High Hydrostatic Stress Research Articles

Formability of dual phase steels in electrohydraulic forming

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. The objective of this paper is to report the results of formability testing of dual phase steels in EHF conditions and to provide an explanation of the observed formability improvement based on analysis of the experimental results and through the use of a modeling technique developed as part of this work for simulating the EHF process. Comparison of the maximum strains resulting from EHF into a conical die and a v-shape die to the maximum strains resulting from quasistatic LDH testing (limiting dome height) indicated that substantially higher strains can be accomplished in the EHF process. In order to obtain the maximum benefits of increased formability with EHF technology, it is necessary to get to the high strain rate regime and the accompanying high hydrostatic stresses in the sheet, which occurs mostly in the contact area between the blank and the die. If the blank decelerates and gets to quasistatic forming conditions before filling the die cavity completely, very limited improvement in formability can be anticipated. A numerical model of the EHF process has been developed, incorporating four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma channel, (3) a model for the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. The relative improvement in plane strain formability in EHF conditions was between 63% and 190%, depending on the grade of dual phase steel. Numerical modeling showed that the peak strain rates occurring in EHF are approximately 20,000 units per second.

Effect of strain path change on mechanical properties and final microstructure of Cu–Al2O3 in equal channel angular pressing

In the present work, the process of equal channel angular pressing of dispersion strengthened Cu–1·1 wt-%Al2O3 containing nanometric alumina particles was investigated by means of mathematical modelling and experimental testing. Through the modelling, deformation parameters such as hydrostatic pressure distribution and strain field were determined, and the effect of deformation path on these parameters was estimated. Equal channel angular pressing as well as mechanical and microstructural evaluations were also conducted to assess mechanical properties, grain structure and void volume fraction after deformation in different routes. The results indicate that distributions of plastic strain and hydrostatic stresses are significantly affected by deformation path route as well as utilised die design. Furthermore, void formation around hard alumina particles may occur in regions with high tensile hydrostatic stresses leading to fracturing during deformation.

Molecular Dynamics Simulation of Metal Cutting with Local Hydrostatic Pressure Field Formation

Improving machined surface integrity is one of the important issues in the precision machining. This study aims to develop a cutting tool, which enables to generate a local hydrostatic pressure field in the vicinity of the cutting point to suppress the extra plastic flow in the workpiece, because it is known that materials including metals never cause plastic flow and fracture no matter how much greater hydrostatic pressure field is given. In this paper, a simple cutting tool with planer jig is proposed and a molecular dynamics simulation of cutting is performed as the first step. As a result, it is confirmed that the reduction of the plastic deformation, mainly in the burr formation become remarkable with the proposed model due to the suppression of extra side plastic flow, and relatively high-hydrostatic stress field is formed in the vicinity of cutting point. However, it is also observed that relatively many dislocations are generated beneath the cutting groove.

Simulation of Hot Extrusion of an Aluminum Alloy with Modeling of Microstructure

In this work a numerical method for the simulation of extrusion processes with modeling of microstructure is presented. Extensive testing was done to provide a basis for the verification of simulation results. Circular rods of AA6005A were extruded by backward and forward extrusion with different extrusion ratios, billet temperatures and product velocities. The extruded rods were cooled either by water or at air to distinguish between dynamic and static recrystallization. Temperature and strain-rate dependent yield stresses were determined from hot compression tests. Special friction tests on cylindrical specimens under high hydrostatic stresses at high temperatures have been performed and the parameters of a friction model were identified from the experiments. The recrystallized volume fraction and grain sizes in the extruded rods were analyzed by means of optical micrographs. The obtained results were used to determine the parameters of a recrystallization model which was implemented in the FE code HyperXtrude. The transferability of the numerical model was checked by simulating forward extrusion tests using the model parameters obtained from backward extrusion tests.

Dielectric characteristic and local phase transition of gallium phosphide nanosolid

The dependences of relative dielectric permittivity, e′r, and tangent of dielectric loss angle, tg δ, of gallium phosphide (GaP) nanosolid on frequency and temperature were investigated. The GaP nanopowders are subglobular in shape, with the average crystallite size of about 50 nm evaluated from Scherrer equation. It can be concluded that the leakage current mechanism plays an important role in the dielectric loss of the GaP nanosolid. The dielectric characteristic of the GaP nanosolid in the range 298–350 K allows to detect an e′r or tg δ peak at 303 K that is due to local phase transitions, probably in the high hydrostatic stress field of dislocations with an edge component. Under the influence of an electric field, the high hydrostatic stress field of dislocations can undergo changes in deformation, accompanied by drastic stress-induced changes in the order parameter near the phase transition temperature, and hence, changes in the Gibbs free energy per unit volume can be found.

