Thermal Actions Research Articles

Effect of Laser and Thermal Annealing on Optical and Structural Properties of Amorphous Metal Alloys

The results of ellipsometric measurements of rough and annealed amorphous alloys of 3d-transition metals (Fe, Ni, and Co) with metalloids are presented. Composition of alloys, directions of steady-state bonds between their compounds, and frequencies of the Auger electron transitions are determined by the methods of Auger spectroscopy and from the Raman spectra. Frequencies of metal-metalloid oscillations are determined and strains arising in nanoregions of the crystalline phase are obtained. Different characters of thermal and laser actions on the optical properties of alloys are established, and changes in the optical properties caused by these actions are compared.

Subsurface layer formation during sliding friction

Considering recent experimental studies and analysis of the literature, the authors substantiate the potential of physical mesomechanics for describing structural degradation of subsurface structures during friction and wear. Physical mesomechanics may be a useful approach for constructing a generalized hierarchical model that has its origin in vortical “shear+rotation” motion of mesovolumes resulting in formation of wear particles. The most important constituents of such a model are mechanical cyclic and thermal actions, both related to the interplay between the surface microirregularities, localization of strain and fragmentation of material at different scale levels.

Fundamental principles of structural behaviour under thermal effects

This paper presents theoretical descriptions of the key phenomena that govern the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undertaken as a part of a research project (The DETR-PIT Project, Behaviour of steel framed structures under fire conditions) to model the full-scale fire tests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by the effects of strength loss caused by thermal degradation, and that large deflections and runaway resulting from the action of imposed loading on a ‘weakened’ structure. Thus ‘strength’ and ‘loads’ are quite generally believed to be the key factors determining structural response (fundamentally no different from ambient behaviour). The new understanding produced from the aforementioned project is that, composite framed structures of the type tested at Cardington possess enormous reserves of strength through adopting large displacement configurations. Furthermore, it is the thermally induced forces and displacements, and not material degradation that govern the structural response in fire. Degradation (such as steel yielding and buckling) can even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when material degradation and loads begin to dominate the behaviour once again. However, because no clear failures of composite structures such as the Cardington frame have been seen, it is not clear how far these structures are from failure in a given fire. This paper attempts to lay down some of the most important and fundamental principles that govern the behaviour of composite frame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single structural elements under a combination of thermal actions and end restraints representing the surrounding structure.

An effect of structural variations on the decomposition of materials under high-intensity thermal actions

An effect of structural variations on the decomposition of materials under high-intensity thermal actions

An effect of nucleation processes on the thermal-destruction rate of materials under intense thermal actions

An effect of nucleation processes on the thermal-destruction rate of materials under intense thermal actions

Influence of variable actions on permanent through–cracking and deflections of prestressed concrete structures

Designers of prestressed concrete structures need simple and effective serviceability design criteria. Emphasis must be placed on the behaviour under permanent loads, but the progressive damaging of the structure under variable traffic loads and thermal actions needs to be taken into account. These variable actions can lead to residual permanent through-cracking of the structure and leave irreversible consequences. Checks at the permanent state must thus account for residual cracking and deformations that are added to the effect of permanent actions. This paper considers these issues in the context of providing sensible design rules for dealing with the main phenomena which govern the serviceability behaviour of this very complex material.

Representative values of thermal actions for concrete bridges

AbstractThe pre‐standard ENV 1991‐2‐5 Thermal Actions was concluded in 1996 and deals with the thermal actions induced in the structures due to climatic effects (air temperature, solar radiation). These climatic conditions cause a non‐linear temperature profile through the depth of the bridge deck. This profile can be subdivided into three parts which are called ‘thermal actions’: bridge effective temperature, linear temperature difference, non‐linear part. For concrete bridges the first two parts play the most important role. According to pre‐ standards ENV 1991‐1 Basis of Design thermal actions are defined as variable actions and following representative values have to be determined: –characteristic value, with return period of 50 years; –infrequent value, with return period of one year; –frequent value, with return period of two weeks; –quasi‐permanent value Three groups of bridge cross sections are investigated: box girders, T‐girders, slabs.The climatic data were obtained from the meteorological station Giessen (Hessen, Germany). This station located in the centre of Germany and Europe and can be considered as a representative place for middle Europe. The record period is from 1981 to 1990.To exclude the problem of autocorrelation the three‐day maximum of climatic effects and extreme value distribution type III are used for statistical analysis. Then the representative values of climatic effects are calculated.For calculation of temperature profile the one‐dimensional heat flow model through concrete bridge deck is used. This model runs off for four different representative values of climatic effects. Afterwards four representative values for uniform temperature component and for linear temperature differences are obtained. The reduction coefficients Ψ are also calculated. The characteristic values of thermal actions are included in the document ENV 1991‐2‐5 Thermal Actions and the Ψ coefficients are included in the document ENV 1991‐3 Traffic Loads on Bridges.

