Constitutive Models Relations between Stress and Strain
3.7 Small Strain, Rate Independent Plasticity: Metals loaded beyond yield
For many design calculations, the elastic constitutive equations outlined in Section 3.1 are sufficient, since large plastic strains are by and large undesirable and will lead to failure. There are some applications, however, where it is of interest to predict the behavior of solids subjected to large loads, sufficient to cause permanent plastic strains. Examples include:
Modeling metal forming, machining or other manufacturing processes
Designing crash resistant vehicles
Plastic design of structures
Plasticity theory was developed to predict the behavior of metals under loads exceeding the plastic range, but the general framework of plasticity theory has since been adapted to other materials, including polymers and some types of soil (clay). Some concepts from metal plasticity are also used in modeling concrete and other brittle materials such as polycrystalline ceramics.
We begin by reviewing the results of a typical tension/compression test on an annealed, ductile, polycrystalline metal specimen (e.g. copper or Al). Assume that the test is conducted at moderate temperature (less than the melting point of the solid e.g. room temperature) and at modest strains (less than 10%), at modest strain rates ( ).
The results of such a test are
For modest stresses (and strains) the solid responds elastically. This means the stress is proportional to the strain, and the deformation is reversible.
If the stress exceeds a critical magnitude, the stressstrain curve ceases to be linear. It is often difficult to identify the critical stress accurately, because the stress strain curve starts to curve rather gradually.
If the critical stress is exceeded, the specimen is permanently changed in length on unloading.
If the stress is removed from the specimen during a test, the stressstrain curve during unloading has a slope equal to that of the elastic part of the stressstrain curve. If the specimen is re-loaded, it will initially follow the same curve, until the stress approaches its maximum value during prior loading. At this point, the stressstrain curve once again ceases to be linear, and the specimen is permanently deformed further.
If the test is interrupted and the specimen is held at constant strain for a period of time, the stress will relax slowly. If the straining is resumed, the specimen will behave as though the solid were unloaded elastically. Similarly, if the specimen is subjected to a constant stress, it will generally continue to deform plastically, although the plastic strain increases very slowly. This phenomenon is known as `creep.’
If the specimen is deformed in compression, the stressstrain curve is a mirror image of the tensile stressstrain curve (of course, this is only true for modest strains. For large strains, geometry changes will cause differences between the tension and compression tests).
If the specimen is first deformed in compression, then loaded in tension, it will generally start to deform plastically at a lower tensile stress than an annealed specimen. This phenomenon is known as the `Bauschinger effect.’
Material response to cyclic loading can be extremely complex. One example is shown in the picture above in this case, the material hardens cyclically. Other materials may soften.
The detailed shape of the plastic stressstrain curve depends on the rate of loading, and also on temperature.
We also need to characterize the multi-axial response of an inelastic solid. This is a much more difficult experiment to do. Some of the nicest experiments were done by G.I. Taylor and collaborators in the early part of the last century. Their approach was to measure the response of thin-walled tubes under combined torsion, axial loading and hydrostatic pressure.
The main conclusions of these tests were
The shape of the uniaxial stress-strain curve is insensitive to hydrostatic pressure. However, the ductility (strain to failure) can be increased by adding hydrostatic pressure, particularly under torsional loading.
Plastic strains are volume preserving, i.e. the plastic strain rate must satisfy
During plastic loading, the principal components of the plastic strain rate tensor are parallel to the components of stress acting on the solid. This sounds obvious until you think about it… To understand what this means, imagine that you take a cylindrical shaft and pull it until it starts to deform plastically. Then, holding the axial stress fixed, apply a torque to the shaft. Experiments show that the shaft will initially stretch, rather than rotate. The plastic strain increment is proportional to the stress acting on the shaft, not the stress increment. This is totally unlike elastic deformation.
Under multi-axial loading, most annealed polycrystalline solids obey the Levy-Mises flow rule, which relates the principal components of strain rate during plastic loading to the principal stresses as follows
In this section, we will outline the simplest plastic constitutive equations that capture the most important features of metal plasticity. There are many different plastic constitutive laws, which are intended to be used in different applications. There are two broad classes:
1. Rate independent plasticity which is used to model metals deformed at low temperatures (less than half the material’s melting point) and modest strain rates (of order 0.01-10/s). This is the focus of this section.
