Deformation behavior and microstructure effect in 2124Al/SiCp composite

Dynamic compression tests were performed by means of a Split Hopkinson Pressure Bar (SHPB). Test materials were 2124Al alloys reinforced with 17% volume fraction of 3, 13 and 37 μm SiC particles, respectively. Under strain rate ε = 2100 l/s, SiC particles have a strong effect on σ_0.2 of the composites and the σ_0.2 increases with different SiC particle size in the following order: 2124Al-alloy → 124Al/SiC_p (37 μm) → 2124Al/SiC_p (13 μm) → 2124Al/SiC_p (3 μm), and the strain hardening of the composites depends mainly on the strain hardening of matrix, 2124A1 alloy. The results of dimensional analysis present that the flow stress of these composites not only depends on the property of reinforcement and matrix but also relates to the microstructure scale, matrix grain size, reinforcement size, the distance between reinforcements and dislocations in matrix. The normalized flow stress here is a function of inverse power of the edge-edge particle spacing, dislocation density and matrix grain size. Close-up observation shows that, in the composite containing SiC particles (3 μm), localized deformation formed readily comparing with other materials under the same loading condition. Microscopic observations indicate that different plastic flow patterns occur within the matrix due to the presence of hard particles with different sizes.

Stress state in front of a crack perpendicular to bimaterial interface

Using a dislocation simulation approach, the basic equation for a crack perpendicular to a bimaterial interface is formulated in this paper. A novel expansion method is proposed for solving the problem. The complete solution for the problem, including the T stress ahead of the crack tip and the stress intensity factors are presented. The stress field characteristics are analyzed in detail. It is found that ahead of the crack tip and near the interface the normal stress, perpendicular to the crack plane, sigma(x), is characterized by the K fields and the normal stress sigma(y) is dominated by the K field plus T stress in the region of 0 < r/b < 0.4 for b/a(0) less than or equal to 0.1, where b is the distance from the crack tip to the interface.

Energy Principle And Nonlinear Electric-Mechanical Behavior Of Ferroelectric Ceramics

Many experimental observations have shown that a single domain in a ferroelectric material switches by progressive movement of domain walls, driven by a combination of electric field and stress. The mechanism of the domain switch involves the following steps: initially, the domain has a uniform spontaneous polarization; new domains with the reverse polarization direction nucleate, mainly at the surface, and grow though the crystal thickness; the new domain expands sideways as a new domain continues to form; finally, the domain switch coalesces to complete the polarization reversal. According to this mechanism, the volume fraction of the domain switching is introduced in the constitutive law of the ferroelectric material and used to study the nonlinear constitutive behavior of a ferroelectric body in this paper. The principle of stationary total potential energy is put forward in which the basic unknown quantities are the displacement u(i), electric displacement D-i and volume fraction rho(I) of the domain switching for the variant I. The mechanical field equation and a new domain switching criterion are obtained from the principle of stationary total potential energy. The domain switching criterion proposed in this paper is an expansion and development of the energy criterion established by Hwang et al. [ 1]. Based on the domain switching criterion, a set of linear algebraic equations for determining the volume fraction rho(I) of domain switching is obtained, in which the coefficients of the linear algebraic equations only contain the unknown strain and electric fields. If the volume fraction rho(I) of domain switching for each domain is prescribed, the unknown displacement and electric potential can be obtained based on the conventional finite element procedure. It is assumed that a domain switches if the reduction in potential energy exceeds a critical energy barrier. According to the experimental results, the energy barrier will strengthen when the volume fraction of the domain switching increases. The external mechanical and electric loads are increased step by step. The volume fraction rho(I) of domain switching for each element obtained from the last loading step is used as input to the constitutive equations. Then the strain and electric fields are calculated based on the conventional finite element procedure. The finite element analysis is carried out on the specimens subjected to uniaxial coupling stress and electric field. Numerical results and available experimental data are compared and discussed. The present theoretic prediction agrees reasonably with the experimental results.

