Modified Model of Crack Tip Stress Field Considering Dislocation Slip Accumulation and Crack Tip Blunting

This study uses the digital image correlation technique to measure the crack tip displacement field at various crack lengths in U71MnG rail steel,and the interpolated continuous displacement field was obtained by fitting with a back propagation(BP)neural network.The slip and stacking of dislocations affect crack initiation and growth,leading to changes in the crack tip field and the fatigue characteristics of crack growth.The Christopher-James-Patterson(CJP)model describes the elastic stress field around a growing fatigue crack that experiences plasticity-induced shielding.In the present work,this model is modified by including the effect of the dislocation field on the plastic zone of the crack tip and hence on the elastic field by introducing a plastic flow factorρ,which represents the amount of blunting of the crack tip.The Levenberg-Marquardt(L-M)nonlinear least squares method was used to solve for the stress intensity factors.To verify the accuracy of this modified CJP model,the theoretical and experimental plastic zone errors before and after modification were compared,and the variation trends of the stress intensity factors and the plastic flow factorρwere analysed.The results show that the CJP model,with the introduction ofρ,exhibits a good blunting trend.In the low plasticity state,the modified model can accurately describe the experimental plastic zone,and the modified stress intensity factors are more accurate,which proves the effectiveness of dislocation correction.This plastic flow correction provides a more accurate crack tip field model and improves the CJP crack growth relationship.

Robust inversion analysis of local gravity anomalies caused by geological dislocation model of faults

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Establishment and validation of standardized animal models of spinal cord injury by normal external force-caused fracture dislocation

The duplication of animal models plays a key role in spinal cord injury research; however, there has been limited study into normal, external force-derived fracture dislocation. This study adopted experimental devices, designed in-house, to construct standardized ventral and dorsal spinal cord injury animal models of 6 g and 17 g falling from a height of 2, 4, and 10 cm, and 15, 30 or 50 g transversal compression on the spinal cord. The results showed that gradual increases in the degree of histopathological injury led to decreased Tarlov and Basso, Beattie and Bresnahan scores for the behavioral test, and increased Ashworth scores for the hind limb. Furthermore, there was a gradual decline in the slope test in the rats with dorsal spinal cord injury that correlated to increases in the falling substance weight or falling height. Similar alterations were observed in the ventral spinal cord injured rats, proportional to the increase in compression weight. Our experimental findings indicate that the standardized experimental rat models of dorsal and ventral spinal cord injury are stable, reliable and reproducible.

A CONSTITUTIVE MODEL FOR CREEP-PLASTICITY INTERACTION BASED ON DISLOCATION DYNAMICS

A glide-plus-climb micromechanism of dislocation evolution with the formation of subgrains is pro-posed for modelling of the creep-plasticity interaction (CPI). The long-range internal stress can be divided intothe resistance for dislocation climb in subgrain boundaries and that for dislocation glide within grains orsubgrains. Their evolution equations are then derived based on dislocation dynamics. Furthermore, a unifiedconstitutive model for CPI is developed from Orowan’s formula. Theoretical calculations on the basis of thismodel show a very good agreement between the model prediction and experimental results of benchmark testsfor 2 1/4 Cr -1 Mo steel at 600℃.

CYCLIC DEFORMATION OF FACE CENTERED CUBIC CRYSTALS AND ITS DISLOCATION INTERACTION MODEL——Ⅱ.DISLOCATION INTERACTION MODEL OF CYCLIC DEFORMATION

A dislocation interaction model has been proposed for cyclic deformation of fcc crystals.Ac-cording to this model,cyclic stress-strain responses and saturation dislocation structures of acrystal are associated with the modes and intensities of dislocation interactions between slipsystems active in the crystal; and,hence,may be predicted by the location of its tensile axis inthe crystallographic triangle.This model has successfully explained the different behaviours ofdouble-slip crystals and multi-slip behaviours of some crystals with orientations usually con-sidered as single-slip ones.

