Instytut Podstawowych Problemów Techniki

Streszczenie:

Research results on both mechanical and acoustic emission (AE) behavior of Mg-Li and Mg-Al alloys matrix composites (AMC) reinforced with ceramic δ-Al2O3 or carbon fibers subjected to the channel-die compression at room and elevated temperatures are presented in this paper. The AE measurements at room temperature showed that, the effect of anisotropy of the fibres distribution (random planar distribution) with respect to the compression axis appeared in the most investigated composites, whereas the AE activity at 140°C revealed a two- range character and the rate of AE events at 140°C was higher than at room temperature. These effects are discussed in terms of both the differences in thermal expansion between the fibres and the matrix as well as the weakening of the coherency between the fibres and the matrix leading to stronger debonding effects at 140°C than at room temperature. The spectral analysis of AE signals was performed with the Windowed Fourier Transform method, which served to plot the spectral density of AE signal as a function of frequency. The alominous and corundum ceramics types were also investigated in order to illustrate the enhanced AE, which was related to the different crack paths in the final stages of the sample degradation. The results were also discussed on the basis of SEM images, including the in-situ observations of microcracking as well as the dislocation strain mechanisms and microcracking ones during the channel-die compression of the Mg-Li-Al AMC.

Słowa kluczowe:

composites, fibres microcracking, acoustic emission, strain mechanisms, dislocations

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Streszczenie:

The paper concerns the application of the acoustic emission (AE) method for the determination of the mechanical properties of continuously cast industrial tin bronze CuSn6P, which reveals tendencies to unstable plastic flow connected particularly with the Portevin–Le Chatelier (PLC) effect. The relations between the jerky flow connected with the PLC effect, AE intensity and the evolution of a fracture of the investigated alloy subjected to the tensile and compression tests have been analyzed. It has been found that the intensity of the oscillation of stresses, observed on the curves σ–ε, coincides with the effect of the instability of plastic deformation PLC type and the recorded signals of the intensity of acoustic emissions. It has also been found that the increase of the tensile temperature in the investigated alloy involves changes in the mechanism of cracking from transcrystalline ductile to brittle intercrystalline, characteristic at the temperature of minimum plasticity (DMT).

Słowa kluczowe:

Copper alloy, Portevin–Le Chatelier phenomenon, Fractography, Acoustic emission, Intercrystalline fracture

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Streszczenie:

The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al and Mg4Li4Zn alloys subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloys and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the relation between plastic flow instabilities and the fracture characteristics. There are discussed the possible influence of the factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries, interaction of solute atoms with mobile dislocations (Cottrell atmospheres) as well as dislocation pile-ups which may lead to the microcracks formation due to the creation of very high stress concentration at grain boundaries. The results show that the plastic flow discontinuities are related to the Portevin-Le Châtelier phenomenon (PL effect) and they are correlated with the generation of characteristic AE pulse trains. The fractography of broken samples was analyzed on the basis of light (optical), TEM and SEM images.

Słowa kluczowe:

lightweight alloys, Acoustic Emission, fracture, Portevin-Le Châtelier phenomenon, twinning, dislocations, shear bands

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Streszczenie:

The subject of this study was investigation of the factors that have a decisive influence on the resistance of siliceous porcelain to degradation processes. There was tested material C 110 type, which is widely used for the production of low-voltage (LV) elements such as insulators and bushings. On the basis of mechanical-acoustic and microscopic research of small-sized samples, which were subjected to compression, there were distinguished successive stages of degradation of the material structure. In the authors’ opinion, they have a reference to the ageing process, taking place during many years of work under operating conditions. Thus, it was possible to assess the structural factors that determine the durability and reliability of LV electroinsulating elements. The results were related to electrical aluminous porcelains.

Słowa kluczowe:

porcelain insulating materials, ageing processes, acoustic emission (AE), optical microscopy

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Streszczenie:

The work concerns the application of the acoustic emission (AE) method in testing the mechanical properties of continuously cast industrial tin bronze CuSn6P, which reveals tendencies to instable plastic flow connected particularly with the Portevin-Le Chatelier (PLC) effect. The relations between the jerky flow connected with the PLC effect, AE intensity and the evolution of a fracture of the investigated alloy subjected to the tensile test at a strain rate (?? ) of about 1.2·10-3s-1 in the range of temperatures (20÷400?C) has been analyzed. It has been found that the highest intensity of the oscillation of stresses, corresponding to the instability of plastic deformation PLC occurred at 200?C, whereas the maximum of the AE activity is at about 200÷250?C. The brittle intergranular fracture starts in the range of equicohersive temperature (TE) of about 200?C. Plastic deformation of the investigated alloy in the range of the temperature of minimum plasticity, amounting to about 400?C, results in intercrystalline fractures on the entire surface of the stretched samples.

