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Ductile Failure Of A Flawed FCC Solid Under Tension
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The work-hardening scenario
You want to break a paper clip. Instinctively, you bend it back and forth until it breaks in two pieces. This works, but you may not appreciate why it does. If you are observant, you notice that with each bend the wire becomes warmer in the region surrounding the bend. There is evidence of local stretching or distortion at the bend, and it appears that this region broadens with further bending. The characteristic flexibility, or ductility, of the metal "stiffens" with each bend, and it is finally transformed into something that can no longer support further deformation. Similar to the behavior of a brittle material, the final bend cracks the metal clip across the flexed region. This process is called "work hardening." What has taken place? Many things suggest that that the metal is undergoing change with each bend. The appearance of heat and the change of surface texture are two such things. The most significant feature is the permanence of the bend; the wire does not return to its original unbent state when the applied force is removed. Unlike a rubber, this indicates that a permanent material change has taken place in the metal. For metals with the face-centered-cubic packing of atoms (fcc), certain dislocations flow easily through the solid when external stress is applied, and this explains the ductility of the solid. Equally important, these dislocations are easily created at imperfections in the solid, especially from the apex of microcracks existing at surfaces and in the bulk. There are typically not enough dislocations originally present in most crystals to account for the extensive slip that can occur in a ductile material. Their creation in vast amounts will occur at severe stress concentrations, such as at crack tips, enabling a stressed metal to be rapidly filled with dislocations needed for material deformation under a steady load or rapid blow.
From this description, we can imagine that by bending a metal at a particular location dislocations are created at solid-state imperfections and flow through the solid. With continual bending, more of the same thing happens. The dislocations can disappear when they come to a surface (forming a step), but not much else happens. While interplanar atomic slippage can explain permanent deformation, the material should remain ductile. However, it does not for an important reason. Colliding dislocations can cause permanent atomic relocation along a line, called a rigid junction or sessile dislocation. These rigid junctions are obstacles to further dislocation mobility. If the junction density is sufficiently high, dislocation mobility becomes insignificant, and ductility of the metal ceases. The metal no longer can bend through dislocation creation and motion, hence it must break under sufficient stress. The ductile metal becomes brittle through this work hardening process.
The computer experiment setup
The computer simulations descriptions
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