Papers by Keyword: Pile-Up

Abstract: The Oliver–Pharr method has extensively been adopted for measuring hardness and Young’s modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. In this work, we characterize the pile-up (shape and height) in steel sheets with different mechanical properties and propose an improved methodology to calculate the real mechanical characteristics of steel sheets with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM).

Abstract: During the early stages of the plastic deformation of a polycrystal, dislocations can pile-up against grain boundaries. Experimental results on large-grained materials have provided excellent verification of this phenomenon. Such a pile-up may activate dislocation slip in the neighbouring grain. Whether this occurs depends on the misorientation between the grains and the resolved shear stresses in the affected grain. Several approximate criteria have been proposed to predict the occurrence of this mechanism. Here, the problem will be assessed directly by calculating the Peach-Köhler force produced by a single dislocation pile-up in one grain on all the possible slip systems in the neighbouring grain, in combination with the effect of the applied external stress as obtained through calculation of the Schmid factor. It will be seen that the problem is significantly more complex than what is generally assumed in basic explanations of the Hall-Petch effect: highly localised stress concentrations are generated for certain misorientations, which are capable of punching out small dislocation loops which may then propagate into the neighbouring grain.

Abstract: The Oliver–Pharr method has extensively been adopted for measuring hardness and elastic modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. This study proposes an improved methodology to calculate the real mechanical characteristics of individual phases in various steels with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM). The effect of grain boundaries on the shape and size of the pile-up lobes was also discussed.

Abstract: Mechanical properties by depth sensing indentation are derived from the indentation load-displacement data used a micromechanical model developed by Oliver & Pharr (O&P). However, O&P analysis on the indentation unloading curve is developed from a purely elastic contact mechanics (sink-in). The applicability of O&P analysis is limited by the materials pile-up. However, when it does, the contact area is larger than that predicted by elastic contact theory (material sinks-in during purely elastic contact), and both hardness H and Youngs modulus E are overestimated, because their evaluation depends on the contact area deduced from the load-displacement data. H can be overestimated by up to 60 % and E by up to 30 % depending on the extent of pile-up [1,2]. It is therefore important to determine the effect of pile-up on obtained mechanical characteristics of the material by depth sensing indentation. The work experimentally analyses the effect of pile-up height on mechanical characteristics H and E, which are determined by O&P analysis. Pile-up height was measured by atomic force microscopy (AFM).

Abstract: This paper presents the results of an experimental investigation on the effects of orientation and grain size on nanoindentation measurements of hardness and modulus of elasticity for three polycrystalline metals: copper, nickel, and iron. Three geometrically different indenter tips were used, and the pile-ups were characterized with a surface probe instrument. The electron backscatter diffraction (EBSD) technique and a scanning electron microscope (SEM) were used to characterize grain orientation and microstructure. It was found that additional contact areas due to pile-ups have a significant effect on determination of mechanical properties by the nanoindenter.

Abstract: Comparing advantages and disadvantages of Oliver-Pharr method, microhardness method and indentation energy method, this article propses the improvement of indentation energy method. The improvement of indentation energy method mainly takes the calculation of energy and pile-up and sink-in phenomenon into account. Using Abaqus to simulate the indentation of aluminum with Vicker indenter, process and analyse data from simulation . By comparing different methods of hardness calculation , analyse the effect of improvement of hardness calculation.

Abstract: Single-point fly cutting and nanoindentation test of quartz glass were performed using three different cutting tools, namely, a V-shaped cutting tool, a Vickers indenter and a spherical indenter, to investigate the elastic and plastic behaviors of quartz glass in ductile-regime machining. It was found that these behaviors depend on tool shape and that the V-shaped cutting tool is most effective for removing quartz glass material followed by the Vickers indenter and spherical indenter.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: The nanoindentation behaviours of single crystalline silicon samples has gained wide attention in recent years, because of the anomaly effects in the loading curve, caused by the pressure induced phase transformation of silicon. To further enlighten the phenomenon bulk, ion-implanted, single crystalline Si samples have been studied by nanoindentation and by atomic force microscopy. The implantation of Si wafers was carried out by P+ ions at 40 KeV accelerating voltage and 80 ions/cm2 dose, influencing the defect density and structure of the Si material in shallow depth at the surface. Our experiments provide Young’s modulus and hardness data measured with Berkovich-, spherical- and cube corner indenters, statistics of the pop-in and pop-out effects in the loading- and unloading process, and interesting results about the piling-up behaviour of the Si material.

Abstract: Although instrumented indentation machines have existed since the 1950s, it is only during the past 20 years or so that there has been a very rapid rise in their use and popularity for determining mechanical properties of bulk solids, thin films and biological materials. Nowadays, machines are available with which indentation experiments are conducted automatically and the data obtained are also analyzed automatically using software, which is based on certain hypotheses and simplifying assumptions. By carrying out several series of comprehensive experiments, using rigid and elastic indenters and elastic substrates, it is shown that the current methods of nanoindentation data analysis are fundamentally flawed, which can lead to incorrect and misleading results. A new experimental approach has been suggested here, which is free from such flaws.