Compressive Hydrostatic Stress Research Articles

Novel Size and Surface Oxide Effects in Silicon Nanowires as Lithium Battery Anodes

With its high specific capacity, silicon is a promising anode material for high-energy lithium-ion batteries, but volume expansion and fracture during lithium reaction have prevented implementation. Si nanostructures have shown resistance to fracture during cycling, but the critical effects of nanostructure size and native surface oxide on volume expansion and cycling performance are not understood. Here, we use an ex situ transmission electron microscopy technique to observe the same Si nanowires before and after lithiation and have discovered the impacts of size and surface oxide on volume expansion. For nanowires with native SiO(2), the surface oxide can suppress the volume expansion during lithiation for nanowires with diameters <∼50 nm. Finite element modeling shows that the oxide layer can induce compressive hydrostatic stress that could act to limit the extent of lithiation. The understanding developed herein of how volume expansion and extent of lithiation can depend on nanomaterial structure is important for the improvement of Si-based anodes.

Pneumatic pressure bioreactor for cyclic hydrostatic stress application: mechanobiology effects on periodontal ligament cells

A bioreactor system was developed to provide high-amplitude cyclic hydrostatic compressive stress (cHSC) using compressed air mixed commercially as needed to create partial pressures of oxygen and carbon dioxide appropriate for the cells under investigation. Operating pressures as high as 300 psi are achievable in this system at cyclic speeds of up to 0.2 Hz. In this study, ligamentous fibroblasts from human periodontal ligaments (n = 6) were compressed on two consecutive days at 150 psi for 3 h each day, and the mRNA for families of extracellular matrix protein and protease isoforms was evaluated by real-time PCR array. Several integrins were significantly upregulated, most notably alpha-3 (6.4-fold), as was SPG7 (12.1-fold). Among the collagens, Col8a1 was highly upregulated at 53.5-fold, with Col6a1, Col6a2, and Col7a1 also significantly upregulated 4.4- to 8.5-fold. MMP-1 was the most affected at 122.9-fold upregulation. MMP-14 likewise increased 17.8-fold with slight reductions for the gelatinases and a significant increase of TIMP-2 at 5.8-fold. The development of this bioreactor system and its utility in characterizing periodontal ligament fibroblast mechanobiology in intermediate-term testing hold promise for better simulating the conditions of the musculoskeletal system and the large cyclic compressive stresses joints may experience in gait, exertion, and mastication.

Negative linear compressibility of hexagonal honeycombs and related systems

Materials exhibiting negative linear compressibility display the very unusual and unexpected property of expanding in at least one direction when placed under compressive hydrostatic stress. Here, it is shown that this property may be manifested by systems having high positive Poisson’s ratios (non-auxetic), including non re-entrant hexagonal honeycombs and wine-rack models where deformation primarily involves changes in the angles between the ribs of the structures.

Damage at negative triaxiality

Abstract The damage process under compressive hydrostatic stress in ductile metals has been observed in many experiments. However, no damage theory explains the physical mechanism of damage due to compressive hydrostatic stress in ductile metals. Recently, one possible physical mechanism of shear damage at low and negative stress triaxiality (The stress triaxiality is defined as ( ( 1 / τ eqv ) ( τ k k / 3 ) ) where τ eqv = ( 2 / 3 ) τ ′ : τ ′ , τ is the Kirchhoff stress tensor. Triaxiality in this paper refers to the stress triaxiality defined above.) was shown to be the development of tensile hydrostatic stress due to grain-to-grain interaction (Kweon, S., Beaudoin, A.J., McDonald, R.J., 2010. Experimental characterization of damage processes in aluminum AA2024-O. Journal of Engineering Materials and Technology 132.) Kweon (2009. Edge cracking in rolling of an aluminum alloy AA2024-O. Mechanical Science and Engineering, University of Illinois, Urbana.) proposed a mesoscale theoretical framework that can be used to quantitatively investigate the amount of shear damage at all triaxiality levels, incorporating the physical mechanism of shear damage. This theoretical framework is based on crystal plasticity and the theory of void growth due to hydrostatic stress. The damage process at negative triaxiality is particularly important since many industrial processes for metals involve a compressive hydrostatic stress state such as rolling. Using the mesoscale theoretical framework, damage at negative triaxiality is theoretically quantified. It is shown that damage does exist at small negative triaxiality, and that the shear deformation component drives damage in the small negative triaxiality regime.

