Papers by Keyword: Nonlinearity

Abstract: Experiments were carried out in which the luminescence of color centers in lithium fluoride crystals was excited by two different femtosecond lasers with significantly different energies, durations and pulse repetition rates. It was established that in all experiments the main center, the luminescence of which was excited nonlinearly, was the F₃⁺ color center. Unusual experimental data were obtained; a laser with low pulse energy (4 nJ) excited triplet luminescence of these centers (570 nm) but did not excite singlet luminescence (540 nm). Another laser with a higher pulse energy (0.3 mJ), on the other hand, excited singlet luminescence and did not excite triplet luminescence. A proposed diagram of energy levels and quantum transitions is presented, explaining the possible mechanisms of nonlinear excitation of luminescence in these experiments.

Abstract: It is proposed a mathematical model of a metamaterial which is represented as a nonlinear elastic two-mass chain with a center of symmetry. The model was continualized. The conditions for the formation of non-sinusoidal strain waves are analyzed.

Abstract: The last accidents of the NPP in Chernobyl and Fukushima give us the new inspiration to verify the safety level of the NPP structures. This paper presents the new requirements to test of the safety and reliability of the NPP structures due to the last accidents in the world. The risk assessment to verify of the safety and reliability of the NPP structures based on probabilistic and nonlinear analysis is presented. The uncertainties of the loads level (long-time temperature and dead loads), the material model (concrete cracking and crushing, behaviour of the reinforcement and liner), degradation effects and other influences following from the inaccuracy of the calculated model and numerical methods were taken into account in the LHS method. The results of the deterministic and probabilistic analysis of the NPP structures are presented.

Abstract: The paper proposes the numerical method of solution the problems of calculation the stress state in thick-walled cylinders and spheres from physically nonlinear inhomogeneous material. The urgency of solved problem due to the change of mechanical properties of materials under the influence of different physical fields (temperature, humidity, radiation, etc.). The deformation diagram describes the three-parameter formula. The numerical method used the method of successive approximations. The results of numerical calculation are compared with the test analytical solutions obtaining the authors with some restrictions on diagram parameters. The obtained results can be considered quite satisfactory.

Abstract: A Damage mechanics finite element numerical computation method was established based on HLC microscopic damage model to solve the problem of damage, crack initialization and growth inside bore during the firing process. The damage and failure process of the bore surface was simulated numerically during multiple rounds of firing. The law of the barrel material performance changing with the number of firing rounds was analyzed during the engraving process of the driving band and compared with the experimental results. It is proved that HLC microscopic damage model can show the complicate damage behavior and predict the cracking defect, which provides a reference for safety design of the gun barrel.

Abstract: A model order reduction (MOR) technique for linear dynamic systems is extended to allow the integration of nonlinear components. The main purpose is to build numerical model of complex systems as support of the design process leading to the exploitation of meta-structures. The selected method is applied to a soil-structure interaction case study.

Abstract: In some cases, due to particular conditions, the project of a bridge requires the construction of high piers. The study of these structural elements needs to go beyond a simple linear analysis, especially considering the second order effects, that contemplate the possibility of instability of the pier due to the slenderness it. In this paper a toolbox that considers the geometrical nonlinearity, the nonlinearity mechanical behaviour of the materials and the time dependent properties of the concrete is presented. The assessment of the pier starts with the possibility of cracking in the section (material non linearity) with consequent loses of stiffness. The second step takes into account the increment of bending moment caused by the P-d effect (geometrical non linearity) and strictly related to the slenderness of the member. The last step is the evaluation of the increment of deformation caused by permanent loads, mainly due to the loss of the concrete stiffness in time for creep effect of concrete. In this paper, a detailed explanation of the automatic procedure is provided and finally the application to a real case of a bridge pier under a critical stage construction condition is presented.

Abstract: This paper describes the nonlinear probabilistic analysis of the failure pressure of the shielding plate of the reactor box of the nuclear power plant under a high internal overpressure and temperature. The scenario of the hard accident in Nuclear power plant (NPP) and the methodology of the calculation of the fragility curve of the failure overpressure using the probabilistic safety assessment PSA 2 level is presented. The fragility curve of the failure pressure was determined using 45 probabilistic simulations using the response surface method (RSM) with the Central Composite Design (CCD) for 10⁶ Monte Carlo simulations for each model and 5 level of the overpressure.

Abstract: An improved nonlinear vibration isolator is proposed in this paper. It uses the exact solution derived load-displacement hysteresis curve formula based on energy method to calculate disc spring, combined with the finite element analysis. The static load capacity and dynamic mechanical property of this new type disc spring vibration isolator is validated through the static load tests or fatigue analysis. Compared with the rubber vibration isolator, the conclusion can be found: the new type vibration isolator has wider scope of vibration isolation, and is better in low frequency vibration isolation. These research achievements are of significant importance in the vibration isolation of heavy load and great shock mechanical equipment.

Abstract: In this contribution a theoretical basis of pendulum oscillations is described. The authors deal also with an experimental investigation of nonlinearity in oscillations of large displacement pendulum. High-speed correlation system was used for the registration of pendulum free motion. The obtained data were consequently processed in Matlab and depicted in a form of graphical visualizations and tables.