1. Introduction
The continuum-level thermo-mechanical problem of predicting the behavior of ductile metallic components subjected to intensive dynamic loading and undergoing plastic flow is of central importance in a number of applications. Consequently, this is a field of study that has received significant attention over at least the last 170 years. However, even with such sustained attention, the problem remains one that is not fully resolved. One reason for this is that closed form solutions of the initial boundary value problem (IBVP) are almost always untenable, and numerical approaches are inherently limited by approximations that the modeler is forced make.
Numerical solution of the IBVP requires the simultaneous solution of a coupled set of governing field equations (typically: conservation of mass, conservation of linear momentum, and conservation of energy), along with an appropriate set of initial and boundary conditions. Note that within the context of plasticy, resultant from intensive dynamic loading, the field equations are typically formulated in rate form. That being the case, they must be solved in incremental fashion as time goes on. Together, the governing field equations form a hyperbolic set, for which a numerical solution may be attempted using Lagrangian, Eulerian, or Augmented Lagrangian–Eulerian (ALE) approaches employing either explicit or implicit numerical time integration schemes depending on the specifics of the IBVP and preferences of the modeler. Numerical approaches include the finite element method and the finite volume method, among others.
If all one has to work with are the governing field equations plus initial and boundary conditions, the IBVP is inherently underconstrained and as such is ill-posed. This is because there are more unknown variables than equations to constrain them. It is through certain constitutive equations that the IBVP attains closure and is well-posed, and the solution of the same becomes tenable. There are a number of constitutive relations that may be critical in this regard, but the focus of this work is on only one—namely that related to the deviatoric Cauchy stress, and it is within the context of classical plasticity that it is herein addressed.
By way of review, classical plasticity consists of the following fundamental parts: (1) a stress–strain relationship (the constitutive equations that are required for closure of the IBVP), and (2) several component parts that feed into the constitutive equations. Those component parts include a yield criterion, a flow rule, and a hardening rule. Note that the hardening rule includes, or should include, mechanisms for softening as well as hardening, so it should perhaps be referred to as a hardening/softening rule, but we shall, out of adherence to convention, refer to it herein as a hardening rule with the understanding that mechanisms for softening are included. The stress–strain equations can be formulated in several different ways. One possibility is an algebraic relationship between the Cauchy stress and Eulerian strain:
where
is the Cauchy stress,
is a fourth order tensor of material properties,
is the Eulerian strain, and the superscript
p is used to denote the plastic part of the strain tensor. Another possibility is to separate the Cauchy tensor into deviatoric and spherical parts and to focus our plasticity-related attention exclusively on the deviator since it is within the deviator that plastic flow characteristics normally manifest. In this case, the spherical part of the Cauchy tensor is determined separately—for example, through something like an equation of state (constitutive relationship for the spherical part of the Cauchy tensor). Taking this approach, as is most common in the field of plasticity resultant from intensive dynamic loading, the constitutive equations are typically formulated, not in terms of the deviator directly, but in terms of a deviatoric stress rate,
, and may be expressed as follows:
where
is the deviatoric stress,
is, as above, a fourth order tensor of material properties,
denotes the deviatoric part of the rate of deformation tensor (deviatoric natural strain rate) [
1], and the superscript
p denotes the plastic part of the rate of deformation tensor. Note that, for isotropic systems,
simplifies to
, where
G is the shear modulus. The deviatoric stress rate is typically cast in terms of a Jaumann stress rate or, alternatively, a Green–Nagdi stress rate [
2]. Note that the choice of formulating the constitutive equations in rate form is natural since the governing field equations, as mentioned earlier, are most often expressed in rate form, and must, consequently, be solved in an incremental fashion over time with an update of the the stress–strain relationship occurring on each time step. Whatever form the constitutive equations take, they should be developed in accordance with four fundamental principles: (1) the principle of local action (if appropriate), (2) the principle of equipresence, (3) the principle of determinism (assures uniqueness), and (4) the principle of frame indifference [
3]. Please note that, although not shown explicitly in Equations (
1) and (
2),
and
are functions of temperature as well as whatever measure of strain is chosen as pertinent.
Let us now briefly address the component parts that feed into the constitutive equations discussed above. The yield criterion is typically formulated in terms of strain, temperature, and possibly some number of internal state variables that evolve over time (i.e.,
). Options for the precise form of the yield criterion are several. One possibility is to express yield in a stress formulation involving the deviator, for example, as follows:
where
F represents the yield surface,
is a fourth order tensor of coefficients that define the shape of the yield surface, and
is the flow stress, which incorporates hardening and softening effects. Note that the yield surface, as expressed here, is a function of deviatoric stress only (as opposed to the full Cauchy tensor) and since
, this is a five dimensional space. Note also that von Mises
plasticity [
2] exists as a simplified subset of Equation (
3) and is expressed as follows:
where
.
As was the case with the yield criterion, the flow rule, which is a rule for the evolution of plastic strain and can be thought of as an internal state growth law, can be formulated in terms of strain, temperature, and perhaps some number of internal state variables (i.e.,
). However, it is most often given in a stress formulation, as in the Levy–Mises flow rule (which is in fact a special case of the more general Prandtl–Reuss flow rule) and is given as:
where
is a history-dependent scalar and all other variables are as already defined. An often-used alternative to the Levy-Mises formulation is what is commonly referred to as an associative flow rule (since it is associated with a particular yield criterion), or the normality condition, and is given as:
However, if we assume that the yield surface is a function of the Cauchy deviator only (as suggested in Equations (
3) and (
4)), then the normality condition is perhaps more appropriately depicted in terms of the deviator as opposed to the full Cauchy tensor:
where all variables are as already defined.
