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Generally, there are components loaded in a wide spectrum of loading rates, however most of design work is based on the data obtained using quasi-statical and uniaxial loading conditions. In the case of inherently brittle materials the situation is all the more complicated because of their brittleness. Cast basalt and soda-lime glass were the main experimental materials used in this investigation as representatives of natural based and structural brittle materials. The main aim of the paper is to investigate influence of strain rate on fracture resistance and to analyze response of the microstructure to high strain rate loading including the change of mechanical properties.

The self-crack-healing ability was systematically studied using flexural strength measurement of beams containing a semi-elliptical surface crack made by Vickers indenter. Beams of Al2O3/SiC ceramic nanocomposite with a good self-crack-healing ability predetermined for four-point and three-point bend test were prepared by ceramographic techniques. Flexural strength and fracture toughness were determined using three- and four-point bend method. Thermal aging in range from 1000 °C up to 1300 °C for 1 h was applied to find optimal conditions to heal the samples pre-cracked by the indentation technique. Fractographical analyses using an optical and/or scanning electron microscopy has been carried out to investigate the surface crack behaviour. Very good self-crack healing ability has been proved for surface micro-cracks having length up to 250 μm and depth no higher than 100 μm.

Fracture toughness is one the most important parameters for design applications and performance assessment of WC–Co cemented carbides (hardmetals). Different from hardness, fracture toughness is commonly a property more difficult to evaluate, particularly in brittle materials. A large number of different testing methods have been introduced to evaluate toughness of hardmetals, but in general all of them have either theoretically debatable issues or important experimental difficulties. In this study, three different fracture toughness testing methodologies are investigated: three-point bending on Chevron notched specimen (“reference” baseline), Palmqvist indentation test, and Hertzian indentation method. The work is conducted in several cemented carbide grades with different microstructures, in terms of both WC grain size and Co binder content. It is found that Chevronnotched three-point bending test yields suitable fracture toughness values for a wide range of cemented carbide grades with varying hardness. Concerning indentation methods, the Hertzian one may be particularly recommended, as compared to Palmqvist method, as far as hardness (HV30) drops below 1300. On the other hand, if HV30 is higher than 1300 Palmqvist indentation procedure yields reliable fracture toughness measurements. Experimental findings are finally analyzed and discussed on the basis of two theoretical models proposed in the literature. Qc 20150224 Master's Thesis project