage fracture.     

Erosion of the GaAs target under irradiation by a high-power pulsed ion beam

The results of examination of the GaAs-target erosion under irradiation by a high-power pulsed ion beam are reported. In the experiments, use was made of a high-power pulsed ion source with the following parameters: ion energy — 250 keV, target current density — 350 A/cm², pulse duration — 80 ns, target energy density — up to 7 J/cm². The target erosion coefficient and its dependence on the number of successive pulses are measured. It is found that the surface roughness parameter is increased with the number of successive beam pulses. A regular structure of surface relief is observed to form in the case where the number of pulses > 20–40. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 2007.     

Attempt at a gauge-theory-based explanation of the mass spectrum and number of light neutrinos

The symplectic group Sp(n/2) of invariance of flavors of n Majorana states (n is even) does not admit the existence of invariant Majorana masses. Only a specific mass matrix involving diagonal and off-diagonal elements is possible. A mass matrix as a result of spontaneous flavor- and chiral-symmetry breaking may appear here only in the case where the number of flavors is n = 6 and only together with spontaneous R- and L-symmetry violation—that is, parity violation. As a result, three light and three heavy Dirac particles (neutrinos) are present if the seesaw mechanism is operative. Special features of the observed spectrum of light neutrinos—in particular, the fact that two states are far off the third one—can be explained by simple properties of the mass matrices arising in Sp(3). The arrangement of states corresponds to an ordinary mass hierarchy. The mixing angles for physical neutrinos cannot be determined without understanding the mechanisms responsible for the formation of the charged-lepton spectrum and the weak current of Majorana states.     

Critical conditions for spontaneous relaxation in self-organizing dissipative film systems

It is established that the necessary conditions for spontaneous relaxation of elastic strain energy in a copper-As₆₀Se₄₀ self-organizing dissipative heterostructure is that the elastic deformation energy and the temperature must reach their threshold values. It is shown that in the temperature range 270–340 K the spontaneous relaxation of elastic deformation energy is accompanied by structural-chemical ordering and anomalous diffusion of copper into the glassy chalcogenide semiconductor layer. The maximum concentration of copper dissolved in the films is 40 at. %. Conductivity inversion from p to n type is observed in doped layers obtained by this method. Zh. Tekh. Fiz. 69, 128–129 (August 1999)     

Effective potentials and threshold anomaly

The strongE andL dependence of the effective elastic channel potentials is shown to be an implicit radial kinetic energy (ε) dependence. It is also shown that this effective potential satisfies the dispersion relation inε variable at the strong absorption radius. Further, the experimental data for both elastic and fusion channels are consistent with thisL-dependence of the corresponding effective potentials. The effective transfer channel potentials derived using CRC code FRESCO are shown to exhibit strong energy dependence as a result of couplings. The energy dependence of effective transfer strength for¹⁶O+²⁰⁸Pb and¹⁶O+²³²Th systems is determined using the experimental transfer angular distributions.     

Measurement of electric field strengths in a waveguide by means of the dynamic Stark effect in hydrogen and neutral helium spectral lines

The dynamic Stark effect of the spectral lines H_β and of the neutral helium lines λ=402.6 nm (2³ P ⁰−5³ D) and λ=438.8 nm (2¹ P ⁰−5¹ D) emitted from a discharge tube was used for probing rf electric fields in a transverse waveguide. Calculations accounting for the pertubation of the atomic states by strong unidirectional fields prove to be suitable in order to interprete the main experimental results. If the waveguide is terminated with a metallic reflector and the plasma in the discharge tube becomes overdense—then representing a slightly permeable mirror—a resonant enhancement of the electric field strength may be achieved by tuning. This enhancement is well recognizable in the spectral line contours.     

Deuteron-proton elastic scattering at intermediate energies

The deuteron-proton elastic scattering has been studied in the multiple-scattering expansion formalism. Primary attention has been given to such relativistic problems as a deuteron wave function in a moving frame and transformation of spin states due to Wigner rotation. Parametrization of the nucleon—nucleon t matrix has been used to take the off-energy-shell effects into account. The vector, A _y , and tensor, A _yy , analyzing powers of the deuteron have been calculated at two deuteron kinetic energies: 395 and 1200 MeV. The obtained results are compared with the experimental data. The text was submitted by the authors in English.     

钢中脆硬粒子裂纹形成机理

钢中的脆硬粒子对钢的解理脆断有直接的影响，解理断裂源大都发生在脆硬粒子上.根据微裂纹形成的热力学模型，利用钢中脆硬粒子开裂时所释放的弹性应变能、位错塞积弹性能，所产生的表面能，对脆硬粒子裂纹形成机理进行分析.模型计算表明，正应力和位错塞积力都是脆硬粒子开裂的必要条件，这与实验事实相符；同时给出脆硬粒子开裂的临界条件计算方法，计算发现，脆硬粒子临界开裂应力不仅取决于脆硬粒子尺寸及表面能，而且与晶粒直径有一定的相关关系，当晶粒直径较小时，这种关系与实验测定的材料解理断裂应力与晶粒尺寸的关系一致，说明整体失稳解 关键词： 解理断裂 裂纹形核 脆硬粒子     

Fracture characteristics of concrete subjected to impact loading

This paper takes concrete as a four-phase composite made of the intact matrix and three mutually perpendicular groups of penny-shaped micro-cracks. The intact matrix is assumed to be elastic,homogeneous and isotropic,and the micro-cracks are penny-shaped. Combined with the failure mechanism of concrete subjected to impact loading,a dynamic constitutive model for concrete is developed based on Mori-Tanaka's average stress concept and Eshelby's equivalent inclusion theory. Experimental results show that concr...     