The final component that feeds into the constitutive equations is the hardening rule. It is a rule for the evolution of the yield surface. This evolution is brought about through the effect of certain internal state varibles, such as the drag stress,
, in the case of isotropic hardening, or the back stress,
, in the case of kinematic hardening, along with whatever internal state variable growth laws (i.e.,
) may be appropriate. In a rather generic form, isotropic and kinematic hardening can be expressed, respectively, by what is given in the two equations that follow:
where the superscripted ′ denotes deviatoric part.
This completes our review of classical plasticity. For a more complex treatment of plasticity—for example, a treatment that includes anisotropy and complications related to material frame indifference—the interested reader may find the publication of Zocher et al. [
4] to be of benefit. To keep things simple, going forward we shall limit our consideration to isotropic materials undergoing
plastic flow in accordance with an associative flow rule and isotropic hardening. Thus, the most pertinent framework for what follows is that represented by Equations (
2), (
4), (
7) and (
8). Within that framework, the flow stress relates to the drag stress and to
, as follows:
In what follows, we shall focus our attention on the hardening model only (i.e., Equation (
10)), or more precisely, on some specific forms that the generic Equation (
10) may take. Those specific forms include the models of Johnson and Cook, [
5] (which we shall refer to in “shorthand” fashion as the JC model, though the model is not universally referred to in this manner), Steinburg, Cochran, and Guinan, [
6] (which we shall refer to in “shorthand” fashion as the SCG model, though the model is not universally referred to in this manner), Follansbee and Kocks [
7] (the MTS model), and Preston, Tonks and Wallace [
8] (the PTW model). Our ultimate objective will be the development of a hardening model for
-phase cerium that is in the style of the SCG model.
Cerium has a very complex phase diagram. There are four solid phases at zero pressure and at least three more high pressure phases. Gamma phase, the stable room temperature zero pressure phase, exhibits unusual hydrostatic constitutive behavior, wherein the bulk modulus decreases with increasing pressure. Cerium is unique in that it is the only pure element possessing a solid–solid critical point. One of the more interesting features of cerium is the isomorphic
-
solid–solid phase transition (fcc to fcc), which occurs at about 0.75 GPa (precipitated by localization/delocalization mechanisms involving 4f electrons). This transition is accompanied by a substantial volume change of almost 15% and has a profound impact upon the dynamic response of cerium. For example, the very large volume collapse that occurs with this transition has the effect of causing the temperature to increase at a rate much higher under dynamic compression than would occur with no volume change. For scientific interests and experimental applications, efforts are underway at Los Alamos National Laboratory and elsewhere to build multi-phase constitutive models for cerium. The present work represents a first step in the development of a multi-phase capability for predicting the deviatoric constitutive response of cerium employing an SCG-type hardening model. A corresponding first step was taken by Plohr et al. [
9], wherein the focus was on a PTW-type hardening model. An important additional factor motivating the present work derives from activity underway by the first author and others in the development of a multi-scale friction model (see [
10]). It turns out, for reasons not pertinent to the present discussion, that the friction model being developed is well suited to the use of a hardening model of SCG-type, but less well suited to the use of some other hardening models. Moreover, since the first author and others wish to test the aforementioned friction model against experiments involving cerium, there is very specific motivation for the present work.
In the following section we shall discuss, very briefly, a few specific hardening models that have appeared in the literature in the “recent” past (in the last 40 years or so). These include the JC model, the SCG model, the MTS model, and the PTW model. Following that, we shall discuss a variant of the PTW model recently developed by Plohr, Burakovsky, and Sjue [
9], specifically for
-phase cerium. We shall develop a characterization of the SCG model that is applicable to cerium. That characterization will include the paramaterization of the melt model of Lindemann [
11] along with models of the shear modulus and flow stress in accordance with SCG. We will then conclude with some summary remarks and recommendations for further study.
3. PTW Model for Cerium
As stated earlier, Plohr, Burakovsky, and Sjue [
9] recently developed a callibration of the PTW model specifically for gamma phase (fcc) cerium. Within this work they treat
and
not as simple constants but as functions of density:
, and
. Following the methodology presented in Burakovsky, Greeff, and Preston, [
13], Plohr et al. initiate the development of equations for
and
with the following definitions:
where
and
are the Grüneisen gammas corresponding to the shear modulus and melt, respectively. These two Grüneisen gammas can be calculated as follows:
where
= 2.62215,
= −5.0, and
= −6400. The interested reader is directed to [
13] for a discussion of the methodology used, in general, to determine
,
, and
. Solving Equations (
32) and (
33) yields the following for
and
:
where the values of
and
were taken from the open literature, and have values of 6.775 and 6.640 g/cm
, respectively. The value of
, for example, was taken from Zinov’ev [
14] (pp. 191–192). At the melting point
= 6.640 on the solid side and 6.687 on the liquid side, hence liquid is denser than the solid, which implies a negative slope on the melting curve. For the analytic models of [
9], it is the value on the solid side that is required (i.e., 6.640 g/cm
). The values of
and
, used in [
9], were 11.0 GPa and 1071 K, respectively.
Reasonable fits of the PTW model that bracket the experimental data available to the authors are given in
Table 1 and
Table 2. Note that parameter set 1 matches parameter set 1 of [
9], whereas parameter set 7 differs from parameter set 2 of [
9]. Using parameter sets 1 and 7, comparisons are made to the empirical data in
Figure 1 for
T = 333 K,
= 6.6877 g/cm
, and
= 4.2 × 10
. The empirical data were provided to the authors by Alexander Petrovtsev [
15]. Two parameter sets have been developed since the empirical data are limited and there is uncertainty associated with any extrapolation beyond the data.