Showing papers on "Deformation (engineering) published in 2001"

Mechanical properties and cell cultural response of polycaprolactone scaffolds designed and fabricated via fused deposition modeling

Abstract: A number of different processing techniques have been developed to design and fabricate three-dimensional (3D) scaffolds for tissue-engineering applications. The imperfection of the current techniques has encouraged the use of a rapid prototyping technology known as fused deposition modeling (FDM). Our results show that FDM allows the design and fabrication of highly reproducible bioresorbable 3D scaffolds with a fully interconnected pore network. The mechanical properties and in vitro biocompatibility of polycaprolactone scaffolds with a porosity of 61 +/- 1% and two matrix architectures were studied. The honeycomb-like pores had a size falling within the range of 360 x 430 x 620 microm. The scaffolds with a 0/60/120 degrees lay-down pattern had a compressive stiffness and a 1% offset yield strength in air of 41.9 +/- 3.5 and 3.1 +/- 0.1 MPa, respectively, and a compressive stiffness and a 1% offset yield strength in simulated physiological conditions (a saline solution at 37 degrees C) of 29.4 +/- 4.0 and 2.3 +/- 0.2 MPa, respectively. In comparison, the scaffolds with a 0/72/144/36/108 degrees lay-down pattern had a compressive stiffness and a 1% offset yield strength in air of 20.2 +/- 1.7 and 2.4 +/- 0.1 MPa, respectively, and a compressive stiffness and a 1% offset yield strength in simulated physiological conditions (a saline solution at 37 degrees C) of 21.5 +/- 2.9 and 2.0 +/- 0.2 MPa, respectively. Statistical analysis confirmed that the five-angle scaffolds had significantly lower stiffness and 1% offset yield strengths under compression loading than those with a three-angle pattern under both testing conditions (p < or = 0.05). The obtained stress-strain curves for both scaffold architectures demonstrate the typical behavior of a honeycomb structure undergoing deformation. In vitro studies were conducted with primary human fibroblasts and periosteal cells. Light, environmental scanning electron, and confocal laser microscopy as well as immunohistochemistry showed cell proliferation and extracellular matrix production on the polycaprolactone surface in the 1st culturing week. Over a period of 3-4 weeks in a culture, the fully interconnected scaffold architecture was completely 3D-filled by cellular tissue. Our cell culture study shows that fibroblasts and osteoblast-like cells can proliferate, differentiate, and produce a cellular tissue in an entirely interconnected 3D polycaprolactone matrix.

Mechanical Properties of Carbon Nanotubes

Abstract: This paper presents an overview of the mechanical properties of carbon nanotubes, starting from the linear elastic parameters, nonlinear elastic instabilities and buckling, and the inelastic relaxation, yield strength and fracture mechanisms. A summary of experimental findings is followed by more detailed discussion of theoretical and computational models for the entire range of the deformation amplitudes. Non-covalent forces (supra-molecular interactions) between the nanotubes and with the substrates are also discussed, due to their significance in potential applications.

Intermittent dislocation flow in viscoplastic deformation

Abstract: The viscoplastic deformation (creep) of crystalline materials under constant stress involves the motion of a large number of interacting dislocations. Analytical methods and sophisticated 'dislocation dynamics' simulations have proved very effective in the study of dislocation patterning, and have led to macroscopic constitutive laws of plastic deformation. Yet, a statistical analysis of the dynamics of an assembly of interacting dislocations has not hitherto been performed. Here we report acoustic emission measurements on stressed ice single crystals, the results of which indicate that dislocations move in a scale-free intermittent fashion. This result is confirmed by numerical simulations of a model of interacting dislocations that successfully reproduces the main features of the experiment. We find that dislocations generate a slowly evolving configuration landscape which coexists with rapid collective rearrangements. These rearrangements involve a comparatively small fraction of the dislocations and lead to an intermittent behaviour of the net plastic response. This basic dynamical picture appears to be a generic feature in the deformation of many other materials. Moreover, it should provide a framework for discussing fundamental aspects of plasticity that goes beyond standard mean-field approaches that see plastic deformation as a smooth laminar flow.

New discoveries in deformed metals

Abstract: Microstructural analysis by advanced and automated methods has allowed deformation microstructures to be quantified in terms of common structural parameters. This quantification has shown for a variety of materials and processing conditions that the microstructural evolution follows a universal pattern of grain subdivision from the macroscale to the nanometer scale. This microstructural evolution has been described empirically and in theoretical models based on general principles for the formation of dislocation structures during plastic deformation by slip. The similarity between the behavior of materials undergoing different deformation patterns forms the basis for future research and development encompassing traditional as well as new materials and processes.

Modelling of TWIP effect on work-hardening

Abstract: Austenitic steels can exhibit both high strength and ductility due to a particularly high work hardening rate. Among all the possible deformation modes for austenitic steels, Twinning Induced Plasticity (TWIP) has the most beneficial effect on the work-hardening. It is believed that deformation twins increase the work-hardening rate by acting as obstacles for gliding dislocations. Many studies have investigated this point experimentally using microscopy. On a physical basis, the purpose of this study is to develop a work-hardening model taking into account the interaction between twinning and dislocation gliding. The results from the model are in good agreement with the tensile test results.

