Volume 54, Issue 4

Kurzfassung Hochfeste Stähle mit einer kubisch raumzentrierten Gitterstruktur können bereits bei sehr geringen Wasserstoffgehalten zur verzögerten Rissbildung neigen. Zum sicheren Einsatz dieser Stähle ist deshalb die Kenntnis des enthaltenen diffusiblen Wasserstoffes notwendig, der sich schadensursächlich auswirken kann. In diesem Beitrag wird die Funk­tionsweise eines neuen Wasserstoffanalyseverfahrens beschrieben, welches sich in der Verfahrensabfolge von konventionellen Verfahren, wie der Trägergasheißextraktion bzw. Trägergasschmelzextraktion, unterscheidet. Bei dem entwickelten Verfahren, der sogenannten Hydrogen Collecting Analysis (HCA), werden die zu analysierenden Proben in Glasküvetten eingeschmolzen und die Wasserstoffextraktion aus der Probe und die Wasserstoffanalytik in zwei konsekutive Verfahrensschritte unterteilt. Mit dem HCA-Verfahren können die Anteile an diffusiblem Wasserstoff in Metallproben im Hundertstel-ppm-Bereich hochgenau analysiert werden.

Fracture toughness (K IC ) of high purity cold isostatically pressed alumina ceramic (CIP-Al 2 O 3 ) was determined from the size of cracks induced by Vickers hardness testing at wide range of loads. The observed cracks appeared at following loads: 4.905 N, 9.81 N, 29.43 N and 49.05 N. The studied material exhibits half-penny crack system at lower load of 4.905 N, but it proceeds as the Palmqvist crack system at higher loads, which was determined computationally. For the fracture toughness calculation different models were compared. It was found that fracture toughness increases by rising applied load for all kind of models, which is explained by the indentation size effect. As the fracture toughness overestimation can lead to product malfunction, it is suggested to use the Niihara model, which gives the smallest fracture toughness values.

The effect of pin penetration depth on the mechanical properties of friction stir spot welded (FSSW) aluminum alloy 1050 and pure copper has been investigated. For a certian tool geometry, constant tool rotation speed and penetration time, the materials have been friction stir spot welded in three different penetration depths, namely 2.8 mm, 4 mm and 5 mm. Tensile shear tests, microhardness measurements and micostructural investigation have been carried out on the welded specimens in order to investigate the effect of penetration depth. 2.8 mm plunge depth has resulted in a weak joint, whereas 4 mm and 5 mm plunge depths offer acceptable tensile shear results. Presence of intermetallic phases had an influence on the tensile shear and hardness values.

In this study, microstructural and mechanical properties of pure aluminum, 5083 and 7075 alloys joined by friction stir welding were investigated. Hardness, tensile, bending and impact tests were applied to the welded samples. In addition, optical and SEM tests were carried out. The effects of welding speed on microstructure and mechanical properties were investigated in these materials. Then, the optimal conditions for friction stir welding were determined for pure aluminum, 5083 and 7075 alloys. The maximum hardness was observed for 7075 while the minimum hardness was observed for pure aluminum.

Production of new types of effective and green softeners for polyvinylchloride (PVC) is one of the important concerns in chemical industry. The paper reports results of investigations on obtaining symmetric esters of a series of fatty acids and triethylene glycol (TEG) and their use as effective PVC plasticizing agents. The esterification process was conducted in the presence of nano-grade titanium dioxide Degussa P-25 and specified by environmentally sound technology. Good compatibility and low volatility of the bio-based type esters in PVC formulations were shown and higher physico-mechanical features of the PVC composites were established. The esters may be recommended as effective and environmentally friendly PVC softeners to replace commonly used phthalates.

Recently, nano-materials have received increasingly more attention for their potential applications as structural and functional materials. The unique mechanical properties of CNTs, their high strength and stiffness and the enormous aspect ratio provide potentials for the improvement of the mechanical properties of structural materials. The aim of this research is to investigate experimentally the effect of nanocom­posite coating on the fatigue life of carbon steel AISI 1045 specimens with different surface finishes. Nanoadhesives of epoxy resin are synthesized and evaluated. They are originally modified by multiwalled carbon nanotubes (MWCNT) with 0.5 wt.-% as reinforcement. Fatigue tests are conducted on the respective specimens by a rotating bending machine of the cantilever type. Comparing the results for specimens coated with 0.5 wt.-% MWCNT-epoxy composition with the base materials it is found that fatigue life increased five times for a roughness of 0.3 µm and three times for an average specimen roughness of 0.8, 1.6 and 2.5 µm, respectively.

In this study, the effect of pre-aging time and temperature on mecha­nical properties of aluminium alloys 6082 and 7075 has been investigated. Pre-aging has been applied at 50, 60, 70 and 80 °C for 24, 72, 144 hours after solution treatment process at 560 °C for 1 hour. Then these samples have been subjected to an aging process at 180 °C and 125 °C. The hardness of samples has been measured during aging process regularly. Eventually, it was determined that if pre-aging process is performed at low temperature, an increase of hardness occurs and hardness decreases with increasing temperature during aging.

Nickel titanium shape memory alloy and stainless steel orthodontic wires were exposed to three different artificial saliva solutions which are modified Fusayama, modified Fusayama with 1g/l NaF addition and modified Fusayama with 1.7 % H 3 PO 4 addition. Under conditions similar to oral cavity temperature, corrosion behaviours were observed by using weight loss method and by macro surface photographic observations. Also, surface roughness of wires were investigated and compared. Consequently, it was determined that corrosion rates of all NiTi wires exposed to the modified Fusayama solutions with NaF addition are higher because of galvanic interaction of NiTi and hydrogen embrittlement of titanium. Also, surface roughness of NiTi wires were increased in modified Fusayama solution with NaF addition.

The present paper focuses on two techniques, namely regression and neural network, for predicting tool wear. Predicted values of tool wear by both techniques were compared with experimental values. Also, the effects of the main machining variables on tool wear have been determined. The metal volume removed (MVR) was taken as response (output) variable and cutting speed, feed rate, depth of cut and hardness were taken as input parameters, respectively. The relationship between tool wear and machining parameters was found out by direct measurement of the tool wear by MVR. The results showed the ability of regression and neural network models to predict the tool wear, accurately.

Adhesive bond strength depends on adherend, adhesive and interphase zone. Adhesive joint includes bonded areas or binding edges across bonded layer. Research of adhesive bond strength focuses on understanding the properties of bulky paper associated with formation of the interphase and adhesive joint failure mode. The key contributors of good adhesive bond strength are paper rough surface necessary for mechanical interlocking, adequate adhesive flow and adaptationfor intimate interaction and acceptable adhesive setting time. Change of parameters has strong influence on final adhesive joint formation. Adhesive bond strength assessment involves adhesive joint strength determination. Pull test method is used for adhesive joint failure mode characterization. Furthermore, adhesive joint strength results directly explain bulky paper bind ability by perfect binding technique. FOGRA recommendation rating directly confirms paperback binding quality with bulky paper application. Understanding of bulky paper bind ability involves comprehensive physiochemical knowledge of material (paper, adhesive). In addition, this binding technique is more unique because of its interest for adhesive joint formation.