Papers by Keyword: Relaxation

Abstract: In this paper, we study convex quadratic relaxation in polynomial programming. The goal is to solve polynomial programs through quadratic reformulation with the factorization method. The quadratization of monomials of degree more than two is carried out by replacing each pair of factors of the monomial with auxiliary variables. In this paper, each pair of factors of a monomial will be considered. The quadratic program obtained is convexified by using eigenvalues. As a result, the quadratic reformulation involving all factors of the monomial strengthens the information of the polynomial function but increases the dimensionality of the variables significantly. The next work is to develop a strategy to reduce the dimensions of the auxiliary variables.

Abstract: Since the material properties of paperboard depend on the processing strain rate and the temperature elevation of the paperboard, the mechanical conditions of the scoring tool (creasing knife) are important for precisely and stably folding the scored zone of the paperboard. When the temperature and the indentation velocity of the creasing knife are changed irregularly during the scoring process, the permanent-indented (residually scored) depth of the paperboard seems to be affected by the temperature and the indentation time of the creasing knife. Although the temperature-dependent and time-dependent behavior of several thin paperboards have been known in the past, their combined behavior was not sufficiently discussed regarding the crease bending characteristics of the paperboard. In this work, the time-dependent and temperature-dependent scoring, and the corresponded bending characteristics of liquid-container-purpose paperboard of basis weight 313g/m² (thickness of t = 0.47mm) were experimentally investigated using a bending (folding) tester, when varying the holding time and the temperature of a flat-edge creasing knife at two levels of the normalized indentation depth d/t = 0.68 and 1.02. As the results, the first peak bending moment M_p1, the first stiffness C₁ (the gradient of bending moment resistance by the folding angle at an angle of 0—4 degrees), and the rating bending moment resistance at the right-angle M_90,1(0) were characterized with the holding time and the temperature elevation of the creasing knife at the pre-stage (scoring) process. Also, some explicit expressions of C₁, M_p1, M_90,1(0) with the permanent scored depth were revealed as a static relationship. It is concluded that the temperature variation and the holding time of the creasing knife are important parameters which must be controlled in the manufacturing process of liquid package.

Abstract: In this study we have synthesized the iron and bismuth co-substituted BaTiO₃ ceramic, with the general formula: Ba_0.95Bi_0.05Ti_1-xFe_xO₃ for x=0.00 to 1.00, by solid state route. The impedance and electrical properties of these materials were investigated. The dispersion in conductivity in these ceramics can be described by Jonscher's power law and suggests a mechanism of conduction that is related to the Correlated Barrier Jump (CBH) model, according to which charge transport occurs between localized states due to a jump of the potential barriers. The conductivity results confirmed the semi-conductor behavior of these ceramics at high frequency region. The Nyquist plots for the different ceramics confirmed the simple electrical relaxation phenomena with the presence of a Debye-type relaxation phenomenon for x<040 of Fe content. While above this rete, the relaxation behavior is transformed into a Non-Debye phenomenon.

Abstract: The study of local dissipative processes by the method of free-damped torsional oscillations excited in polyvinyl alcohol (PVA) showed the presence of three loss peaks in the internal friction spectrum in different temperature ranges of this spectrum [1].

Abstract: It is known that epoxy resins (ES) and materials based on them are widely used in various areas of natural economy due to their valuable properties: low shrinkage during curing, high adhesion to various materials, chemical resistance, good physical and mechanical properties, and excellent dielectric properties [1].

Abstract: In this paper, trap levels around the glass transition temperature (T_g) of polymers have been characterized using Thermally Stimulated Current (TSC) technique. Deconvolution on α-peaks of the T_g for PE (-104 °C), plasticized PVC (-35 °C), PMMA (90 °C) and PET (96 °C) were carried out based on the first-order kinetic theory for non-Debye relaxation. Using temperature, T from TSC experimental data, we have successfully separated the α-peaks of the thermoplastic polymers. It is found that the complex curve of α-peaks can composed of four (4) to eight (8) sub peaks. Dominant sub peaks were identified at T_max = -105 °C, -34 °C, 89 °C and 92 °C for PE, pPVC, PMMA and PET, respectively. These peaks show activation energy, E_a of shallow and deep trap centers ranged from 0.3 eV to 4.6 Ev where they represent the depolarization of localized dipoles and space charges relaxations in the polymers.

