Key Engineering Materials Vol. 586

Abstract: The acoustic or sonic pulse-propagation technique for the measurement of dynamic elastic modulus has the advantage of not being dependent on the sample cross-sectional characteristics. This technique also gives a direct measure of modulus rather than the indirect measure in the form of load versus extension. The sonic tests are relatively simple to apply and are nondestructive. The values of sonic modulus of fibrous structures are dependent on the orientation of components and packing density as well. The main aim of this work is to quantify effect of yarn twist on the sonic modulus of staple yarns from polypropylene fibers. The results are compared with selected models of yarn twist influence on the mechanical properties of staple yarns. The correlation between yarn orientation factor defined by Pan and sonic modulus are shown. The sonic modulus is compared with tensile modulus of yarns.

Abstract: The high temperatures induced during the welding process cause transient thermal stresses and non-continuous plastic strains around the weld. Uneven heating and cooling processes together with these plastic strains result in residual (welding) stresses, [1]. This paper deals with the development of plastic zones, related to welding stresses and their effects on the bearing capacity from a Civil-engineering perspective.

Abstract: The use of nanoindentation techniques enhances the capabilities of qualitative analysis of elasto-plastic characteristics, especially, estimating mechanical properties of relatively small specimens in their surface layers. The results are in agreement with macromethods, which gather the information over the higher volume of the material. It was confirmed, that hardening of double phase Ti-6Al-4V alloy by quenching from beta temperatures (above beta-transus), reduces the elastic modulus by about 8 % due to increased ratio of low-modulus beta phase from 8 to 34 %.

Abstract: Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

Abstract: The nanohardness of WC – Co hardmetals has been investigated using instrumented indentation and Berkovich tip indenter. The nanohardness, HIT, and indentation modulus, EIT, of Co phase and individual WC grains and the influence of their crystalographic orientation have been studied. SEM, AFM and EBSD methods were used for the characterization of the microstructures and indents and for the identification of crystallographic orientation of WC grains, respectively. Strong indentation load-size effect and significant influence of the crystallographic orientation of WC crystals on HIT and EIT have been found. The nanohardness of Co binder was approximately 10 GPa and that of WC grains varied between 25 and 50 GPa, depending on the grain orientation and load. The nanohardness values of the basal and prismatic planes of individual WC grains at load of 10 mN were 40.4 ± 1.6 GPa and 32.8 ± 2.0 GPa, respectively.

Abstract: As the hardest and one of the most durable load bearing tissues of the body, enamel has attracted considerable interest from both material scientists and clinical practitioners due to its excellent mechanical properties. The aim of this investigation is to determine the influence of different loading conditions on the deformation behavior of human enamel using instrumented indentation and Berkovich indenter. The used samples were fresh intact human premolars, extracted due to orthodontic reasons. Hardness tests were performed with different loading regimes. To study the influence of loading rates tree different loading rates have been used with 10, 100 and 1000 mN/min. at maximum applied load of 200 mN. The indentation size effect (ISE) was studied using loads from 5 mN to 400 mN without holding time. The indents have been studied using atomoc force microscopy and scanning electron microscopy.

Abstract: The most commonly used method for the analysis of instrumented indentation test (Oliver-Pharr) is based on isotropic elastic solution of contact problem which is not necessarily valid when indenting at the scale of one (anisotropic) grain. In this paper, we performed the grid indentation method at the sub-micron scale (at low indentation load and depth of penetration) on an area containing several grains with different crystallographic orientation which was simultaneously characterized by electron back-scattered diffraction. Measured dependencies of hardness and indentation modulus on crystallographic orientation were compared with analytical solution and finite element simulations.

Abstract: The nanoindentation and tribological measurements were performed on DC and RF magnetron sputtered W-C coatings to optimize the deposition conditions to obtain maximum nanohardness and to compare their properties and behavior with earlier studied PECVD W-C coatings. Despite number of similarities, some consequences of a tribo-chemical transfer film formation are inconsistent with the existing wear model for W-C coatings with controlled carbon content. Transfer films in the friction contact consist from iron- and tungsten oxides and carbon generated during carbide oxidation. The role of carbon in the transfer films leading to high coefficients of friction has to be principally different from the expected lubricative role of carbon in W-C coatings and needs further study.

Abstract: Final properties of formed steel or another alloy pieces are affected even by plastic deformation of material. Therefore it is needful to know detail structure changes of material under conditions of plastic deformation caused by forming, grinding, drilling etc. Estimation of the deformation based on its observable macroscopic effects doesn’t correspond fully with microscopic structural changes in whole volume of deformed parts and such estimation is quite impossible in case if only surface layers are deformed. It is possible to obtain value of strain by measurement of grain boundary deformation. Effect of grains boundaries self-orientation caused by grains deformation can be identified on metallographic cut. Stereological measurement of degree of grain boundary orientation is relatively simple if axes of orientation are known (as it is in most of deformation processes). Preferred direction of grains boundaries orientation is the same as direction of deformation; however orientation is not the same thing as deformation. Therefore model of conversion of degree of grain boundary orientation to deformation based on an idealized shape of grains has been proposed. This conversion model is independent on an initial grain size – strain depends only on the shape of the grain and does not depend on its dimension. It allows experimental estimation of local plastic deformation by means of measurement of grain boundary orientation in various areas of plastically deformed parts.