Initial Compressive Stress Research Articles

The local crystallization in nanoscale diamond-like carbon films during annealing

The local crystallization during annealing at 600 °C in nanoscale diamond-like carbon coatings films grown by pulsed vacuum-arc deposition method was observed using modern techniques of high-resolution transmission electron microscopy. The crystallites formed by annealing have a face-centred cubic crystal structure and grow in the direction [01¯1¯] as a normal to the film surface. The number and size of the crystallites depend on the initial values of the intrinsic stresses before annealing, which in turn depend on the conditions of film growth. The sizes of crystallites are 10 nm for films with initial compressive stresses of 3 GPa and 17 nm for films with initial compressive stresses of 12 GPa. Areas of local crystallization arising during annealing have a structure different from the graphite. Additionally, the investigation results of the structure of nanoscale diamond-like carbon coatings films using Raman spectroscopy method are presented, which are consistent with the transmission electron microscopy research results.

Failure temperatures of steel H-section columns under elevated temperatures

This paper introduces the failure temperatures and stresses of H-section columns under elevated temperatures. The failure tests using the heating furnace were performed at the Fire Insurers Laboratories of Korea. The eight specimens fabricated of SS400 steel were tested and the slenderness ratio, the width-thickness ratio of the flange, and the width-thickness ratio of the web of all specimens were fixed at 45.4, 10.0 and 26.4, respectively. The specimens loaded by constant compression were heated until failure occurred. Four types of compressive stresses were selected as initial loading conditions;, these compressive stresses corresponded to 50, 60, 70, and 80% of the yield strength at ambient temperature. The specimens are classified into four groups according to the initial compressive stress. The compressive stresses which are initially loaded in the tests are also defined as the failure stresses at the failure temperatures of specimens. The failure stresses are compared with the linear elastic and nonlinear buckling stresses and the yield stresses (which respond to the failure temperatures) by using the Eurocode 3 Part 1.2 which defines the material property of steel under elevated temperatures. The linear local and overall buckling stresses were higher than the yield stresses at the failure temperatures and the failure phenomenon was caused by nonlinear buckling occurring before yielding failure. The failure stresses were lower than the yield stresses obtained by the EC3 at the failure temperatures. The failure stress ratios of the four groups of specimens loaded as 50, 60, 70, and 80% of ambient yield strength were evaluated as 0.78, 0.89, 0.88, and 0.92 of the yield stresses given in EC3, respectively. The nonlinear buckling stresses defined by EC3 showed about 0.65 of the failure stresses. The four groups of specimens failed at 543.5, 534.5, 489.9, and 457.9°C, respectively.

New physically based model for thermal induced initial stress in 3D for silicon germanium films after deposition

Purpose – The purpose of this paper is to provide a new three dimension physically based model to calculate the initial stress in silicon germanium (SiGe) film due to thermal mismatch after deposition. We should note that there are many other sources of initial stress in SiGe films or in the substrate. Here, the author is focussing only on how to model the initial stress arising from thermal mismatch in SiGe film. The author uses this initial stress to calculate numerically the resulting extrinsic stress distribution in a nanoscale PMOS transistor. This extrinsic stress is used by industrials and manufacturers as Intel or IBM to boost the performances of the nanoscale PMOS and NMOS transistors. It is now admitted that compressive stress enhances the mobility of holes and tensile stress enhances the mobility of electrons in the channel. Design/methodology/approach – During thermal processing, thin film materials like polysilicon, silicon nitride, silicon dioxide, or SiGe expand or contract at different rates compared to the silicon substrate according to their thermal expansion coefficients. The author defines the thermal expansion coefficient as the rate of change of strain with respect to temperature. Findings – Several numerical experiments have been used for different temperatures ranging from 30 to 1,000°C. These experiments did show that the temperature affects strongly the extrinsic stress in the channel of a 45 nm PMOS transistor. On the other hand, the author has compared the extrinsic stress due to lattice mismatch with the extrinsic stress due to thermal mismatch. The author found that these two types of stress have the same order (see the numerical results on Figures 4 and 12). And, these are great findings for semiconductor industry. Practical implications – Front-end process induced extrinsic stress is used by manufacturers of nanoscale transistors as the new scaling vector for the 90 nm node technology and below. The extrinsic stress has the advantage of improving the performances of PMOSFETs and NMOSFETs transistors by enhancing mobility. This mobility enhancement fundamentally results from alteration of electronic band structure of silicon due to extrinsic stress. Then, the results are of great importance to manufacturers and industrials. The evidence is that these results show that the extrinsic stress in the channel depends also on the thermal mismatch between materials and not only on the material mismatch. Originality/value – The model the author is proposing to calculate the initial stress due to thermal mismatch is novel and original. The author validated the values of the initial stress with those obtained by experiments in Al-Bayati et al. (2005). Using the uniaxial stress generation technique of Intel (see Figure 2). Al-Bayati et al. (2005) found experimentally that for 17 percent germanium concentration, a compressive initial stress of 1.4 GPa is generated inside the SiGe layer.