Crack propagation in the power-law nonlinear viscoelastic material

An analysis on crack creep propagation problem of power-law nonlinear viscoelastic materials is presented. The creep incompressilility assumption is used. To simulate fracture behavior of craze region, it is assumed that in the fracture process zone near the crack tip, the cohesive stress sigma(f) acts upon the crack surfaces and resists crack opening. Through a perturbation method, i. e., by superposing the Mode-I applied force onto a referential uniform stress state, which has a trivial solution and gives no effect on the solution of the original problem, the nonlinear viscoelastic problem is reduced to linear problem. For weak nonlinear materials, for which the power-law index n similar or equal to 1, the expressions of stress and crack surface displacement are derived. Then, the fracture process zone local energy criterion is proposed and based on which the formulas of cracking incubation time t

Electromechanical influence of crack velocity at bifurcation for poled ferroelectric materials

The piezoelastodynamic field equations are solved to determine the crack velocity at bifurcation for poled ferroelectric materials where the applied electrical field and mechanical stress can be varied. The underlying physical mechanism, however, may not correspond to that assumed in the analytical model. Bifurcation has been related to the occurrence of a pair of maximum circumferential stress oriented symmetrically about the moving crack path. The velocity at which this behavior prevails has been referred to as the limiting crack speed. Unlike the classical approach, bifurcation will be identified with finite distances ahead of a moving crack. Nucleation of microcracks can thus be modelled in a single formulation. This can be accomplished by using the energy density function where fracture initiation is identified with dominance of dilatation in relation to distortion. Poled ferroelectric materials are selected for this study because the microstructure effects for this class of materials can be readily reflected by the elastic, piezoelectic and dielectric permittivity constants at the macroscopic scale. Existing test data could also shed light on the trend of the analytical predictions. Numerical results are thus computed for PZT-4 and compared with those for PZT-6B in an effort to show whether the branching behavior would be affected by the difference in the material microstructures. A range of crack bifurcation speed upsilon(b) is found for different r/a and E/sigma ratios. Here, r and a stand for the radial distance and half crack length, respectively, while E and a for the electric field and mechanical stress. For PZT-6B with upsilon(b) in the range 100-1700 m/s, the bifurcation angles varied from +/-6degrees to +/-39degrees. This corresponds to E/sigma of -0.072 to 0.024 V m/N. At the same distance r/a = 0.1, PZT-4 gives upsilon(b) values of 1100-2100 m/s; bifurcation angles of +/-15degrees to +/-49degrees; and E/sigma of -0.056 to 0.059 V m/N. In general, the bifurcation angles +/-theta(0) are found to decrease with decreasing crack velocity as the distance r/a is increased. Relatively speaking, the speed upsilon(b) and angles +/-theta(0) for PZT-4 are much greater than those for PZT-6B. This may be attributed to the high electromechanical coupling effect of PZT-4. Using upsilon(b)(0) as a base reference, an equality relation upsilon(b)(-) < upsilon(b)(0) < upsilon(b)(+) can be established. The superscripts -, 0 and + refer, respectively, to negative, zero and positive electric field. This is reminiscent of the enhancement and retardation of crack growth behavior due to change in poling direction. Bifurcation characteristics are found to be somewhat erratic when r/a approaches the range 10(-2)-10(-1) where the kinetic energy densities would fluctuate and then rise as the distance from the moving crack is increased. This is an artifact introduced by the far away condition of non-vanishing particle velocity. A finite kinetic energy density prevails at infinity unless it is made to vanish in the boundary value problem. Future works are recommended to further clarify the physical mechanism(s) associated with bifurcation by means of analysis and experiment. Damage at the microscopic level needs to be addressed since it has been known to affect the macrocrack speeds and bifurcation characteristics. (C) 2002 Published by Elsevier Science Ltd.

A field model interpretation of crack initiation and growth behavior in ferroelectric ceramics: change of poling direction and boundary condition

Strain energy density expressions are obtained from a field model that can qualitatively exhibit how the electrical and mechanical disturbances would affect the crack growth behavior in ferroelectric ceramics. Simplification is achieved by considering only three material constants to account for elastic, piezoelectric and dielectric effects. Cross interaction of electric field (or displacement) with mechanical stress (or strain) is identified with the piezoelectric effect; it occurs only when the pole is aligned normal to the crack. Switching of the pole axis by 90degrees and 180degrees is examined for possible connection with domain switching. Opposing crack growth behavior can be obtained when the specification of mechanical stress sigma(infinity) and electric field E-infinity or (sigma(infinity), E-infinity) is replaced by strain e and electric displacement D-infinity or (epsilon(infinity), D-infinity). Mixed conditions (sigma(infinity),D-infinity) and (epsilon(infinity),E-infinity) are also considered. In general, crack growth is found to be larger when compared to that without the application of electric disturbances. This includes both the electric field and displacement. For the eight possible boundary conditions, crack growth retardation is identified only with (E-y(infinity),sigma(y)(infinity)) for negative E-y(infinity) and (D-y(infinity), epsilon(y)(infinity)) for positive D-y(infinity) while the mechanical conditions sigma(y)(infinity) or epsilon(y)infinity are not changed. Suitable combinations of the elastic, piezoelectric and dielectric material constants could also be made to suppress crack growth. (C) 2002 Published by Elsevier Science Ltd.

