Uticaj kontaktnog otpora pri određivanju toplotne provodnosti metodom zaštićene tople ploče

U radu je opisana aparatura za merenje toplotne provodnosti slaboprovodnih materijala, u koje spadaju stakla, plastike, gume, keramike i sl. Metoda se primenjuje u Labaratoriji za termotehniku i energetiku Instituta za nuklearne nauke “Vinča” od kraja 60-ih godina i daje dobre i pouzdane rezultate u opsegu od 10 do 200 °C. Tokom poslednje godine metoda je podvrgnuta reviziji i unapređena izradom programa za automatizovanu akviziciju, prikaz i obradu podataka pod kontrolom računara. Primena unapređenja ilustrovana je eksperimentima na referentnim uzorcima od Ilford stakla. Nekoliko uzastopnih eksperimenata sa različitim kontaktnim medijumom na površinama uzorka ispitan je uticaj kontaktnog otpora na rezultat merenja

Application of stereometric deformation measurement method to assessment actual stress - strain diagram of heterogeneous welded joints

Одређивање стварног дијаграма напон-деформација представља вишедеценијски проблем јер методе одређивања деформација, по све три осе, у реалном времену тек од недавно представљају реалност научног испитивања. Напретком оптичких метода за одређивање деформација, приликом испитивања затезањем дошло се до могућности мерења деформација узоркa у реалном времену. Ову могућност је обезбедила примена стереометријског начина мерења деформација. Дакле применом две камере и софтверских пакета за обраду добијених резултата, коначно постоји прилика да се одреди стварни дијаграм затезања тј. стварни дијаграм напон-деформација. У оквиру ове дисертације приказана је методологија одређивања стварног дијаграма напон-деформација за ’’undermatching’’ заварени спој, на мартензитном челику. Коришћена је корелација дигиталних слика (DIC) и метода коначних елемената (FEM) уз употребу софтверских пакета Aramis и Abaqus. Дијаграм затезања који се најчешће користи назива се ''инжењерски дијаграм'', јер напони и деформације који су представљени на њему нису стварни тј. добијени су употребом формула које важе само до тренутка када на епрувети почне стварање ''врата''. Поред овог дијаграма често се користи и ''стварни'' дијаграм затезања, који се добија употребом коригованих формула којима је израчунат инжењерски дијаграм. Ни овај приступ не даје тачне вредности напона нити деформација јер не узима у обзир сужење епрувете и локалне промене димензија као ни концентрацију напона која се јавља на месту сужења. У оквиру ове дисертације, размотрен је приступ да троосно стање напона, које се јавља након стварања ''врата'', доводи до пораста напона ком је епрувета изложена и представљен је начин добијања потребних параметара за одређивање стварног дијаграма који узима у обзир и концентрацију напона на месту сужења која је, како ће се испоставити, доминантан параметар за пораст напона поред смањења површине попречног пресека епрувете. Представљена напонска анализа је урађена тако да нема утицаја основних и додатних материјала на добијене резултате напона. Може се закључити да је предложена метода погодна за одређивање параметара чврстоће и код осталих типова заварених спојева као и основних материјала у инверзном поступку. Дакле, приликом испитивања материјала код којих нису познате почетне вредности потребних димензија (''overmatching'' заварени спојеви или основни материјали), може се установити инверзна метода преко мерења крајњих димензија и израчунавања почетних, чиме би се добиле све потребне димензије за употребу предложене методе.Determining the actual stress-strain diagram is a decades-long problem, because real-time strain determination methods on all three axes have only recently become a reality of scientific testing. Advances in optical methods for determining strains during tensile testing have made it possible to measure sample strains in real time. This possibility was provided by the application of the stereometric method for strain measurement. Therefore, by applying two cameras and a software package for processing the obtained results, we are finally able to determine the actual tensile diagram, ie. Actual stress-strain diagram. This disertation presents the methodology for determininig the actual stress-strain diagram for undermatching welded joints on martensitic steel. Used methods are digital image correlation (DIC) and FEM with software packages Aramis and Abaqus. The tensile diagram which is commonly used is called an engineering diagram, because the stresses and strains presented are not true, ie. they are obtained using formulas that are valid only until cross section of the specimen begin to change, necking of the sample ocurs. In addition to the engineering diagram, there is also a ’true’ diagram obtained by using corrected formulas. This principle to, does not provide true results because it does not take into account the reduction of cross section and stress concentration that occurs at the necking site of the specimen. This disertation considered an approach that the triaxial stress state that occurs after the formation of the neck leads to an increase in the stress to which the specimen is exposed and presented a method of obtaining the necessary parameters for determining the actual diagram, which includes the stress concentration at the necking site of the specimen. The presented stress analysis was done so that there is no influence of basic and filler materials on the obtained results of stresses. It can be concluded that the proposed method is suitable for determining the strength parameters in other types of welded joints as well as basic materials in the inverse process. Therefore, when testing materials for which we do not have initial values of required dimensions (overmatching welded joints or basic materials), we can establish an inverse method by measuring the final dimensions of the sample and calculating the initial ones, which would give all the necessary dimensions for using the proposed method

