Key Engineering Materials Vol. 586

Abstract: The spherical indentation response of pressure vessel reactor steel with austenitic cladding is investigated both experimentally and numerically. The instrumented indentation test was performed for both materials at a sufficient distance from the bi-material interface, thus the results can be compared with the bulk data obtained from the standard tensile and compression tests. The stress – plastic strain curve for austenitic cladding estimated by a simplified inverse analysis of the indentation load – penetration curve is shifted to a harder response compared with that determined from the tensile test. One of the possible reasons, anisotropy of the cladding metal, was experimentally observed during the compression tests performed in the longitudinal orientation of the tensile test specimens and in the transverse orientation identical with the direction of the material indentation.

Abstract: The paper summarizes principles of similarity and shows where they loose their validity in material tests by indentation and what should be considered in their preparation and evaluation.

Abstract: Indentation hardness of homogeneous materials should be constant. However, at very small depths, the apparent hardness often increases with decreasing imprint size. The paper discusses various cases of this indentation size effect in metals and ceramics and explains the extrinsic and intrinsic reasons.

Abstract: Nowadays, nanoindentation is commonly applied to various materials to assess micromechanical properties. Often, exact microstructure of the material building blocks is not properly analyzed which may introduce large discrepancies in the data obtained from different tests. It is shown in the paper, that different deformation mechanisms in tension and compression take place for the tested materials which is demonstrated by large differences between the measured nanoindentation moduli and macroscopic tensile elastic moduli. The situation is illustrated on several types of biological and man-made fibers. Differences ~44-57% in elastic moduli evaluated from the two tests appear in case of biological fibers, ~68% difference was found for high strength PVA fibers and 767% (!) for carbon fibers.

Abstract: This paper presents a case study of instrumented indentation for assessment of mechanical properties of a specific Almigo Hard aluminium alloy. It shows that conventional microhardness is not suitable for local testing of this material and that instrumented indentation can reveal significantly more information about the tested material such as Young’s modulus and elastic-plastic characteristics. The study compares mechanical and elastic-plastic properties of the Almigo Hard, aircraft Al alloy and reactor vessel ferritic steel by single peak load and cyclic indentations in order to demonstrate superior properties of the Almigo Hard alloy.

Abstract: The article is focused on local stress simulation as well as local deformation analysis in a cylinder of piston combustion engine. Design of the exhaust pipe system has to fulfill special requirements with regard to the combined loading due to high pressures and temperatures. There is a special role of the exhaust pipe barrier, which is situated in the exhaust gas pipe. This barrier separates the main air hole from the secondary lateral auxiliary air holes. The exhaust pipe barrier seems to be a critical point of the cylinder. The finite-element analysis applied on the silumin-cast cylinder is presented in the paper in order to prove the critical places with high stress concentration. The local strength of the cylinder material was estimated using a material model based on relation between the strength and the mean distance between the secondary dendrite arms (SDAS) in the as-cast microstructure. Due to local heterogeneities there are differences in the local strength value at the various places of the microstructure. The simulation of stress distribution was performed by use of the finite-element method. Based on the relation „local stress/local strength“ the individual critical places in the cylinder were defined and located.

Abstract: This research work aims to model the failure initiation in dual-phase (DP) steel. A microstructure based approach by means of representative volume elements (RVE) is employed to evaluate the microstructure deformation and the failure initiation on the mesoscale. In order to determine cohesive parameters for martensite cracking, a two level approach has been performed experimentally. First, in-situ bending test in SEM with EBSD measurements before and after the test showed that the crack initiation occurs in martensite islands. Then, mini tensile tests with DIC technique were carried out to identify macroscopic failure initiation strain values. RVE modeling combined with extended finite element method (XFEM) was utilized to model martensite cracking on mesoscale. The identified parameters were validated by comparing the predictions with the experimental results.

Abstract: Sandwich construction is a composite material structure combining low weight, high strength and good dynamic properties. Typically a sandwich composite consists of three main parts: two thin, stiff and strong facing layers separated by a thick, light and weaker inner core. The faces are adhesively bonded to the core to obtain a load transfer between the components. By this way the properties of each separate component is utilized to the structural advantage of the whole assembly leading to a very high stiffness-to-weight and high bending strength-to-weight ratio. As a result sandwich components achieve the same structural performance as conventional materials with less weight. The material characterization described in the paper is aimed at the structural design of the end cub for a high speed train made of composite sandwich materials. A sandwich structure was considered, made of glass fibber polyester face sheets with a polymeric foam core. Initially, the material properties and the rate sensitivity of the skin and core materials were investigated through a series of static and quasi-static tests. Static and dynamic impact tests were then run on the sandwich structure. For all materials tested, no significant strain-rate effects were observed over the range of test conditions investigated in the study. Results show that the structural response of the sandwich depends primarily on the strength properties of the foam core material. Sandwich peel test is intended for determining the comparative peel resistance of adhesive bonds between facing and cores of sandwich constructions tested under specified test conditions. One method, the climbing drum peel method, is most applicable when the peeled facings are relatively thin, but it can not be applicable in tested composite structures. Peel resistance was tested by modified experimental setup.

Abstract: At the present time bonding has spread into almost all sectors of practice and it would be very difficult to find an industry in which there is no need to use this technology of joining a wide range of materials. In comparison with conventional joining methods (riveting, welding and screwing) provides bonding new combination of options and allows obtaining special shapes and properties which cannot be created by conventional methods of coupling. For the formation of quality bonded joint it is important that the adhesive bonding surface is well wetting. Wettability is characterized by the contact angle of wetting. The liquid must have a lower surface tension than the solid in order to be able to wetting the solid substance. This article describes the effect of beta irradiation on the contact angle of wetting, on the surface energy and on the final strength of bonded joints of HDPE.

Abstract: This work presents an investigation of basic mechanical properties (hardness, Young modulus) of metal matrix composites (MMC) reinforced with non-oxide ceramic nanoparticles by means of nanoindentation measurements. The evaluated materials were manufactured by selective laser melting (SLS/M). As a matrix 316L stainless steel and as a reinforcing phase TiC nanoparticles were used. The influence of nanoscale reinforcement on the mechanical properties of MMC manufacturing with SLS/M process was examined. In this case statistical evaluation of hardness and Young modulus based on nanoindentation data was performed.