Statistical Analysis of Thermal Actions on a Concrete Segmental Box-Girder Bridge

A method is presented for statistical treatment of thermal actions measured on concrete bridges during long-term monitoring. The method is based on recognising that thermal actions are not entirely random, but contain deterministic components due to the Earth’s rotation and revolution. Greater precision and reliability could be attained in calculating frequent, quasi-permanent and 50-year return-period values for thermal actions by applying separate statistical analyses to these random and deterministic parts. The procedure is used to check the validity of the empirical relationships in Eurocode 1 between the effective mean temperature of a bridge and the shade air temperature.

Statistical Analysis of Thermal Actions for Concrete Bridge Design

Thermal effects have often been associated with concrete bridge damage. The non-linear temperature distributions that arise in bridges are difficult to estimate and are often considered in a simplified way by designers, which may lead to structural behaviour problems. In this paper, a methodology is presented, based on a statistical analysis of temperature distributions, that may be used to define the differential and uniform variations of temperature. Examples are presented for the climatic conditions in Portugal. Temperature distributions were obtained by considering a numerical technique for resolution of the Fourier heat-transfer equation, taking into account the geometry of the bridge cross section, the thermal properties of concrete and asphalt, and climatic conditions. The validity of this technique was confirmed using experimental data from instrumented bridges. By considering the available meteorological data, a statistical analysis was performed to obtain characteristic and combination values.

Environmental effects on the mechanical properties of glass-FRP and aramid-FRP rebars

In the paper, the experimental results on the effect of temperature and moisture on the mechanical properties of FRP (Fiber-Reinforced Polymer) reinforcements are presented. FRP rebars made from glass and aramid fibers were subjected to cyclic thermal actions at temperatures ranging between 20 and 70°C, typical of natural hot-climate environments. Tensile tests were also carried out on FRP rebars. The effect of moisture was investigated by cyclic wetting and drying the FRP rebars under laboratory conditions before their testing in tension. Finally, the elastic modulus and tensile strength of the FRP rebars exposed to these cyclic actions were compared with those obtained for unexposed ones, in order to evaluate the mechanical damage caused by environmental conditions.

On the use of the P–TFE method for panel flutter optimization

A numerical approach for structural analysis based on the concept of parameter transfer finite elements (P–TFE) was proposed by the authors in previous papers Barboni R, Castellani A, Mannini A. Un metodo analitico-numerico per l'analisi strutturale. Atti del Dipartimento Aerospaziale, Roma, Novembre, 1988; Barboni R, Gaudenzi P, Mannini A, Santini P. An application to the dynamics of large space structures of a new semi-analytic method for structural analysis. 17th Congress ICAS, Stockholm, September, 1990; Barboni R, Gaudenzi P, Mannini A. Parameter-transfer finite element method for structural analysis. AIAA Journal 1993; 31:923–9; Barboni R, Mannini A, Scarponi C. Structural optimization with parameter-transfer finite element. Meccania 1995; 30: 291–304. Such elements are able to take into account, like a transfer function, the whole behaviour of the structure under concern, subject to dynamic, aerodynamic or thermal actions, and to produce, in analogy with the finite element method, a numerical technique of discretization of a continuum. The purpose of this paper is to apply this methodology to optimization problems, in particular to the design of a minimum-weight monodimensional panel under critical flutter pressure constraint. The use of P–TFE in this aeroelastic optimization problem overcomes most difficulties of the usual techniques of solution, in particular, the evaluation of the sensitivity matrix, the terms of which can be calculated analytically in the frame of the proposed approach. Moreover, the critical conditions are directly imposed by the solution of a non-linear system of two equations in the eigenfrequency and the dynamic pressure unknowns. Several examples relevant to different boundary conditions are presented and the results compared with those obtained by other authors, using different methodologies.