The Rockburst Experiment Usage of Similar Material and the Mechanism of the Energy Releasing

As one of the geological disasters, rockburst is often generated in underground engineering under excavation unloading conditions in high geostress areas. Taking advantage of the simulation test especially for the rock mass under high hydrostatic stress, the dynamic failure rockburst style resulted from instantaneous unloading in one direction was studied, whose peak velocity and acting time are analyzed. The results indicate that the formula for calculating peak velocity and the kinetic energy is proper and the kinetic energy is little portion of the recovery strain energy, most of which is dissipation in different ways; the unloading time at different positions apart from the free surface is nearly the same and more larger than the unloading wave’s disturbance time.

An attempt to produce ex situ TTS to understand their mechanical formation conditions – The case of an ultra high purity iron

High pressure torsion (HPT) tests can be used to transform the structure of materials. Several studies in the literature highlight that such tests are a means of investigating certain formation mechanisms of Tribological Transformed Structures (TTS) outside contacts, as they reproduce conditions that are close to those found inside them (quasi-hydrostatic pressures and shearing) under conditions that are easier to control than in a real contact. As there is no unique path leading to deformation with TTS in real contacts, it has been decided to simplify the history of deformation as the first step in investigating the occurrence of TTS. In this study, a Bridgman anvil apparatus, which allows shearing samples in torsion under pressure, is used to test ultra high purity iron (99.99999%) with an initial grain size of 100 μm, under a mean pressure of 0.5 GPa. The microstructural investigations led to establishing a map identifying the refined zones within the volume of the sample and linked to contact conditions. This work considers these zones with the finest microstructure (size grains of about 220 nm) as “TTS”. The experimental study is coupled with finite element analysis to investigate the specificity of the stress and strain cycles of the microstructure produced. Although further work is needed to validate the suggested comparison to friction induced transformed layers, the results showed that “TTS” are produced in a specific sample volume linked to sliding/adhesion contact conditions. The coupling between experiments and modeling shows that “TTS” formation can be associated with high hydrostatic stress components, high local plastic strains and high strain gradients.

Modelling of Reaction of Metallic Nanospheres with Gas

The stoichiometric MpXq hollow nanospheres are produced by reaction of metallic M nanospheres with the gaseous X phase. In the first stage a sufficiently thick MpXq nanoshell on the metallic core of phase M is formed. During this stage high supersaturation of vacancies in the M core or very high hydrostatic stress in the M core, due to the misfit between the core and the nanoshell, are developed and provide favourable conditions for the hollow nucleation. The misfit is caused by the Kirkendall effect. Based on the application of the thermodynamic extremal principle a kinetic model of MpXq nanoshell formation is derived. The kinetics is driven by the change of the chemical energy due to reaction of M and X components, of the interface and surface energies, and of the elastic strain energy due to misfit strain of the whole system. The model is used for simulation of the Cu2O shell growth kinetics due to oxidation of a Cu nanosphere, and the results of simulations are discussed.

Finite element modeling of hydrostatic stress distribution in copper dual-damascene interconnects

Hydrostatic stresses of copper dual-damascene interconnects are calculated by a commercial finite element software in this paper. The analytical work is performed to examine the effects of different low-k (k is permittivity) dielectrics, barrier layer and aspect ratio of via on hydrostatic stress distribution in the copper interconnects. The results of calculation indicate that the hydrostatic stresses are highly non-uniform throughout the copper interconnects and the highest tensile hydrostatic stress exists on the top interface of lower level interconnect near via. Both the high coefficient of thermal expansion and the low elastic modulus of the low-k dielectrics and barrier layer can decrease the highest hydrostatic stress on the top interface, which can improve the reliability of the copper interconnects.