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Streszczenie:

The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al alloy subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloy and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the possible influence of factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries or solute atoms along dislocations (Cottrell atmospheres) or dislocation pile-ups at grain boundaries which create very high stress concentration leading to fracture. The results show that the plastic instabilities are related to the Portevin–Le Châtelier phenomenon (PL effect) and they are correlated with the generation of AE peaks. The fractography of breaking samples was analyzed on the basis of light (optical), TEM and SEM images.

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Streszczenie:

The purpose of these investigations was to determine the effect of the strain rate on the phenomenon of a heterogeneous plastic deformation of the Portevin–Le Chatelier type while testing free compression of CuZn30 single crystals with a crystallographic orientation of [139] at 300°C. Moreover, the relations between the work-hardening curve s–e displaying the PLC effect and the characteristics of the signals of the acoustic emission generated in the uniaxial-compression test were determined. It was found that the process of plastic deformation of the tested single crystals in the analyzed range of the frequencies up to 35 kHz generates differentiated sources of acoustic-energy emission, mainly the impulsive emission generated by signal events, correlated with the oscillations of the stresses on the work-hardening curves s–e. The strain rate mainly causes the changes in the intensity of the oscillation typical for the PLC effect.

Słowa kluczowe:

plastic strain, Portevin–Le Chatelier effect (PLC), single crystals, copper alloys, compression test, acoustic emission (AE)

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Streszczenie:

The paper presents the investigation of the relation between the acoustic emission (AE) and instability of plastic deformation type Portevin-Le Chatelier (PLC) of single-phase brass CuZn30 monocrystals with crystallographical orientation [1¯3 9]. The monocrystals have been investigated applying the method of free compression at a constant strain rate and the temperature within the range from 200°C to 400°C, simultaneously recording PLC phenomenon by means of acoustic emission. During hot axial compression tests the correlation between work-hardening curves σ - ε, which display PLC effect and characteristic of acoustic emission signals has been found. Moreover, it was proved that in the range of the PLC effect, the acoustic signal is an impulse a character of cyclic repeatability, distinctly correlated qualitatively with the stress oscillations on the curves σ - ε. The analysis of the obtained results leads to the conclusion that in the tested monocrystals the effect PLC is probably controlled by complex processes similar to the phenomenon of dynamic strain ageing (DSA), which are described by diffusion models.

Słowa kluczowe:

plastic strain, Portevin - Le Chatelier effect (PLC), monocrystals, copper alloys, compression test, acoustic emission (AE)

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Streszczenie:

The paper presents the microscopic and mechanoacoustic study of degradation processes of the porcelain material C 110 type. Small-sized samples, derived from the low voltage insulator, were subjected to a slow, quasi-static compression, with simultaneous recording of acoustic emission descriptors. There were distinguished consecutive stages of the material degradation. Obtained results were compared with the images of the microstructure of low-voltage insulator materials after many years of operation. On this basis, there were distinguished the factors determinant of the short-term strength of porcelain and its resistance to ageing processes under operating conditions.

Słowa kluczowe:

electrical porcelain, microscopic analysis, acoustic emission, degradation of the porcelain

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The paper presents the evaluation of the mullite hypothesis explaining the strength of porcelains. There was researched the influence of the mullite phase on the short- and long-term mechanical strength of electrotechnical porcelain of different types (C 110, C 112, C 120 and C 130). The total mullite phase content, the size and distribution of precipitates and dispersed single crystals in the glassy matrix were considered. Mechanoacoustic and microscopic techniques as well as ultrasonic testing were used during the investigation. The role of the mullite phase in the increase of strength and resistance to aging processes of electrical porcelain of various types was described. Mullite hypothesis is valid in reference to siliceous (C 110), cristobalite (C 112) and aluminous C 120 type materials of a typical raw components content and phase composition. In the case of modern strengthened C 120 and C 130 type aluminous materials advantageous are only dispersed fine mullite needles. The precipitates of mullite phase, especially of bigger size, are undesirable.

Słowa kluczowe:

electrotechnical porcelain, porcelain degradation, acoustic emission (AE), microscopic analysis

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Streszczenie:

The paper deals with the problems related to the diagnostics of selected parts of modern Diesel engines. The evolution of mechanical properties of four alloys of Mg-Li-Al system and four composites made on the base of the alloys mentioned above, caused by variation of its composition was presented. The Acoustic Emission (AE) method applied to monitoring of degradation of mechanical properties of the alloys and composites was described. Moreover, the results of the investigation of failures occurring in the injectors of Common Rail Diesel engines per formed with the application of AE method were also reported.

