Showing papers on "Deformation (engineering) published in 1997"

Bending and buckling of carbon nanotubes under large strain

Abstract: The curling of a graphitic sheet to form carbon nanotubes produces a class of materials that seem to have extraordinary electrical and mechanical properties. In particular, the high elastic modulus of the graphite sheets means that the nanotubes might be stiffer and stronger than any other known material, with beneficial consequences for their application in composite bulk materials and as individual elements of nanometre-scale devices and sensors. The mechanical properties are predicted to be sensitive to details of their structure and to the presence of defects, which means that measurements on individual nanotubes are essential to establish these properties. Here we show that multiwalled carbon nanotubes can be bent repeatedly through large angles using the tip of an atomic force microscope, without undergoing catastrophic failure. We observe a range of responses to this high-strain deformation, which together suggest that nanotubes are remarkably flexible and resilient.

Phase transformations of silicon caused by contact loading

Abstract: Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 μm, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I...

Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H–SiC

Abstract: The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for the E2(1), A1(TO), E1(TO), and E2(2) optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV.

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Abstract: Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

Nondilatant brittle deformation of serpentinites: Implications for Mohr-Coulomb theory and the strength of faults

Abstract: We conducted deformation experiments to investigate the strength, deformation processes, and nature of the brittle-ductile transition of lizardite and antigorite serpentinites. A transition from localized to distributed deformation occurs as confining pressure increases from ∼200 to ∼400 MPa at room temperature. Deformation in both brittle (localized) and ductile (distributed) regimes is accommodated by shear microcracks, which form preferentially parallel to the (001) cleavage. Axial microcracks (mode I) are infrequently observed. Volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant, consistent with the shear-dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal plastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracking. The weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlee's law. If brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end-member style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present.

Nanoindentation creep of single-crystal tungsten and gallium arsenide

Abstract: An extensive study of indentation creep on the nanometre scale has been made on single-crystal indium, tungsten and gallium arsenide. We use the force modulation technique which gives a direct measure of contact stiffness and, being insensitive to thermal drift, allows the accurate observation of creep in small indents to be carried out over long time periods: We show that strain rate indices similar to those for macroscopic creep can be obtained for indium. Stress relaxation negative creep is also observed in a manner similar to macroscopic tests. Indentation of tungsten and gallium arsenide shows a distinct pop-in at a critical load, before which the deformation is essentially elastic and after which it is elastoplastic with significant dislocation multiplication. The creep behaviour is quite different before and after pop-in, clearly demonstrating the role of mobile dislocations in creep, even in nanometre-sized volumes of materials.

Influence of a Hard Surface Layer on the Limit of Elastic Contact—Part I: Analysis Using a Real Surface Model

Abstract: A numerical simulation technique for calculating the pressure distribution and the deformed geometry of an elastic half space which has a hard surface layer in contact with a rigid indenter with a rough surface is presented. In order to reduce the computing time, the Conjugate Gradient Method ( CGM ) is applied to solve a set of linear equations for unknown pressures. In each iteration of the CGM, the Fast Fourier Transform (FFT) is used for the task of multiplying a direction vector by an influence coefficient matrix. An FFT-based scheme for evaluating subsurface stresses in the layer and the substrate is also presented. As an example, the pressure distribution and the deformed geometry of a steel surface coated with a TiN layer in contact with a rigid rough indenter are calculated. The subsurface stresses are also compared with von Mises yield criterion to investigate the deformation mode at the asperity contacts. The results show that the limit of elastic contact is highly dependent on the layer thickness and the surface roughness.

Strain-softening of concrete in uniaxial compression

Abstract: 0025-5432/97 © RILEM tory cast its own specimens following a prescribed recipe The pre-peak behaviour was found to be independent of specimen slenderness when low friction loading platens were used However, for all loading systems a strong increase of (post-peak) ductility was found with decreasing specimen slenderness Analysis of the results, and comparison with data from literature, showed that irrespective of the loading system used, a perfect localization of deformations occured in the post-peak regime, which was first recognised by Van Mier in a series of uniaxial compression tests on concrete between brushes in 1984 Based on the results of the Round Robin, a draft recommendation will be made for a test procedure to measure strain softening of concrete under uniaxial compression Although the post-peak stress-strain behaviour seems to be a mixture of material and structural behaviour, it appears that a test on either prismatic or cylindrical specimens of slenderness h/d = 2, loaded between low friction boundaries (for example by inserting sheets of tef lon between the steel loading platen and the specimen), yields reproducible results with relatively low scatter For normal strength concrete, the closed-loop test can be controlled by using the axial platen-to-platen deformation as a feed-back signal, whereas for high-strength concrete either a combination of axial and lateral deformation should be used, or a combination of axial deformation and axial load FOREWORD An extensive Round Robin test programme on compressive softening was carried out by the RILEM Technical Committee 148-SSC “Test methods for the Strain Softening response of Concrete” The goal was to develop a reliable standard test method for measuring strain softening of concrete under uniaxial compression The main variables in the test programme were the specimen slenderness h/d and the boundary restraint caused by the loading platen used in the experiments Both high friction and low friction loading systems were applied Besides these main variables, which are both related to the experimental environment under which softening is measured, two different concretes were tested: a normal strength concrete of approximately 45 MPa and a higher strength concrete of approximately 75 MPa In addition to the prescribed test variables, due to individual initiatives, the Round Robin also provided information on the effect of specimen shape and size The experiments revealed that under low boundary friction a constant compressive strength is measured irrespective of the specimen slenderness For high friction loading systems (plain steel loading platen), an increase of specimen strength is found with decreasing slenderness However, for slenderness greater than 2 (and up to 4), a constant strength was measured The shape of the stress-strain curves was very consistent, in spite of the fact that each laboraRILEM TC 148-SSC: TEST METHODS FOR THE STRAIN-SOFTENING RESPONSE OF CONCRETE

