The effects of systematic variations in the size and volume fraction of reinforcing phase on the mechanical properties of and fracture processes in silicon carbide particlereinforced aluminium matrix composites have been studied. Tensile tests to failure have been performed to determine the mechanical properties of the composites. A simple model has been proposed for this behaviour. The micromechanisms of fracture have been investigated by a combination of fractographic and dynamic techniques. Matched fracture halves have been obtained from the composites and the fracture processes elucidated. Fracture proceeded by a ductile void nucleation, growth and coalescence mechanism. Void nucleation occurred at the reinforcing phase, with a change in nucleation mechanism on varying the micrstructural parameters. A simple critical stress criterion has been proposed for the nucleation process. Support for this proposal has been obtained by the study of sections through the failed tensile specimens. In situ scanning electron microscopy fracture studies have been performed. These revealed void nucleation before the onset of macroscopic cracking. Crack propagation has been shown to occur by the concurrent formation of microcracks ahead of the crack tip and failure of the joining matrix ligaments. The magnitude of matrix deformation has been shown to determine the extent of microcracking. Acoustic emissions have been monitored during tensile straining. Void nucleation events have been recorded from the onset of plastic deformation and continuing throughout the plastic régime until final failure. The suppression of void coalescence by the constaint imposed on matrix flow by rigidly-bonded interfaces has been proposed to account for the extended void growth in materials containing fractured particles. The importance of the local values of the microstructural parameters on the far-field strain at nucleation has been shown.

<p>U disertaciji je predstavljen postupak spajanja topljenjem nezavarljive legure aluminijuma EW AN 7049A u stanju T 652 postupkom zavarivanja trenjem -<br />miješanjem. Korišćenjem pet različitih tipova alata za zavarivanje, varijacijom brzine zavarivanja i broja obrtaja, zavareni spoj dostiže efikasnost od 70% do 80 % zatezne čvrstoće osnovnog materijala. Karakterizacija zavarenog spoja izvedena je standardnim metodama za eksperimentalno određivanje energije udara i udarne žilavosti na Charpy epruvetama, a ispitivanja mehanike loma zavarenih spojeva savijanjem u tri tačke SENB epruveta (ASTM E 1820), poprečno na pravac zavarivanja u zoni najviše pogođenom termomehaničkim uticajem alata.<br />Kriterijumima elasto-plastične mehanike loma ( CTOD-otvaranje vrha prsline i J-<br />integrala) utvrđena je veća otpornost prema širenju prsline u različitim mjestima zavarenog spoja (centru, strani napredovanja i na povratnoj strani metala šava) nego u osnovnom materijalu.</p> / <p>The thesis presents the procedure of joining by melting of unweldable alloy of Al EW AN 7049A of status T 652 by the procedure of friction-stir welding. By using five various types of tools for welding, variation of welding velocity and number of revolutions, the welded joint shall reach the efficiency of 70% -80% of tensile strenght of basic material. Characterization of welded joint was made by standard methods for experimental determination of impact energy and impact toughness on Charpy tubes, and breakage mechanical properties by bending in three points of SENB tubes (ASTM E 1820), transversely to direction of welding in the zone of highly affected thermo mechanical impact of tool. Criteria of elastoplastic mechanics of the fracture (CTOD crack opening displacement and J-integral) determined bigger resistivity to cracking at various locations of welded joint (center, side of advancing and on retreating side of the seam metal) than on the basic material.</p>

The usage of more inexpensive silicon feedstock for the crystallization of multicrystalline silicon blocks promises cost reduction for the photovoltaic industry. Less expensive substrates made out of metallurgical silicon (MG-Si) are used as a mechanical support for the epitaxial solar cell. Moreover, conventional inert solar cells can be produced from up-graded metallurgical silicon (UMG-Si). This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Solar cell processing steps can also exert a change in the values of mechanical strength of processed multicrystalline silicon wafers until the fabrication of a solar cell. Bending tests, fracture toughness and dynamic elastic modulus measurements are performed in this work in order to research the mechanical behavior of multicrystalline silicon crystallized with different qualities of silicon feedstock. Bending tests and residual stress measurements allows the quantification of the mechanical strength of the wafers after every solar cell processing step. The experimental results are compared with theoretical models found in the classical literature about the mechanical properties of ceramics. The influence of second phase particles and thermal processes on the mechanical strength of silicon wafers can be predicted and analyzed with the theoretical models. Metals like Al and Cu can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, amorphous silicon oxide particles (SiOx) lower the mechanical strength of multicrystalline silicon due to thermal residual stresses and elastic mismatch with silicon. Silicon nitride particles (Si3N4) reduce fracture toughness and cause failure by radial cracking in its surroundings due to its thermal mismatch with silicon. Finally, silicon carbide (SiC) and crystalline silicon oxide (SiOx) introduce thermal residual stresses but can have a toughening effect on the silicon matrix and hence, increase the mechanical strength of silicon wafers if the particles are smaller than a certain size. The surface of as-cut wafers after multi-wire sawing presents sharp micro-cracks that control their mechanical behavior. Subsequent removal of these micro-cracks by texture or damage etching approximately doubles the mechanical strength of silicon wafers. The mechanical behavior of the wafers is then governed by defects like cracks and particles formed during the crystallization of multicrystalline silicon blocks. Further thermal processing steps have a minor impact on the mechanical strength of the wafers compared to as-cut wafers. Finally, the mechanical strength of final solar cells is comparable to the mechanical strength of as-cut wafers due to the high residual thermal stress introduced after the formation of the metallic contacts which makes silicon prone to crack.

The diploma thesis is focused on the evaluation of mechanical fracture parameters of concrete after exposure to high temperatures. In the introductory theoretical part general principles of fracture mechanics with the concentration on a linear elastic fracture mechanics and non-linear fracture models for the concrete are summarized. The meaning of the three-point bending fracture test used for determination of fracture parameters is also explained. Further the influence of high temperatures on the partial components of concrete and general modeling of temperature loading is described. The practical part is concerned with the evaluation of fire experiments on the concrete panels including numerical simulations using GiD and ATENA software. The evaluation of data obtained from the three-point bending test carried out on specimens with initial stress concentrator taken from concrete panels is a main part of the diploma thesis. The values of modulus of elasticity, effective fracture toughness, work of fracture and fracture energy are determined from the measured F–d and F–CMOD diagrams after their proper corrections in the GTDiPS application. The evaluation of the selected mechanical fracture parameters was performed by StiCrack software using effective crack model and work of fracture method and DKFM_BUT software using the double-K fracture model. Finally, the attention is paid to the analysis of the obtained data.