2. Rate dependent plasticity, or viscoplasticity used to model high temperature creep (steady accumulation of plastic strain at contstant stress) and also to model metals deformed at high strain rates (100/s or greater), where flow strength is sensitive to deformation rate. Viscoplasticity will be discussed in Section 3.3.
There are also various different models within these two broad categories. The models generally differ in two respects (i) the yield criterion; (ii) the strain hardening law. There is no completely general model that describes all the features that were just listed, so in any application, you will need to decide which aspect of material behavior is most important, and then choose a model that accurately characterizes this behavior.
Key ideas in modeling metal plasticity
Five key concepts form the basis of almost all classical theories of plasticity. They are
1. The decomposition of strain into elastic and plastic parts;
2. Yield criteria, which predict whether the solid responds elastically or plastically;
3. Strain hardening rules, which control the way in which resistance to plastic flow increases with plastic straining;
4. The plastic flow rule, which determines the relationship between stress and plastic strain under multi-axial loading;
5. The elastic unloading criterion, which models the irreversible behavior of the solid.
These concepts will be described in more detail in the sections below.
For simplicity, we will at this stage restrict attention to infinitesimal deformations.
Consequently, we adopt the infinitesimal strain tensor
as our deformation measure. We have no need to distinguish between the various stress measures and will use to denote stress.
It is also important to note that the plastic strains in a solid depend on the load history. This means that the stress-strain laws are not just simple equations relating stress to strain. Instead, plastic strain laws must either relate the strain rate in the solid to the stress and stress rate, or else specify the relationship between a small increment of plastic strain in terms of strain, stress and stress increment . In addition, plasticity problems are almost always solved using the finite element method. Consequently, numerical methods are used to integrate the plastic stress-strain equations.
Experiments show that under uniaxial loading, the strain at a given stress has two parts: a small recoverable elastic strain, and a large, irreversible plastic strain, as shown in the picture. In uniaxial tension, we would write
Experiments suggest that the reversible part is related to the stress through the usual linear elastic equations. Plasticity theory is concerned with characterizing the irreversible part.
For multiaxial loading, we generalize this by decomposing a general strain increment into elastic and plastic parts, as
The elastic part of the strain is related to stress using the linear elastic equations (discussed in detail in 3.1)
The yield criterion is used to determine the critical stress required to cause permanent deformation in a material. There are many different yield criteria here we will just list the simplest ones. Let be the stress acting on a solid, and let denote the principal values of stress. In addition, let denote the yield stress of the material in uniaxial tension. Then, define
VonMises yield criterion
Tresca yield criterion
In both cases, the criteria are defined so that the material deforms elastically for , and plastically for . The yield stress may increase during plastic straining, so we have shown that Y is a function of a measure of total plastic strain , to be defined in Section 3.2.5
An alternative form of VonMises criterion. For a general stress state, it is a nuisance having to compute the principal stresses in order to apply von Mises yield criterion. Fortunately, the criterion can be expressed directly in terms of the stress tensor
are the components of the `von Mises effective stress’ and `deviatoric stress tensor’ respectively.
These yield criteria are based largely on the following experimental observations:
(1) A hydrostatic stress (all principal stresses equal) will never cause yield, no matter how large the stress;
(2) Most polycrystalline metals are isotropic. Since the yield criterion depends only on the magnitudes of the principal stresses, and not their directions, the yield criteria predict isotropic behavior.
Tests suggest that von Mises yield criterion provides a slightly better fit to experiment than Tresca, but the difference between them is very small. Sometimes it simplifies calculations to use Tresca’s criterion instead of von Mises.
Any arbitrary stress state can be plotted in `principal stress space,’ with the three principal stresses as axes.
The VonMises yield criterion is plotted in this way in the picture to the right. The yield criterion is a cylinder, radius , with its axis parallel to the line
If the state of stress falls within the cylinder, the material is below yield and responds elastically. If the state of stress lies on the surface of the cylinder, the material yields and deforms plastically. If the plastic deformation causes the material to strain harden, the radius of the cylinder increases. The stress state cannot lie outside the cylinder this would lead to an infinite plastic strain.
