Tensile Deformation Process Research Articles

Deformation Dynamics and Young's Modulus of Silver Nanocontacts

We observed the tensile deformation process of silver nanocontacts (Ag NCs) in situ at room temperature by high-resolution transmission electron microscopy. Simultaneously, the conductance and the tensile force of the NCs were measured. Stress-strain relationship of Ag NC was investigated. Typical saw-tooth curves showing elastic and plastic deformations of NCs were observed. The Young's modulus of NCs was estimated from the stress-strain curve. [DOI: 10.1380/ejssnt.2009.621]

Size-dependent shear fracture and global tensile plasticity of metallic glasses

The tensile ductility or brittleness of metallic glasses is found to depend strongly on the critical shear offset. Based on experimental observations, the tensile shear fracture processes of metallic glasses can be divided into three stages: multiplication and coalescence of the free volume, formation of void and the final fast propagation of a shear crack. Accordingly, the size effect on the tensile shear deformation processes of metallic glass can be well understood: with decreasing specimen size smaller than the equivalent critical shear offset, the shear deformation of metallic glass is changed from unstable to stable, which leads to a transition from global brittleness on the macroscale to large global plasticity or even necking on the microscale. These results are fundamentally useful in understanding the physical nature of tensile shear deformation of various metallic glasses and even in the design of new metallic glass materials with good plasticity.

Characteristics of the tensile flow behavior of Ti–46Al–9Nb sheet material – Analysis of thermally activated processes of plastic deformation

Flow behavior, strain hardening and activation parameters, i.e. activation volume, stress exponents and normalized free enthalpy of activation, of Ti–46Al–9Nb sheet with near-gamma microstructure have been investigated in tension tests between 700 and 1000 °C. The dependence of yield stress on temperature and strain rate, the course of the strain hardening curves and the values of activation parameters show that thermally activated dislocation mechanisms are mainly involved in the tensile deformation process of the investigated material. At constant temperature the value of the activation volume depends both on plastic strain and strain rate. The activation volume generally decreases with increasing strain. The decrease is particularly well observable for higher strain rates, thus indicating a growing role of thermally activated climb mechanisms governing the process of dynamic recovery. The activation volume calculated for a constant plastic strain (2% in case of this study) is a function of temperature and strain rate. At lower deformation rates, or alternatively at higher temperatures, the activation volume increases. Such behavior indicates a decrease in dislocation density due to the onset of dynamic recrystallization. The analysis of stress exponents and the obtained free enthalpy of activation confirm that different thermally activated processes are acting during deformation under the tensile test conditions studied.

Elastic Modulus of the Crystalline Regions of Poly (p-phenylene terephthalamide) Single Fiber Using SPring-8 Synchrotron Radiation

The elastic modulus, El, of the crystalline regions of poly (p-phenylene terephthalamide) (PPTA) single fiber in the direction parallel to the chain axis was measured by X-ray diffraction using synchrotron radiation at beamline BL46XU of SPring-8. The El value of the PPTA single fiber was obtained as 156 GPa at room temperature from the initial slope of the stress–strain curve of the crystal lattice. However, the inclination of the stress–strain curve changed at a tensile stress of around 1000 MPa, which produced an El′ value of 199 GPa in a higher stress region. This indicates that the deformation mechanism in the crystal lattice at higher tensile stress was different from that in the initial stage of the tensile deformation. Stress hardening of PPTA was also observed macroscopically during the tensile deformation process. Stress hardening in the crystal lattice was found to directly affect macroscopic stress hardening of the PPTA fiber.

Microstructural Stability of Friction Stirred AA 2218 Alloy on Tensile Properties at Elevated Temperature

The microstructural features of Al-Cu 2218 alloy after FSP were composed mainly of equiaxed 4 μm grains as well as a much more random orientation in SZ comparing to the extruded base metal. In this investigation, tensile properties of FSP specimens displayed an advanced performance than base metal. Furthermore, it should be noted that the FSP specimens possessed great elongation which even more than 100% at 400 °C. As the test temperature over 450°C, the deterioration of the elongation could be responsible to second phase particles coarsening during tensile deformation process as well as abnormal grain growth. Ductility was commonly degenerated in both base metal and FSP specimens at 500°C, whole grains within FSP sample grew into large ones that conducted the seriously descent of ductility. It is rational to deduce that the tensile strain promoted the coarsening of particles that would resulted from Ostwald ripening phenomenon. The declination of elongation could be ascribed mainly to the microstructural stability problem at elevated temperature.

