Thermal expansion and stress relaxation of metal-matrix composites

The coefficient of thermal expansion (CTE) of a series of Al-6%Si matrix samples, with reinforcements of carbon, SiC, Al₂O₃, or boron fibres, cloths, or ceramic particles was measured in the range 60°–220°C with a dilatometer. The anisotropy of the CTE was measured and found to be very large for specimens unidirectionally reinforced with carbon fibre. Relaxation of stresses due to the different thermal expansion of matrix and reinforcement was studied by using the bending of asymmetrically reinforced samples and the magnitude of the stresses evaluated using bending beam theory.     

Thermal hysteresis in nonlinear media

One-dimensional thermal relaxation processes in media with nonlinear thermophysical properties are treated. Dynamic hysteresis is investigated theoretically in continuous and discontinuous nonstationary temperature fields. Boundary conditions are analyzed, for which a high-flow hysteresis process is realized. A quantitative estimate is given of irreversible variations in the material thermal state. Examples are given of constructing hysteresis branches. New properties are established for thermal shock waves propagating along the relaxing background.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 1, pp. 149–155, July, 1990.     

Measurement of stress relaxation in broken fibers embedded in epoxy using Raman spectroscopy

Raman spectroscopy was used to study the stress relaxation in broken fibers in a unidirectional composite. A single-fiber model composite consisting of a high modulus PAN-based carbon fiber and an epoxy resin matrix was loaded incrementally until the fiber got broken.Then the stress profile in the broken fiber was monitored under constant overall strain for 1000 hours by determining fiber stress through the stress dependence of the 2700 cm–1 Raman band peak position. Three experiments were done at different overall strains.It was observed that the stress profile in each broken fiber changed only a little even after 1000 hours whereas matrix normal stress in the fiber direction relaxed to about a quarter of the initial value in about 200 hours. It is shown that this result does not support linear viscoelastic solutions based on perfect bonding at interface since the present experiments had interfacial debonding and matrix shear yielding around fiber breaks.     

Thermal stress relaxation in magnesium matrix composites controlled by dislocation breakaway

In metal matrix composites (MMCs) thermal stress relaxation can be achieved either by interface debonding, crack propagation or by dislocation motion. The present paper shows that in the case of magnesium matrix, interface thermal stresses are relaxed by dislocation motion. Moreover the results obtained by mechanical spectroscopy prove that this dislocation motion is controlled by a solid friction mechanism, which is not thermally activated. This point is very interesting for the development of MMCs, which exhibit a high damping capacity over a wide frequency range. Dislocation hysteretic motion in the magnesium matrix is evidenced by the dependence of the mechanical loss on the stress amplitude. The obtained relationship obeys perfectly to the Granato–Lücke model for dislocation breakaway.     

Thermal hysteresis in ferrite memory cores

The effects of different writing and reading temperatures on the switching voltage waveform, permeability, and flux state of standard nickel-copper-manganese and copper-manganese ferrite memory cores have been investigated over a temperature range of 0 to 70°C. These temperatures are well below the Curie temperatures of the materials being tested. It has been found that writing at a higher temperature than the read temperature leads to increased low temperature voltage switching thresholds, higher low frequency (<40 MHz) permeabilities, lower high frequency (>40 MHz) permeabilities, and flux states almost equal to the equilibrium value for the lower temperature. Read current pulse rise time and duration have been found to have no effect on the increase in threshold with temperature. Externally produced stresses have been used to simulate the effects of temperature changes on the core.     

Dielectric relaxation of epoxy polymer in composites cured in an electric field

Results of a study of the effect of various fillers (discrete aluminum-oxide fibers and powdered barium titanate) on the molecular mobility of the epoxy polymer in composites cured under the action of an electric field are given. Institute of General and Inorganic Chemistry, Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fisicheskii Zhurnal, Vol. 69, No. 4, pp. 589–597, July–August, 1996.     

Correlation between magnetic hysteresis and magnetic relaxation in YBaCuO single crystals

The dependence of the magnetic momentm obtained from the hysteresis loops on the speed of the magnetic field sweep =dH _ext/dt is explained on the basis of Anderson's interpretation of the magnetic flux creep. In addition, a phenomenological model is suggested which predicts a linear dependence ofm on ln with the slope m/ ln , numerically equal to the relaxation rate m/ ln(t) from the usual magnetic relaxation. Such linear relations betweenm and ln were observed experimentally in single crystals of YBaCuO. Preliminary experiments on the complementary time dependent relaxation ofm after a simulated step change ofH _ext gave mostly relaxation rates close to the predicted values. The model here presented also enables one to compare the critical state in the superconductor at a field sweep rate with the critical state at some timet _eff after a step change ofH _ext. The values of analyzed in our experiments actually correspond to the critical state at timest _eff between0.04 and4 sec after an imaginary large step change ofH _ext.     

Write-noise relaxation in magnetic film memories

In magnetic film memories incorporating a metal ground plate, the write noise, i.e., the sense-line voltage caused by application of the bit-current pulse, has a long "tail." By an analysis of pulse propagation along a conductor with rectangular cross section above a stratified ground plate, it is shown that 1) the slow decay of the write noise is due to current diffusion into the ground plate, and 2) the time derivative of the write noise, which should be as small as possible for best rejection by the sense system, can be considerably reduced by a) an appropriate stratification of the ground plate, and b) keeping the ground plate at a low temperature. The most effective stratified ground plate consists of a top layer of copper separated from the substrate by a layer of a ferromagnetic conductor such as Permalloy. The relative permeability of the Permalloy layer μrneed not be greater than 100. Ifmu_{r} = 100, a Permalloy thickness of ≳ 1.5 microns makes the electrical properties of the ground plate independent of the substrate material. The optimum thickness of the copper layer is a function of the time interval between the end of the bit pulse and readout. A thick copper ground plate at liquid-nitrogen temperature is almost as effective in reducing the noise-voltage slope as the best stratified ground plate at room temperature.     