Low energetic possibilities for control of crystal growth

Multi-parametric experimental and computer analyses has shown that unsteady low energy thermal control actions “thermal waves”, induced by periodic variations of heater temperature, changes of crystal and crucible rotation speeds and low frequency vibrations, are able to eliminate temperature fluctuations at the crystallization front and can be used as potential tools for the growth of crystals with high homogeneity.

Natural Killer Cell Activity Reduced by Microwave Exposure during Pregnancy Is Mediated by Opioid Systems

We have previously demonstrated immunosuppression including reduced splenic natural killer cell activity (NKCA) in pregnant rats exposed to microwaves produced mainly by their thermal action. To examine the involvement of opioid systems in reduced NKCA in pregnant rats exposed to microwaves at a relatively low level (2 mW/cm2incident power density at 2450 MHz for 90 min), we assayedβ-endorphin (βEP) in blood, pituitary lobes, and placenta as well as splenic NKCA in virgin and/or pregnant rats. Although microwaves elevated colonic temperatures by 0.8°C for virgin and 0.9°C for pregnant rats, andβEP in blood and anterior pituitary lobes (AP) significantly, it did not change blood corticosterone as an index of hypothalamic-pituitary adrenal axis. There were significant interactions between pregnancy and microwave exposure on splenic NKCA,βEP in both blood and AP, and blood progesterone. Intra-peritoneal administration of opioid receptor antagonist naloxone prior to microwave exposure increased NKCA, blood, and placentalβEP in pregnant rats. Alterations in splenic NKCA,βEP and progesterone in pregnant rats exposed to microwaves may be due to both thermal and nonthermal actions. These results suggest that NKCA reduced by microwaves during pregnancy is mediated by the pituitary opioid system.

A study of the thermoelastic behavior of a cylinder of low electrical conductivity in a uniform steady external electromagnetic field

We determine and study the temperature fields and stresses in a cylinder of low electrical conductivity resulting from force and thermal actions on it by a uniform external electromagnetic field. We study resonance phenomena and the behavior of the unknown fields in the vicinity of the resonance frequencies of the electromagnetic field.

The thermally stressed state of a thickwalled hollow composite cylinder

We propose an approach to the determination of the temperature, stress, and displacement, fields in laminated hollow composite cylinders. The cylinders are studied in three-dimensional formulation; the material of each lamina has the properties of thermal and elastic orthotropy. We study the influence of the heat flow passing through the end surfaces and nonuniform heating of the cylinder to its thermally stressed state. We exhibit peculiarities in the stress and displacement distributions caused both by anisotropy of the mechanical and thermophysical properties and by the nature of the thermal actions applied.

Structural optimization with parameter-transfer finite element

A mathematical method to solve structural problems, using parameter-transfer finite elements (P-TFE) was recently proposed by the authors [1] [2] [3]. The proposed transfer finite element approach is able to create a mathematical model of a structure, taking into account directly the whole behaviour of the structure under dynamic, aerodynamic, and thermal actions, and not by assembling, in a separate fashion, the stiffness and the mass matrix on one side and the external load vector as performed by the classical finite element procedure.