Atomistic dynamics of deformation, fracture, and joining of individual single-walled carbon nanotubes

The tensile deformation process of individual single-walled carbon nanotubes (SWCNTs) was in situ observed with measurements of the mechanical properties by transmission electron microscopy combined with the functions of scanning probe microscopy. The values of Young's modulus, tensile stress and strain at the fracture of SWCNTs are $950\ifmmode\pm\else\textpm\fi{}130\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $25\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and $0.30\ifmmode\pm\else\textpm\fi{}0.03$, respectively. It was shown that two tips of fractured SWCNTs can be recovered to produce a single tubular structure by contact under compressive force and bias voltage. The in situ observations also demonstrated the formation of carbon monatomic wires during the fracture process of SWCNTs.

高炭素冷延鋼板の延性におよぼすミクロ組織の影響

The effect of void initiation due to microstructural factors, including the size and distribution of spheroidized cementite and ferrite grain size, on elongation in JIS S35C and JIS S65C high carbon cold-rolled steel sheets were investigated.Among mechanical properties after 2nd annealing, yield strength (YS) and tensile strength (TS) both increased as cold-rolling reduction increased, and total elongation (T.EI.) also increased simultaneously. This resulted in large improvement in the TS-T.EL. Balance. As mirostructural changes in this case, with the increase in cold-rolling reduction, the ferrite grain diameter decreased and cementite on the ferrite grain boundary increase.According to the results of a detailed study of S35C, when the microstructure was composed of fine ferrite and cementite on the ferrite grain boundary, elongation was high. In the tensile deformation process, destruction of the grain boundary cementitie was frequently observed. Voids were initiated from these points of origin, and fracture then occurred rapidly. On the other hand, when the microstructure was composed of coarse ferrite grains and fine cementite existed within the ferrite grain, elongation was low. In particular, uniform elongation was markedly decreased. In this deformation process, voids were initiated from cementite on deformation bands, then joined to form cracks, resulting in fracture.

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.

Effect of Cyclic Fatigue Conditions on Nonlinear Dynamic Viscoelasticity and Fatigue Behaviors for Short Glass-Fiber Reinforced Nylon 6

Fatigue behaviors of short glass-fiber reinforced nylon 6 (GF/Ny6) under three cyclic fatigue conditions, tension–tension (T–T), tension–compression (T–C) and compression–compression (C–C), were investigated on the basis of nonlinear dynamic viscoelastic analysis. A nonlinear viscoelastic parameter (NVP) was used for quantitative evaluation of nonlinear dynamic viscoelasticity for the (GF/Ny6) during the fatigue process. The magnitude of NVP increased with a decrease in the fatigue lifetime under three cyclic fatigue conditions. Under the T–T type cyclic deformation, glass-fibers that lay parallel to the cyclic deformation direction were broken during the fatigue process. After NVP attained a constant magnitude, cracks propagated perpendicularly to the cyclic deformation direction from the glass-fiber ends and ultimately, the (GF/Ny6) was fractured. The stress distribution in the matrix nylon6 became more inhomogeneous due to both glass-fiber breakage and crack propagation, resulting in an increase in NVP. In the case of the C–C type cyclic deformation, debonding at the (glass-fiber/matrix nylon6) interface occurred during the fatigue process, and the magnitude of NVP was strongly related to the progress of interfacial debonding between glass-fibers and the matrix nylon 6. Under the T–C type cyclic deformation, the magnitude of NVP increased accompanying crack growth during the tensile deformation process and also, interfacial debonding during the compressive deformation process.