Micromechanics of coalescence—I. Synergistic effects of elasticity, plastic yielding and multi-size-scale voids

Abstract: Void growth and coalescence under physical states similar to those found in highly stressed regions ahead of a crack is investigated. The analysis introduces a representative material volume containing several large voids and a population of microvoids present from the very beginning, all of which are modeled as discrete entities. Plastic yielding has pervaded the material volume of interest. The underlying micromechanics of final rupture is dominated by a succession of rapidly growing microvoids. This involves the synergistic interaction between elasticity associated with high stress triaxiality, stiffness softening caused by plastic yielding and a rich supply of length scales arising from voids of vastly different sizes. A primary feature of the coalescence phase is an unstable deformation mode whereby a minute, benign void rapidly enlarges reaching a size set by the characteristic length of the locally elevated stress field. The process begins with a large void growing in concert with the plastic strain. Simultaneously, a local zone of high stress concentration emanates from the large void and spreads across the material raising the stresses at nearby microvoids. As a result, the hydrostatic stress surrounding one or more microvoids is raised to a level that activates an unstable deformation mode in which the stored elastic energy drives the plastic expansion of the microvoid. Although the overall stress decreases rapidly, small zones of high stress concentration are generated near growing voids—causing even smaller nearby microvoids to grow rapidly. This process continues until the submicron ligament fails by microcleavage or by shearing along crystallographic planes. Plastic yielding plays a crucial role in the above process by lowering the stress level required for the unstable-like growth mode of microvoids. The process outlined above appears to be the main operative mechanism in several observed failure modes in metal alloys. The morphologies of fracture surfaces dominated by flat dimpled rupture and voidsheet formation can be elucidated by the present work. For high-strength metals, our studies suggest that microvoid cavitation and link-up will increasingly dominate the failure process resulting in a brittle-like ductile rupture mode in which very little energy is expended.

Shear localization and recrystallization in high-strain, high-strain-rate deformation of tantalum

Abstract: Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-generated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved generated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (eeff 2.5, T > 1000 K). Whereas the post-deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the deformation time is not sufficient to generate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (eeff > 1). Residual tensile ‘hoop’ stresses are generated near the central hole region upon unloading; this resulted in ductile fracturing along shear localization bands.

Temperature dependence of yield stress, deformation mode and deformation structure in single crystals of TiAl (Ti−56 at.% Al)

Abstract: The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at.% Al has been studied in compression as a function of crystal orientation in the temperature range from −196 to 1100°C. The profile of yield stress-temperature curves for all orientations studied can be divided into three temperature regions; the yield stress rapidly decreases with increasing temperature at both low and high temperatures and an anomalous increase in yield stress is observed at intermediate temperatures, exhibiting a peak at 700–1000°C depending on crystal orientation. Both ordinary and [101] superlattice slip exhibit an anomalous increase in critical resolved shear stress (CRSS). However, the extent of the anomaly associated with the former slip is much smaller than that for the latter slip. The CRSS for [101] slip depends on crystal orientation. In the anomalous temperature region, the CRSS exhibits very small strain-rate sensitivity and thermal reversibility24Jun1996. Superlattice disloc...

Deformation and viscoelastic behavior of polymer gels in electric fields

Abstract: “Smart” polymer gels actively change their size, structure, or viscoelastic properties in response to external signals. The stimuli-responsive properties, indicating a kind of intelligence, offer the possibility of new gel-based technology. The article attempts to review the current status of our knowledge of electromechanical effects that take place in smart polymer gels. Deformation and the mechanism of polyelectrolyte gel behavior in electric fields are first studied experimentally and then theoretically. In particular, the swelling or bending is discussed in detail. Particulate composite gels whose modulus of elasticity can vary in electric fields are revealed as a new smart material. The driving force causing varying elastic modulus in electric fields is explained by a qualitative model based upon polarized particles. Finally, applications of the two electromechanical effects are presented.

