PDF《pure metals》

1 Modelling and Simulation in Materials Science and Engineering The role of the mobility law of dislocations in the plastic response of shock loaded pure metals Be?at?Gurrutxaga-Lerma1 Department of Mechanical Engineering, Imperial College London, Exhibition Rd, SW7 2AZ London, UK E-mail: bg374@cam.

ac.uk Received

3 May 2016, revised

6 June

2016 Accepted for publication

21 June

2016 Published

11 July

2016 Abstract This article examines the role that the choice of a dislocation mobility law has in the study of plastic relaxation at shock fronts. Five different mobility laws, two of them phenomenological fits to data, and three more based on physical models of dislocation inertia, are tested by employing dynamic discrete dislocation plasticity (D3P) simulations of a shock loaded aluminium thin foil. It is found that inertial laws invariably entail very short acceleration times for dislocations changing their kinematic state. As long as the mobility laws describe the same regime of terminal speeds, all mobility laws predict the same degree of plastic relaxation at the shock front. This is used to show that the main factor affecting plastic relaxation at the shock front is in fact the speed of dislocations. Keywords: dislocations, plasticity, mobility law, shock loading, elastic?precursor decay (Some figures?may appear in colour only in the online journal) 1.?Introduction The plastic shielding of a shock front is the fundamental process behind the attenuation of the dynamic yield point. Gurrutxaga-Lerma et?al (2015) [1] showed that attenuation of the dynamic yield point (otherwise known as the '

elastic precursor decay'

) is the result the Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

1 Current address: Trinity College Cambridge, CB2 1TQ, Cambridge UK. 0965-0393/16/065006+21$33.00? ?

2016 IOP Publishing Ltd? Printed in the UK Modelling Simul. Mater. Sci. Eng.

24 (2016)

065006 (21pp) doi:10.1088/0965-0393/24/6/065006

2 accumulated interference of elastic waves emanating from shielding dislocations that are gen- erated at the shock front. This phenomenon is greatly affected by the motion of these dislocations. Dislocations are generated in shielding and anti-shielding pairs. The shielding dislocations move frontward, and as their speed approaches the transverse speed of sound, the elastodynamic fields they radi- ate are magnified ahead of the dislocation core in the direction of motion. The anti-? shielding dislocations move in the direction opposite to the front, and as their speed approaches the transverse speed of sound, the magnitude of their elastodynamic fields behind the core in the direction of motion is weakened. This weakens the anti-shielding effect, and results in an enhanced plastic shielding of the shock front [1]. Thus, the plastic relaxation of the shock front appears to be greatly affected by the way in which dislocations move at the shock front. In continuum elasticity descriptions of plasticity and dislocation dynamics, dislocation motion is described in terms of mobility laws [2]. Mobility laws express dislocation motion in their slip planes as a force or energy balance in which the action of an external stimulus (typi- cally, an external stress) is balanced by the crystalline lattice'

s natural resistance to its motion (the dislocation '

drag'

), and by the need to change the dislocation'