Strain measurement of pressure equipment components using 3D Digital Image Correlation method

Pressure equipment has widespread application in various industrial sectors. Due to this variety, pressure equipment can have complex structure and is subjected to different working loads (static, dynamic, thermal etc.) during the operation life that can cause failure. Strain measurement of complex structure has always been a huge challenge for researchers. Conventional experimental methods (e.g. strain gauges) give only limited data sets regarding measurement on critical areas with high geometrical discontinuities. 3D Digital Image Correlation method is an optical method that enables full-field strain measurement of critical areas on structural components. Sphere/cylinder junction is common geometrical discontinuity on pressure equipment and globe valve housing was chosen as representative example. In this paper, globe valve housing was subjected to external axial loading caused by pipeline dilatations. Highest measured von Mises strain values around 0.15 % were recorded on cylinder/sphere intersection. Determining strain state of critical areas enables better understanding of complex structures and provides an opportunity for further development and improvement for practical industrial application

Analysis of strain distribution in overmatching V groove weld using digital image correlation

Welded joints are treated as critical sites when constructing and calculating welded structures due to inhomogeneity and anisotropy of materials at the welded joint site. Because of the change in the geometry of the elements, the welded joint is viewed as the location of the stress concentration and therefore as a place that weakens the overall load capacity of the structure. Due to this approach in the design of welded structures, practice requires that in most cases welded joints are performed with better mechanical properties compared to the base metal. A welded joint that is made so that the mechanical properties of the weld metal exceed the mechanical properties of the base metal is called the overmatching welded joint. This paper presents the measurement of the strain over the 'overmatching' welded V joint using the DIC (Digital Image Correlation) method and the analysis of the strain distribution is provided. The strain distribution obtained is interesting because it shows that in the stress concentration points the strain have almost minimal values

Application of the subsecond calorimetry technique with both contact and radiance temperature measurements: case study on solid phase tungsten at very high temperatures

This work presents an application of the subsecond calorimetry technique at very high temperatures, which uses both contact and radiance temperature measurements. This technique is normally applied for thermophysical characterization of high temperature solid phase materials in the temperature range from ambient up to about 2600 K, which is the limit of the standard tungsten-rhenium thermocouple use. Simultaneously with contact temperature measurements, noncontact or radiance temperature detection may be performed in the approximate range from 1000 to 2600 K in order to acquire information on spectral normal emissivity of specimen under test. In this study, however, the specimen is heated above 2600 K and, then, the temperature is measured only by the noncontact mean. In the extended temperature range, the obtained values of the spectral normal emissivity are extrapolated for each experimental run, which makes possible a conversion from radiance to absolute specimen temperature. In order to test this application, a pure polycrystalline specimen of tungsten in the form of rod, 3 mm in diameter and 200 mm in length, has been used. The specimen has been heated in vacuum environment of about 10–4 mbar by short pulses of high DC current with a gradual increase of the total heating time from about 0.5–2.5 s. During the specimen heating and the beginning of the cooling period, four sets of experimental data have been recorded and reduced by using the corresponding data reduction procedure. Obtained results of specific heat and specific electrical resistivity from ambient to 3700 K, total hemispherical emissivity from 1000 to 3700 K and spectral normal emissivity from 1000 to 2600 K (extrapolated to 3700 K) are presented, discussed and compared with related literature data