Energy principle of ferroelectric ceramics and single domain mechanical model

Many physical experiments have shown that the domain switching in a ferroelectric material is a complicated evolution process of the domain wall with the variation of stress and electric field. According to this mechanism, the volume fraction of the domain switching is introduced in the constitutive law of ferroelectric ceramic and used to study the nonlinear constitutive behavior of ferroelectric body in this paper. The principle of stationary total energy is put forward in which the basic unknown quantities are the displacement u (i) , electric displacement D (i) and volume fraction rho (I) of the domain switching for the variant I. Mechanical field equation and a new domain switching criterion are obtained from the principle of stationary total energy. The domain switching criterion proposed in this paper is an expansion and development of the energy criterion. On the basis of the domain switching criterion, a set of linear algebraic equations for the volume fraction rho (I) of domain switching is obtained, in which the coefficients of the linear algebraic equations only contain the unknown strain and electric fields. Then a single domain mechanical model is proposed in this paper. The poled ferroelectric specimen is considered as a transversely isotropic single domain. By using the partial experimental results, the hardening relation between the driving force of domain switching and the volume fraction of domain switching can be calibrated. Then the electromechanical response can be calculated on the basis of the calibrated hardening relation. The results involve the electric butterfly shaped curves of axial strain versus axial electric field, the hysteresis loops of electric displacement versus electric filed and the evolution process of the domain switching in the ferroelectric specimens under uniaxial coupled stress and electric field loading. The present theoretic prediction agrees reasonably with the experimental results given by Lynch.

Numerical study of bifurcation solutions of spherical Taylor-Couette flow

The steady bifurcation flows in a spherical gap (gap ratio sigma=0.18) with rotating inner and stationary outer spheres are simulated numerically for Re(c1)less than or equal to Re less than or equal to 1 500 by solving steady axisymmetric incompressible Navier-Stokes equations using a finite difference method. The simulation shows that there exist two steady stable flows with 1 or 2 vortices per hemisphere for 775 less than or equal to Re less than or equal to 1 220 and three steady stable flows with 0, 1, or 2 vortices for 1 220

Sidewall flow stability in the MHD channel flows

The velocity distribution between two sidewalls is; M-shaped for the MHD channel flows with rectangular cross section and thin conducting walls in a strong transverse magnetic field. Assume that the dimensionless numbers R(m) much less than 1, M, N much greater than 1, and sigma

Molecular dynamics studies on the dislocation gliding near a tilt boundary

The gliding behavior of edge dislocation near a grain boundary(QB) in copper under pure shear stresses is simulated by using molecular dynamics(MD) method. Many-body potential incorporating the embedded atom method (EAM) is used. The critical shear stresses for a single disocation to pass across GB surface are obtained at values of sigma(c)=23MPa similar to 68 MPa and 137 MPa similar to 274 MPa for Sigma=165 small angle tilt GB at 300 K and 20 K, respectively. The first result agrees with the experimental yield stress sigma(y)(=42 MPa) quite well. It suggests that there might be one of the reasons of initial plastic yielding caused by single dislocation gliding across GB. In addition, there might be possibility to obtain yield strength from microscopic analysis. Moreover, the experimental value of sigma(y) at low temperature is generally higher than that at room temperature. So, these results are in conformity qualitatively with experimental fact. On the other hand, the Sigma=25 GB is too strong an obstacle to the dislocation. In this case, a dislocation is able to pass across GB under relatively low stress only when it is driven by other dislocations. This is taken to mean that dislocation pile-up must be built up in front of this kind of GB, if this GB may take effect on the process of plastic deformation.

