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Mechanical behaviors in MD simulations and experiments: loading rate

Kejie Zhao's picture

As well known that most materials properties and behaviors are loading rate sensitive. Even with the current fastest computations, however, the MD method is still too time-consuming to simulate the experiments in laboratories, or to simulate the deformation process lasting longer than 1us. As result, the MD method deem to be suitable for high loading rate (10^6~10^12 /s) or extremely short time process. Of couse this does not invalidate the great value for MD to provide helpful insights on the mechanism of macro deformation, even at some 'unrealistic' conditions (seen Xi Chen's discussion) It seems that MD method can never be applied in simulating the quasi-static cases in laboratory. Actually many contiuum models have not involved loading rate factor. But, is it the case? Or can we give a point-point relation for atomistic loading rate <->experimental rate, for example, maybe someday we can say the applied strain rate 10^5~10^6 in MD can be seen as the exact quasi-static case (10^-2 /s) in experiments? 

I take two simple examples for instance.

  strain rate VS strength_triaxial                 strain rate VS strength_uniaxial

                      Fig.1                                                                       Fig. 2

Fig. 1 and Fig.2 show the applied strain rate dependence of incipient yield strength, for nano-voided Cu under hydrostatic loading and uniaxial loading, respectively. We can see from two figures an apparent loading rate dependence, the higher one behaves stronger, partly due to atomic inertia effect. However, lower than the values given, this trend becomes moderate. Is it saying this decrease will convert when applied strain rate is 10^4 or higher? So that the MD simulated value could be comparable with quasi-stactic experimental value? Of cource this guess has not been proven by super computation, but hope to hear from you.  

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