Tensile deformation behaviour of high isotactic polypropylenes

The tensile flow behaviour and deformation processes during uniaxial loading of high isotactic (high crystallinity) polypropylenes have been investigated at various strain rates. The strain rate sensitivity index indicated that long chain high isotactic (high crystallinity) polypropylene (iPP-L) is relatively less sensitive to strain rate than short chain high isotactic (high crystallinity) polypropylene (iPP-S). Deformation bands formed at low strain contributed to the separation of fibrils/microfibrils, and on subsequent increase in tensile strain, the deformation bands developed into an array of closely spaced crazes that multiplied with an increase in strain and strain rate, and grew inwards. Three fracture morphologies were identified in iPP-L: brittle fracture, crazing - tearing, and brittle fracture with ductile pulling of fibrils/microfibrils (quasi-cleavage). In iPP-S, the predominant mode of failure at all displacement rates was brittle fracture. However, at lower displacement rates, smooth and circular voids associated with nucleating particle were randomly present on the brittle fracture surface of iPP-S. Occasional wedges were also observed on the tensile deformed surface of iPP-S. The deformation processes have been depicted in terms of strain rate - strain deformation diagrams.

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers.

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.

Microstructural evolution during tensile deformation of polypropylenes

Tensile deformation processes occurring at varying strain rates in high and low crystallinity polypropylenes and ethylene–propylene di-block copolymers have been investigated by scanning electron microscopy. This is examined for both long and short chain polymeric materials. The deformation processes in different polymeric materials show striking dissimilarities in spite of the common propylene matrix. Additionally, the deformation behavior of long and their respective short chain polymers was different. Deformation mechanisms include crazing/tearing, wedging, ductile ploughing, fibrillation, and brittle fracture. The different modes of deformation are depicted in the form of strain rate-strain diagrams. At a constant strain rate, the strain to fracture follows the sequence: high crystallinity polypropylenes<low crystallinity polypropylenes<ethylene–propylene di-block copolymers, indicative of the trend in resistance to plastic deformation.

Acoustic Emission during the Tensile Straining of Paper from Hardwood Pulp

Acoustic emission (AE) that occurred during tensile straining of paper from hardwood pulp with and without dry strength resin was measured and analysed to investigate the process and mechanism of tensile deformation up to maximum load. Maximum amplitude distribution of the AEs that occurred during the straining manifested some changes of micro failures with increases in beating degree and addtion level of dry strength resin. The increase in bonding area by beating and increase in specific bonding strength by addition of the resin cause an increase in failure of strong fiber bonds. Although almost all of micro failures that occurred during the straining up to maximum load are attributed to those of weak fiber bonds, some strong fiber bonds and/or fibers themselves fail just before maximum load for the handsheets from beaten pulp. With addition of the resin, strong fiber bonds and fibers themselves begin to fail even in mid of the plastic deformation.

Ｔｉ／ＴｉＣｐ系焼結複合材料の引張性質に及ぼすＴｉＣ粒子の空間分布の影響

The composites of the different dispersion state were prepared from the titanium powder and the TiC with the different particle sizes. The effect of the difference of spatial distribution of the TiC particles on the tensile properties and the behavior of TiC particles' breaking in the tensile deformation process have been quantitatively investigated. In tensile deformation, TiC particles begin to break just after the yielding of the material. Finally the connection of cumulated cracks of particles brings the propagation of a main crack and fracture of the composite material. The material of which the spatial distribution of the second phase particles is close to random distribution has larger tensile strength and tensile ductility than the material of stronger clustering tendency. The reason for this is that the destruction of TiC particles in the materials of stronger clustering tendency is more frequent from the initial stage of the deformation, because they have more regions of larger local volume fraction and larger local particle density. The tendency, showing that the particles in the clustered regions are easy to be broken, was explained from the result of the elastic-plastic analysis by finite element method.

Correlation between elongation at fracture and strain rate for dynamic loading

Strain rate sensitivity is known to act stabilising on a tensile deformation process. As soon as neck formation starts, the local strain rate in the neck zone increases rapidly. the local flow stress increases as well, so that higher tensile forces are needed for the continuation of extension. Other specimen regions undergo additional deformation, so that the elongation at fracture increases with increasing strain rate sensitivity and extension rate. The flow behaviour of iron, steel, copper, bronze, Tantalum, Al-Alloy and Tungsten is investigated in dynamic tensile and compression tests using a Split-Hopkinson-bar. The elongation at fracture of the tensile specimens of all test materials is found to increase with increasing strain rate. Correlations are formulated of the dependency of elongation at fracture on strain rate as well as on the strain hardening exponent and the viscous damping parameter. A simple model is applied to determine these dependencies quantitatively.