Molecular dynamics studies on the local disordering of σ3 and σ11 grain boundaries in aluminium bicrystal

Molecular dynamics (MD) simulations using Morse interaction potential are performed in studies of [110] symmetrical tilt grain boundary (GB) structures with mis-orientation angles 50.5 degrees(Sigma 11), 129.5 degrees(Sigma 11), 70.5 degrees(Sigma 3) and 109.5 degrees(Sigma 3) at various tempratures. The GB structures are found to start local disordering at about 0.5T(m)(T-m is the melting point of aluminium) for 50.5 degrees(Sigma 11), 0.32T(m) for 129.5 degrees(Sigma 11) and 0.38T(m) for 70.5 degrees(Sigma 3), respectively. These results agree with conclusions deduced from the anelastic measurements. But, for twin-boundary structure 109.5 degrees(Sigma 3), this disordering has not been found even when temperature increases up to 0.9T(m).

Singular behaviour near the tip of a sharp V-notch in a power law hardening material

This paper presents an asymptotic analysis of the near-tip stress and strain fields of a sharp V-notch in a power law hardening material. First, the asymptotic solutions of the HRR type are obtained for the plane stress problem under symmetric loading. It is found that the angular distribution function of the radial stress sigma(r) presents rapid variation with the polar angle if the notch angle beta is smaller than a critical notch angle; otherwise, there is no such phenomena. Secondly, the asymptotic solutions are developed for antisymmetric loading in the cases of plane strain and plane stress. The accurate calculation results and the detailed comparisons are given as well. All results show that the singular exponent s is changeable for various combinations of loading condition and plane problem.

Fatigue crack-growth from a circular notch under high-levels of biaxial stress

A series of experiments have been conducted on cruciform specimens to investigate fatigue crack growth from circular notches under high levels of biaxial stress. Two stress levels (Δ&sigma;₁= 380 and 560 MPa) and five stress biaxialities (λ=+1.0, +0.5, 0, −0.5 and −1.0; where λ=&sigma;₂/&sigma;₁ were adopted in the fatigue tests in type 316 stainless steel having a monotonic yield strength of 243 MPa. The results reveal that fatigue crack growth rates are markedly influenced by both the stress amplitude and the stress biaxiality. A modified model has been developed to describe fatigue crack growth under high levels of biaxial stress.

Effect of grain size on slow fatigue crack propagation and plastic deformation near crack tip

Fatigue crack growth and its threshold are investigated at a stress ratio of 0.5 for the three-point bend specimen made of Austenitic stainless steel. The effect of grain size on the crack tip plastic deformation is investigated. The results show that the threshold value Δkth increases linearly with the square root of grain size d and the growth rate is slower for materials with larger grain size. The plastic zone size and ratio for different grain sizes are different at the threshold. The maximum stress intensity factor is kmax and &sigma;ys is the yield strength. At the same time, the characteristics of the plastic deformation development is discontinuous and anti-symmetric as the growth rate is increased from 2·10—8 to 10−7 mm/cycle.

Proceedings of the Second PICES Workshop on the Okhotsk Sea and Adjacent Areas [Nemuro, Japan, November 9-12, 1998]

