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Influence of loading rate on dislocation

Source:   Date:2014-10-28 15:46:34   Views:

When the external load conditions change, such as the loading rate is increased or temperature changes on material properties and the past has been very different. Such as low temperatures, lower intensity, brittle material; original case under normal circumstances does not appear at this time will suddenly occur to produce a great deal of security and safety risks. Many such examples before it happened. With the increase of the loading speed, fracture toughness of the material does not change much, but tends to decrease, but only slightly. After exceeding a certain speed, the fracture toughness of the material will rapidly rise, the material tends to plasticity. Whether it is plastic deformation or damage and failure occur, are due at the microscopic motion of dislocations caused. Now we analyze the effects of loading rate on dislocation motion.
Dislocation is the force acting on the material within the material atoms in such relative movement, leaving the original arrangement of atoms in a linear lattice defect shifted neat form. Make the crystal along the slip plane when the external force to continue to promote the role of dislocation motion relative movement area continues to expand. When out of crystal dislocations will give a slip, causing permanent deformation of the material, the result is a lot of plastic deformation dislocation motion. This is normal loads, the dislocation movement speed when loading some changes will occur.
Under the sealer high speed loading density of dislocations formed to be higher than normal cases and more uniform distribution, already mentioned above, the main reason for the increase in density is very high rate of formation of dislocations formed when loading, and dynamic recovery rate is relatively low, Another sealer to increase speed when loading high amount of stress leads to partial dislocation sources for several days. Besides loads of time and intensity also has a great influence on the formation of dislocations. With increasing pressure, the dislocation density will also increase. Dislocation density is proportional to the square root of the load and force, but when the load pressure reaches 100 GPa, high temperatures due to the load generated in a short time can not be passed out so the relationship will no longer apply.
With the increase of the speed of the load, the deformation dislocation region becomes small, the branch to reduce the dislocation. The main reason for this situation is that the load loading faster, increasing the dislocation is generated at this time, to accelerate the movement of dislocations, the movement of dislocations concentrated in the load direction. Loading rate is increased, the dislocation linear velocity is substantially increased. Since the relatively short duration of action, reduction of the dislocation to the transmitter in the other direction, the occurrence of width reduction movement of dislocations, plastic deformation regions of the material decreases.
During high-speed packaging tip seal cutting machine cutting, sealing and cutting machine shear rate is relatively high. Since the role of clamping force, the packaging of each part by force, only near the tip of cutting edge material with serious stress concentration, resulting in the dislocation slip is confined within a small range near the cutting edge. With the sliding dislocation, then the grain boundary becomes a slip dislocation is further barriers blocked large accumulation of dislocations, when the number of bit error accumulation reaches a certain value, the material will be concentrated stress crack formation. High-speed sealer sealing and cutting machine shearing shear rate is relatively high when the moving speed of dislocations increases, the crack tip dislocations primarily edge dislocations, too late to spread around in a short time, the cracks along the cutting edge linear extension direction of movement, to ensure relatively flat section. In addition, the plastic deformation of the material is also due to the movement of dislocations formed during sealing and cutting machine to cut, crack tip dislocation to spread around to reduce the crack tip plastic deformation region is also reduced accordingly. Therefore, the plastic deformation region in the vicinity of the package is only cutting edge, and to a lesser extent. Thus, packaging plastic deformation sealer to reduce shear when the material tends to be brittle fracture. Sealing and cutting machine shearing section corresponding increase in quality.
From the above analysis, we can draw the following conclusions: Package sealing and cutting machine shearing, the sealer shear failure was mainly due to the stress intensity factor K11 cutting edge tip attachment material is greater than the critical stress intensity factor of the material Km, resulting in the formation of crack further expansion until the final fracture separation. In doing high-speed sealer shear analysis, the critical stress intensity factor Kn} material can not be calculated using the calculation method of the stress intensity factor at low speeds because of the critical stress intensity factor fracture toughness of the material that is affected by the speed of the impact occurs changes in fracture toughness at high speed significantly lower than the fracture toughness of the material at low speed, the speed has little effect on the fracture toughness by the material at low speed, when a speed higher than the fracture toughness sharply with the increase in speed increased, namely the existence of a minimum fracture toughness, there are also corresponding to the minimum speed when the fracture toughness. Packaged in this high speed of sealer shear, fracture toughness minimum basic material for the whole fracture material brittle fracture, fracture surface quality at this time is best. In the micro level, within the material under high-speed loads sealer lattice dislocation slip generate increased number and density more uniform. Generation of dislocations in the load of sliding, dislocation slip accumulation phenomenon occurs when the grain boundary dislocations encountered obstacles, causing stress concentration. When the stress concentration reaches a certain value, it will transgranular fracture occurs at grain boundaries. Dislocation slip speed with increasing load speed increases, the formation of dislocation reduction in diffusion in the other direction in a short time. Therefore, the crack tip plastic zone is small, the material tends to be brittle fracture, better quality section.