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Abstract 4/2006

MECHANISMS OF THE EARLY FATIGUE DAMAGE IN METALLIC MATERIALS
Jaroslav Polák - Jiří Man - Martin Petrenec

The early stages of the fatigue damage in f.c.c. and b.c.c. metals are studied using high resolution techniques. The localization of the cyclic plastic strain results in formation persistent slip bands (PSBs) with specific dislocation structure. Characteristic surface relief is formed at locations where PSBs emerge on the surface. It consists of extrusions and intrusions separated by the original flat surface. Atomic force microscopy is used to study the details of the surface relief. The experimental findings are discussed in terms of the point defect model of fatigue crack initiation.

QUANTITATIVE FRACTOGRAPHY - WELL SPRING OF INTIMATE KNOWLEDGE IN FATIGUE CRACK GROWTH HISTORY
Jiří Kunz - Jan Siegl - Ivan Nedbal

Quantitative fractography of fatigued structure parts can offer very useful and valuable information on the fatigue process. One of main contribution of this experimental method is a detailed description of fatigue crack growth in time and in space. The fractographic reconstitution of fatigue process history is conditioned by the existence and detectability of a fractographic feature characteristics of which is correlated with the fatigue crack growth rate. In the paper presented, three various fractographic features are used for the fractographic reconstitution - striations (in the case of constant amplitude loading), beach marks (for simple program loading) and special inserted fracture marks (for complex program loading). All three methods described are illustrated on case studies, all from the area of testing and development of aircraft structure parts.

THERMAL AND MECHANICAL FATIGUE IN A QE22 MAGNESIUM ALLOY REINFORCED WITH SHORT SAFFIL FIBRES
Zuzanka Trojanová - Pavel Lukáč

Using non-destructive methods changes in the microstructure of QE22 metal matrix composites (MMC) due to thermal and mechanical cycling have been investigated. Thermal stresses induced in composites due to a considerable difference between thermal expansion coefficients of the matrix and ceramic reinforcement may create new dislocations on cooling from elevated to ambient temperature. Thermal stresses can achieve the yield stress of the matrix and micro-glide of newly created dislocations as well as their annihilation can occur. Thermodynamic processes in the matrix influence these effects. Keywords: Metal matrix composites, Squeeze casting, Dislocations, Internal friction, Acoustic properties, Thermal properties.

AN EFFECT OF MICROSTRUCTURE DEFECTS ON FATIGUE RESISTANCE OF GLASS FIBRE REINFORCED POLYMER COMPOSITES USED FOR SPRINGS OF FREIGHT VEHICLES
Ivo Černý - Rayner M. Mayer - Ivan Fürbacher

Fatigue properties of glass reinforced polymer (GRP) composite materials and advanced components manufactured using GRP are discussed with an emphasis on scale and microstructure effects. Results of an experimental investigation of fatigue damage mechanisms under bending and shear cyclic loading and effects of microstructural defects resulting from an insufficient resin wet out are presented and analysed. It was shown that if microstructural defects are present in the component, initiation of fatigue damage is localised into these areas, resulting in sudden, premature failure. Fatigue life can be then reduced by more than three orders in comparison with the perfect material. The presence of defects is affected particularly by properties of the constituents and moulding parameters. Suitable changes of the process parameters resulted in a significant improvement of fatigue resistance.

CONSTRAINT ASPECTS OF THE EVALUATION OF FATIGUE TEST RESULTS IN PARIS REGION
Zdeněk Knésl - Pavel Hutař - Stanislav Seitl

The growth of the fatigue crack is dependent on the crack growth resistance of the material as a bulk property. The fatigue crack growth properties of a material are usually described by the correlation between a crack propagation rate, and a corresponding range of the stress intensity factor (Paris-Erdogan law). Formulated modified Paris-Erdogan law estimates quantitatively the changes of the fatigue crack propagation rate due to different level of constraint. In-plain constraint was described by T-stress value. The validity of the approach suggested has been proved by the comparison of measured and calculated values of the crack propagation rate and it was found that experimental values correspond well with those numerically predicted. The results presented makes it possible to relate experimentally measured data obtained from specimens with different geometries and thus contribute to more reliable estimates of residual fatigue life of structures.

