The behavior of screw dislocations dynamically emitted from the tip of a surface crack during loading and unloading

1995 ◽  
Vol 10 (1) ◽  
pp. 183-189 ◽  
Author(s):  
C.C. Huang ◽  
C.C. Yu ◽  
Sanboh Lee
Keyword(s):  
Surface Crack ◽  
Crack Tip ◽  
Friction Stress ◽  
Critical Stress Intensity Factor ◽  
Dislocation Model ◽  
Dislocation Emission ◽  
Screw Dislocations ◽  
Free Zone ◽  
Discrete Dislocation ◽  
Loading And Unloading

The behavior of screw dislocations dynamically emitted from the tip of a surface crack during loading and unloading has been investigated using a discrete dislocation model. The critical stress intensity factor at the crack tip for dislocation emission is a function of friction stress, core radius of dislocation, and dislocations near the crack tip. During motion, the velocity of dislocation is assumed to be proportional to the effective shear stress to the third power. The effect of crack length and friction stress on dislocation distributions, plastic zone, and dislocation-free zone during loading and unloading was examined.

Continuum Mechanics-Discrete Defect Modeling and Bubble Raft Simulation of Cracked Specimen Response in Nanoscale Geometries

2002 ◽  
Vol 740 ◽  
Author(s):  
Michael J. Starr ◽  
Walter J. Drugan ◽  
Maria d. C. Lopez-Garcia ◽  
Donald S. Stone
Keyword(s):  
Fracture Behavior ◽  
Crack Tip ◽  
Crack Size ◽  
Dislocation Model ◽  
Dislocation Emission ◽  
Crack Location ◽  
Discrete Dislocation ◽  
Emission Behavior ◽  
The Continuum ◽  
Continuum Elasticity

ABSTRACTIn a continuation of prior work, a new group of Bragg bubble model experiments have been performed to explore the effects of nanoscale crack size and nanoscale structural geometry on atomically-sharp crack tip dislocation emission behavior. The experiments have been designed to correspond to the theoretical limits that bound the expected crack tip response. Continuum elasticity analyses of these situations have also been carried out, in which the leading-order terms in the Williams expansion (the K and T terms) are determined, and the predictions of these continuum analyses coupled with discrete dislocation theory are compared with the experimental results. The experiments exhibit fascinating changes in crack tip dislocation emission direction with changing crack and structural size, crack location and loading conditions, as well as substantial changes in the magnitude of the resolved shear stress that drives dislocation emission. These changes are predicted well by the continuum elasticity-discrete dislocation model down to extremely small dimensions, on the order of a few atomic spacings. Preliminary experiments were performed with layered and two-atom basis rafts to establish crucial comparisons between theory and experiment that validate the applicability of continuum elasticity theory to make predictions directly related to nanoscale fracture behavior.

Film Effects On Ductile/Brititle Behavior In Stress-Corrosion Cracking

1995 ◽  
Vol 409 ◽  
Author(s):  
Tong-Yi Zhang ◽  
Wu-Yang Chu ◽  
Ji-Mei Xiao
Keyword(s):  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Critical Stress ◽  
Fracture Behavior ◽  
Applied Load ◽  
Crack Tip ◽  
Critical Stress Intensity Factor ◽  
Corrosion Cracking ◽  
Dislocation Emission ◽  
Mode Iii

AbstractThe present work analyzes the effects of a passive film formed during stress corrosion cracking on ductile/brittle fracture behavior, considering the interaction of a screw dislocation with a thin film-covered mode III crack under an applied remote load. Exact solutions are derived, and the results show that the crack stress field due to the applied load is enhanced by a harder film or abated by a softer film. The critical stress intensity factor for dislocation emission from the crack tip is greatly influenced by both the stiffness and thickness of the filn. A dislocation is more easily to be emitted from the crack tip if the covered film has a shear modulus larger than that of the substrate. The opposite is also true, i.e., a softer film makes dislocation emission more difficult. Both phenomena become more significant when the film thickness is smaller.

