Key Engineering Materials Vols. 592-593

Abstract: Micromechanical calculations and elasticity standard relations are used to predict the elastic properties of porous alumina, zirconia and kaolin-based ceramics, as well as the high-temperature Young moduli of alumina-zirconia and alumina-mullite composites. The predictions are compared with experimental results obtained via impulse excitation. It is found that the Young moduli of highly porous (cellular) alumina ceramics can be predicted via the Gibson-Ashby power-law relation, whereas for partially sintered kaolin-based ceramics our exponential relation, albeit better than the Gibson-Ashby relation, does not give a satisfactory prediction. However, once the Young moduli are known, the shear and bulk moduli can be reliably predicted in both cases, based on rough information on the Poisson ratio. The temperature dependence of the Youngs moduli of two-phase composites can be quite precisely predicted as soon as the master curves of the constituent phases and the type of porosity (convex, concave, or saddle-point) are known.

Abstract: A Standing Contact Fatigue (SCF) test set up has been developed in order to facilitate quick testing of contact fatigue resistance of material surfaces. In this method the sample is pressed against a hard ball rapidly and the resulting crack formation is studied in order to evaluate the SCF resistance. Induction hardened surfaces of cam-ring steel and steels with pearlitic, martensitic, bainitic, ausferritic and quench and tempered microstructures have been studied. Characterization was performed by optical microscopy, scanning electron microscopy and hardness measurements. Ring-cone cracks were found at the edge of the indentations but inside the indent in the surface hardened cam-ring steel samples. Sectional views revealed that these cracks also grow underneath the indentation. Radial cracks were found in non-surface hardened samples. The test of the SCF resistance of steels with different microstructures showed that the ausferritic microstructures tested shoved better SCF resistance than the quench and tempered samples with similar hardness. A comparison between different tempering temperatures of surface hardened steels showed that samples tempered at the higher temperature 240 °C resulted in better SCF resistance.

Abstract: Eutectic Al-Si alloys are typically used for the production of internal combustion engine pistons. A high-cycle, high-temperature fatigue characterization of AlSi12 alloy performed using specimens extracted from actual pistons is presented and discussed. Fatigue strength at 10⁷ cycles were obtained at test temperatures of 250 °C, 300 °C and 350 °C. The fatigue strength reduction was quantified. The micro structural features were quantified by quantitative metallography and fatigue fracture surfaces inspected to identify the initiation causes.

Abstract: Fatigue crack growth (FCG) rates in an EA4T railway axle steel heat treated by two different methods in near threshold and stable regions of growth were evaluated. Quite significant differences were observed, when the obtained results were compared with those published in the literature. Participation of the laboratory in an Exova (GE Aviation) FCG measurement qualification round robin programme with very good results practically excluded errors in the experimental methodology used. Strength of the two different evaluated series of the experimental material was equivalent. Nevertheless, there were substantial differences in fatigue crack growth rates, about 5-times in stable FCG region and even more than 10-times in the near threshold region, when oil quenching and air hardening treatments were compared. The differences were explained by different microstructures of the two groups of materials. Some minor differences between the character of the FCG curve in the threshold region evaluated using SEN(B) and M(T) specimens, published in the literature, are discussed considering crack closure phenomenon.

Abstract: The correlation between fracture toughness parameter K_Jc and cleavage initiation distance (CID) for the three point bending (3PB) pre-crecked Charpy type specimens of VVER-1000 reactor pressure vessel base and weld metals was observed. Two types of brittle fracture origin sites were found: nonmetallic inclusions and grain or subgrain boundaries. It was shown that K_Jc values are shifted to the higher temperature area for weld metal with respect to base metal data. In case when the initiation origin is grain or subgrain boundary, the K_Jc values are higher for base metal at the same CID values. This indicates the higher crack resistance of base metal.