Mechanobiological prediction of proximal femoral deformities in children with cerebral palsy

Children with cerebral palsy (CP) walk with altered gait and frequently exhibit proximal femoral deformities, such as anteversion and coxa valga. The objective of this research was to investigate the effect of specific gait patterns on the femoral morphology in CP. In this study, the mechanobiological principles were implemented on a 3D finite element (FE) model of the proximal femur in order to predict changes in morphology over time in healthy and CP children. This model relies on the assumption that cyclic octahedral shear stress promotes growth and cyclic hydrostatic compressive stress inhibits growth. Growth was simulated over 16 iterations, representing approximately 5 months of growth. The FE model predicts an increase in the femoral anteversion and coxa valga for CP loading conditions when compared with healthy ones. Understanding the role of loading in skeletal morphogenesis may help prevent bone deformities and improve function in children with gait abnormalities.

Elastic fields generated by a semi-spherical hydride particle on a free surface of a metal and their effect on its growth

The formation of a metal hydride is associated with a large increase of volume relative to the parent metal and therefore in large strain energies. Effects of elastic energy on the hydriding of metals are revealed in the microstructural evolution and kinetics of hydride growth on free surfaces. In the present work, we study in detail the elastic fields set up by a semi-spherical hydride particle growing at a free surface of metal with cubic symmetry, with and without an oxide layer. These systems combine geometric (structural) and material anisotropies.Three stages along the microstructural evolution on the surface of some hydride forming metals exposed to hydrogen at constant pressure were described experimentally. For these stages along hydride growth, correlations with the elastic fields are suggested as follows. (a) A hydride particle at the free surface generates regions of tensile and compressive hydrostatic stress in the surrounding matrix. This may induce a preferred nucleation of new hydrides and formation of clusters of hydrides precipitates, which is indeed observed experimentally. (b) Clustering, on the other hand, may contribute to the cease of growth due to competition on hydrogen. In addition, as the particle grows, changes in the stress fields may retard further diffusion from the surface and be another contribution to the cease of growth. (c) A growing hydride increases the stress in the oxide layer and may finely break it. Then the elastic energy per unit volume drops to its minimum value and the growth may accelerate. The formation of such “growth center” is favored for that hydride precipitate that grow alone and not in a cluster.

Ultraprecision ductile mode machining of glass by micromilling process

Glass is considered as a difficult-to-machine material because of its susceptibility to brittle fracture during machining. To avoid brittle fracture and achieve superior surface finish, glass must be machined in ductile mode. Mostly, ductile mode machining is performed by a single point cutting tool. This paper presents the results of an experimental study to achieve ductile mode machining of glass by micromilling process. Fracture-free slots have been cut in soda-lime glass workpiece by micro-endmilling. Experimental results have established that fracture-free slots can be machined in glass by micromilling process within the controlled set of cutting conditions. These cutting conditions are conducive to highly compressive hydrostatic stresses developed in the cutting zone to suppress the crack propagation during machining. The feed per edge and the axial depth of the cut have been identified as the critical factors for the ductile–brittle transition in microcutting of glass by milling process.

Manufacture of a β-titanium hollow shaft by incremental forming

Excellent mechanical properties and corrosion resistance combined with low weight qualify β-titanium materials for lightweight applications in aviation, automotive and energy engineering. Thus far, actual applications of these materials have been limited due to high material costs and limited processing knowledge. One approach for developing resource-efficient manufacturing methods is the application of incremental forming methods. This article focuses on the development of the incremental spin extrusion process, which creates hollow profiles from solid bars. This method allows hollow shape manufacturing with a much higher flexibility than other forming methods and a significantly improved material utilization in comparison to machining methods, such as deep hole drilling. Beta-titanium alloys basically have very good cold forming suitability and the resulting material properties can be controlled. The application of incremental forming methods with high hydrostatic compressive stress is a promising manufacturing approach. The β-titanium Ti-10V-2Fe-3Al material has an excellent combination of the properties strength, ductility and fatigue strength. In order to utilize these properties the forming conditions and the temperature control need to be optimized. The investigations show that the Ti-10V-2Fe-3Al material can be formed only in a narrow semi-hot forming temperature window. The paper describes the investigation and presents results on the design of partial forming process sequences, forming properties, microstructure formation and failure prevention. The process design objective is a very fine microstructure with a homogeneous secondary α-phase and very small grained β-phase.

Analysis of densification and swelling of solids using pressure dependent plasticity criteria

We consider certain constitutive laws for analyzing the elastic-plastic behavior of granular material, which is subjected to compressive hydrostatic stresses and concomitantly undergoing swelling. ...

