Elastic interaction between a moving screw dislocation and a surface crack
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A moving screw dislocation near a surface crack was investigated using dislocation modeling. Motion directions parallel (x direction) and perpendicular (y direction) to the crack surface were considered. Due to the free surface, the net Burgers vector inside the crack is zero. After obtaining the dislocation distribution in the crack, we calculated the stress field in the medium. Relative to a static screw dislocation, the magnitude of cryz due to the moving screw dislocation decreases with increasing velocity Vx. Generally, the effect of dislocation shielding on fracture is reduced if the velocity Vx increases. The magnitude of the image force of the dislocation also decreases with increasing velocity Vx. The effect of velocity along the y direction on the stress intensity factor and image force has the opposite trend to that along the x direction. The present result can reduce to a moving dislocation near a semi-infinite crack and a static dislocation near a surface crack.
I. INTRODUCTION Dislocations in the vicinity of the crack tip play an important role in fracture, resulting in much research. The effect of a static dislocation shielding near a semiinfinite crack was studied by Rice and Thomson1 and by Shiue and Lee2 using a thermodynamic approach, and by Majumdar and Burns3 using the method of conformal mapping. The effect of a static dislocation shielding on a surface crack was explored by Juang and Lee 4 using dislocation modeling and by Chu5 using conformal mapping. A static screw dislocation near an internal crack was analyzed by Eringen6 using nonlocal elasticity, and by Kirchner and Michot7 using the weight function. A moving screw dislocation has an analogy with a line charge. Based on the analogy between screw dislocation and line charge, Eshelby8 suggested the Lorentz force on the moving screw dislocation. This issue was discussed by Stroh,9 Nabarro,10 and Lothe." Weertman,12 and Weertman and Weertman13 investigated the moving dislocations in detail. Lund 1415 studied a moving string-like dislocation and the Lorentz force. The dislocation emission criteria and criteria for fracture were studied by Lin and Thomson16 using the static dislocation and by Brock17 using the moving dislocation. Zhao, Dai, and Li18 studied the dynamic emission of dislocations from a semi-infinite crack tip. Zhao and Li19 and Huang, Lee, and Yu20>21 studied the unloading behavior of dislocations emitted from a semi-infinite crack and a finite crack, respectively. Their calculation18"21 was based on the assumption that a dislocation moves with a velocity proportional to the effective shear stress to the third power. However, their 2668
J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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effective stresses were obtained by using a static dislocation. Their effective shear stress seems to be modified by using a moving dislocation. Although we studied a moving screw dislocation near a semi-infinite crack,22 the effect of a moving dislocation on fracture toughness depends on crack g
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