Pillar Compression Research Articles

Influence of bulk pre-straining on the size effect in nickel compression pillars

Micro-compression tests were performed on pre-strained nickel (Ni) single crystals in order to investigate the influence of the initial dislocation arrangement on the size dependence of small-scale metal structures. A bulk Ni sample was grown using the Czochralski method and sectioned into four compression samples, which were then pre-strained to nominal strains of 5, 10, 15 and 20%. Bulk samples were then characterized using transmission electron microscopy (TEM), micro-Laue diffraction, and electron backscatter diffraction. TEM results show that a dislocation cell structure was present for all deformed samples, and Laue diffraction demonstrated that the internal strain increased with increased amount of pre-straining. Small-scale pillars with diameters from 200nm to 5μm were focused ion beam (FIB) machined from each of the four deformed bulk samples and further compressed via a nanoindenter equipped with a flat diamond punch. Results demonstrate that bulk pre-straining inhibits the sample size effect. For heavily pre-strained bulk samples, the deformation history does not affect the stress–strain behavior, as the pillars demonstrated elevated strength and rather low strain hardening over the whole investigated size range. In situ TEM and micro-Laue diffraction measurements of pillars confirmed little change in dislocation density during pillar compression. Thus, the dislocation cell walls created by heavy bulk pre-straining become the relevant internal material structure controlling the mechanical properties, dominating the sample size effect observed in the low dislocation density regime.

Plastic deformation and residual stresses in amorphous silica pillars under uniaxial loading

We have carried out uniaxial compression of micron-scale amorphous silica pillars. We have measured load–displacement curves and observed the morphology of the pillars after unloading, providing strong evidence for large plastic deformations. Minor cracking is also observed, with a well-defined pattern. We find that the van Mises stress in compression is comparable to the intrinsic tensile strength of silica. Precise analysis of the deformation of the pillars has been carried out by finite element modeling (FEM) using the constitutive equation determined previously (G. Kermouche et al., Acta Materialia, 56 (2008) 3222), which quantitatively takes into account densification, shear flow and strain hardening. The residual stress distribution we predict by FEM matches the observed crack pattern well. Finally the calculated stress fields in pillar compression and cone indentation are compared. We propose an interpretation of the contrasts in terms of confinement.

MODELLING AND SIMULATION OF STRENGTH AND DAMPING OF THE SUPPORT PILLAR WELDED BY LONGITUDINAL WELD

In some fields of industry, such as stowage works, strongly safety assessment should be ensured, so it is important to design constructions, like platforms, which would withstand heavy static loading and, in emergences situations, to withstand the same bulk, dropped from the predicted height, with the ability to damp it. In order to perform a FEM structural analysis of such structure, the accurate material characteristics should be known. For this reason, four types of specimens - standard flat, tube contoured and the same ones containing longitudinal weld, welded in the longitudinal direction using electrodes CT36, were manufactured from a stainless 321 steel sheet and tested in order to define mechanical properties of the material. Using the obtained true stress-strain curves, the static analysis of pillar compression was carried out and the simulation of its behaviour, if a heavy weight structure will be dropped from a height of 5 meters, was performed. The static analysis showed that, when the static load is 40 tons, the pillar structure satisfies high safety requirements. The dynamic impact simulation showed that the most reasonable load for the pillar is 9 – 11 tons and, if it is not overloaded, the longitudinal weld does not influence an adverse effect and the pillar deforms smoothly without cracks.DOI: http://dx.doi.org/10.5755/j01.mech.18.2.1575

Free-surface enhanced continuum model predicts size-effect for pillar compression at micro- and nano-scale

Abstract It has recently been observed that micro- and nano-sized pillars under uniaxial compression and tension attain a higher flow-stress than samples of macroscopic size. Here it is hypothesized that such a mechanical size-effect is driven by the interaction of dislocations with the free surfaces. An extended continuum plasticity theory is presented to account for such an effect by considering the free-surface imagestresses on the dislocation motion immediately following dislocation nucleation. The model employs only fundamental material parameters, Young's modulus, Poisson's ratio, bulk yield-strength, and the Burgers vector. The model predicts the magnitude of the size-effects observed in the compression of Au and Cu micro- and nano-pillars.

The hardness and strength of metal tribofilms: An apparent contradiction between nanoindentation and pillar compression

After sliding contact of a hard spherical counterface on a metal surface, the resulting wear scar possesses a complex microstructure consisting of dislocations, dislocation cells, ultrafine or nanocrystalline grains, and material that has undergone dynamic recovery. There remains a controversy as to the mechanical properties of the tribolayer formed in this wear scar. To investigate the properties of this thin layer of damaged material in single crystal nickel, we employed two complementary techniques: pillar compression and nanoindentation. In both techniques, the tests were tailored to characterize the near surface properties associated with the top 500nm of material, where the wear-induced damage was most extensive. Pillar compression indicated that the worn material was substantially softer than neighboring unworn base metal. However, nanoindentation showed that the wear track was substantially harder than the base metal. These apparently contradictory results are explained on the basis of source limited deformation. The worn pillars are softer than unworn pillars due to a pre-straining effect: undefected pillars are nearly free of dislocations, whereas worn pillars have pre-existing dislocations built in. Nanoindentation in worn material behaves harder than unworn single crystal nickel due to source length reduction from the fine-grained wear structure.

