Structure analysis, fatigue testing, and lifetime prediction of composite steels

Composite steels prepared by technology of powder metallurgy are widely used as low cost parts with good resistance to wear, fracture, and corrosion. The development of powder composite steels is directly related to strength under vibration, fatigue stabilizing, and accurate lifetime prediction for actual composite topology. The fatigue behavior of powder steels was studied by experimental and numerical methods of composite mechanics and materials sciences. The chemical composition of composite steel is a pure iron powder as the base material and a handful of carbon, chromium, nickel, or phosphorus powders. The powder multi-component mixture is compacted by cold isostatic pressing to a rectangular form. The compactants are sintered in protective atmosphere. The microscale examination of the composite structure included an METAM-RV-21 metallographic optic microscope with a high-resolution ScanNexIIc scanner and an image processing package on the PC platform. The phase composition of powder steels has complex disordered topology with irregular ferrite/austenite grains, iron carbide inclusions, and pores. The microstructure images are treated according to the theory of stochastic processes as ergodic probability functions; statistical moments and a structural covariance function of the composite steels are given. The microscale stress-strain state of the composite steel is analyzed by finite element methods. The stiffness matrix of the composite steel is also presented together with stiffness matrices of ferrite/austenite grains, iron carbide inclusions, and pores as zero matrices. Endurance limits of the microstructural components are described by the Basquin or Coffin-Manson laws, respectively, as high and low cycle fatigue; cumulative microdamage in loading with a variable amplitude is taken from the Palmgren-Miner rule. Planar specimens were tested by console bending. Symmetric fatigue cycling was performed at a stable frequency of 20 Hz with endurance limits up to 5·10⁶ cycles. The probabilistic S-N curves were studied for various types of the composite steels. The fatigue properties of the structural components such as ferrite/austenite grains and carbide particles were defined by the microscale stress-strain modeling. Structural impact on the fatigue lifetime was computed; the probabilistic fatigue curves of the composite steels of various phase compositions are given. The prediction of cyclic lifetime and fatigue testing show good agreement for the powder composite steels studied.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Perm' State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 371–382, May–June, 1998.     

Long-Term Durability of Glass-Fiber-Reinforced Composites under Operation in Pulp and Reactant Pipelines

Composite pipes 215 mm in diameter with a 6-mm wall, 3-mm functional layer, and ±60° glass-fiber-reinforced plastic (GFRP) layers were examined by means of microstructural analysis and mechanical testing. Three types of pipes were considered: unused, after a five-year operation at a pressure of 0.6 MPa and temperature of 70°C, and after a five-year operation at 0.6 MPa and 20°C. The GFRP load-carrying layer and the interfaces were investigated by a metallographic optical microscope, a computerized microscanning equipment, and a software image processing package. The initial and accumulated damages in the microstructure of the composite and interfaces were examined. The mechanical properties of the pipes after a long-term operation were examined on two-layer specimens in three-point bending. The stiffness and strength characteristics were measured in the axial and circumferential directions under tensile and compressive loads at elevated temperatures. Composite pipes, 8 m in length and 215 mm in diameter, were simulated numerically as shells of revolution. The shell model was corrected by a refined Timoshenko theory and a dual-modulus temperature-dependent model of stiffness for the multilayer composite structure. The strength margin of the composite pipes was determined based on real strength properties.     

Influence of high pressure on the electrical properties of amorphous chalcogenides of the Ag-Ge-As-S system

New AgGe_{1 + x} As_{1 − x} S₃ (x = 0.1, 0.4–0.9) silver chalcogenides are synthesized and certified. Their electrical properties are studied at pressures of up to 45 GPa by means of impedance spectroscopy. Regions of significant variations in electrical properties are detected via analysis of an impedance hodograph, the baric dependencies of resistance, and the dielectric loss tangent.     

Holographic interferometry study of deformation of cement and concrete under mechanical and thermal loads

Compressive strain of concrete is accompanied by rotation of the rigid aggregate and by local shifts of the cement matrix, which by analogy with local deformation of metals is the cause of a decrease of the real strength of the material. It is shown that deformation of concrete with haydite and granite aggregates in the presence of a heat supply (within limits of positive operating temperatures) is distinguished by damping of deformations in the first case and by local deformation of the aggregate in the second.Presented at the Ninth International Conference on Mechanics of Composite Materials, Riga. October, 1995.Kharkov State Technical Academy of Railroad Transport, Ukraine. Kharkov State Technical Construction and Architecture University, Ukraine. Kharkov Fire Safety Institute Ukraine. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 2, pp. 202–208, March–April, 1996.     

Comparative study of the high-pressure conductivity of graphite and fullerene

The electrical resistance of polycrystalline fullerene and graphite was studied at pressures up to 50 GPa within the temperature range 77–450 K. The temperature dependences of the resistance of both materials behave in a qualitatively similar manner. However, the resistances of these materials differ by several orders of magnitude. This shows that fullerene and graphite retain some specific features of their microstructure in the pressure and temperature ranges covered.     

Spatial soliton-like structures in a thin-film system with a transverse photonic crystal in the feedback loop

The problem of existence and stability of soliton-like structures in a system consisting of a thin layer with resonance nonlinearity, a feedback mirror, and a photonic crystal located between them is studied. It is shown that, depending on the parameters of the photonic crystal, its presence can lead both to a change in the background on which localized structures are excited and to an increase in the interval of test pulse amplitudes necessary for the excitation of these structures.     

High pressures, low temperatures, and magnetic field effects on AgFeAsSe3 and AgFeSbSe3 properties

A procedure for synthesizing AgFeAsSe₃ and AgFeSbSe₃ is presented, and their electric and magnetic properties are investigated over a wide range of temperatures, pressures, and magnetic field variation. At 100–400K, the samples are characterized by semiconductor properties. Under pressures of ～25 and ～24 GPa, the electric properties of AgFeAsSe₃ and AgFeSbSe₃ change greatly.     

Electrical conductivity and thermal EMF of CsI at high pressures

Abstract

The pressure dependence of thermal EMF and the resistivity-temperature dependence of CsI has been measured at pressures 20-50 GPa. In CsI non-monotonous change of resistivity, thermal EMF and activation energy of charge carriers has been observed at pressures above 40 GPa. The sign of thermal EMF corresponds to the electron conductivity. At pressures below 47 GPa the resistivity-temperature dependence is of the type characteristic of non-degenerate semiconductors, at pressure above 49 GPa it is characteristic of degenerate semiconductors (or metals). The observed properties are connected probably with the continuous distortion of B2 to an hcp-like phase.