Deformation behavior and plastic instabilities of ultrafine-grained titanium

Abstract: Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals.

The influence of stacking fault energy on the mechanical behavior of Cu and Cu-Al alloys : Deformation twinning, work hardening, and dynamic recovery

Abstract: The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78 ergs/cm2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm2, respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and 6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys (low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al alloys.

FTIR AND DSC STUDIES OF MECHANICALLY DEFORMED β-PVDF FILMS

Abstract: Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the β-phase were studied by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The main goal of this study was to improve the understanding of the structural changes that occur in β-PVDF during a mechanical deformation process. FTIR spectroscopy was used to examine the structural variations as a function of strain. DSC data allowed measurement of the melting temperatures and enthalpies of the material before and after deformation, providing information about the changes in the crystalline fraction. After the molecular vibrations were assigned to the corresponding vibrational modes, we investigated the energy and intensity variations of these vibrations at different deformations. A reorientation of the chains from perpendicular to parallel to the stress direction was observed to occur in the plastic region. During the deformation, a decrease in the degree of crystallinity of the material was observed, but ...

Three-dimensional maps of grain boundaries and the stress state of individual grains in polycrystals and powders

Abstract: A fast and non-destructive method for generating three-dimensional maps of the grain boundaries in undeformed polycrystals is presented. The method relies on tracking of micro-focused high-energy X-rays. It is verified by comparing an electron microscopy map of the orientations on the 2.5 × 2.5 mm surface of an aluminium polycrystal with tracking data produced at the 3DXRD microscope at the European Synchrotron Radiation Facility. The average difference in grain boundary position between the two techniques is 26 µm, comparable with the spatial resolution of the 3DXRD microscope. As another extension of the tracking concept, algorithms for determining the stress state of the individual grains are derived. As a case study, 3DXRD results are presented for the tensile deformation of a copper specimen. The strain tensor for one embedded grain is determined as a function of load. The accuracy on the strain is Δ∊ ≃ 10−4.

Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

Abstract: The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models. © 2001 Elsevier Science B.V. All rights reserved.

Mixed-mode fracture analyses of plastically-deforming adhesive joints

Abstract: A mode-dependent embedded-process-zone (EPZ) model has been developed and used to simulate the mixed-mode fracture of plastically deforming adhesive joints. Mode-I and mode-II fracture parameters obtained from previous work have been combined with a mixed-mode failure criterion to provide quantitative predictions of the deformation and fracture of mixed-mode geometries. These numerical calculations have been shown to provide excellent quantitative predictions for two geometries that undergo large-scale plastic deformation: asymmetric T-peel specimens and single lap-shear joints. Details of the deformed shapes, loads, displacements and crack propagation have all been captured reasonably well by the calculations.

Applying Anand Model to Represent the Viscoplastic Deformation Behavior of Solder Alloys

Abstract: A unified viscoplastic constitutive law, the Anand model, was applied to represent the inelastic deformation behavior for solders used in electronic packaging. The material parameters of the constitutive relations for 62Sn36Pb2Ag, 60Sn40Pb, 96.5Sn3.5Ag, and 97.5Pb2.5Sn solders were determined from separated constitutive relations and experimental results. The achieved unified Anand model for solders were tested for constant strain rate testing, steady-state plastic flow and stress/strain responses under cyclic loading. It is concluded that the Anand model can be applied for representing the inelastic deformation behavior of solders at high homologous temperature and can be recommended for finite element simulation of the stress/strain responses of solder joints in service. @DOI: 10.1115/1.1371781#

Dynamic Recrystallization in Pure Magnesium

Abstract: Microstructural evolution of commercial grade pure magnesium was studied during plastic deformation by torsion under high pressure at ambient temperature and by compression at temperatures ranging from 293 to 773 K and at a strain rate of 3 x 10 -3 s -1 . Grain refinement takes place by operation of dynamic recrystallization (DRX) at all examined temperatures. The mechanisms of DRX change with temperature and strain. As a result, unusual dependencies of recrystallized grain size against strain and recrystallized volume fraction against temperature are observed. In the temperature interval of 293-623 K the deformation twinning results in twin mechanism of DRX, which processes strain softening at an initial stage of deformation. At T ≤ 423 K the other mechanism of low temperature DRX takes place at high strains. Such DRX is accompanied by strain hardening. In contrast, continuous DRX (CDRX) yielding a steady-state flow operates frequently at temperatures ranging from 523 to 773 K. CDRX occurs mainly in overall recrystallization process at elevated temperatures. Discontinuous DRX (DDRX) takes place by bulging of boundaries of coarse recrystallized grains evolved from twins at T = 723 K. DDRX occurs repetitively, but gives an insignificant contribution into total recrystallization process. The present results suggest that the mechanisms of DRX and the deformation mechanisms are closely related.

