Vol. 6, No. 2, 2018

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ISSN: 2325-3444 (e-only)
ISSN: 2326-7186 (print)
A strain gradient variational approach to damage: a comparison with damage gradient models and numerical results

Luca Placidi, Emilio Barchiesi and Anil Misra

Vol. 6 (2018), No. 2, 77–100
Abstract

The global response of experimental uniaxial tests cannot be homogeneous, because of the unavoidable presence of localized deformations, which is always preferential from an energetic viewpoint. Accordingly, one must introduce some characteristic lengths in order to penalize deformations that are too localized. This is what leads to the concept of nonlocal damage models. The nonlocal approach employs nonlocal terms in the internal deformation energy in order to control the size of the localization region. In phase-field models and, in general, in gradient models, dependence of the internal energy upon the first gradient of damage is assumed, while in our approach the nonlocality is given by the dependence of the internal energy upon the second gradient of the displacement field. A discussion of the advantages and challenges of using the gradient of damage and of using the second gradient of the displacement field will be addressed in the present paper. A variational inequality is formulated and partial differential equations (PDEs), boundary conditions (BCs), and Karush–Kuhn–Tucker (KKT) conditions will be derived within the framework of 2D strain gradient damage mechanics. A novel dependence of the stiffness coefficients with respect to the damage field will also be discussed. Further, an explicit derivation of the damage field evolution in loading conditions will be provided. Finally, a numerical technique based on commercial software has been introduced and discussed for a couple of standard problems.

Keywords
strain gradient, damage mechanics, variational inequality, dissipation, regularization
Mathematical Subject Classification 2010
Primary: 74C05, 74R99
Milestones
Received: 4 October 2017
Accepted: 26 April 2018
Published: 29 May 2018

Communicated by Jean-Jacques Marigo
Authors
Luca Placidi
Facoltà di Ingegneria
Università Telematica Internazionale Uninettuno
Roma
Italy
Emilio Barchiesi
Dipartimento di Ingegneria Strutturale e Geotecnica
Università degli Studi di Roma “La Sapienza”
Roma
Italy
Anil Misra
Civil, Environmental and Architectural Engineering Department
University of Kansas
Lawrence, KS
United States