Analyses on gravity variation before and after the Lijiang earthquake based on a finite rectangular dislocation model

The methods were discussed to calculate the gravity variation due to crustal deformation based on a model of dis-location on a finite rectangular plane. Taking the Lijiang MS=7.0 earthquake as an example the calculating princi-ple of fault parameters were determined, and the results were given. Of particular interests were the characteristics of the gravity variations in different dislocation types. With comparison between the calculated results and the practical measurements, it was found that the model could to some extent account for the observations. But it failed to give explanations to the more far spatial gravity variation.

DISLOCATION EMISSION AND EQUATION OF ENERGY BARRIERS AT MODEL I CRACK TIP

The propagation for the model I crack in aluminum single crystal has been directly studied by in-situ TEM observation.The equation of energy barrier of the dislocation building-up and emission at the model I crack tip has been established by means of Peierls-Nabarro dislocation model and starting from angle of energy.By means of calculation,the critical value of spontaneous emission of the dislocations from tip of the model I crack was obtained.

Properties of multi-Gaussian Schell-model beams carrying an edge dislocation propagating in oceanic turbulence

Based on the theory of coherence, the model of multi-Gaussian Schell-model(MGSM) beams carrying an edge dislocation generated by the MGSM source is introduced. The analytical cross-spectral density of MGSM beams carrying an edge dislocation propagating in oceanic turbulence is derived, and used to study the evolution properties of the MGSM beams carrying an edge dislocation. The results indicate that the MGSM beam carrying an edge dislocation propagating in oceanic turbulence will evolve from the profile with two intensity peaks into a flat-topped beam caused by the MGSM source, and the beam will evolve into the Gaussian-like beam due to the influences of oceanic turbulence in the far field.As the propagation distance increases, the MGSM beam carrying an edge dislocation propagating in oceanic turbulence with the larger rate of dissipation of mean-squared temperature(χT) and ratio of temperature to salinity contribution to the refractive index spectrum(?) or the smaller rate of dissipation of kinetic energy per unit mass of fluid(ε) evolves into the flat-topped beam or a Gaussian beam faster.

Microstructure, Dislocation Density and Thermal Expansion Behavior Using Thermo Elastic Models of Zircon Sand Reinforced as Cast ZA-27 Composites

In the present work stir casting route is used to fabricate the ZA27 Metal matrix composites containing 3 wt%, 6 wt%, 9 wt%, and 12 wt%. Zircon sand particulates of size 100 mesh. Microstructure studies using Optical Microscopy, SEM-EDAX are carried out to ascertain the distribution and morphology of particulates in the composites. Effect of zircon sand as reinforcement on bulk density, porosity, of the fabricated composites is studied. SEM studies are carried out to understand the behavior of as-cast ZA27 alloy reinforced with zircon sand. The dislocation density of the fabricated composite affects the strength of the composites and depends on the strain due to thermal mismatch and is found to increase with increase in weight% of zircon sand. However, it does not consider casting defects of voids/clustering observed in micrographs of the fabricated composite. Porosity in composites does not have influence on Coefficient of thermal expansion (CTE) of the ZA27 composites studied using thermoelastic models like Kerner and turner model and rule of mixtures of composite.

Dislocation Model and Morphology Simulation of bcc<->fcc Martensitic Transformation

By using molecular dynamics computer simulation at atomic level, the effects of single dislocation and dipole dislocations on nucleation and growth of martensitic transformation have been studied. It was found that only the location of tension or compression stress fields of the dislocations are favorable for martensite nucleation in NiAl alloy and the dislocations can move to accommodate partly the transformation strain during the nucleation and growth of martensite. Combined with the molecular dynamics simulation, a two dimensional simulation for martensite morphology based on a dislocation model has been performed. Many factors related to martensitic transformation were considered, such as supercooling, interface energy, shear strain, normal strain and hydrostatic pressure. Different morphologies of martensites, similar to lath, lenticular, thin plate, couple-plate and lenticular couple-plate martensites observed in Fe-C and Fe-Ni-C alloys, were obtained.