Because the yield criterion defines a surface in stress space, it is often referred to as a yield surface. The yield surface is often drawn as it would appear when viewed down the axis of the cylinder, as shown below. The Tresca yield criterion can also be plotted in this way. It looks like a cylinder with a hexagonal cross section, as shown.
Experiments show that if you plastically deform a solid, then unload it, and then try to re-load it so as to induce further plastic flow, its resistance to plastic flow will have increased. This is known as strain hardening.
Obviously, we can model strain hardening by relating the size and shape of the yield surface to plastic strain in some appropriate way. There are many ways to do this. Here we describe the two simplest approaches.
Rather obviously, the easiest way to model strain hardening is to make the yield surface increase in size, but remain the same shape, as a result of plastic straining.
This means we must devise some appropriate relationship between Y and the plastic strain. To get a suitable scalar measure of plastic strain we define the accumulated plastic strain magnitude
(the factor of 2/3 is introduced so that in a uniaxial tensile test in which the specimen is stretched parallel to the direction. To see this, note that plastic strains do not change volume, so that and substitute into the formula.)
Then we make Y a function of . People often use power laws or piecewise linear approximations in practice. A few of the more common forms of hardening functions are
Perfectly plastic solid:
Linear strain hardening solid:
Powerlaw hardening material:
In these formulas, , h and m are material properties. These functions are illustrated in the figures below
An isotropic hardening law is generally not useful in situations where components are subjected to cyclic loading. It does not account for the Bauschinger effect, and so predicts that after a few cycles the solid will just harden until it responds elastically.
To fix this, an alternative hardening law allows the yield surface to translate, without changing its shape. The idea is illustrated graphically in the picture. As you deform the material in tension, you drag the yield surface in the direction of increasing stress, thus modeling strain hardening. This softens the material in compression, however. So, this constitutive law can model cyclic plastic deformation. The stress-strain curves for isotropic and kinematic hardening materials are contrasted in the figure below.
To account for the fact that the center of the yield locus is at a position in stress space, the Von-Mises yield criterion needs to be modified as follows
Here, Y is now a constant, and hardening is modeled by the motion of the yield surface. To do so, we need to relate to the plastic strain history somehow. There are many ways to do this, which can model subtle features of the plastic response of solids under cyclic and nonproportional loading. The simplest approach is to set
This hardening law predicts that the stress-plastic strain curve is a straight line with slope c. This is known as linear kinematic hardening.
A more sophisticated approach is to set
where c and are material constants. It’s not so easy to visualize what this does it turns out that that this relation can model cyclic creep the tendency of a material to accumulate strain in the direction of mean stress under cyclic loading, as illustrated in the figure on the right. It is known as the Armstrong-Frederick hardening law.
There are many other kinematic type hardening laws. New ones are still being developed.
To complete the plastic stress-strain relations, we need a way to predict the plastic strains induced by stressing the material beyond the yield point. Specifically, given
1. The current stress applied to the material
2. The current yield stress (characterized by for isotropic hardening, or for kinematic hardening)
3. A small increase in stress applied to the solid
we wish to determine the small change in plastic strain .
The formulas are given below, for isotropic and kinematic hardening. These are just fits to experiment (specifically, to the Levy-Mises flow rule). The physical significance and reason for the structure of the equations will be discussed later.
The plastic strains are usually derived from the yield criterion f defined in 3.6.3, and so are slightly different for isotropic and kinematic hardening. A material that has its plastic flow law derived from f is said to have an `associated’ flow law i.e.the flow law is associated with f.
Isotropic Hardening (Von-Mises yield criterion)
denotes the Von-Mises yield criterion, and is determined from the condition that the yield criterion must be satisfied at all times during plastic straining. This shows that
Here, is the slope of the plastic stress-strain curve. The algebra involved in differentiating f with respect to stress is outlined below.