Orientation dependence of stress-induced phase transformation and dislocation plasticity in NiTi shape memory alloys on the micro scale

Abstract NiTi shape memory alloys can be used as micro actuators and small scale pseudoelastic components. Therefore there is a need to characterize their mechanical properties on the micro scale. In several previous studies, such tests (nanoindentation, pillar compression) were performed for different NiTi alloys. However, no consistent results concerning the coupling between plastic deformation and martensitic transformation were obtained. Moreover it is unclear whether the material's response to loading on the micro scale reflects its large scale mechanical anisotropy. In this study, we investigate a binary, solution annealed precipitate free NiTi alloy and compress small pillars in –, – and –directions. Mechanical results are analyzed in the light of SEM and post-mortem TEM investigations. We identify deformation mechanisms and show that there is deformation anisotropy. We show that micro pillar testing yields results which are in good qualitative agreement with previous work from macroscopic investigations.

Deep Drawing Simulations Based on Microstructural Data

AbstractFor the optimization of process chains in sheet metal forming it is required to accurately describe each partial process of the chain, e.g. rolling, press hardening and deep drawing. The prediction of the thickness distribution and the residual stresses in the blank has to be of high reliability, since the subsequent behavior of the semi‐finished product in the following subprocesses strongly depends on the process history. Therefore, high‐quality simulations have to be carried out which incorporate real microstructural data [1,2,3]. In this contribution, the ferritic steel DC04 is analyzed. A finite strain crystal plasticity model is used, for the application of which micro pillar compression tests were carried out experimentally and numerically to identify the material parameters of DC04. For the validation of the model, a two‐dimensional EBSD data set has been discretized by finite elements and subjected to homogeneous displacement boundary conditions describing a large strain uniaxial tensile test. The results have been compared to experimental measurements of the specimen after the tensile test. Furthermore, a deep drawing process is simulated, which is based on a two‐scale Taylor‐type model at the integration points of the finite elements. At each integration point, the initial texture data given by the aforementioned EBSD measurements is assigned to the model. By applying this method, we predict the earing profiles of differently textured sheet metals. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Compression of micron-sized pillars of anodic aluminium oxide nano-honeycomb

Micro-pillars of anodic aluminium oxide with nano-sized honeycomb channels along the pillar axis exhibit compressive stress–strain response with large excursions corresponding to discrete, inhomogeneous deformation events. Each excursion is found to associate with the severe distortion of a material layer at the pillar’s head, whereas the remaining of the pillar remains intact. The stresses at which these excursions occur do not exhibit any significant dependence on the pillar size. A simple model is proposed to describe the response of pillars under compression, which energetically, as well as kinetically, explains as to why the localized deformation always takes place at the pillar head. Predictions on the occurrence of instability events from this model also quantitatively agree with the experimental observations.

Size Independent Shape Memory Behavior of Nickel–Titanium

AbstractWhile shape memory alloys such as NiTi have strong potential as active materials in many small‐scale applications, much is still unknown about their shape memory and deformation behavior as size scale is reduced. This paper reports on two sets of experiments which shed light onto an inconsistent body of research regarding the behavior of NiTi at the nano‐ to microscale. In situ SEM pillar bending experiments directly show that the shape memory behavior of NiTi is still present for pillar diameters as small as 200 nm. Uniaxial pillar compression experiments demonstrate that plasticity of the phase transformation in NiTi is size independent and, in contrast to bulk single crystal observations, is not influenced by heat treatment (i.e., precipitate structure).

Research Trends for Mechanical Characterization of MEMS Materials in Korea: Research Needs and New Developments

AbstractThe mechanical properties of microelectromechanical system (MEMS) materials are the basic parameters for designing MEMS devices and evaluating their reliability. The methods used to measure them, however, are not yet well established, and several unresolved issues require further investigation. In this paper, we introduce recent work on the mechanical characterization of MEMS materials in Korea with an emphasis on research needs and new developments. After a description of the research needs of MEMS industries, research activities on tensile and fatigue tests are introduced. Because of a strong interest in simpler test methods in Korea, their concepts and technical issues are described, especially for indentation, strip‐bending, and pillar compression tests. Some techniques based on atomic force microscope (AFM) are also introduced as new research activities. © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Rock mass—mine workings interaction model for Polish copper mine conditions

Geological and mining conditions characteristic for the Polish Legnicko–Glogowski Okreg Miedziowy (LGOM) copper mines as well as various exploitation systems utilized in that area are presented. The historical background of mining systems development as well as their classification are also reviewed. Presently, due to difficult mining conditions, a new more universal analytical tool for mine workings geometry selection is required. Therefore, a very useful in design process multi-plate analogy based physical model of rock mass—mine workings interaction has been developed and presented in the paper. The model utilizes the analytical approximation of pillar compression including the effect of pillar critical and residual strength on roof strata deflection resulting in the definite bump hazard level. The research performed in the past which dealt with the effect of size and slenderness on pillar strength is reviewed. A new approach for pillar strength calculation and its utilizing in a general problem of overburden–mine workings interaction is proposed. The problem has been illustrated by numerical examples concerning a rock mass static and dynamic behavior in the area of one of the underground Polish copper mines. By solving the numerical model based on the finite element method formulated in three dimensions, the effect of extraction path on mine workings safety in a static load domain has been proved. The dynamic load transfer from mine workings to a shaft lining has also been determined.

Numerical analysis of elastic-plastic compression of pillars accounting for material hardening and softening

Numerical analysis of elastic-plastic compression of pillars accounting for material hardening and softening

Numerical analysis of elastic-plastic compression of pillars accounting for material hardening and softening

An elastic-plastic model of rock or soil is assumed for which the yield condition depends on hydrostatic pressure and varying material density or porosity. The problem of compression of a finite rectangular pillar between two rigid plates is treated numerically in plane strain conditions by applying the ‘initial stress’ finite element incremental technique. The development of softening and hardening zones within the pillar is studied and related to the load carrying capacity of the pillar.