Dependence between the Favourable Initial (Residual) Stresses and Durability of the Ball Screws

This work is based on theoretical and experimental researches on increasing the carrying capacity and durability of the ball screws used on machine tools by creating favourable initial compression stresses (non-conventional treatment) in the surface layer of the elements into contact, so that these stresses keep their effectiveness during usage.

Effect of smooth moving punch in an initially stressed monoclinic magnetoelastic crystalline medium due to shear wave propagation

This work studies the effect of a semi-infinite smooth moving punch on the shear wave propagation in an initially stressed magnetoelastic monoclinic strip. The closed form expression of dynamic stress concentration due to the punch for the force of a constant intensity has been established. The speed of moving punch associated with shear wave speed in the medium, vertical compressive initial stress, vertical tensile initial stress and monoclinic magnetoelastic coupling parameter have a substantial effect on the dynamic stress concentration owing to a semi-infinite smooth moving punch. Numerical computation and graphical demonstration has been carried out for quartz crystal with monoclinic symmetry to observe the profound effect of these affecting parameters on dynamic stress concentration. In order to highlight some of the important peculiarities of the problem, a comparative study of dynamic stress concentration in an initially stressed magnetoelastic monoclinic strip and initially stressed magnetoelastic isotropic strip has been made. Weiner-Hopf technique indicated by Matczynski has been adopted to solve the problem. In particular, some special cases have been deduced from the obtained expression of dynamic stress concentration due to the smooth moving punch.

In Depth Study of Ge Impact on Advanced SiGe PMOS Transistors

SiGe material brings outstanding advantages for pMOS transistors [1,2]. Low Vt values are much easier to control thanks to specific band structure. The strain induced by engineering of lattice mismatch with silicon leads to enhanced mobility and performances. However a lot remains to understand in these processes and performances. This paper puts the light on two effects: (1) an unexpected Vt shift induced by Ge and (2) the relative impact of Ge, strain and defects on the mobility gain.Many SiGe processes and architectures have been studied to allow clear and reproducible conclusions: (1) pMOS with a given Ge content and various stack thicknesses; (2) pMOS with a given stack and various Ge compositions; (3) SiGe pMOS performed by epitaxy or by Ge-enrichment technique, from unstrained or strained Si substrate, leading to various combinations of stress and Ge content. Both bulk and SOI devices have been processed. Data have been analyzed in a global and coherent view, thus avoiding addressing non pertinent data. Extra Ge-induced Vfb shift: Advanced simulations have been performed using UTOX simulator [3,4]. 6×6 kp simulations and Poisson Schrodinger allow to simulate band structures with strain effects for all %Ge as well as electrical C(V) responses. Comparison between simulations and experiments indicates an extra Vt shift which is not expected from the band structure (Fig.1). This shift is proportional to %Ge content in the channel; it has been observed on SOI and Bulk devices.The analysis of the various gate stack thicknesses rules out an effect of charges in the stacks or at the interfaces. All the electrical data lead to a behavior explained with a Ge induced dipole at the SiGe/SiON interface. Detailed SIMS analysis (Fig.2) as well as STEM, EELS, HAADF experiments show a slight Ge diffusion in SiON probably explaining the dipole at this interface. Notice that this diffusion does not reach the SiON/HfO2 interface and the other existing dipole at this interface [5]. This result is essential for future devices design and modeling, as the extra shift is in the range of the expected Vt (~0.2V). Mobility on strained pMOS: The described processes induce an initial biaxial compressive stress in SiGe layer. This stress improves significantly the channel hole mobility. Meanwhile Ge atoms degrade the SiGe/SiON interface and increase the interface states density Dit (Fig.3). These Dit are related to Ge diffusion in SiON and induce dipoles. However they have no direct effect on Vt.Using the various technological splits available we have compared splits with similar final strain but different concentrations of Ge (Fig.4). This shows that the strain generated by lattice mismatch has a 1storder effect. The percentage of Ge itself as well as generated dipoles and Dit play a minor role, at least at the low %Ge (≤30%). At higher %Ge this is no more true and the gain by strain is counter-balanced by the interface degradation. An improvement of process is required to maintain a good interface quality.Real devices have 10-20 nm length and width. In this case the strain can relaxed depending on directions and may significantly increase or decrease the performances. A careful strain engineering is then expected. This will be discussed in the final paper.