COMPARISON OF A NEW EQUATION DESCRIBING FATIGUE CRACK GROWTH CURVES WITH THE NASGRO EQUATION
Jan Kohout - Stanislav Věchet - Vojtěch Hrubý

The so-called NASGRO equation allows a very good description of fatigue crack growth curves, i.e. the dependence of crack length increase per one fatigue cycle da/dN on the stress intensity factor range delta K. In 1999 the first author of this paper published quite similar equation, which written for given loading cycle asymmetry with positive stress ratio R contains the same parameters having similar meaning as in the NASGRO equation. In most cases studied by the authors this equation leads to a better fit than the NASGRO equation, above all when the experimental curve contains relatively long Paris straight line and/or relatively sharp bend from this line to the threshold stress intensity factor range. The generalization of the NASGRO equation for various values of stress ratio R was made in a quite complicated way. The new equation was generalized using the Walker model based on the relation delta K(R) = delta K(0)(1 - R)-m which is valid also for threshold value delta Kth but not for critical stress intensity Kc being a constant independent of R . Then the shift of the growth curves with the change of R ratio is described only by one parameter m (0 < m < 1) both for positive and for negative values of R . It means that the crack closure models are very important for explanation and deep study of fatigue crack growth mainly in negative R region but play no crucial role in a phenomenological description of the growth in the case of short-term tests in non-aggressive media.

INFLUENCE OF TECHNOLOGICAL FACTORS ON FATIGUE PROPERTIES OF STEEL SHEETS
Marián Buršák - Otakar Bokůvka

Treatment technology can influence the fatigue process positively or negatively. The paper analyzes the influence of cold levelling of strips from coils of steel grade E 700TS (Re = 700 MPa) on the fatigue crack initiation and the fatigue limit under one-sided bending (R = 0.1). It indicates a positive effect on the above-mentioned characteristics if strips are loaded under one-sided bending from the side where residual compressive stress is formed during levelling. By blasting a sheet made of steel grade 11 375.1, the fatigue limit under symmetrical flat bending was increased due to the hardening and formation of residual compressive stress. The paper also indicates risks connected with blasting, influencing the fatigue life. Key words: Internal stress, steel sheets, levelling, blasting, mechanical properties, fatigue limit.

INFLUENCE OF ATMOSPHERIC CORROSION ON THE SERVICE LIFE OF STEEL SHEETS
Marián Buršák - Ján Michež

The paper analyses the influence of the duration of atmospheric corrosion on basic mechanical and fatigue properties of steel sheet 2.16-4.46 mm thick, made of low-carbon steel with higher atmospheric corrosion resistance. A decrease of mechanical properties as a function of the exposure time is mainly due to a weight loss and the fatigue limit decrease under flat bending is due to increased surface roughness weathering steel too. The fatigue limit is a crucial characteristic for the service life of products, since after 5 years of exposure it decreased by 36%, while the ultimate tensile strength decreased by as little as 8%. Key words: Low-carbon steel, corrosion resistance, surface roughness, mechanical properties, fatigue limit.

FATIGUE BEHAVIOUR OF NODULAR CAST IRON AT VARIOUS LOADING CYCLE ASYMMETRY
Stanislav Věchet - Jan Kohout - Klára Hanzlíková - Vojtěch Hrubý

Fatigue behaviour of nodular cast irons is described in specialized literature quite well but the majority of fatigue tests in high cycle region has been made at symmetrical bending or at symmetrical tension-compression loading. The role of loading cycle asymmetry is very important because the fatigue limit of nodular cast irons is substantially influenced by mean static stress. Unfortunately, the number of papers describing the influence of mean static stress is quite low. The aim of the presented paper consists in presenting the results of long-term research of nodular cast irons at high cycle region at different loading cycle asymmetry. Several heats with various heat treatments leading to various structures of matrix were used for fatigue tests performed mainly at symmetrical tension-compression loading and at repeating tensile loading. From S-N curves the fatigue limits for 107 cycles were determined and then used for the calculation and construction of the Haigh and the Smith diagrams. Nodular cast irons seem to be more sensitive to mean static stress than structural steels. It was found that, in contrast to many papers, the dependence of upper stress on mean stress is not linear but parabolic with the exponent lower than 1 whose value is linear function of UTS.

