International Journal of Engineering Science, 41 (2003), 1553-1602.
COMPARISON OF INTEGRAL-TYPE NONLOCAL PLASTICITY MODELS
FOR STRAIN-SOFTENING MATERIALS
Milan Jirásek and Simon Rolshoven
Swiss Federal Institute of Technology
LSC - ENAC, EPFL,
1015 Lausanne, Switzerland
Abstract
The paper analyzes and compares a number of softening plasticity models regularized
by nonlocal averaging. To highlight the fundamental properties and gain insight
into the regularizing effect of various formulations, the localization problem
is examined in the one-dimensional setting. It is shown that some of the
theoretically appealing formulations are not genuine localization limiters,
and that a localized plastic zone of nonzero measure is obtained only with
softening laws that take into account the effect of both the local and the
nonlocal cumulative plastic strain. The evolution of the plastic zone is
studied, and formulations suitable for the description of the entire deformation
process up to the complete loss of cohesion are identified. The effect of
boundaries on the shape of the plastic strain profile and on the dissipated
energy is analyzed. Attention is also paid to the thermodynamic aspects of
nonlocal plasticity, especially to the consistent extension of the concept
of generalized standard materials to nonlocal continua.
Summary and Conclusions
A number of nonlocal formulations of softening plasticity have been scrutinized,
with attention to their localization properties, boundary effects, and thermodynamic
admissibility. The main results can be summarized as follows:
- The nonlocal plasticity models proposed by Eringen do not act as localization
limiters. The first model does not prevent localization into a set of measure
zero, and the second model does not allow any localization at all (in the
one-dimensional setting).
- Models with the yield stress dependent only on the nonlocal cumulative
plastic strain (basic nonlocal formulation, model of Borino et al., and Nilsson's
model) provide only a partial regularization and are essentially equivalent
to a cohesive zone model. Plastic strain is localized into a set of zero
measure but, in contrast to the local formulation, the global structural
response in terms of the load-displacement diagram and work spent during
the failure process is captured correctly. In finite element simulations
in multiple dimensions, such models are likely to exhibit mesh-induced directional
bias, since the plastic yielding would localize into one layer of elements.
- Models that combine in a suitable way the effect of the local and
nonlocal cumulative plastic strain on the current yield stress act as true
localization limiters and lead to a nonzero size of the localized plastic
zone. This is true for the Vermeer-Brinkgreve model, models motivated by
ductile damage (integral-type version of the implicit gradient plasticity
model due to Geers et al. and nonlocal extension of the Gurson model), and
the thermodynamically motivated model proposed by Svedberg and Runesson.
The model proposed by Bazant and Lin has similar properties. At the first
bifurcation from a uniform strain state, all these models are essentially
equivalent. The subsequent evolution of the plastic strain profile and the
stress transmitted by the plastic zone depend on the specific form of the
softening law.
- If the softening process needs to be simulated until the complete
loss of material integrity (zero residual yield stress), the model should
be selected with great care. At late stages of the softening process, certain
formulations produce pathological effects such as stress locking or spatial
expansion of the plastic zone. Such formulations have a limited scope and
they should be combined with appropriate tools for the description of highly
localized strain and of the transition to fracture. Only the Vermeer-Brinkgreve
model and the ductile damage models seem to be suitable for a pure continuum-based
description of the complete failure process.
- The models of Borino et al. and of Svedberg and Runesson comply with
the postulate of maximum plastic dissipation (in a modified form for the
entire body) and the dissipation is guaranteed to be nonnegative. An alternative
formulation due to Nilsson, also motivated by thermodynamic considerations,
is not really consistent. The evolution laws cannot be derived from a maximum
dissipation postulate and, in some particular cases (e.g., in the presence
of compressive stresses within the interaction distance from the plastic zone
yielding under tension), the model can give negative dissipation.
- Nonlocal extension of the postulate of maximum plastic dissipation
leads to theoretically appealing models with a symmetric structure. These
models require a double application of the nonlocal averaging operator, which
complicates their numerical implementation. Also, at present there does not
seem to be any model of this type that would act as a true localization limiter
and at the same time could describe the complete failure process without
any locking effects.
- For nonlocal models formulated ad hoc, without any recourse to thermodynamics,
it is not trivial to check their thermodynamic admissibility. For the basic
nonlocal model, it is possible to prove that, if the free-energy potential
is assumed in the same form as for the thermodynamically motivated model due
to Borino et al., the dissipation is always nonnegative (but the evolution
law for the softening variable is of course not associated). For models with
the softening law contaning both local and nonlocal terms, the construction
of a suitable free-energy potential is more tricky and will be the subject
of further research.
- If the nonlocal weight function is scaled in the proximity of boundaries,
as is routinely done, the solution with a plastic region localized at the
boundary usually dissipates much less energy than if the plastic region localizes
inside the body. Since the solution that would actually occur is that with
the steepest descent of the post-peak branch of the load-displacement diagram
(Bazant and Cedolin, 1991), the boundary acts as a weak layer that attracts
localization. Whether this is physically realistic depends on the actual
structure of the material in the boundary layer. For the thermodynamically
motivated models with double nonlocal averaging, the largest plastic strain
does not develop directly on the boundary but at a finite distance from it
(for the Svedberg-Runesson model this is true if the nonlocal hardening modulus
exceeds the magnitude of the local softening modulus by at least 25%).
The complete paper can be downloaded in PDF format.
EPFL / 2 June 2003 / milan.jirasek@epfl.ch