Abstract: The pyrolysis process of polysiloxane resin conducted in the temperature range 400 – 700 °C results in hybrid materials owning some polymeric (thermosetting) behaviour. A certain level of elastic recovery and/or viscoelastic flow showed at various steps of pyrolytic transformation was monitored using the instrumented Vickers hardness method. Determined indentation force-indentation depth curves reflect the mechanical response and the level of the transformation; however, the relaxation behaviour is not covered by this method fully. An extensive indentation relaxation was revealed in the material partially pyrolysed at 400 °C, about 16 % and 8 % when the HV 0.1 and the HV 0.2 loading were applied, respectively. Materials pyrolysed from 500 to 650 °C which exhibited the indentation relaxation below 1 % and the mostly elastic response on the loading were observed. Above the pyrolysis temperature of 600 °C a rapid onset of mechanical properties, namely indentation elastic modulus and hardness, was observed. The short-term indentation relaxation was evaluated via the indentation force relaxation method in the regime of constant indentation depth obtained at the moment of reaching an initial force of 0.981 N or 1.962 N. The obtained indentation force relaxation curves were analysed on the basis of a logarithmic function. The significant effect of the pyrolysis temperature as well as the influence of loaded volume was described.

Abstract: In the result of application of the load in different sections of the structure elastic or plastic deformation may occur. In its turn, the removal of the applied load can lead to the development of relaxation processes, caused by the intention of the material to return to equilibrium. There is a subtle change of body shape (warping), which depends on its properties, the type of deformation and its nature in the flow of time. In this regard, the consideration of this factor is necessary in the manufacture of various parts and components of LCM using the methods of metal forming (stamping, bending, calibration, etc.) [1-6].

Abstract: A single phase perovskite, Y_xSr_1−xTi_0.6Fe_0.4O_3-δ (x= 0.06-0.09), was fabricated at 1350°C in air by sol-gel method. The effects of Y-and Fe-doping into SrTiO₃ on phase structure, electrical conductivity, ionic conductivity and its impedance behavior were investigated. The optimized Y_0.07Sr_0.93Fe_0.4Ti_0.6O_3-δ sample exhibits an electrical conductivity of 0.135 S·cm^-1 at 800 °C. Y-doping decreases the migration energy for oxygen ions, leading to a significant increase in ionic conductivity. The ionic conductivity of Y_0.09Sr_0.91Ti_0.6Fe_0.4O_3-δ sample varies from 0.0052 S· cm^-1 at 600°C to 0.02 S·cm^-1 at 800°C. Impedance characteristics over a wide frequency range of 0.01Hz-100 KHz reveal that the resistance of ionic conduction is predominantly influenced by grain boundary, the relaxation time of which decreases with increase of Y-doping amount.

Abstract: Atomistic simulations of the structure, energy and relaxation under the action of high frequency cyclic straining are carried out for columnar nickel nanocrystals with [112] column axis, the grain boundaries (GBs) of which are in a nonequilibrium state caused by the presence of extrinsic grain boundary dislocations (EGBDs). A special method of introducing EGBDs is used to create initial structures with nonequilibrium GBs. Energy of GBs as a function of the degree of nonequilibrium is evaluated and qualitatively compared to the results of dislocation and disclination modeling. It is shown that under loading by symmetrically oscillating stresses the nonequilibrium GBs generate lattice dislocations, which travel across the grains and are absorbed by opposite GBs thus resulting in a relaxation of the structure, long-range stress fields and the energy of GBs.