THE RELATION BETWEEN MICROSTRUCTURE COMPOSITION AND FATIGUE PROPERTIES OF ADI
Klára Hanzlíková - Stanislav Věchet - Jan Kohout

The presented work deals with a high-strength variant of nodular cast iron, so-called ADI (austempered ductile iron). This material ranks among prospective structural materials and is more and more applied in all important branches of machine industry. Recently it has even been applied to castings for cyclically loaded components, i.e. gears and traversing wheels, crankshafts of motor-cars, vans and trucks, swivel pins, rail brakes, pressure pipes in oil industry etc. The microstructure composition of an ADI matrix consists especially of bainitic ferrite and stabilized austenite. However, in dependence on transformation dwell some amount of martensite can appear as well. In this work the relation between mechanical properties and microstructure composition of the matrix mixture was studied in detail for the case of ADI transformed at 380 °C, with emphasis on fatigue properties.

INFLUENCE OF ANNEALING ON STRUCTURE AND FATIGUE PROPERTIES OF MICROALLOYED STEEL
Gejza Rosenberg

Advanced microalloyed steels (HSLA steels) contain microstructural components whose mechanical properties can be altered by exposure to elevated temperatures, for example, in application of welding cycles. The objective of this paper is to identify the relative influence of annealing (annealing at 650°C for 25 min) on microstructure and fatigue properties variability of HSLA steels. The results obtained in this work suggest that in spite of no or very small positive effect of annealing on the fatigue limit of HSLA steels with no stress concentrator in its presence it can be expected that the annealing has a negative influence either on fatigue crack initiation or fatigue crack growth. Key words: fatigue strength; crack initiation; fatigue crack growth, low-temperature annealing; pipeline steel; microstructure.

A NEW APPROACH TO THE INTERPRETATION OF SIGNALS FROM TEMPERATURE SENSORS
Milan Blagojevic - Dejan Petkovic

A basic approach in adaptive modeling of data acquisition is based on the comparison of real time data with the data previously predicted from the adequate numerical model. Our research was initiated to find a better algorithm for prediction of fire growth in enclosed spaces by means of interpretation of signals from fire sensors. The data obtained from temperature sensors are compared in a real time with predicted data and are used to adjust the numerical model for prediction so that it matches the reality. This adaptive approach has the following primary goals: early warning of fire growth, better alarm decision making, realization of an adaptive threshold (a threshold changing in the time according to the input signal), very small false alarm rate, etc. In this paper we introduce a method that uses smooth spline polynomials for approximation of the collected data and the time sliding window principle. The length of time sliding window varies in the real time and depends on the calculated error. Keywords: data acquisition, time sliding window, smoothing spline approximation, adaptive modeling.

COMMUNICATIONS: THE WAY OF RANSMITTING MESSAGES AND COMMUNICATION SKILL
Zoran Čekerevac - Slobodan Ristić

Communication is woven in almost all branches of economy and art, either as a separate skill or as one of the segments in the interdisciplinary approach. It is also a demonstrative and irreplaceable link in the study of various disciplines of management and marketing, as well as in public relations. This paper, after a short basic characteristic of communication, gives detailed analysis about the way of transmitting messages and communication skill. Special attention was paid to the understanding of the sender and the recipient, through the process of coding and decoding of the information. At the end, a communication network established by combining verbal and non-verbal communication systems is analyzed. Key words: communication, coding, noise, speech, rhetoric, information.

 
   

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