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1

Criticality of self-heating in degradation processes of polymeric composites subjected to cyclic loading: A multiphysical approach

Katunin A., Wronkowicz A., Bilewicz M., Wachla D.

Archives of Civil and Mechanical Engineering

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2017

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Vol. 17, no. 4

806--815

EN

In this paper, the criticality of the self-heating effect accompanying the fatigue process of polymeric composites is studied by monitoring various physical parameters, which reflects degradation progress in a direct or indirect way. The occurring self-heating effect, resulted from the mechanical energy dissipation due to the viscoelastic nature of a polymeric matrix of composites, under certain loading conditions, may dominate the fatigue process, causing significant intensification of degradation and thermal failure at temperature often higher than the glass-transition temperature. The aim of this study is to determine the critical values of the self-heating temperature, which exceeding results in damage initiation and, in consequence, intensive degradation and failure. Additionally, performed tests enable evaluation of sensitivity of particular techniques as well as obtaining more accurate results with physical justification. Following the obtained results, the critical value of a self-heating temperature, at which domination of the fatigue process by the self-heating effect is observed, is at a level of 65–70 °C. Information about the obtained critical values has a great importance both during the design stage of composite structures subjected to cyclic loading as well as their further operation.

2

Mechanoacoustic and microscopic study of aluminous porcelain resistance to structural degradation

Ranachowski P., Rejmund F., Ranachowski Z., Pawełek A., Piątkowski A., Kudela S.

Archives of Metallurgy and Materials

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2011

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Vol. 56, iss. 4

1127-1233

EN

The paper presents mechanoacoustic and microscopic testing of degradation processes of modern C 120 electrotechnical porcelain of domestic medium voltage line insulator. Samples of small dimensions, cut off from the rod of insulator, were subjected to compressive loading, with recording of acoustic emission descriptors. Microscopic analysis enabled determining the advancement of degradation effects. Three stages of the structure degradation were distinguished. The effectiveness of dispersive and fibrous reinforcement of modern aluminous porcelain C 120 type has been described. Structural strengthening by corundum grains and mullite needle shaped crystals improves mechanical parameters and distinguishes this material from typical aluminosilicate ceramics. The presented results enable drawing up the conclusions concerning the resistance of investigated material to the ageing degradation process development during long term operation.

PL

W pracy przedstawiono mechanoakustyczne i mikroskopowe badania procesów degradacji w nowoczesnym tworzywie porcelanowym rodzaju C 120 liniowego izolatora SN. Małogabarytowe próbki, wycięte z pnia izolatora, były quasi-statycznie ściskane z jednoczesną rejestracją deskryptorów emisji akustycznej. Badania mikroskopowe ściskanych próbek wykorzystano do opisu mechanizmu degradacji tworzywa. Wyróżniono trzy kolejne etapy degradacji struktury tworzywa. Stwierdzono wysoką krótko- i długotrwałą odporność materiału izolatora w porównaniu do typowych tworzyw rodzaju C 120. Jest to wynikiem skutecznego dyspersyjnego i włóknistego wzmocnienia struktury czerepu badanej porcelany

3

Aluminous porcelain degradation study using mechanoacoustic and microscopic methods

Ranachowski P., Rejmund F., Ranachowski Z., Pawełek A., Piątkowski A.