Linear Kinematic Hardening (Von-Mises yield criterion)
where the yield criterion is now
and as before is determined from the condition that the yield criterion must be satisfied at all times during plastic straining. This shows that
Recall that for linear kinematic hardening the hardening law is
Comparison of flow law formulas with the Levy-Mises flow rule
The Levy-Mises flow law (based on experimental observations) states that principal values of the plastic strain increment induced by a stress increment are related to the principal stresses by
It is straightforward to show that this observation is consistent with the predictions of the flow law formulas given in this section. To see this, suppose that the principal axes of stress are parallel to the directions. In this case the only nonzero components of deviatoric stress are
The flow law
Thus, we see that
with similar expressions for other components. Some trivial algebra then yields the Levy-Mises flow law.
Differentiating the yield criterion
Differentiating the yield criterion requires some sneaky index notation manipulations. Note that
Now, recall that
and further that
However, observe that
There is one final issue to consider. Experiments show that plastic flow is irreversible, and always dissipates energy. If the increment in stress is tangent to the yield surface, or brings the stress below yield, as shown in the picture then there is no plastic strain.
For an isotropically hardening solid, this unloading condition may be expressed as
For kinematic hardening,
In both cases, the solid deforms elastically (no plastic strain) if the condition is satisfied.
We conclude by summarizing the complete elastic-plastic stress strain relations for an isotropic solid with Von-Mises yield surface.
Isotropically hardening elastic-plastic solid
The solid is characterized by its elastic constants and by the yield stress as a function of accumulated plastic strain and its slope
In this case we have that
These may be combined to
It is sometimes necessary to invert these expressions. A straightforward but tedious series of index notation manipulations shows that
This constitutive law is the most commonly used model of inelastic deformation. It has the following properties:
It will correctly predict the conditions necessary to initiate yield under multiaxial loading
It will correctly predict the plastic strain rate under an arbitrary multiaxial stress state
It can model accurately any uniaxial stressstrain curve
It has the following limitations:
It is valid only for modest plastic strains (<10%)
It will not predict creep behavior or strain rate sensitivity
It does not predict behavior under cyclic loading correctly
It will not predict plastic strains accurately if the principal axes of stress rotate significantly (more than about 30 degrees) during inelastic deformation
Linear Kinematically hardening solid
The solid is characterized by its elastic constants and by the initial yield stress and the strain hardening rate c. Then,
Finally, the evolution equation for is
This constitutive equation is used primarily to model cyclic plastic deformation, or plastic flow under nonproportional loading (where principal axes of stress rotate significantly during plastic flow). It has the following limitations:
It is valid only for modest plastic strains (<10%)
It will not predict creep behavior or strain rate sensitivity
It does not predict the shape of the stress-strain curve accurately
Unlike elastic constants, the plastic properties of metals are highly variable, and are also very sensitive to alloying composition and microstructure (which can be influenced by heat treatment and mechanical working). Consequently, it is impossible to give accurate values for yield stresses or hardening rates for materials. The table below (again, taken from `Engineering Materials,’ by M.F. Ashby and D.R.H. Jones, Pergamon Press) lists rough values for yield stresses of common materials these may provide a useful guide in preliminary calculations. If you need accurate data you will have to measure the properties of the materials you plan to use yourself.
The constitutive law outlined in the preceding section has an important property, known as the principle of maximum plastic resistance.
Statement of the principle: Let be a stress state which causes plastic deformation, let be a small change in and let be the resulting strain increment. Now, let be any other stress that can be imposed on the specimen that either does not reach yield, or else just satisfies the yield criterion, i.e. with .
Interpretation: The Principle of Maximum Plastic Resistance is a mathematical statement of the following ideas:
(i) The Mises yield surface is convex
(ii) The plastic strain rate is normal to the yield surface.
It is best to illustrate these ideas graphically. In principal stress space, the product is represented by the dot product of the stress and plastic strain rate vectors. The statement
is equivalent to the requirement that the angle between the vectors formed by and must be greater than 90 degrees for all stresses and strain rates. This is only possible if the yield stress is convex and the strain rate is normal to the yield surface.
The Principle of Maximum Plastic Resistance is important because it is the basis for a number of very important theorems concerning plastic deformation in solids. For example, it can be shown that the stress field in a material that obeys the Principle is always unique. In addition, the principle leads to clever techniques to estimate collapse loads for elastic-plastic solids and structures.
Proof of the principle of maximum plastic resistance
Our goal is to prove that . The simplest way to do so is to show that , while , where is the plastic strain magnitude, and Y is the yield stress. To this end:
1. Recall the plastic flow rule
2. Multiply both sides by stress
where we have noted that .