Modification of single-walled carbon nanotubes and its influence on the interfacial strength in polymeric composites

Single-walled carbon nanotubes was purified and modified with dichlorocarbene, HNO3 or octadecylamine and incorporated in polymeric matrices of polycarbonate and epoxy. Exposing the composite material to a tensile strain and measuring the shift of the G′-band revealed that higher loads could be transferred to the HNO3-modified carbon nanotubes. Exfoliation was regarded as the main reason for the increased interfacial strength resulting in larger specific surface area and higher hydrostatic compressive stresses on the nanotube bundles.

Measurement of residual thermal stress in WC–Co by neutron diffraction

The temperature dependence of residual stresses in a WC–17.8vol.%Co cemented carbide was measured by neutron diffraction. The comparison of the WC lattice parameter within the WC–Co and within stress-free WC reference provides a measurement of lattice elastic strains and, using Hooke’s law, stresses. WC is found to be under hydrostatic compressive stresses of about −400 MPa at room temperature, which decrease monotonically with temperature to a near-zero value at 800 °C. Residual stresses in cobalt also decrease with increasing temperature, but show an apparent increase above 800 °C, which is attributed to an increase in lattice parameter due to W dissolution in the Co phase.

Formation of ultrafine-grained microstructure in HSLA steel profiles by linear flow splitting

Linear flow splitting is a new cold forming process for the production of branched sheet metal structures in integral style. It induces extremely high deformation degrees without formation of cracks in the split sheets due to hydrostatic compressive stresses. Investigations on a HSLA steel (ZStE 500) show the formation and fragmentation of a dislocation cell structure in the severely deformed regions of the steel sheet. This results in ultrafine-grained microstructures and improved mechanical properties, similar to SPD processes as Equal Channel Angular Pressing (ECAP) or High Pressure Torsion (HPT). EBSD measurements reveal a gradient in grain size with an increase in direction perpendicular to the surface, whereas micro hardness decreases in the same direction. Based on these results, basic principles of linear flow splitting and its expected potential are discussed.

Finite-element analysis of V-ring indenter mechanism in fine-blanking process

In fine-blanking process, V-ring indenter is very important in increasing the edge quality of the blanked part. Previous research in this field was mainly experiment-based; therefore, the theoretical mechanism was not clarified. In this study, the V-ring indenter mechanism was investigated using the finite element method. The mechanism and effects of the V-ring indenter could be theoretically clarified on the basis of material flow analysis and stress distribution. The application of the V-ring indenter increased compressive stress before the cutting phase, and significantly suppressed the increased rotation of material flow during the cutting phase. This, in turn, caused increased hydrostatic compressive stress, resulting in a larger smooth cut surface.

Stressed solid-phase epitaxial growth of ion-implanted amorphous silicon

The kinetics of stressed solid-phase epitaxial growth (SPEG), also referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial recrystallization, and solid-phase epitaxial crystallization, of amorphous (α) silicon (Si) created via ion-implantation are reviewed. The effects of hydrostatic, in-plane uniaxial, and normal uniaxial compressive stress on SPEG kinetics are examined in intrinsic (0 0 1)Si. Particular emphasis is placed on unifying the results of different experiments in a single-stress-dependent SPEG model. SPEG kinetics are observed to suffer similar exponentially enhanced growth rates in hydrostatic and normal uniaxial compressive stress. However, there are discrepancies between researchers in terms of the influence of in-plane stress on growth rates. Two different stress-dependent SPEG models are thus advanced, each with different physical bases. The model advanced by Aziz et al. proposes SPEG can be modeled as a single-atomistic process while the model advanced by Rudawski et al. suggests that stress influences the nucleation and migration processes of growth differently and that SPEG cannot be modeled as a single step. The basis for the Rudawski et al. model is based on the crystal island and ledge migration model of SPEG advanced by others. Morphological instabilities of the growing α/crystalline interface with in-plane compression are also addressed within the context of both the Aziz et al. and Rudawski et al. models. Finally, using the Rudawski et al. model, it is possible to examine, characterize, and isolate the different atomistic processes during growth. Calculation of the activation energies for nucleation and migrations processes suggests that the activation energy of 2.7 eV observed for the growth rate in stress-free SPEG by Olsen and Roth is representative of the activation energy for the single-atomistic process of crystal island nucleation. Thus, the study of stressed SPEG provides a new atomistic picture of the nature of growth.

Equilibrium shape of a liquid intergranular inclusion in a stressed elastic solid

The influence of applied stress on the equilibrium shape of a liquid intergranular inclusion is derived to show that (i) results differ significantly from those for a void, and (ii) this difference explains the experimental data if liquid inclusions behave as voids under tensile stress and as liquid inclusions under hydrostatic compressive stress.