Effect of stress on the characteristics of elliptical hollow core optical fiber

Propagation characteristics of an elliptical hollow core optical fiber (EHOF) have been investigated under the application of high lateral and hydrostatic stresses. The simulation results exhibit significant effects of stress on birefringence, polarization mode dispersion (PMD), group velocity dispersion, effective area, and power confinement. High birefringence (~10−3), negative dispersion (−22.07 ps/km-nm), low PMD, small effective area, and ultrahigh value of power density (~103) have been found for EHOF under different stresses. The inclusion of central air hole and asymmetry in geometrical structure of the fiber under stress are responsible for different changes in propagation characteristics.

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.

Further processing of ultrafine-grain materials

Linear flow splitting enables the production of bifurcated profiles out of sheet metal. Included in the Collaborative Research Centre (SFB) 666 the new roll forming procedure uses obtuse angled splitting rolls and supporting rolls to increase the surface of the band edge. High hydrostatic stresses in the deformed zone lead to an elevated formability of the high strength steel ZStE 500. As a result of the process, the flanges consist partially of an UltraFine-Grain microstructure (UFG). Produced parts are characterized by increased stiffness and high surface hardness. The new structure influences the formability in an extensive way. The investigations presented in this paper discuss the further processing of the flanges from linear flow split profiles. Therefore three point bending tests and roll forming experiments are carried out. A finite element analysis to characterize the damage is set up as well.

Assessment of uncertainties in life prediction of fatigue crack initiation and propagation in welded rails

The risk for initiation of fatigue cracks in the rail head and web in the weld zone of a rail is studied. The interaction between the welding residual stress field and the stress field caused by service loads is simulated in a nonlinear finite element (FE) analysis where the welding residual stress distribution (shape) and magnitude, the service load magnitude and the material parameters used in the fatigue life estimation are varied. The initiation of fatigue cracks is assessed using the shear-stress-based multiaxial fatigue criterion proposed by Dang Van, and the propagation of fatigue cracks in the rail web is carried out using a Paris-type crack growth law. A discussion is presented of the interpretation of using the Dang Van criterion to assess crack initiation for stress–response load cycles with low shear stress amplitudes and high hydrostatic stress levels. In addition, the accuracy in the fatigue life assessment is evaluated by statistical uncertainty analysis where the variances according to the Gauss approximation formula are studied. The risk for fatigue crack initiation and propagation in the rail head and rail web, respectively, is enhanced due to the welding residual stress, and the uncertainty in load level dominates the uncertainty in the fatigue assessments.

43 COMPARISON OF THE EFFECTS OF PRE-TREATMENT WITH SODIUM CHLORIDE, SUCROSE AND TREHALOSE ON DEVELOPMENTAL COMPETENCE OF PORCINE OOCYTES

Modified environmental stress was reported to improve the developmental competence and cryotolerance of porcine oocytes, such as high hydrostatic pressure (HHP; Du et al. 2008 Cloning Stem Cells, Epub ahead of print) and osmotic stress (Lin et al. 2008 Reprod. Biomed. Online, in press). HHP also improved the cryotolerance of bovine and murine blastocysts (Pribenszky et al. 2005a Reprod. Dom. Anim. 40, 338–344; Pribenszky et al. 2005b Anim. Reprod. Sci. 87, 143–150). In the present study we compared the effects of NaCl with that of concentrated solutions of two non-permeable osmotic agents, sucrose and trehalose on in vitro maturated oocytes. A total of 2050 slaughterhouse-derived porcine cumulus–oocyte complexes (COCs) were matured for 41–42 h, and then put into 800 μL T2 (HEPES-buffered TCM-199 [Earle’s salts] with 2% cattle serum) supplemented with additional NaCl, sucrose or trehalose with the same osmotic level (588 mOsmol) in 4-well dishes and incubated for 1 h at 38.5°C in air. COCs incubated in T2 under the same conditions without supplementation were used as controls. Subsequently COCs were incubated in IVM medium for 1 h at 38.5°C in 5% CO2 with maximum humidity. After this recovery period cumulus cells were removed with 1 mg mL–1 hyaluronidase and pipetting, and oocytes were used as recipients for somatic nuclear transfer with handmade cloning (HMC) method. Porcine fetal fibroblasts were used as nuclear donor cells. Embryo culture was performed in PZM-3 medium (Yoshioka et al. 2002 Biol. Reprod. 66, 112–119) in 5% CO2, 5% O2 and 90% N2 and maximum humidity. Cleavage and blastocyst rates were checked on Day 1 and Day 6, respectively. Cell numbers were counted after fixation in glycerol containing 20 μg mL–1 Hoechst 33342 fluorochrome on Day 6. t-test was performed for statistical calculations with SPSS 11.0 program (SPSS, Chicago, IL, USA). Results are shown in Table 1. Osmotic stress with both permeable and non-permeable agents increased developmental competence of porcine IVM oocytes. NaCl seems to be more appropriate for the purpose, as the other two components resulted in decreased cell number in blastocysts after somatic cell nuclear transfer (SCNT). In conclusion, a simple NaCl pre-treatment of oocytes has improved the in vitro efficiency of porcine SCNT. Table 1.Developmental competence of porcine HMC embryos derived from oocytes treated with different agents The authors thank Ruth Kristensen, Anette Pedersen, Janne Adamsen and Klaus Villemoes for their help and excellent technical assistance.