Jerky flow behavior in a rapid solidification processed Al–Li alloy

The specific jerky flow behavior in a rapid solidification processed (RSP) Al–Li alloy has been studied systematically. The results show that stress–strain curves of the as-quenched RSP Al–Li alloy tensioned with a moderate strain rate (6.9×10 −4 s −1) show a typical plateau step feature with obvious physical yielding; the width of each plateau (except for the yield plateau) increases linearly with increasing total strain, while the height of the steps is almost the same for one specimen. Small and regular serrations usually begin to appear beyond the third plateau of transverse specimens, and the size of serrations in each plateau is equal, while that on the latter plateau is larger than that on the former. The specific jerky flow behavior of the RSP Al–Li alloy is suggested to be formed by initiation and propagation of a local deformation band during the tensile deformation process, which is caused by the combined effects of static and dynamic strain aging of Li and Mg atoms. The yield plateau is thought to be related to the static strain aging that occurred in the time period between aging treatment and tensile tests. Other plateaus are suggested to be caused by the first stage of dynamic strain aging (DSA I), while small and regular serrations on plateaus are attributed to the second stage of dynamic strain aging (DSA II). High Li content, high volume fraction of δ′ phase and very fine grains in the RSP Al–Li alloy are major causes leading to the specific jerky flow behavior. The plateau step characteristic in stress–strain curves of longitudinal specimens is not so typical as that of transverse specimens. The influence of artificial aging and tensile strain rates on the jerky flow behavior is also discussed.

Ｉｎ‐ｓｉｔｕ粒子分散型複合材料ＴｉＣ／Ｔｉ‐６Ａｌ‐４Ｖ‐１１Ｃｒの延性破壊

Tensile deformation and ductile fracture process are investigated for titanium alloy matrix composite reinforced with TiC particles in comparison with Ti-6Al-4V alloy. Material used in this work is TiC/Ti-6Al-4V-11Cr composite produced by vacuum arc remelting process. TiC/Ti-6Al-4V-11Cr composite has strong interface between matrix and TiC particles because of in-situ reaction process. Tensile tests for smooth and 1mmR notched round bar specimens are performed to investigate ductile fracture process and influence of stress triaxiality on it. Degradation in ductility due to the stress triaxiality is more significant in TiC/Ti-8Al-0.5Mo-0.5V composite than in Ti-6Al-4V alloy. Most of micro-voids in composite are nucleated from cracking of TiC particles. Unit cell model combined with FEM analysis has revealed that the cracking of TiC particles are caused by mismatching with the matrix due to plastic strain and high stress triaxiality. Difference in process of micro-void coalescence between smooth and notched specimens has been demonstrated in fractographic observation of fracture surfaces.

Acoustic emission studies on welded and thermally treated AISI 304 stainless steel during tensile deformation

The present investigations are planned to study the influence of prior martensites formed due to cold treatment as 77K in AISI 304 SS welded specimens, on strain-induced martensites occurred during tensile deformation using AE technique. AE parameters like count rate and root mean square (r.m.s.) voltage have been used to characterize AE activities generated during tensile deformation process in as-welded and welded-treated samples. Frequency spectrum analysis of AE signals captured from the samples has been done to understand the dynamic behavior of the martensite phase formation. Tensile properties of these samples have also been reported. Volume fraction of the magnetic phase (martensite and delta ferrite) formed in these samples are measured before and after straining. X-ray diffraction (XRD) technique has been used to support the presence of delta ferrite (formed during welding) and martensite in the weld region.

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure.

Transmission electron microscope in-situ observations of twinning in the lamellar microstructure in a TiAl based alloy

The process of tensile deformation of the two-phase lamellar γ(L1 0) α 2(DO 19 ) microstructure in a TiAl based alloy was investigated by in-situ transmission electron microscopy. The strain contrast in “soft” α 2 lamella and twinning in “hard” γ lamella are observed when the lamellar structure is loaded. It is suggested that the relaxation of the twinning strain by α 2 plays an important role in ductility improvement of the lamellar structure and the twinning is an important deformation mechanism in the two-phase TiAl alloys.