Ab initio DFT study of ideal shear strength of polytypes of silicon carbide

Ab initio density functional calculations are performed to investigate the ideal shear deformation of SiC polytypes (3C, 2H, 4H, and 6H). The deformation of the cubic and the hexagonal polytypes in small strain region can be well represented by the elastic properties of component Si4C-tetrahedrons. The stacking pattern in the polytypes affects strain localization, which is correlated with the generalized stacking fault energy profile of each shuffle-set plane, and the ideal shear strength. Compressive hydrostatic stress decreases the ideal shear strength, which is in contrast with the behavior of metals.

New Forming Processes for Sheet Metal with Large Plastic Deformation

Due to the high effort involved, bifurcated constructions in mass market products made from sheet metal remained largely unused. Extruded profiles with cross-sections containing bifurcations show the possibility to increase the stiffness and allow modern lightweight design using load optimized structures as well as in box strap, sandwich and stringer constructions or different profiles. The two new forming processes linear flow splitting and linear bend splitting developed at the PtU enable the production of bifurcated profiles in integral style made of sheet metal without joining, lamination of material or heating of the semi-finished product. These forming processes use obtuse angled splitting rolls and supporting rolls to transform the sheet metal at ambient temperature. Whereas the linear flow splitting process increase the surface of the band edge and forms the band into two flanges. At linear bend splitting a bended sheet metal as semi finished product is used. Thereby bifurcations at nearly any place of a sheet metal can be produced. Both processes induce high hydrostatic compressive stresses in the local forming zone during the process which leads to an increased formability of the material and thereby to the realization of large strains. Parts produced are characterized by increased stiffness, high surface hardness and low surface roughness. Experimental investigations have shown an increasing of the band edge surface at maximum splitting depth up to 1800%. By a following forming process new multi-chambered structures and integral stringer construction can be realized with thin walled cross-sections from steel of higher strength.

Ab initio DFT simulation of ideal shear deformation of SiC polytypes

We perform ab initio density functional calculations to investigate the ideal shear deformation of SiC polytypes (3C, 2H, 4H and 6H). The deformation of the cubic and hexagonal polytypes in the small-strain region can be well represented by the elastic property of component Si4C-tetrahedrons. The stacking pattern in the polytypes affects strain localization, which is correlated with the GSF energy profile of each shuffle-set plane and the ideal shear strength. Compressive hydrostatic stress decreases the ideal shear strength but does not much affect the shear elastic coefficient. Tersoff classical interatomic potential can represent the deformation behaviour of SiC crystals in the small-strain region but cannot be applied to largely sheared and compressed situations.

Basics of linear flow splitting

Linear flow splitting enables the forming of bifurcated profiles in integral style out of sheet metal without joining, lamination of material or heating of the semi-finished part. The new roll forming process uses obtuse angled splitting rolls and supporting rolls to increase the surface of the band edge. The high hydrostatic compressive stresses induced in the local forming zone during the process lead to an increased formability of the material and enable the realization of large strains. Produced parts are characterized by increased stiffness, high surface hardness and low surface roughness. The basic principle of the process and the occurring phenomena are described in this paper.

X-ray microtomographic characterisation of pore evolution during homogenisation and rolling of Al–6Mg

The effect of homogenisation heat treatment and subsequent hot rolling on the evolution of porosity in a direct chill cast Al–6Mg (wt-%) alloy was studied. Porosity was quantified using conventional 2D optical metallography and X-ray microtomography (XMT) which allowed 3D imaging. Metallographic observations show an increase in maximum pore length during heat treatment and at the centre line during rolling. These observations together with the high temperatures might suggest a classical interpore Ostwald ripening mechanism is operative. However, XMT revealed that the pores have a highly tortuous shape, which when sectioned metallographically is not apparent and can lead to misinterpretation. X-ray microtomography observations proved that these tortuous 3D shapes spheroidised during homogenisation owing to localised coarsening of high curvatures within the complex, branching structure of each pore, termed intrapore coarsening. Accelerated centreline intrapore coarsening was observed during the initial rolling passes when relatively low reduction ratios were used. Finite element modelling was used to demonstrate that under the geometric rolling conditions employed the central region of the billet experienced a tensile rather than a compressive hydrostatic stress. This combined with deformation enhanced diffusion is proposed as the reason for the accelerated intrapore spheroidisation. X-ray microtomography was critical to revealing the true 3D pore shape and the mechanisms of pore coarsening, which could have been misinterpreted if only 2D metallography was used.