Atomistic Study of Hydrogen Occupation Sites around {112} ‹111› Edge Dislocation in Alpha Iron

Despite the extensive investigations and the recent remarkable progress in experimental equipments, the effects of hydrogen on materials properties have not been sufficiently clarified yet. The difficulties to reveal the essential effects of hydrogen are mainly attributed to the characteristics of hydrogen such as ppm order extremely low concentration and high diffusivity in metals. In this study, we performed molecular statics (MS) analyses of hydrogen occupation sites around a {112} ‹111› edge dislocation in alpha iron. The stable hydrogen occupation region, where the hydrogen trap energy is strong, is not consistent with the result obtained by continuum theory of elasticity that hydrogen concentrate at the high hydrostatic stress region. According to the MS calculations, hydrogen trap energy has the different characteristics depending on the occupation sites under shear stress as well as hydrostatic stress. Furthermore, this results suggest that the hydrogen concentration on the slip plane around a {112} ‹111› dislocation can be extremely high.

An investigation to the influence of bogie direction reversal on equalizing rail vehicle wheel wear

Railway wheels are subject to high hydrostatic pressure and tangential stresses resulting in plastic deformation and wear. The present study focuses on the initial shakedown, i.e., running-in, period of wheel material and on a procedure of bogie direction reversal. This paper aims to show the importance of two domains of moving a wheelset, which are contact mechanics and operational aspect in the wear rate and wear pattern. The authors present the field test results highlighting the significance of changing the direction of the coach on the reduction of wheel wear resulting in a symmetric wear pattern.

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.

Damage mechanisms at room and high temperature in notched specimens of Al6061/Al 2O 3 particulate composites

The aim of the present study was the investigation of the effects of macroscopic stress triaxiality on the presence and evolution of damage in particulate MMC. Two Al6061/Al 2O 3/10p and 20p particulate reinforced composites were investigated on smooth and differently notched specimens pulled in tension at 20 and 250 °C. Fractographic analyses and microstructural observations were carried out in order to identify and to map damage mechanisms. Particle fracture was the prominent form of material damage at 20 °C, while debonding was the main damage mechanism at 250 °C. The distribution of the two damage forms within the specimen volume was interpreted on the basis of the results of finite element analyses. At a given level of triaxiality (considered as the hydrostatic to Von Mises stress ratio), particle fracture localized in regions of high maximum principal stress, generally associated to the high hydrostatic stress. It was further observed that matrix/interface debonding concentrated in the same regions, but developed on other particles at higher/lower load level depending on test temperature.

Continuum damage modelling of environmental degradation in joints bonded with E32 epoxy adhesive

A mesh-independent continuum damage model has been proposed to predict the residual strength of adhesively-bonded joints by introducing a displacement-based damage parameter into the constitutive equation of the damaged materials. This approach was originally developed for EA9321-bonded single-lap joints and this paper extends it to butt joints bonded with a different ductile adhesive (E32). This involves not only a different adhesive and joint configuration but the high hydrostatic stress requires a more realistic yielding model. A dry and a partially saturated steel joint were used to calibrate the moisture dependent damage parameters of the adhesive E32, and then these parameters were used without further modification to predict the failure of the other environmentally degraded steel butt joints and the aluminum butt joints. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. A von Mises yield model was used initially. Then, a linear Drucker–Prager plasticity model was used, as this could incorporate the hydrostatic stress dependency found in the adhesive. The predicted joint residual strengths agreed well with the corresponding experimental data, which exhibited a significant reduction in strength on exposure to moisture but still retained a cohesive failure mode in the adhesive. The mesh independence of the model was also demonstrated.

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.