Creep at Elevated Temperatures and High Vacuum

High vacuum, high temperature, creep tests on copper and silve

Crack growth behavior of AISI-4340 steel during environmental exposure

AISI-4340 is observed to undergo stress corrosion cracking when subjected to a constant load during exposure to a 3.5% NaCl solution. Crack initiation, nucleation, and growth has been monitored as a function of time. Stepped regions consisting of fast and slow crack growth periods are shown to correspond to microstructural changes observed in the fracture surface of the steel. These regions of fast and slow crack rate variations with time show that the crack growth rates do not increase continuously with an increase in the stress intensity

Application of a new processing method to post-LDL-apheresis data.

Our aim was to elaborate a method to optimise treatment intervals for the individual low-density lipoprotein (LDL)-apheresis treated patients. After each treatment, plasma LDL concentrations show a time-related increase with a decreasing speed until a maximum level.We searched to interpret the post- LDL-apheresis experimental data trend as the physical process that produces the observed curve, so that the fitting presupposed theoretical function is a direct consequence of the physic process, because to establish the better time. Applying the proposed fitting method to a succession of 15 samples obtained from the mean of six plasmapheresis executed on five different subjects, small estimate standard error (5 mg/dl) and relative error (1.7%) with a dispersion evidently related to the experimental error were observed. Obviously, applying the same method to a single case, the dispersion is more marked (relative error ,5%), with a SE of 10–13 mg/dl, even though the aspect of a casual phenomenon is conserved. Our physical interpretation appears to be a practical model to predict the LDL-rebound kinetic of the single patient

Continuous and discontinuous phase transitions and partial synchronization in stochastic three-state oscillators

We investigate both continuous (second-order) and discontinuous (first-order) transitions to macroscopic synchronization within a single class of discrete, stochastic (globally) phase-coupled oscillators. We provide analytical and numerical evidence that the continuity of the transition depends on the coupling coefficients and, in some nonuniform populations, on the degree of quenched disorder. Hence, in a relatively simple setting this class of models exhibits the qualitative behaviors characteristic of a variety of considerably more complicated models. In addition, we study the microscopic basis of synchronization above threshold and detail the counterintuitive subtleties relating measurements of time averaged frequencies and mean field oscillations. Most notably, we observe a state of suprathreshold partial synchronization in which time-averaged frequency measurements from individual oscillators do not correspond to the frequency of macroscopic oscillations observed in the population

energetic and economic analysis of a new concept of solar concentrator for residential application

Abstract Renewable energy penetration is increasing in last years, covering a more and more important role in both electrical and thermal supply. Nowadays, the photovoltaic conversion is a consolidated technology and can be efficiently combined with solar concentration. In this study, a new concept of photovoltaic solar concentrator based on non-conventional mirrors coupled with high efficiency triple-junctions cells is described and discussed. More in details, as for the optical design, deformations are applied to classical spherical mirrors to control solar aberrations and boost efficiency of a receiver consisting in a dense array of cells. The efficiency enhance is obtained by high matching between the collected solar irradiance and the receiver electrical features. The concentrator is able to produce both electrical and thermal energy: the system requires in fact an active cooling circuit to maintain the cells performance. This behavior makes the system suitable for combined heat and power applications with particular reference to high direct irradiance environments. An analytical study, considering a residential utility has been performed in order to understand the energetic and economic performance of the system. In particular, a simulation has been carried out by the use of an in-house-developed calculation code considering a whole year of operation

Study on the State Feedback Selection and Measurement for the Application of an LQRI Secondary Voltage Regulator to a Transmission System

The electrical power system is being significantly affected by the climate change mitigation actions. The power generation, originally centralized, is transitioning towards a more decentralized paradigm due to the coal-fired power plants shut off and the increase in renewable power. Issues in transmission system's voltage control may arise, if the voltage regulation architecture is not modified accordingly. To this aim, in this paper it is investigated the use of a Linear Quadratic Regulator with Integral action (LQRI) for the secondary voltage regulation, aimed at exploiting several reactive power sources as actuators. Being the LQR class of regulators requiring the system state to correctly operate, and being a transmission system a complex system, a critical investigation must be done. In particular, it is needed to identify the variables that are directly measured in a real system, determine if they can be useful for the LQRI state feedback, and finally study the effect of the different possible feedback selection on the regulation performance

Reactive Power Resources Management in a Voltage Regulation Architecture Based on LQRI Control

The Italian transmission system's voltage control is based on its subdivision into decoupled control areas, where a hierarchical regulation architecture is applied. However, the structure and the voltage regulation of the electrical power system are being significantly impacted by the actions being taken to limit climate change. The increase in renewable energy sources exploitation is leading to a more-distributed and converter-based energy production. In addition, the forthcoming coal-fired plants shut-off will force the shift from providing regulation capability with a small number of big power plants, towards using a big number of smaller resources. Thus, in the near future a decrease in the effectiveness of the present voltage control architecture is expected. To solve such issue, a new voltage control architecture is needed, involving the more-distributed and converter-based energy production systems, as well as no longer relying on physically decoupled control areas. Therefore, in this paper a coordinated LQRI secondary voltage control is presented, able to use each grid-available reactive power source as an actuator. Furthermore, a bumpless transfer technique is proposed to solve the problem of managing a varying number of actuators (due to the reactive power resources' connection and disconnection)

Incubation during laser ablation with bursts of femtosecond pulses with picosecond delays

Abstract: We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results