Archives of Electrical Engineering

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2011

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Vol. 60, nr 2

109-128

EN

This experimental paper comprises the results of acoustic emission (AE), microscopic and ultrasonic measurements of samples subjected to slowly increasing compressive stress. On the basis of conducted measurements the successive stages of the material structural degradation have been recognized. The objects of study were samples made of C 120 aluminous porcelain. The investigated material has found at present the application in the fabrication of technical elements like overhead power line insulators. In the case of such objects, not only high mechanical strength, but especially elevated durability as well as operational reliability are required. The expected "life time" of net insulators during exploitation is about 40 years. The analysis of obtained mechanoacoustic dependences pointed out a complex mechanism of degradation of the material. Microscopic investigation of samples, which were stressed to different levels of load, enabled to specify the development of gradual growth of microcracks and successive crushing out of elements of the structure. These processes appear to be similar to the ageing processes occurring in the material during long period of exploitation under a working load. Three stages of the structure degradation were distinguished. The preliminary and subcritical ones show low or moderate intensity of AE signals and considerable variety for the particular samples. The critical stage directly precedes the destructtion of samples. Its range is relatively narrow and reveals the AE activity of high energy. The effectiveness of dispersive and fibrous reinforcement of modern aluminous porcelain C 120 type has been described. Structural strengthening by corundum grains and mullite needle shaped crystals improves mechanical parameters and distinguishes this material from typical aluminosilicate ceramics. The presented results enable drawing up the conclusions concerning the resistance of investigated material to the ageing degradation process development during long term operation.

4

Comparison of Acoustic Emission and Structure Degradation in Compressed Porcelain and Corundum Materials

Ranachowski P., Rejmund F., Ranachowski Z., Pawełek A., Piątkowski A.

Archives of Acoustics

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2009

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Vol. 34, No. 4

655-676

EN

The paper presents the results of acoustic emission (AE), microscopic and ultrasonic measurements of the samples subjected to slowly increasing compressive stress. On the basis of performed measurements, the successive stages of the materials structural degradation have been recognized. The object of research and comparison were the samples made of the electrotechnical materials – aluminous porcelain and corundum ceramics. Both investigated materials have wide technical application. The analysis of obtained mechanoacoustic characteristics pointed out a complicated mechanism of degradation of the materials. Microscopic investigation of samples, which were stressed to different levels of load, enabled to specify the various course of the gradual growth of microcracks and successive crushing out of elements of the structure. These effects appear to be similar to the aging processes occurring in the materials during long periods of exploitation under working load. For the two tested materials there were distinguished three stages of the structure degradation. The preliminary and subcritical ones show low or moderate intensity of AE signals and considerable variety for the particular samples. The critical stage directly precedes destruction of the samples. Its range is relatively narrow and contains AE activity of high energy. The presented results enable to draw up conclusions concerning the resistance of investigated materials to the aging degradation processes development during long-term operation. Comparison of the results of mechanical, acoustic and microscopic studies have revealed that the differences registered for the strength and characteristics are due to inhomogeneities of the materials in the semi-macro as well as in the micro-scale. The effectiveness of dispersive and fibrous reinforcement of aluminous porcelain was described. Strengthening of porcelain by corundum grains and mullite needleshaped crystals improves mechanical parameters and distinguishes this material from typical aluminosilicate ceramics. The occurrence of groupings of bigger grains in the structure of the corundum material represents most probably the intermediate state, leading to the known effect of the abnormal grain growth (AGG).

5

Structure degradation, textureand acoustic emission in compressed corundum ceramics

Ranachowski P., Rejmund F., Pawełek A., Piątkowski A.

Archives of Metallurgy and Materials

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2008

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Vol. 53, iss. 3

703-709

EN

The paper presents the results of microscopic, ultrasonic and acoustic emission (AE) iiwestigations of corundum ceramic materiał C 799 kind samples, exposed to compressive stress. On the basis of AE measurements of slowly compressed samples, the successive stages of structural degradation have been registered. Microscopic analysis enabled to specify the processes of the gradual growth of microcracks and crushing out of corundum grains, similarly to the aging processes occurring in materiał after long period of exploitation under mechanical stress. The analysis of AE results revealed diversified strength and mechanical-acoustic characteristics of the particular samples. It was found that this effect was due to technological defects, especially textural inaccuracies - different grain size and their non-uniform spatial distribution. The structure analysis of the samples compressed to advanced subcritical stage revealed the presence of micro- and macrocracks. The most of the cracks underwent propagation along grain boundaries, whereas only few within the grains. It was found that the centers of joined bigger grains are especially susceptible to the destruction process. During the critical stage of destruction the propagation of cracks occurs at great velocity and throughout all the elements of the structure. lnvestigations revealed that differences registered for the strength and the mechanical-acoustic characteristics are due to the inhomogeneity of the materiał in the semi-macro as well as in the micro scalę. The occurrence of groupings of grains in the structure of the corundum materiał represents most probably the intermediate state, leading to the known effect of the abnormal grain growth (AGG).