3 The tissue and thermal effects of electrosurgery in the uterine cavity

A brief outline of the physics associated with the production of electrosurgical wave-forms has been described. The conversion of electrical to thermal energy in electrosurgical techniques has demonstrable tissue and thermal actions and these were discussed in greater detail. In vitro electroresection was associated with a narrow ZTN underlying the electrode that did not vary with power output, but was related to duration of exposure. For blend 2 cutting output the mean ZTN ranged from 0.97 to 1.40 mm for a single sweep of the electrode. With longer time increments this ZTN increased to 2.20 mm. Coagulation by desiccation, in vitro, resulted in a depth of destruction of 3.24-3.49 mm that did not vary with power output or duration of exposure. Fulguration only achieved a depth of destruction of up to 1.78 mm that was related to time. This confined-ZTN in the fulguration mode was felt to be too narrow to destroy the deeply seated myometrial gland elements. Thermal transmission through blocks of human uterine tissue, in vitro, has been demonstrated. Resection generated a maximum rise of 5.8 degrees C at 12 mm from the electrode, with desiccation, a maximum of 4.0 degrees C. It was also demonstrated that resection and desiccation have differing thermal profiles, the latter being more confined in nature. Both power output and duration of exposure in resection are important factors, in vitro, with regard to heat generation. Duration of exposure during desiccation is important in terms of heat generation, but not power output. Repetitive electrosurgical insults have a cumulative effect on the thermal profile in vitro. The underlying damage to uterine tissue during electrosurgery in vivo has been established. In electroresection the zone of thermal necrosis was related to duration of exposure, whereas in electrodesiccation the damage appeared confined, self-limiting, and not time-related. The lack of clinically significant demonstrable thermal transmission through the uterus was important. However, excessively prolonged application of electrosurgical energy (greater than 5 seconds) in the narrow regions of the uterus, especially with electroresection, could still be potentially dangerous. The use of the desiccation mode in the cornual regions is recommended for three reasons; the loop electrode has greater potential for perforation in this thin region (because of its diameter); the desiccating mode appeared to have a clearly defined, consistent ZTN that did not alter appreciably; and the thermal imaging of the ball electrode, in vitro, demonstrated the more confined nature of its thermal profile compared to the loop electrode.(ABSTRACT TRUNCATED AT 400 WORDS)

Thermodynamic instabilities in machining processes. II. Numerical investigation of stability

A numerical investigation of methods that intensify machining by additional mechanical and thermal actions is performed. Criteria describing the formation of stagnant dissipative structures with thermodynamic instabilities of the technological system and a calculation procedure for machining regimes that ensures stable parameters of the process are proposed.

Simulation of the thermal behaviour of thermal flow sensors by equivalent electrical circuits

Abstract The usage of equivalent electrical circuits for the modelling of thermal actions allows the application of standard network simulation programs. This paper shows the analogy between thermal and electrical problems. It presents a method to handle the moving medium using a stationary electrical equivalent circuit by the introduction of controlled current sources. The results of the simulation are compared with measurements made with an integrated Si-based flow sensor built at the Lehrstuhl fur Technische Elektronik, Technische Universitat Munchen.

Contribution to the study of thermal erosion on Mars

Similar to the coldest terrestrial periglacial regions like Yakutia (Siberia), the planet Mars exhibits wide outflow channels. This analogy and the presence of permanent ground ice suggest that thermal erosion have existed during Martian fluvial outbursts 3000 m.y. ago. Thermal erosion here is considered to be the result of ground thawing produced by heat transfer between the water flow and the frozen ground, followed by a transport of unfrozen sediments. This paper proposes a qualitative explanation of thermal erosion with the help of some physical models. From the quantitative point of view, two thermal mathematical models are proposed and discussed. The zones undergoing the most efficient erosion are river banks. The first model studied here describes extreme thermomechanical actions: a constant thermal flux on the ground surface produces ground thaw, and the unfrozen sediments are immediately removed by water flow. It is an ablation model. It allows a theoretical evaluation of the river bank recession. Validity of theoretical estimations is acquired by comparing the calculations to the observed backwearing of the banks of some Siberian rivers which can reach 25 m yr−1. Thermal erosion acting upon the bottom of wide rivers (on Mars or in Siberia as well) is a very complex mechanism. Indeed, the mathematical modeling requires the study of interaction between thermal processes and transport of sediments laws. These laws are not yet well defined and are very difficult to establish. Thus another model presented in this paper concerns only one‐dimensional thermal actions of water flow upon the bottom of wide rivers. It considers fluvial flow at positive Celsius temperature producing a thermal flux from the bottom to the initially frozen semi‐infinite ground. This model considers a constant convective heat transfer coefficient and allows an estimation of the thickness of the thawed ground beneath the river (talik). Calculated thickness of the melted ground is given in the paper for different periods of time.