Magneto-electro-viscoelastic torsional waves in aeolotropic tube under initial compression stress

This study examines the effect of electric and magnetic field on torsional waves in hetrogeneous viscoelastic cylindrically aeolotropic tube subjected to initial compression stresses. A new equation of motion and phase velocity of torsional waves propagating in cylindrically aeolotropic tube subjected to initial compression stresses, nonhomogeneity, electric and magnetic field have been derived. The study reveals that the initial stresses, nonhomogeneity, electric and magnetic field present in the aeolotropic tube of viscoelastic solid have a notable effect on the propagation of torsional waves. The results have been discussed graphically. This investigation is very significant for potential application in various fields of science such as detection of mechanical explosions in the interior of the earth.

FEM Analysis of Residual Stress Distribution and Cutting Forces in Orthogonal Cutting with Different Initial Stresses

Machining induced residual stress is influenced by many factors. Extensive studies on the influence of cutting parameters, tool parameters, as well as basic properties of materials have been carried out during the past decades, while another important factor, initial stress distribution in workpiece, was often ignored. In this paper a relatively complete FEM simulation on the formation mechanism of machining induced residual stress in high speed machining is carried out, illustrating the three stress zones affected by mechanical and thermal loads, and their influence on ultimate residual stress. And the influence of initial compressive stress on stress formation and cutting forces is analyzed. Initial compressive stress weakens the tensile effect caused by the shear deformation, and the residual stress tend to be more compressive with larger initial compressive stress. Cutting force becomes larger with the increase of initial compressive stress. And the results in this FEM study can be used to explain some unaccounted experimental phenomena in former researches.

Analysis of a Compressed Bistable Buckled Beam on a Flexible Support

The bistable snap-through behavior of a compressed beam is modeled and measured experimentally as its supporting surface is bent through positive and negative curvatures. When the supporting angle of the beam exceeds a critical angle, bistability is lost and only one stable state is supported. The critical angle is controlled only by the initial compressive stress in the beam, and we report a nondimensionalized calculation method for this angle. This large-deflection nonlinear model provides design rules for low-power sensors and actuators that can measure and control surface curvature from the micro- to macroscale.

Finite Element Simulation of Brittle Fracture of Bulletproof Glass System

The thickness and weight of a bulletproof glass material can be reduced using strengthened glass possessing current protective abilities. In this study, numerical simulations are used to estimate the protective ability of strengthened borosilicate glass used in bulletproof glass systems. High-velocity impacts and perforation behaviors are well described by a dynamic brittle fracture model. A parametric study of the material model of glass is conducted by comparing test results of individual impacts with corresponding numerical estimations; size of back-surface spall, morphology of perforated surface, and fractured areas are compared. Material parameters of strengthened and nonstrengthened borosilicate glasses are determined. Numerical simulations using a material model with these parameters well describe the overall fracture behavior of bulletproof glass. The main parameters that affect the protective ability are initial compressive yield stress and fracture stress. Furthermore, the protective ability of strengthened borosilicate glass is ~20% better than that of nonstrengthened borosilicate glass.

