| The characteristic of materials will be changed when the grain size is refined to the range of100nm to1000nm, which is considered as Ultra-Fine Grain (UFG) materials, such as increasingthe strength, lowered the melt point, enhancing the toughness, and so on. All these are the apparentphysical properties of UFG materials. The other mechanic properties, such as the formation ofadiabatic shear band (ASB) and fracture, of these materials will be further researched in theprocess of engineering application.In order to analyze the formation of ASB under higher loading strain rate in the UFGmaterials, one-dimensional characteristic-line method is used to simulate the formation of ASBunder different loading condition in the UFG copper, as well as considering the affection of grainsize, the affection of strain rate and the influence of the ambient temperature. To the affection ofgrain size, the rotational softening model proposed by Joshi and Ramesh[93-94]is considered in ourconstitute model. To ensure the reliability of our model used in the numerical calculation, theresults of the stress vs. strain curve from the experiments and the numerical calculation iscompared, which shows a greatly similarity in whole, indicating that the model used here isreliable. Based on this, the response behaviors of the UFG copper (FCC) and UFG steel (BCC)under the same loading strain rate are compared, finding that the influence of rotational softeningto different types of material is so great when they are different in lattice configuration. To the firstmaterial, the rotation of grain is main reason for unloading of stress rather than a formation of ASBstrictly. To the latter one, the rotation of grain is just unloading the stress in some degree while heatsoftening is the main factor for stress unloading, which is leading to a real ASB. According theprevious literatures, we can get the information that BCC materials are easier to form ASB thanFCC materials, which is accompany with the calculated results. In order to make a furtherunderstanding of the influence of different factors in the formation of ASB in UFG copper, thevariation regulation of each variable for the materials in different grain sizes, a list of loading strainrate and ambient temperature is simulated, showing that:(1) Refining the grain size and increasingthe loading strain rate is surely to accelerating the rotation speed of grain, which are to reduce the strain when the grain turns to the softest direction.(2) Elevating the ambient temperature is tolower down the rotation speed, leading a higher strain when the grain turns to the softest direction.Based on the research of single ASB, multi-ASB is studied while increasing the regulardistribution defects in the sample calculated in the paper, with a same distribution for each defectsas the single one. Since the unloading of stress in the single ASB research is not presenting a realASB, the results of changing grain size, loading strain rate and ambient temperature is similar tothat of the single one when increasing the defects in process of researching multi-ASB.Two-dimensional characteristic-line method is attempted for the sake of the propagation ofASB in the plate. In this paper, the recurrence relations of the variables on each node in the plateare obtained when the double characteristic-line method is employed to disperse the equations inthe condition of plane strain problem, adopting the visco-elasticity constitute model provided byPerzyna[109-112]. Some examples under elasticity present that it is useful to show the propagation ofstress wave in the plate. But the calculated results also tell that this method will bring some loss ofequations on the boundary corners, which is producing some errors in these nodes, accumulated asmistakes, leading to the instability of the last results. Therefore, a further improvement is needed tocomplete this method to finally analysis the propagation of ASB in the plate. |
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