Application of the Laser Pulse Method of Measuring Thermal Diffusivity to Thin Alumina and Silicon Samples in a Wide Temperature Range

Paper presents results of measuring thermal diffusivity of translucent or partially transparent thin discs of non-metals such as alumina and silicon using most widely spread experimental technique, the standard laser pulse method. Difficulties in its application to such materials are discussed. The thermal diffusivity has been measured from room temperature up to 900 degrees C for alumina, and to 1200 degrees C for silicon. Obtained results are analyzed and compared with available literature data and existing recommended functions

Optimal parameterization in the measurements of the thermal diffusivity of thermal barrier coatings

The paper presents an estimation procedure for the measurement of the thermal diffusivity of thermal barrier coatings deposited on thermal conductive substrates using the laser flash method when the thermal contact resistance between the coating and substrate is unknown. The procedure is based on the application of the optimal parameterization technique and Gauss minimization algorithm. It has been applied on the experimental data obtained by using two different samples, one made of PTFE (polytetrafluoroethylene) coating deposited on a stainless steel substrate and the other made of PVC (polyvinylchloride) deposited on a copper substrate

Numerical Modelling of Y Joints of Trusses Made of Steel Hollow Sections

The use of welded structural hollow sections in civil engineering is relatively new. Constructing and dimensioning joints of steel trusses made of welded structural hollow sections requires a more specified approach, compared to traditional joints, achieved by gusset plates. Stress and local deformations at the contact between elements are non-linear and very complex. In this paper, the FEM modelling of the Y-joint was performed, accounting for the non-linear behaviour of steel. The ultimate bearing capacities of the joint were determined numerically, by applying different failure criteria. The results showed very good agreement with the experimental data

PERFORMANCE ANALYSIS OF MRC-SC MACRODIVERSITY RECEPTION OVER GENERALIZED FADING CHANNELS

This paper shows a detailed statistical characterization of a specific system configuration consisting of one multibranch maximal-ratio-combining (MRC) and one selection-combining (SC) micro-level base station, and SC back processing unit at macro level. Primarily, the scenario of the independent and identically distributed generalized-K fading channels is investigated. After that, the correlated branches at SC-based micro-level are assumed. The outage probability and the error probability performance for both cases are defined. According to the presented analytical analysis, numerical results are obtained. Also, the impact of the number of MRC and SC input branches, the impact of the fading/shadowing factor, the predefined outage threshold, the average signal-to-noise ratios and the correlation coefficient on the specified system performance is shown. Simulations validate the accuracy of the proposed analytical analysis

Temperature non-uniformity due to heat conduction and radiation in the pulse calorimetry technique

The paper presents an assessment of the unwanted temperature non-uniformity found in high temperature applications of the pulse calorimetry technique. Specimens in the form of a solid cylinder undergoes fast electrical heating and an intense heat radiation at high temperatures, coupled with the heat conduction the specimens’ cold ends, make them having a highly non-uniform temperature distribution, both in their radial and axial directions. By using finite element method simulations of a typical pulse calorimetry experiment, the temperature non-uniformity across the specimen diameter and along the specimen effective length has been estimated for different specimen dimensions and materials, as well as for different heating rates. The obtained results suggest that an optimization of experimental parameters, such as the specimen diameter, specimen total and effective length and heating rate, is needed for minimization of the temperature non-uniformity effect