The adiabatic correction factor for deformation heating during the uniaxial compression test

Abstract: The isothermal uniaxial compression test is a common method to determine the flow stress of metals. For accurate flow stress data at strain rates >10−3 s−1, the data must be corrected for flow softening due to deformation heating. The first step in the correction is to determine the increase in temperature. An adiabatic correction factor, η, is used to determine the temperature between strain rates of 10−3 to 101 s−1. The adiabatic correction factor is the fraction of adiabatic heat retained in the workpiece after heat loss to the dies, η=(ΔT ACTUAL)/(ΔT ADIABATIC), where ΔT ADIABATIC=(0.95 f σdɛ)/(ρC p ). The term η is typically taken to be constant with strain and to vary linearly (0 to 1) with log ( $$\dot \varepsilon $$ ) between 10−3) and 101 s−1. However, using the finite element method (FEM) and a one-dimensional, lumped parameter method, η has been found to vary with strain, die and workpiece thermal conductivities, and the interface heat-transfer coefficient (HTC). Using the lumped parameter method, an analytical expression for η was derived. In this expression, η is a function of the die and workpiece thermal conductivities, the interface heat-transfer coefficient, workpiece heat capacity, strain, and strain rate. The results show that an increase in the HTC or thermal conductivity decreases η.

Hydrogen induced shear localization of the plastic flow in metals and alloys

Abstract: Hydrogen enhanced localized plasticity (HELP) is a viable mechanism for hydrogen embrittlement supported by experimental observations. According to the HELP mechanism, hydrogen induced premature failures result from hydrogen induced plastic instability which leads to hydrogen assisted localized ductile processes. The objective of this work is to reveal the role of hydrogen in possibly localizing the macroscopic deformation into bands of intense shear using solid mechanics methodology. The hydrogen effect on material deformation is modeled through the hydrogen induced volume dilatation and the reduction in the local flow stress upon hydrogen dissolution into the lattice. Hydrogen in assumed to reside in both normal interstitial lattice sites (NILS) and reversible traps associated with the plastic deformation. The analysis of the plastic deformation and the conditions for plastic flow localization are carried out in plane strain uniaxial tension. For a given initial hydrogen concentration in the unstressed specimen, a critical macroscopic strain is identified at which shear localization commences.

A high-strain-rate superplastic ceramic

Abstract: High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10-2 to 10-1 s-1 and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based and magnesium-based alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10-5 to 10-4 s-1 for most oxides and nitrides with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and alpha-alumina phases exhibits superplasticity at strain rates up to 1 s-1. The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s-1. The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.

Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification

Abstract: Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The porosity depends on the melting temperature of copper melt, while the orientation of the pores is controlled by the freezing direction. The anisotropy in the uniaxial tensile behavior of the porous copper is examined. The ultimate tensile strength and the yield strength of the porous copper with the cylindrical pores orientated parallel to the tensile direction decrease linearly with increasing the porosity. For the porous copper whose pore axes are perpendicular to the tensile direction, the ultimate tensile strength decreases significantly with increasing the porosity at low porosity, while the yield strength reaches a maximum at a low porosity and then decreases with increasing the porosity. These results are explained in terms of the stress concentration induced in the vicinity of the pores during tensile deformation.

In situ transmission electron microscopy studies of shear bands in a bulk metallic glass based composite

Abstract: In situ straining transmission electron microscopy (TEM) experiments were performed to study the propagation of the shear bands in the Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5 bulk metallic glass based composite. Contrast in TEM images produced by shear bands in metallic glass and quantitative parameters of the shear bands were analyzed. It was determined that, at a large amount of shear in the glass, the localization of deformation occurs in the crystalline phase, where formation of dislocations within the narrow bands are observed.

Shear localization and recrystallization in dynamic deformation of 8090 Al-Li alloy

Abstract: The microstructural evolution in localized shear deformation was investigated in an 8090 Al-Li alloy by split Hopkinson pressure bar (strain rate of approximately 10(3) s(-1)) at ambient temperature and 77 K. The alloy was tested in the peak-, over-, under-, and natural-aged conditions, that provide a wide range of microstructural parameters and mechanical properties. Two types of localized shear bands were distinguished by optical microscopy: the deformed shear band and the white-etching shear band. They form at different stages of deformation during localization. There are critical strains for the occurrence of deformed and white-etching localized shear deformation, at the imposed strain rate. Observations by transmission electron microscopy reveal that the white-etching bands contain fine equiaxed grains; it is proposed that they are the result of recrystallization occurring during localization. The deformed-type bands are observed after testing at 77 K in all heat treatment conditions, but they are not as well defined as those developed at ambient temperature. Cracking often occurs along the localized shear at ambient temperature. The decrement in temperature is favorable for the nucleation, growth and coalescence of the microcracks along the shear bands, inducing fracture. (C) 2001 Elsevier Science B.V. All rights reserved.