Słowa kluczowe:

Diesel engines diagnostics, light alloys and composites, Common Rail fuel system, Acoustic Emission method

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Streszczenie:

W pracy przedstawiono badania wpływu fazy mulitowej na krótko- i długotrwałą wytrzymałość mechaniczną elektroporcelan rodzaju C 110, C 112, C 120 oraz C 130. Rozpatrywana była sumaryczna zawartość fazy mulitowej, wielkość i rozłożenie jej wydzieleń oraz niezaglomeryzowanych kryształów w osnowie szklistej. W badaniach wykorzystano metodę mechanoakustyczną, techniki mikroskopowe (MO i SEM) oraz metodę ultradźwiękową. Przedstawiono rolę fazy mulitowej w podwyższeniu odporności na procesy starzeniowe porcelany elektrotechnicznej różnego rodzaju.

Słowa kluczowe:

porcelana elektrotechniczna, degradacja tworzyw porcelanowych, emisja akustyczna, badania mikroskopowe

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The paper deals with the effect of Al content in two-phase Mg-10Li-xAl (x = 0, 1, 5) alloys on their straining behavior before and after severe plastic deformation by high pressure torsion. Both coarse-grained and ultrafine-grained alloys were compression strained in channel-die device with simultaneous acoustic emission monitoring. It has been observed that Hall-Petch strengthening in ultra-fine grained alloys superposes with the strengthening resulting from Al alloying (solution hardening, particulate strengthening). Superposition of these strengthening mechanisms becomes stronger with increase in Al content, presumably due to enhanced grain refining effect.

Słowa kluczowe:

Mg-Li-Al alloys, grain refinement, high pressure torsion, acoustic emission, Hall-Petch strengthening

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Streszczenie:

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.

Słowa kluczowe:

aluminous porcelain, structural degradation, acoustic emission (AE), microscopic analysis

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Streszczenie:

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.

Słowa kluczowe:

aluminous porcelain, ceramic insulators, insulator testing, acoustic emission, microscopic analysis, structural degradation

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Streszczenie:

The paper presents the concept of mechanoacoustic testing of degradation processes of ceramic electrotechnical materials. Samples of small dimensions were subjected to slowly increasing compressive loading with simultaneous recording of acoustic emission descriptors. The process was continued to the destruction or it was stopped at various stages of degradation of the material structure. Microscopic analysis of samples enabled determining the effects of stress action.

Słowa kluczowe:

acoustic emission (AE), material degradation, electrotechnical ceramics

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Streszczenie:

This paper presents the results of acoustic emission (AE) measurements of samples under compressive stress. Additional research, concerning structure and parameters of the material, was performed using microscopic and ultrasonic methods. The object of the investigation were samples of porcelain material C 130 type of domestic long-rod insulator LP 75/31W. This kind of aluminous porcelain demonstrates high mechanical and electrical strength as well as resistance to aging degradation processes. It finds wide application in modern electrotechnical engineering. The aim of this study was to recognize stages of degradation processes of the porcelain structure. The present work constitutes a continuation of the study concerning the influence of technological faults, existing in porcelain material structure, on long-term mechanical strength. These results were presented at the OSA Conference in 2006. The analysis of mechanical-acoustic characteristics pointed out diversified strength and complicated mechanism of the material degradation. The effectiveness of the dispersive and fibrous reinforcement of aluminous porcelain C 130 type was ascertained. It was stated during the microscopic investigation that the structure of the material is different from that in the case of typical aluminosilicate ceramic materials. The main reason for this effect is due to densely dispersed corundum and mullite fine crystals in glassy matrix.

Słowa kluczowe:

porcelain material C 130 type, acoustic emission, compressive strength, structural degradation

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Streszczenie:

The object of investigation was the behaviour of acoustic emission (AE) during channel-die compression of the Mg-Li-Al alloys matrix composites (AMC) reinforced with ceramic δ-Al2O3fibres at room and at elevated temperature. The results of AE measurements at room temperature showed that in the majority of the investigated compositions the effect of anisotropy of the fibres distribution (planar random distribution) appeared with respect to the compression axis, whereas the AE activity at 1400C revealed a two-range character and the level of the rate of AE events was higher than thatat room temperature. These effects are discussed in terms of both, the differences in thermal expansion as well as weakening of the coherency between the fibres and the matrix. The results of AE measurements served to plot the spectral characteristics of the registered AE signals, i.e. the spectral density of AE signal as a function of its frequency. The spectral analysis of AE signals generated during the micro-cracking process of ceramic fibres in channel-die compressed Mg-Li-Al AMC was performed with the Windowed Fourier Transform method. The results are also discussed on the basis of optical and scanning microstructure images including these in-situobservations of microcracking fibres as well as in the context of the dislocation strain mechanisms and microcracking ones during the channel-die compression of the Mg-Li-Al AMC.

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