Ti掺杂及Ti应力缓和层对类金刚石薄膜附着力的影响

研究了Ti掺杂对磁控溅射类金刚石(DLC)薄膜附着力及硬度的影响,同时在Ti掺杂类金刚石(Ti-DLC)薄膜的基础上,通过引入Ti应力缓和层制备了Ti/Ti-DLC/Ti/Ti-DLC……软硬交替多层薄膜,研究了Ti应力缓和层对进一步提高薄膜附着力特性的作用.采用纳米划痕仪和显微硬度计分析测试了薄膜的附着力和硬度.研究表明,金属Ti的掺杂有利于DLC薄膜附着力特性的改善,但对硬度有一定的影响.Ti应力缓和层的导入进一步改善了Ti-DLC薄膜的附着力特性,使其达到或超过了TiN薄膜的水平,对于附着力的改善Ti应力缓和层存在最佳的厚度值.采用特殊的变周期多层结构设计即在应力集中的膜基界面附近采用较小的调制周期,薄膜项层附近采用较大的调制周期不但可以保持足够的附着力,还可维持Ti-DLC薄膜原有的硬度.     

Ultrafast excitation relaxation in titanylphthalocyanine thin film

Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump–probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5–5 ps, a fast and a long-lived processes with lifetimes of about 5–10 ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton–exciton annihilation process with a t^−1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton–exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm².     

Thermal expansion and linear-dimension hysteresis for polycrystalline silicon carbide

Length measurements have been made on polycrystalline silicon carbide on cooling from 290 to 23 K, and the linear-expansion coefficient has been determined. The lack of reproducibility in the length after cooling has been examined. The irreproducibility after the first cooling is 10^–5, but after four cycles of cooling at low rates, the linear dimensions stabilize. The temperature dependence has been derived for the thermal relative length change in equilibrium silicon carbide specimens.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 6, pp. 958–961, December, 1990.     

Thermal conductivity of epoxy resin-aluminium

Journal of Materials Science -     

Experimental study of low-temperature long-time relaxation in epoxy resin

The power (T ₁,T ₀,t) released after rapid temperature change from initialT ₁ (1.1T ₁25K) to finalT ₀ (1.1T ₀4.2 K) for 0.2t700 hr was measured. At low temperature,T ₁,T ₀<2 K, the results agree well with the tunneling theory and the corresponding density of states of two-level systems =1.0×10³⁹/Jg. For higher temperatures, significant deviations from the tunneling theory are observed.On leave of absence from the Institute of Physics of Czechoslovak Academy of Sciences, Prague, Czechoslovakia.On leave of absence from the Institute for Solid State and Material Research of Academy of Sciences of German Democratic Republic, Dresden, GDR.On leave of absence from the Technical University, Dresden, GDR.     

Thermal stress analysis of a trilayer film/substrate system with weak interfaces

Trilayer film/substrate systems with weak interfaces are investigated in this article, and the closed-form analytical solution is presented, which can be readily used to evaluate the thermal residual stresses induced by temperature change. A fourth-order differential equation governing the curvature is derived and its solution is obtained. The present analytical results can be degenerated to the ones for a system with perfect interfaces, as well as those for a bilayer system with weak interface as reported previously. A quite different feature from our previous work is that certain force equilibrium conditions in the longitudinal direction must be used for trilayer systems, while they are satisfied automatically in the analysis of bilayer systems.     

Thermal stress analysis of a trilayer film/substrate system with weak interfaces

Trilayer film/substrate systems with weak interfaces are investigated in this article, and the closed-form analytical solution is presented, which can be readily used to evaluate the thermal residual stresses induced by temperature change. A fourth-order differential equation governing the curvature is derived and its solution is obtained. The present analytical results can be degenerated to the ones for a system with perfect interfaces, as well as those for a bilayer system with weak interface as reported previously. A quite different feature from our previous work is that certain force equilibrium conditions in the longitudinal direction must be used for trilayer systems, while they are satisfied automatically in the analysis of bilayer systems.     

Biaxial fatigue behavior of an epoxy polymer with mean stress effect

Biaxial fatigue behavior of an epoxy polymer was investigated under cyclic shear and proportional axial-shear combined loadings with mean strains. Axial and shear strains were simultaneously measured by a non-contact real-time strain measurement system during fatigue process. Then the mechanical parameters (stress/strain components, mean stress/strain, strain energy densities, etc.) of the specimens during entire fatigue life were able to be quantitatively retrieved from the recorded stress-strain data. Multiaxial fatigue life prediction models were established upon stress-, strain- and energy-based approaches with consideration of mean stress/strain effect, where better agreement was achieved in stress-based and energy-based approaches.     

Thermal fringing at turn-over in an amorphous chalcogenide thin film sandwich structure

Experimental evidence has been obtained which demonstrates the importance of the thermal properties of the substrate with regard to the onset of negative differential resistance or threshold switching effects in thin film amorphous chalcogenide sandwich structure devices. Because the turn-over voltage V_t depends on the substrate thermal conductance, device geometries which enhance lateral heat flow in the substrate will have higher values of V_t than geometries which do not. Experiments have been performed on thin film devices using a-As₂SeTe₂ as the active material. The circular devices were prepared with various radii on substrates in which lateral heat flow could take place. It was observed that V_t increased in a well-behaved manner with decreasing radius, thus showing that Joule heating plays a strong role in determining V_t for the chosen chalcogenide. Additional evidence for thermal spreading was provided by thin film thermometry.