Aseismic negative dislocation model and deformation analysis of crustal horizontal movement during 1999--2001 in the northeast margin of Qinghai-Xizang block

Through numerical simulation for GPS data, aseismic negative dislocation model for crustal horizontal movement during 1999～2001 in the northeast margin of Qinghai-Xizang block is presented, combined with the spatial distribution of apparent strain field in this area, the characteristics of motion and deformation of active blocks and their boundary faults, together with the place and intensity of strain accumulation are analyzed. It is shown that: a) 9 active blocks appeared totally clockwise motion from eastward by north to eastward by south. Obvious sinistral strike-slip and NE-NEE relative compressive motion between the blocks separated by Qilianshan-Haiyuan fault zone was discovered; b) 20 fault segments (most of them showed compression) locked the relative motion between blocks to varying degrees, among the total, the mid-east segment of Qilianshan fault (containing the place where it meets Riyueshan-Lajishan fault) and the place where it meets Haiyuan fault and Zhuanglanghe fault, more favored accumulation of strain. Moreover, the region where Riyueshan-Lajishan fault meets north boundary of Qaidam block may have strain accumulation to some degree. c) Obtained magnitude of block velocities and locking of their boundaries were less than relevant results for observation in the period of 1993～1999.

Negative Dislocation Model and the Movement Pattern of a Fault:Determination of Potential Earthquake Sources in Western Yunnan

To determine the locked and slip parts on a fault,we simulated the movement pattern of the fault with a combined model consisting of a negative dislocation in the locked area superimposed on a relative movement between two sides of the fault.In this way,we determined the locked parts of faults by inversion with multiple epochs of geodetic data.In this paper,we have determined the movement patterns of four main faults in western Yunnan by inversion with two epochs of geodetic surveys and some crossing-fault observations.From the inversion result,we have identified the areas around Dali and Xiangyun to be the locked area,which is regarded as a sign of potential earthquake sources.

REPRESENTATION OF PLASTICITY AT AN INTERFACE CRACK BY INCLINED STRIP YIELD SUPERDISLOCATION MODEL

The basic equations of the representation of plasticity at an interfacecrack by an inclined strip yield superdislocation model are derived.With a specialcombination of the material properties and small-scale yielding case,the problem isreduced to an algebraic equation in an unknown,the ratio of the plastic zone size.Somediscussions on the limitations of this model are presented.

Unusual Behaviour of Dislocation Velocity Exponent in Dislocation Multiplication Model

The chaotic behavior of dislocation multiplication process is investigated. The change of Lya-punov exponent which is used to determine the stability of quasi-periodic and chaotic behavior as well as that of equilibrium points and periodic solution is reported using an iteration model of dislocation multiplication. An unusual behavior of Lyapunov exponent and Feigenbaum ex-ponent which respond to the geometric convergence of orbit from bifurcation to chaos is shown by dislocation velocity exponent m and there is a distinction on the tendency of convergence for the dislocation multipIication model when it is compared with togistic map. lt is reasonable for the difference to be analyzed from the materials viewpoint.

Effects of Earth's curvature and radial heterogeneity in dislocation studies:Case studies of the 2008 Wenchuan earthquake and the 2004 Sumatra earthquake

Recently,effects of Earth's curvature and radial heterogeneity on coseismic deformations are often investigated based on the 2004 Sumatra earthquake.However,such effects are strongly related to earthquake types.As a low dip angle event,the 2004 Sumatra earthquake is not a good seismic case for such a topic since the effects for moderate dip angle events are much bigger.In this study,the half-space and spherical dislocation theories are used,respectively,to calculate coseismic displacements caused by the 2008 Wenchuan earthquake and the 2004 Sumatra earthquake.Effects of Earth's curvature and stratification are investigated through the discrepancies of results calculated using the two dislocation theories.Results show that the effects of Earth's curvature and stratification for the 2008 Wenchuan earthquake are much larger than those for the 2004 Sumatra earthquake.Ignoring the effects will cause errors up to 100%-200% in far field displacements for a moderate dip angle event like the 2008 Wenchuan earthquake.Such great effects are much bigger than those conclusions of previous studies.Besides,comparison with observations verifies that spherical dislocation theories yield better results than half-space ones in far fields.