Dislocation emission criterion: Grain boundary effect

1993 ◽  
Vol 8 (8) ◽  
pp. 1853-1857 ◽  
Author(s):  
Sham-Tsong Shiue ◽  
Tong-Yi Zhang ◽  
Sanboh Lee
Keyword(s):  
Grain Size ◽  
Stress Intensity ◽  
Grain Boundary ◽  
Plastic Zone ◽  
Critical Stress ◽  
Boundary Effect ◽  
Critical Stress Intensity Factor ◽  
Dislocation Emission ◽  
Brittle Behavior ◽  
Free Zone

Based on the results of Shiue and Lee [J. Appl. Phys. 70, 2947 (1991)], the effect of plastic zone and grain boundary on the dislocation emission criterion was investigated. The emission criterion is based on the concept of spontaneous emission. The critical stress intensity factor for dislocation emission increases with the increasing size of dislocation-free zone and the number of piled-up dislocations in the plastic zone, but decreases with increasing grain size. The ductile versus brittle behavior of material was determined by the competition of critical stress intensity factors for dislocation emission and crack propagation. A material with larger grain size is easier to emit dislocation and allows more dislocations to be piled up, so that it behaves more ductile.

Effect of nanoscale amorphization on dislocation emission from a semi-elliptical surface crack tip in nanocrystalline materials

2021 ◽  
pp. 108128652110451
Author(s):  
Fujun Jiang ◽  
Min Yu ◽  
Xianghua Peng ◽  
P.H. Wen
Keyword(s):  
Numerical Analysis ◽  
Crack Length ◽  
Surface Crack ◽  
Nanocrystalline Materials ◽  
Crack Tip ◽  
Emission Angle ◽  
Curvature Radius ◽  
Complex Method ◽  
Dislocation Emission ◽  
The Impact

An impact analysis model is built to describe the effect of nanoscale amorphization on dislocation emission from a surface semi-elliptical crack tip in nanocrystalline materials. The nanoscale amorphization is formed by the splitting transformation of grain boundary(GB)disclinations caused by the motion of GBs. The analytical solution of the model is obtained by using the complex method, and the influence of nanoscale amorphization, dislocation emission angle, crack length, and curvature radius of surface crack tip on the critical stress intensity factor (SIF) of the first dislocation emission is investigated through numerical analysis. The numerical analysis shows that the impact of nanoscale amorphization on the critical SIF corresponding to dislocation emission depends on the dislocation emission angle, the position and the size of the nanoscale amorphous, the curvature radius, and the length of surface crack. As the curvature radius of surface crack tip and the crack length increase, the normalized critical SIF increases. When the nanoscale amorphization size is small, it has a great impact on the critical SIF for dislocation, but when the size is relatively large, the effect becomes small. The effect of the increasing strength of the nanoscale amorphization on dislocation emission from the surface crack tip is related to the distance between the nanoscale amorphization and the crack tip, and there is a critical crack-junction for which the increase of dislocation strength has little effect on dislocation emission.

The unloading behavior of screw dislocations emitted from a surface crack tip

1993 ◽  
Vol 140 (2) ◽  
pp. 369-379 ◽  
Author(s):  
C. C. Huang ◽  
S. Lee ◽  
C. C. Yu
Keyword(s):  
Surface Crack ◽  
Crack Tip ◽  
Screw Dislocations

Atomistic Study of Cleavage and Dislocation Emission in NiAl

1994 ◽  
Vol 364 ◽  
Author(s):  
M. Ludwig ◽  
P. Gumbsch
Keyword(s):  
Finite Element ◽  
Mixed Mode ◽  
Crack Tip ◽  
Mode I ◽  
Crack Plane ◽  
Dislocation Emission ◽  
Dislocation Generation ◽  
Embedded Atom ◽  
Atomistic Study ◽  
Eam Potential

AbstractThe atomistic processes during fracture of NiAl are studied using a new embedded atom (EAM) potential to describe the region near the crack tip. To provide the atomistically modeled crack tip region with realistic boundary conditions, a coupled finite element - atomistic (FEAt) technique [1] is employed. In agreement with experimental observations, perfectly brittle cleavage is observed for the (110) crack plane. In contrast, cracks on the (100) plane either follow a zig-zag path on (110) planes, or emit dislocations. Dislocation generation is studied in more detail under mixed mode I/II loading conditions.

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