Plastic behavior of nanophase Ni: A molecular dynamics computer simulation

Abstract: We report molecular dynamics computer simulations of low temperature-high load plastic deformation of Ni nanophase samples with several mean grain sizes in the range of 3–5 nm. The samples are polycrystals nucleated from different seeds, with random location and orientation. Among the mechanisms responsible for the deformation, grain boundary sliding and motion, as well as grain rotation are identified. No dislocation activity is detected, in contrast to the behavior of coarse grain metals. Interpreting the results in terms of grain boundary viscosity, a linear dependence of strain rate with the inverse of the grain size is obtained.

Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels

Abstract: Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15-30)wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α ' - and £-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy γ fcc and the free energy ΔG γ-e for the γ→e phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γ fcc and therefore strongly suppress the γ→e transformation while silicon decrease γ fcc and sustains the γ-e transformation. The γ→e phase transformation takes place in alloys with γ fcc 20 mJ/m 2 . The stacking fault energy of the Fe-25Mn-3Si-3Al alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm -3 combine high tensile ductility up to 80 % at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extremely high strain rates of about e = 10 3 s -1 results in an extraordinary shock resistence and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates.

Structural features in a brittle–ductile wax model of continental extension

Abstract: Structural features produced during the rifting of continents depend on the layered rheological properties of the crust and lithosphere and, in particular, on the presence of any transitions between brittle and ductile behaviour1. Here we use a wax model to explore the gross structural response of continental lithosphere under pure shear extension in the presence of a continuous brittle–ductile transition. The wax models were deformed under various boundary conditions to reflect a variety of different regions, most notably the Basin and Range province of North America. Our experiments show the development of listric normal faults, structures common to regions of continental extension. We also observe the formation of distributed and discrete rifting, and intrusion and occlusion of the upper brittle layer by the ductile lower layer. The factor controlling deformation style in each case appears to be the relative thickness of the brittle and ductile layers, although a relatively high rate of strain generally promotes discrete rifting.

Deformation of elastomeric ethylene-octene copolymers

Abstract: The elastomeric behavior of low-crystallinity ethylene−octene copolymers prepared by Dow's INSITE constrained geometry catalyst technology is described. Deformation in uniaxial tension was examined as a function of comonomer content and molecular weight. Within the melting range of copolymers, temperature was used as an experimental variable to reveal the relationship between crystallinity and stress response. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions provided the structural basis for analysis of the elastic behavior. The rubber modulus scaled with crystallinity. Furthermore, the dimension of the fringed micellar junction obtained from the modulus correlated well with the average crystallizable sequence length of the copolymer. Because classical rubber theory could not account for the large strain dependence of the modulus, a theory which incorporates the contribution of entanglements to the network response was considered. Slip-lin...

The plasticity of icosahedral quasicrystals

Abstract: The experimental aspects of the plastic deformation of icosahedral quasicrystals are reviewed. Macroscopic experiments, which involve the general investigation of stress-strain curves and the determination of the thermodynamic activation parameters of the deformation process are described. Investigations of the microstructure of plastically deformed samples, studied by means of transmission electron microscopy are presented. Important parameters such as the dislocation density, the Burgers vectors of dislocations, and slip systems are analyzed. Additionally, the results of in-situ straining experiments giving direct insight into the dynamics of the deformation process are presented. Direct conclusions on the nature of the plastic deformation process are drawn, and the current view of the deformation mechanism based on the specific structure of this class of materials is consistently discussed in terms of a qualitative ‘cluster friction model’.

On the formulation of enhanced strain finite elements in finite deformations

Abstract: Presents recent advances obtained by the authors in the development of enhanced strain finite elements for finite deformation problems. Discusses two options, both involving simple modifications of the original enhancement strategy of the deformation gradient as proposed in previous works. The first new strategy is based on a full symmetrization of the original enhanced interpolation fields; the second involves only the transposed part of these fields. Both modifications lead to a significant improvement of the performance in problems involving high compressive stresses, showing in particular a mode‐free response, while maintaining a simple and efficient (strain driven) numerical implementation. Demonstrates these properties with a number of numerical benchmark simulations, including a complete modal analysis of the elements.

Theoretical tensile stress in tungsten single crystals by full-potential first-principles calculations

Abstract: Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength is performed for the first time using full-potential LAPW method. As a specific example, tensile strength of single-crystalline tungsten loaded uniaxially along the (001) and (111) directions is analyzed. Although tungsten is elastically nearly isotropic ( C 44 ≈ C′), theoretical tensile strength exhibits a marked anisotropy ( σ 001 th = 0.289 Mbar, σ 111 th = 0.401 Mbar). This anisotropy is explained in terms of structural energy differences between bcc, fcc and simple cubic structures which occur on the calculated deformation paths. Theoretical results compare favorably with experimental value of 0.247 ± 0.036 Mbar obtained for tungsten whiskers grown along the (110) direction.