PL

W pracy przedstawiono wyniki badań mikroskopowych, ultradźwiękowych oraz emisji akustycznej (EA) próbek poddanych wolno narastającemu obciążeniu ściskającemu. Przedmiotem badań były małogabarytowe kształtki z tworzywa korundowego rodzaju C 799. Zawartość tlenku glinu w badanym materiale wynosiła 99,7%. Z uwagi na szerokie zastosowanie we współczesnej technice, badania procesów degradacji tworzyw korundowych mają istotne znaczenie poznawcze. Przeprowadzone badania umożliwiły rozpoznanie tekstury tworzywa oraz monitorowanie kolejnych etapów degradacji struktury, aż do całkowitego jej zniszczenia. Zastosowana metodyka badań pozwala na odniesienie wyników do procesów starzeniowych, zachodzących w trakcie wieloletniej eksploatacji elementów korundowych. Wykonane pomiary wykazały zróżnicowaną wytrzymałość oraz charakterystykę mechaniczno-akustyczną poszczególnych próbek. Obserwacje mikroskopowe struktur w różnych fazach degradacji ujawniły, że zróżnicowane właściwości próbek są konsekwencją defektów technologicznych, wielomodalnego rozkładu wielkości ziarn oraz niejednorodnego ich rozmieszczenia (tekstury materiału). Autorzy uważają, że rejestrowana struktura próbek odpowiada wstępnemu etapowi znanego efektu nadnaturalnego wzrostu ziarn (abnormal grain growth).

6

Mechanical-acoustic and structural investigations of degradation processes of aluminous insulator porcelain C 130 type

Ranachowski P., Rejmund F., Pawełek A., Piątkowski A.

Archives of Metallurgy and Materials

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2007

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Vol. 52, iss. 4

641-654

EN

Presented paper aims to estimate the resistance of porcelain C 130 type to aging degradation processes during long-lasting period of exploitation. The objects of investigation were specimens made of high mechanical strength aluminous porcelain of C 130 type. This kind of material is used in the production of reliable electrotechnical elements such as insulators of overhead power lines. In case of the insulators not only high mechanical strength but especially elevated durability as well as operational reliability are required. The paper comprises the results of microscopic, ultrasonic as well as acoustic emission (AE) measurements of samples subjected to slowly increasing compressive stress. It concerns problems connected with exploitation, production technology and ultrasonic measurements of specially prepared samples of porcelain. The samples prepared for examination were divided into three groups: first . without any defects, second . with faults of smaller or medium intensity, and third . containing numerous defects in structure. The acoustic parameters of not loaded samples and the ones loaded to various values of compressive stress were measured. Moreover, some of the specimens were additionally subjected to 200.C temperature action. This enabled verifying the insulator material resistance to temperature increase as a result of leakage currents. The analysis of the obtained results revealed that the mechanism of ultrasonic wave propagation in porcelain of C 130 type is different from that in the case of typical aluminosilicate ceramic materials. This is the consequence of the effective reinforcement of the material structure by densely dispersed corundum and mullite phases in glassy matrix. Measurements of the attenuation coefficient offer better possibility to estimate the structure degradation of the porcelain material than ve- locity of ultrasonic wave propagation. Subsequently, the effect of the structural defects, introduced into the material, on the mechanical-acoustic behaviour and on the strength of the samples was presented. The occurrence of the pseudoplasticity effect, directly proportional to the presence of technological defects in the structure of the material, was observed. The acousto-mechanical measurements were completed by microscopic analysis of the porcelain material. The phases�f content was recognized. There were described the structures of samples belonging to three groups containing technological defects and with different advancement of material degradation. Presented results enable to drawn up conclusions concerning the resistance of the ceramic material to the aging degradation processes development during long term exploitation.