Abstract: Rapidly solidified aluminium alloys have many interesting properties such as higher thermal stability and strength, when compared with conventional cast alloys. Due to these properties, RS alloys seem to be prospective for using in automotive or aircraft industry. Aim of this work was to compare the differences in microstructure of alloys containing Fe, Ni and Cr which were prepared by different solidification rate. Alloys were prepared by melt spinning, melting with follow-up quenching into the water and by conventional casting with pouring into brass mould. Microstructure of prepared alloys was investigated by scanning electron microscope; phase composition was determined by x-ray diffraction. In this experiment, microhardness was measured in the initial state of all types of alloys; rapidly solidified alloys were also annealed to determine thermal stability by microhardness measurement. Results indicate that higher solidification rate refines the microstructure which is composed of supersaturated solid solution of alloying elements in aluminium and stable and meta-stable intermetallic phases. Hardness of the alloys increases and microstructure refines with solidification rate.

Abstract: Low cost, low material waste and good accuracy in components with complex geometry are the main reasons for powder metallurgy to be considered as a promising manufacturing process for the future. Like wrought steel, sintered steel can also be heat treated to increase surface hardness and to improve strength. This paper compares mechanical properties of the hardened sintered steel with the sintered steel of the same powder metal SINT D30. Firstly, the static strength of both samples is determined by quasi-static tensile tests. Results are compared in stress strain diagram and they show that the tensile strength of the hardened sintered steel SINT D30 can surpass 700 MPa. The main focus of this study is however fatigue behaviour of the sintered steel. Both sets of samples are tested on a pulsating test machine with the load ratio of R = 0. The first sample is subjected to a load that corresponds to 90 % of the yield strength and is then gradually lowered to achieve one million stress cycles without breakage. Obtained results are then presented as Wöhler curves and compared in S-N diagram.

Abstract: Cement – polymer composites are nowadays widely used in repair systems not only in case of concrete or reinforced concrete constructions but also in masonry. Polymers addition for example already at 5% m.c. modifies the structure of the cement – polymer composite in a way that many of the mechanical properties such as flexural strength, tensile strength or adhesion to substrates are improved. The paper presents the results of tests such as flexural, compressive or adhesion strength to ceramic substrate of hardened cement mortars with different composition, as well as selected cement mortars modified by two polymers: polyvinyl alcohol and styrene – butadiene polymer dosed at 5 % m.c. Four types of cement mortars modified by lime (component used in historical constructions as well as in contemporary masonry mortars) are also examined for comparison.

Abstract: The influence of fly ash quality and quantity on abrasion resistance of hardened concretes with siliceous fly ash addition is analysed in the paper. Abrasion resistance was measured in two standard tests according to EN 1338: 2005: reference test of the Wide Wheel and alternative test of the Bohme. Cement was replaced with 20, 35, and 50% of Class F siliceous fly ash in three categories of losses of ignition A, B and C by mass. The water to binder ratio, the air-entraining and the workability of mixtures were maintained constant at 0.38, 4.5% and 150 mm respectively. Test results indicated that in both methods, all tested concretes according to EN 1338: 2005 could be classified to 4-the highest class of abrasion resistance. In reference test of the Wide Wheel fly ash quality and quantity not influences abrasion resistance. However, in alternative, Böhme test abrasion resistance lowering with growth quantity of fly ash in binder, while loss of ignition of fly ash no influenced abrasion resistance. There were no correlation between the abrasion resistance and the compressive strength.

Abstract: In the present work, high density polyethylene based composites filled with glass spheres, talc and calcite particles were prepared. Fillers contents in the HDPE were 5, 10, 15, and 20 wt%. The mechanical, morphological and tribological properties of the polymer composites were investigated. Substantial improvements in the some mechanical properties were obtained by the addition of filler. For example, the results showed that the elasticity modulus of composites improved with increasing the filler content. The addition of fillers to the HDPE changed significantly the friction coefficient and wear rate of the composites. HDPE filled with a high level content of fillers showed higher wear rate than pure HDPE under dry sliding. The structure and properties of the composites are characterized using a scanning electron microscopy (SEM).