Influence of Initial Residual Stress on Material Properties of Bearing Steel During Rolling Contact Fatigue

Rolling contact fatigue (RCF) experiments were performed on AISI M50 bearing balls using a single ball test rig to investigate the evolution of the material properties within the RCF-affected subsurface region. Using a combination of micro-indentation and miniature compression testing methods, the influence of contact stress, initial residual stresses, and the number of contact cycles on the resulting evolution of material properties was investigated. It was found that the balls with initial residual compressive stresses show less change in material properties after RCF loading than the balls without such initial residual stresses. The formation of a light etching region (LER) is shown to not correlate with a decrease in material strength and hardness, but it does serve as a predictor for failure due to spall.

Numerical Modelling of the Initial Stress and Upward Deflection of Glulam Beams Pre-Stresseed by Compressed Wood

A new approach to reinforce glulam timber beams has been developed by using compressed wood (CW) which is made of a lower grade wood through densification processes. In the reinforcing practice, compressed wood blocks are inserted into pre-cut holes on the top of glulam beams to produce pre-camber and to generate initial tensile and compressive stresses on the top and the bottom extreme fibre of the glulam beam. In order to optimize the size, the number and the location of CW blocks, 3-D finite element models have been developed. 3D non-linear finite element models have been developed to simulate the pre-camber of Glulam beams locally reinforced by compressed wood blocks. The models developed have also produced the initial tensile and compressive stresses at the top and bottom extreme fibres with building-up moisture-dependent swelling on the CW blocks. With the pre-camber and the initial stress state that cancel out proportions of working deflection and stresses.

Transverse panel-to-panel connections for full-depth precast concrete bridge deck panels on continuous steel girder bridges: Part 2, time-dependent analysis

In this article, the authors seek to determine the initial compressive stress to avoid cracking and leaking in precast, prestressed concrete bridge deck panels. They use an age-adjusted effective modulus method to evaluate time-dependent stress redistribution in composite systems. The evaluation found an initial precompression of 200 to 300 psi to be adequate for single span bridges, 200 to 700 psi for three-span bridges, and 100 psi for two-span bridges. They also determine that accelerated construction schedules for continuous span bridges make it necessary to substantially increase initial compressive stress.

Finite element simulation of brittle fracture of bulletproof glass system

The thickness and weight of a bulletproof glass material can be reduced by using strengthened glass that possesses current protective capabilities. In this study, numerical simulations are performed to estimate the protective capability of strengthened borosilicate glass used in bulletproof glass systems. High-velocity impacts and perforation behavior are well described by a dynamic brittle fracture model. A parametric study of the material model of glass is conducted by comparing test results of individual impacts with corresponding numerical estimations; the size of back-surface spall, morphology of perforated surface, and fractured areas are compared. Material parameters of strengthened and non-strengthened borosilicate glasses are determined. Numerical simulations that use a material model considering these parameters well describe the overall fracture behavior of bulletproof glass. The main parameters that affect protective capability are initial compressive yield and fracture stresses. The protective capability of strengthened borosilicate glass is ∼20% better than that of non-strengthened borosilicate glass.

Dispersion of Dilatation Wave Propagation in Single-Wall Carbon Nanotubes Under Initial Stress Using Nonlocal Scale Effects

In this paper we extend our previous work to show that there is an effect of initial stress on the dilatation wave propagation in single-wall carbon nanotubes. The nanotube structures are treated within the multilayer thin shell approximation with the elastic properties taken to be those of the graphene sheet. The equation of motion of the dilatation wave is obtained using the nonlocal elastic theory. The phase velocity and the group velocity are derived, respectively. The dispersion relation is analyzed with different wave numbers, scale coefficients and initial stress parameter. It can be observed from the results that the dispersion properties of the dilatation wave are induced by small scale effects, which will disappear in local continuous models. The dispersion degree can be strengthened by increasing the scale coefficient and the wave number. In particular, the group velocity of dilatation wave are sensitive to initial compression stress for higher frequencies and insensitive for lower frequencies. The investigation presented may be helpful in the application of CNTs, such as ultrahigh-frequency resonators, electron emission devices, high-frequency oscillators and mechanical sensors.