3. Recall that causes yield, and so must satisfy the yield condition . This shows that
4. Now consider
5. Note that
6. Now, recall again that causes yield, while can be at or below yield. The yield criterion therefore requires that . Substituting these inequalities into (5) shows that . Finally, this shows
Thus , proving the principle.
Constitutive models of inelastic behavior are based largely on experimental observations of plastic flow in laboratory specimens. Similar constitutive laws are used to describe very different materials, including metals, ceramics, glasses, soils and polymers. The mechanisms of deformation in these materials are very different, so it is surprising that their response is similar.
One perspective on the structure of constitutive laws for inelastic solids was developed by Drucker in the 1950s. Drucker introduced the idea of a stable plastic material, as follows. Consider a deformable solid, subjected to boundary tractions , which induce some displacement field . Suppose that the tractions are increased to , resulting in an additional displacement . The material is said to be stable in the sense of Drucker if the work done by the tractions through the displacements is positive or zero for all :
It can be shown that, for a plastic material to be stable in this sense, it must satisfy the following conditions:
The yield surface must be convex
The plastic strain rate must be normal to the yield surface
The rate of strain hardening must be positive or zero
Furthermore, a material that is stable in the sense of Drucker must satisfy the principle of maximum plastic resistance.
This does not really explain why the constitutive law should have this structure, but materials that do not satisfy the Drucker stability criterion tend to be difficult to work with in calculations, so there is a strong incentive for choosing a constitutive law that meets the condition. It is not surprising, then, that the people developing constitutive laws ended up with a form that satisfies Drucker stability.
It is possible to obtain some insight into the structure of the constitutive laws for metals by considering the micromechanisms responsible for plastic flow.
Plastic flow in metals is caused by dislocation motion. Dislocations are line defects in crystalline solids you can think of a dislocation as an extra plane of atoms inserted within a perfect crystal, as shown in the picture on the right.
When the crystal is subjected to stress, these defects move through the solid and rearrange the crystal lattice. For example, if the model crystal shown above is subjected to a shear stress, the atoms rearrange so that the top part of the crystal is shifted to the right relative to the bottom part.
Because the crystal lattice is distorted near a dislocation, only a modest shear stress is required to drive the dislocation through the solid, causing permanent plastic deformation.
Experiments and atomistic simulations suggest that dislocation motion obeys Schmidt’s law: a dislocation moves through a crystal if the shear stress on its glide plane exceeds a critical magnitude .
It can be shown that a material which deforms by dislocation glide, and which obeys Schmidt’s law, will satisfy the principle of maximum plastic resistance. This, in turn, implies that the yield surface for the solid must be convex and the plastic strain rate must be normal to the yield surface.
The notion of a yield surface and convexity for a material which deforms by dislocation glide can be illustrated with a simple double-slip model. Consider a single crystal, which contains two dislocation glide planes oriented at to the principal axes of stress as shown in the picture on the right. Assume plane stress conditions, for simplicity.
The picture below shows side views of the two slip planes. As an exercise, you should verify that the shear stresses (tangential component of traction) acting on the two slip planes are
The solid reaches yield if . The resulting yield surface is sketched below: the planes of the yield surface have been color coded to indicate which slip plane is active.
Observe that the yield surface is convex: this is a consequence of Schmidt’s law.
Now, suppose that slip is activated on one of the glide planes. Let denote the tangent to the slip plane, and let denote the normal. To compute the strain produced by slip on a single slip system, consider the deformation of an infinitesimal line element dx under shear .
The deformed line element is given by
or, in index notation
The deformation gradient follows as
and the Lagrange strain tensor is
For small , we can approximate the Lagrange strain tensor by the infinitesimal strain tensor
Now, suppose that the stress satisfies
as marked on the yield locus shown below. This activates slip as shown in the picture. The normal and tangent to the appropriate slip plane are
The strain rate therefore follows as
Thus, , showing that the plastic strain rate is normal to the yield locus (see the figure).You could verify as an exercise that that if the stress reaches the other limiting surfaces of the yield locus, the resulting strain rate will be normal to the yield locus.
(c) A.F. Bower, 2008