Design and Performance Analysis of Axial Flux Permanent Magnet Machines with Double-Stator Dislocation Using a Combined Wye-Delta Connection

Conventional fractional slot concentrated winding three-phase axial flux permanent magnet machines have an abundance of armature reaction magnetic field harmonics which deteriorate the torque performance of the machine.This paper presents a double-stator dislocated axial flux permanent magnet machine with combined wye-delta winding.A wye-delta(Y-△)winding connection method is designed to eliminate the 6 th ripple torque generated by air gap magnetic field harmonics.Then,the accurate subdomain method is adopted to acquire the no-load and armature magnetic fields of the machine,respectively,and the magnetic field harmonics and torque performance of the designed machine are analyzed.Finally,a 6 k W,4000 r/min,18-slot/16-pole axial flux permanent magnet machine is designed.The finite element simulation results show that the proposed machine can effectively eliminate the 6 th ripple torque and greatly reduce the torque ripple while the average torque is essentially identical to that of the conventional three-phase machines with wye-winding connection.

Dislocation parameters of Gonghe earthquake jointly inferred by using genetic algorithms and least squares method

The Second Crustal Deformation Monitoring Center, China Seismological Bureau, has detected a marked upliftassociated with the Gonghe Ms=7.0 earthquake on April 26, 1990, Qinghai Province. From the observed verticaldeformations and using a rectangular uniform slip model in a homogenous elastic half space, we first employ genetic algorithms (GA) to infer the approximate global optimal solution, and further use least squares method to getmore accurate global optimal solution by taking the approximate solution of GA as the initial .parameters of leastsquares. The inversion results show that the causative fault of Gonghe Ms=7.0 earthquake is a right-lateral reversefault with strike NW60, dip SW and dip angle 37, the coseismic fracture length, width and slip are 37 km , 6 kmand 2.7 m respectively. Combination of GA and lean squares algorithms is an effective joint inversion method,which could not only escape from local optimum of least squares, but also solve the slow convergence problem ofGA after reaching adjacency of global optimal solution.

Numerical analysis of interaction between solute atom and extended dislocation using force multipoles

The interaction between a solute atom and an extended dislocation was investigated using a continuum approximation method with force multipoles.The dislocation core structure of extended dislocation was modeled with the Peierls-Nabarro model discretized with a number of infinitesimal Volterra dislocations.The interaction energy and force between a nickel solute atom and perfect and extended dislocation in copper were successfully calculated using the force multipoles.The results clearly show that the core structure of extended dislocation weakens the interaction with solute atoms.The interaction energy and force for extended dislocations are almost the half of those for perfect dislocations.

EFFECT OF TEMPERATURE ON DISLOCATION EMISSION FROM CRACK TIP FOR BCC METAL MO

The effect of thermally activated energy on the dislocation emission from a crack tip inBCC metal Mo is simulated in this paper.Based on the correlative reference model on which the flexi-ble displacement boundary scheme is introduced naturally,the simulation shows that as temperature in-creases the critical stress intensity factor for the first dislocation emission will decrease and the totalnumber of emitted dislocations increase for the same external load.The dislocation velocity and exten-sive distance among partial dislocations are not sensitive to temperature.After a dislocation emission,two different deformation states are observed,the stable and unstable deformation states.In the stabledeformation state,the nucleated dislocation will emit from the crack tip and piles up at a distance faraway from the crack tip,after that the new dislocation can not be nucleated unless the external load-ing increases.In the unstable deformation state,a number of dislocations can be emitted from thecrack tip continuously under the same external load.