Tuning the locally resonant phononic band structures of two-dimensional periodic electroactive composites

The band structures of two locally resonant phononic crystals (LRPCs) with periodic multilayered cylindrical inclusions embedded in an elastic matrix are investigated by the finite-element method. The inclusions consist of electroactive polymer layer(s). Tunability of the band gaps of the phononic crystals by applying electric field upon the electroactive layer is demonstrated. A simple analytical expression is presented on the relationship between the stop band boundaries and the electric field. Good agreement between the analytical and numerical predictions is obtained. The effects of initial stress on the band structures are explored. It is found that tensile initial stress shifts up the band gaps while compressive initial stress shifts down or even closes them.

The influence of buffer layer coalescence on stress evolution in GaN grown on ion implanted AlN/Si(111) substrates

The effect of AlN buffer layer morphology on the evolution of growth stress in GaN epilayers deposited by metalorganic chemical vapor deposition on N+ ion-implanted AlN/Si(111) substrates was investigated. AlN buffer layers were grown using either a continuous or pulsed source flow process which altered the grain size and extent of coalescence of the films. In situ stress measurements revealed that substrate implantation reduced the initial compressive stress in the GaN epilayers likely due to a decoupling of the AlN lattice from the underlying crystalline Si substrate. The buffer layer morphology was found to significantly alter the influence of ion-implantation on the film properties. GaN epilayers grown on ion-implanted AlN/Si(111) substrates prepared with the pulsed conditions exhibited a 63% decrease in threading dislocation (TD) density compared to unimplanted substrates. In addition, these films were observed to grow under a low overall stress compared to the other samples which exhibited a more typical compressive to tensile stress transition during growth. The low overall growth stress of the GaN grown on the implanted pulsed AlN/Si(111) was explained in terms of a reduced strain gradient from dislocation inclination.

Hydration and radiation effects on the residual stress state of cortical bone

The change in the biaxial residual stress state of hydroxyapatite crystals and collagen fibrillar structure in sections of bovine cortical bone has been investigated as a function of dehydration and radiation dose using combined small- and wide-angle X-ray scattering. It is shown that dehydration of the bone has a pronounced effect on the residual stress state of the crystalline phase, while the impact of radiation damage alone is less dramatic. In the initial hydrated state, a biaxial compressive stress of approximately −150MPa along the bone axis exists in the hydroxyapatite crystals. As water evaporates from the bone material, the stress state moves to a tensile state of approximately 100MPa. The collagen fibrillar structure is initially in a tensile residual stress state when the bone is hydrated and the state increases in magnitude slightly with dehydration. Radiation dose in continually hydrated samples also reduces the initial biaxial compressive stress magnitude in the hydroxyapatite phase; however, the stress remains compressive. Radiation exposure alone does not appear to affect the stress state of the collagen fibrillar structure.

Propagation of P- and S-waves in initially stressed gravitating half space

The present paper contributes in studying the phase velocities of P- and S-waves in a half space subjected to a compressive initial stress and gravity field. The density and acceleration due to gravity vary quadratically along the depth. The dispersion equation is derived in a closed form. It is shown that the phase velocities depend not only on the initial stress, gravity, and direction of propagation but also on the inhomogeneity parameter associated with the density and acceleration due to gravity. Various particular cases are obtained, and the results match with the classical results. Numerical investigations on the phase velocities of P- and S-waves against the wave number are made for various sets of values of the material parameters, and the results are illustrated graphically. The graphical user interface model is developed to generalize the effect.

Influence of braking on the microstructure and mechanical behavior of railroad wheel steels

We study the changes in the microstructure, micromechanism of fracture, and characteristics of cyclic crack resistance of high-strength (KP-T-type) and medium-strength (KP-2-type) railroad wheel steels under the conditions simulating the process of braking in a wheel–rail couple. It is shown that, in the contact zone, their initial pearlitic structure is transformed into martensite and the initial compressive residual stresses of the second kind are replaced by tensile stresses, and the higher the carbon content of steel and its cooling rate, the more intense are these changes. The analyzed processes lead to the realization of low-energy cleavage intergranular mechanism of fracture of steels under cyclic loads and to a significant (almost twofold) decrease in their cyclic fracture toughness $$ \varDelta {K_{{\operatorname{fc}}}} $$ .