Table of Contents [pdf, 0.01 Mb] Preface [pdf, 0.01 Mb] Masaaki Aota Long-term tendencies of sea ice concentration and air temperature in the Okhotsk Sea coast of Hokkaido [pdf, 0.05 Mb] Hajime Ito & Miki Yoshioka Geography of the seasonally ice covered seas [pdf, 0.5 Mb] George V. Shevchenko & Victor F. Putov On wind and tide induced sea-ice drift on the northeastern shelf of Sakhalin Island (analysis of radar data) [pdf, 0.96 Mb] Boris S. Dyakov, A.A. Nikitin, L. S. Muktepavel & T.A. Shatilina Variability of the Japan and Okhotsk Seas ice cover depending on geopotential field H500 over the Far-Eastern region [pdf, 0.10 Mb] Aleksandr G. Petrov & Nikolay A. Rykov Intermediate cold layer and ice cover in the Sea of Okhotsk [pdf, 0.37 Mb] Vladimir Ponomarev, Olga Trusenkova, Elena Ustinova & Dmitry Kaplunenko Interannual variations of oceanographic and meteorological characteristics in the Sea of Okhotsk [pdf, 0.16 Mb] George V. Shevchenko & Akie Kato Seasonal and interannual changes of atmospheric pressure, air and water temperature in the area of the Kuril Ridge [pdf, 0.13 Mb] George V. Shevchenko & Vladimir Yu. Saveliev Spatial variability of the wind field in the area of the Kuril Islands [pdf, 0.15 Mb] Alexander L. Figurkin & Igor A. Zhigalov Seasonal variability and specifity of the oceanological conditions in the northern Okhotsk Sea in 1997 [pdf, 1.04 Mb] Igor A. Zhabin Ventilation of the upper portion of the intermediate water in the Okhotsk Sea [pdf, 0.80 Mb] Vladimir A. Luchin & Alexander L. Figurkin Oceanographic conditions over the Kashevarov Bank [pdf, 0.61 Mb] Toshiyuki Awaji, Tomohiro Nakamura, Takaki Hatayama, Kazunori Akimoto & Takatoshi Takizawa Tidal exchange through the Kuril Straits [pdf, 2.01 Mb] Tomohiro Nakamura, Toshiyuki Awaji, Takaki Hatayama, Kazunori Akimoto, Takatoshi Takizawa & Masao Fukasawa Vertical mixing induced by tidally generated internal waves in the Kuril Straits [pdf, 0.83 Mb] Katsuro Katsumata & Ichiro Yasuda Water exchange between the Okhotsk Sea and the North Pacific Ocean estimated by simple models [pdf, 0.97 Mb] Konstantin A. Rogachev Oyashio west path culmination as the consequence of a rapid thermohaline transition in the Pacific Subarctic [pdf, 0.22 Mb] Yasuhiro Kawasaki On the year-to-year change in subarctic water characteristics around the Kuril Islands [pdf, 0.39 Mb] Alexander L. Figurkin & Evgeniy E. Ovsyannikov Influence of oceanological conditions of the West Kamchatka shelf waters on spawning grounds and on pollock egg distribution [pdf, 0.97 Mb] Igor E. Kochergin & Alexander A. Bogdanovsky Transport and turbulence characteristics for the northeastern Sakhalin shelf conditions [pdf, 0.08 Mb] Igor E. Kochergin, Alexander A. Bogdanovsky, Valentina D. Budaeva, Vyacheslav G. Makarov, Vasily F. Mishukov, S.N. Ovsienko, Victor F. Putov, L.A. Reitsema, J.W. Sciallabba, O.O. Sergucheva & P.V. Yarosh Modeling of oil spills for the shelf conditions of northeastern Sakhalin [pdf, 0.32 Mb] Valentina D. Budaeva & Vyacheslav G. Makarov A peculiar water regime of currents in the area of eastern Sakhalin shelf [pdf, 0.66 Mb] Nikolay A. Rykov The oceanographic databases on the Sakhalin shelf [pdf, 0.27 Mb] Akifumi Nakata, Iori Tanaka, Hiroki Yagi, Tomomi Watanabe, Gennady A. Kantakov & Andrew D. Samatov Formation of high-density water (over 26.8 sigma-t) near the La Perouse Strait (the Soya Strait) [pdf, 0.09 Mb] Minoru Odamaki & Kouji Iwamoto Currents and tidal observations by Hydrographic Department of Maritime Safety Agency, off the Okhotsk coast of Hokkaido [pdf, 0.16 Mb] Yasushi Fukamachi, Genta Mizuta, Kay I. Ohshima, Motoyo Itoh, Masaaki Wakatsuchi & Masaaki Aota Mooring measurements off Shiretoko Peninsula, Hokkaido in 1997-1998 [pdf, 0.19 Mb] Mikhail A. Danchenkov, David Aubrey & Stephen C. Riser Oceanographic features of the La Perouse Strait [pdf, 0.91 Mb] Iori Tanaka & Akifumi Nakata Results of direct current measurements in the La Perouse Strait (the Soya Strait), 1995-1998 [pdf, 0.06 Mb] Gennady A. Kantakov & George V. Shevchenko In situ observations of Tsushima and West-Sakhalin currents near La Perouse (Soya) Strait [pdf, 0.79 Mb] Irina Y. Bragina Geographical and biological characteristics of the net zooplankton in the southwestern part of the Sea of Okhotsk during 1987-1996 [pdf, 0.27 Mb] List of corresponding authors [pdf, 0.01 Mb] (Document pdf contains 193 pages)