Interfacial adhesion between semi-crystalline polymers (polypropylene and nylon-6): in situ compatibilized interface and fracture mechanism

The interfacial fracture toughness between semi-crystalline polymers (polyamide/polypropylene) were studied to understand the failure mechanisms at the interface, especially when the interface was reinforced by an in situ compatibilizer. Based on the observation of the interface using scanning electron microscopy and wide angle X-ray spectroscopy, it was revealed that crystalline structure of polypropylene was not affected by the in situ compatibilizer at the interface. The reinforcing mechanism could be qualitatively identified by investigating the evolution of fracture toughness as a function of annealing time and temperature. The adhesion strength increased with the annealing time. Depending on the annealing temperature, the fracture toughness passed a peak value and then reached a plateau after some bonding time. As long as the chain length of the compatibilizer is long enough to form entanglements with the molecules at both bulk sides, the fracture at the interface is decided by the balance between adhesion strength at the interface and cohesive strength in the weak modulus side; the failure locus follows the lower one. Thus, adhesive failure occurred first when the reaction at the interface did not occur long enough to provide high adhesive strength at the interface, but the cohesive failure occurred in the crack propagation side after the adhesive strength value became higher than the cohesive strength value.     

Crack propagation in amorphous polymers and their composites

The fracture energy of a polymer depends strongly on the viscoelastic responses of the material, and therefore is a function of temperature and crack velocity. The toughness of a composite is determined by the way in which the reinforcing filler modifies the energy dissipating mechanisms of the polymeric matrix.

The fracture toughness of a variety of polymeric glasses and their composites with glass beads, glass fibers, and rubber particles was measured. The velocity of rapidly moving cracks and the crack propagation rates under controlled loading conditions were also measured.

It was found that the crack propagation velocities in unfilled and glass bead filled materials were controlled by the longitudinal stress waves in the matrix and that the only effects of the glass beads were to blunt the crack tip and limit the viscous deformation. The effect on fracture toughness was relatively small and either positive or negative, depending on which of the above two factors dominated.

The presence of rubber particles as a second phase lowered terminal crack propagation velocities and greatly increased the fracture toughness, indicating a crack retarding effect of the rubber particles. This is related to the induction of crazes in the matrix by the rubber phase.

Glass fibers had a tendency to bridge the tip of a propagating crack, thereby greatly increasing the fracture toughness. In this case the work of fracture comes from a combination of the elastic strain energy stored in the fibers, the energy dissipated in debonding the fibers from the matrix, and the fracture energy of the matrix itself.     

Fullerene–epoxy nanocomposites-enhanced mechanical properties at low nanofiller loading

In this study, we characterized the mechanical properties of fullerence (C₆₀) epoxy nanocomposites at various weight fractions of fullerene additives in the epoxy matrix. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. All of the above properties of the epoxy polymer were significantly enhanced by the fullerene additives at relatively low nanofiller loading fractions (~0.1 to 1% of the epoxy matrix weight). By contrast, other forms of nanoparticle fillers such as silica, alumina, and titania nanoparticles require up to an order of magnitude higher weight fraction to achieve comparable enhancement in properties.     

The wideband attenuation of short radio waves on mid-latitude paths during X-ray flares in January 2005

We present the results of the ionosphere oblique chirp sounding on the Cyprus—Nizhny Novgorod, Cyprus—Rostov-on-Don, and Moscow—Rostov-on-Don mid-latitude paths during X-ray flares in January 17, 19, and 20, 2005. It is found that during strong flares the blackout of short radio waves was observed over the entire frequency range of chirp sounding on the Cyprus—Nizhny Novgorod and Cyprus—Rostov-on-Don paths. Modeling of the electron-density profiles in the lower ionosphere based on absorption of short radio waves on the Moscow—Rostov-on-Don path at different stages of the decay of the X-ray radiation intensity is carried out. It is shown that at the instant corresponding to the maximum value of the flare radiation flux, the electron density in the lower ionosphere at altitudes 60–80 km increased by a factor of about 10 and 100 for flares with radiation flux densities 5·10⁻² and 3·10⁻¹ erg/(cm ²·s) in the wavelength range 0.5–4.0 Å which took place in January 19 and 20, respectively. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 1–8, January 2007.     

Observation of a single laser-excited center at the point of a crystalline needle

The points of lithium fluoride needles are investigated by laser photoelectronic projection microscopy. A situation in which a single atomic-size source of electrons — an F ₂ color center — is observed in the region near the point is realized. As a result of the good fluorescence properties of these centers, these needles can be used as the active element of a scanning fluorescence microscope employing resonance transfer of electronic excitation energy. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 441–444 (25 March 1997)