PL

Praca ma na celu ocenę odporności tworzywa rodzaju C 130 na procesy degradacji starzeniowej podczas wieloletniej eksploatacji. Obiekt badań stanowi porcelana wysokoglinowa rodzaju C 130 o dużej wytrzymałości mechanicznej. Tworzywo tego typu stosowane jest do produkcji odpowiedzialnych elementow elektrotechnicznych jak izolatory napowietrznych linii energetycznych. W przypadku tych wyrobow wymagana jest nie tylko duża wytrzymałość, lecz przede wszystkim wysoka trwałość i niezawodność. Przedstawiono wyniki badań mikroskopowych, ultradźwiękowych oraz emisji akustycznej (EA) probek poddanych wolno narastającym naprężeniom ściskającym. Omowione zostały zagadnienia eksploatacyjne tworzywa, technologia wytwarzania oraz badania ultradźwiękowe specjalnie wytworzonych probek porcelany. Przygotowane probki materiału podzielone zostały na trzy grupy - kształtki pozbawione wad technologicznych, probki zawierające defekty strukturalne o mniejszym lub średnim nasileniu oraz kształtki posiadające liczne wady. Mierzone były parametry akustyczne probek nieobciążonych oraz po przyło- żeniu naprężeń ściskających o rożnej wartości. Kilka probek było ponadto poddanych działaniu temperatury 200?C. Miało to na celu sprawdzenie odporności tworzywa na temperaturę, podwyższoną w wyniku prądow upływu. Pomiary ultradźwiękowe wykazały, że mechanizm propagacji fal w porcelanie rodzaju C 130 jest odmienny niż w ty- powych glinokrzemianowych materiałach ceramicznych. Wynika to z występowania gęsto rozłożonych ziarn korundu oraz sieci drobnych igłowych kryształow mulitu w matrycy tworzywa wysokoglinowego. Badania ultradźwiękowe i mikroskopowe potwierdziły ważną rolę tych faz w strukturalnym wzmocnieniu materiału oraz wysoką odporność osnowy na powstawanie i propagację pęknięć. Stwierdzono, że pomiary wspołczynnika tłumienia dają lepszą możliwość oceny stopnia degradacji materiału w porownaniu do prędkości propagacji fal ultradźwiękowych. Zasadniczą część pracy stanowią badania wpływu wprowadzonych do materiału wad struktury na charakterystykę mechani- czno-akustyczną oraz wytrzymałość mechaniczną probek poddanych wolno narastającym obciążeniom ściskającym. Wyniki ba- dań mechaniczno-akustycznych skorelowane zostały z analizą mikroskopową struktury probek o rożnym stopniu zaawansowania procesu degradacji materiału. Określono wpływ defektow technologicznych o rożnym stopniu nasilenia na postępujący proces rozwoju mikropęknięć oraz szczelin. Dokonano oceny wpływu poszczegolnych faz tworzywa na przebieg procesu degradacji struktury czerepu. Na podstawie przedstawionych badań sformułowano wnioski odnośnie odporności tworzywa na rozwoj procesow degradacji starzeniowej podczas wieloletniej eksploatacji.

7

Assessment of structural changes at IN-519 cast steel reformer tubes

Łabanowski J.

Advances in Materials Science

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2005

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Vol. 5, nr 1-3

14-20

EN

Catalytic tubes are the most important parts in reformer furnaces at ammonia chemical plants. A steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide and carbon dioxide. This reaction proceeds at a temperature range 800-900 °C and under pressure 3 - 4 MPa. This severe working condition causes a structural damage in the tubes. It is necessary to develop reliable methods for inspection of tube degradation and for realistic prediction of its residual life. The paper presents effects of long-term service at elevated temperatures on microstructural changes of the alloy IN-519 (24%Cr, 24%Ni, Nb).