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Chapter 5
Analytical techniques and solutions for linear elastic solids
5.4 Solutions to 3D static problems in linear elasticity
The field equations of linear elasticity are much more difficult to solve in 3D than in 2D. Nevertheless, several important problems have been solved. In this section, we outline a common representation for 3D problems, and give solutions to selected 3D problems.
5.4.1 Papkovich-Neuber Potential representations for 3D solutions for isotropic solids
In this section we outline a general technique for solving 3D static linear elasticity problems. The technique is similar to the 2D Airy function method, in that the solution is derived by differentiating a potential, which is governed by a PDE. Many other potential representations are used in 3D elasticity, but most are simply special cases of the general Papkovich-Neuber representation. Assume that
The Papkovich-Neuber procedure can be summarized as follows: 1. Begin by finding a vector function
as well as boundary conditions
2. Calculate displacements from the formula
3. Calculate stresses from the formula
HEALTH WARNING: Although the displacements and stresses that solve a linear elasticity problem are unique, the Papkovich-Neuber potentials that generate a particular solution are not. Consequently, if you find several different sets of potentials in the literature that claim to solve the same problem, don’t panic. It is likely that they really do solve the same problem.
5.4.2 Demonstration that the Papkovich-Neuber solution satisfies the governing equations
We need to show two things: 1. That the displacement field satisfies the equilibrium equation (See sect 5.1.2)
2. That the stresses are related to the displacements by the elastic stress-strain equations To show the first result, differentiate the formula relating potentials to the displacement to see that
Substitute this result into the governing equation to see that
Finally, substitute the governing equations for the potentials
and simplify the result to verify that the governing equation is indeed satisfied. The second result can be derived by substituting the formula for displacement into the elastic stress-strain equations and simplifying.
5.4.3 Point force in an infinite solid
The
displacements and stresses induced by a point force
where
5.4.4 Point force normal to the surface of an infinite half-space
The
displacements and stresses induced by a point force
where
The displacements and stresses follow as
5.4.5 Point force tangent to the surface of an infinite half-space
The
displacements and stresses induced by a point force
The displacements and stresses can be calculated from these potentials as
5.4.6 The Eshelby Inclusion Problem
The Eshelby problem (Eshelby, Proc Roy Soc A 241 p. 376) is posed as follows: 1. Consider an infinite, isotropic, linear elastic
solid, with (homogeneous) Young’s modulus and Poisson’s ratio 2. The solid is initially stress free, with displacements,
strains and stresses 3. Some unspecified external agency then induces a
uniform `transformation strain 4. Because the ellipsoid is encapsulated within the
surrounding elastic solid, stress, strain and displacement fields are induced
throughout the elastic solid. These
fields must be defined carefully because the initial configuration for the
solid could be chosen in a number of different ways. In the following,
Inside the ellipsoid, the total strain consists of
the transformation strain (which does
not induce stress); together with an additional elastic strain
The Eshelby solution gives full expressions for these fields. It has proved to be one of the most important solutions in all of linear elasticity: it is of some interest in its own right, because it provides some insight into the mechanics of phase transformations in crystals. More importantly, a number of very important boundary value problems can be solved by manipulating the Eshelby solution. These include (i) the solution for an ellipsoidal inclusion embedded within an elastically mismatched matrix; (ii) the solution for an ellipsoidal cavity in an elastic solid; (iii) solutions for circular and elliptical cracks in an elastic solid. In addition, the Eshelby solution is used extensively in theories that provide estimates of elastic properties of composite materials.
The displacement field is generated by Papkovich-Neuber potentials
where
the integral is taken over the surface of the ellipsoid,
is the transformation stress (i.e. the stress that would be induced by applying an elastic strain to the inclusion that is equal to the transformation strain). The stresses outside the inclusion can be calculated using the standard Papkovich-Neuber representation given in Section 5.4.1. To calculate stresses inside the inclusion, the formula must be modified to account for the transformation strain, which gives
For the general ellipsoid, the expressions for displacement and stress can be reduced to elliptic integrals, with the results
Solution inside the ellipsoid: Remarkably, it turns out that the stresses and strains are uniform inside the ellipsoid. The displacements, strains and stresses can be expressed as: (i) Displacement (ii) Strain (iii) Stress Here,
where
are elliptic integrals of the first and second kinds. Then
The
remaining components of
For certain special
shapes the expressions given for
Oblate spheroid
Prolate spheroid
Sphere
Additional terms follow
from the symmetry conditions
Cylinder
Additional terms follow
from the symmetry conditions
Solution outside the ellipsoid: The solution outside the ellipsoid can also be expressed in simplified form: J.
D. Eshelby “The Elastic Field Outside an Ellipsoidal Inclusion,” Proceedings
of the Royal Society of
where
and
The displacements follow as
where
The remaining displacement components can be calculated by cyclic permutations of (1,2,3), and strains and stresses can be calculated by differentiating the displacements appropriately. The results are far too complicated to write out in full, and in practice the algebra can only be done with the aid of a symbolic manipulation program. Some special results can be reduced to a tractable form, however:
Displacements far from the ellipsoid
Solution outside a spherical inclusion: For this case the Papkovich-Neuber potentials can be reduced to
The displacements and stresses follow as
where
5.4.7 Elastically mismatched ellipsoidal inclusion in an infinite solid subjected to remote stress
The
figure shows an ellipsoidal inclusion, with semi-axes
The solution is constructed by superposing the Eshelby solution to the uniform stress state. To represent the Eshelby solution, we introduce: 1. The Eshelby transformation strain 2. The Eshelby tensor 3. The displacement induced by the Eshelby
transformation 4. The stresses induced by the Eshelby transformation The
functions
The solution for the solid containing the inclusion follows as
where the transformation strain is calculated by solving
for
is the stiffness of the matrix, with a similar expression for the stiffness of the inclusion.
5.4.8 Spherical cavity in an infinite solid subjected to remote stress
The
figure shows a spherical cavity with radius a in an infinite, isotropic linear elastic solid. Far from the
cavity, the solid is subjected to a tensile stress
The solution is generated by potentials
The displacements and stresses follow as Derivation: This solution can be derived by superposing two solutions:
The unknown coefficients A and B must be chosen to satisfy the traction free boundary condition
Substituting back into the Eshelby potentials and simplifying yields the results given. The same approach can be used to derive the solution for a rigid inclusion in an infinite solid subjected to remote stress, as well as the solution to an elastically mismatched spherical inclusion in an infinite solid.
5.4.9 Flat ended cylindrical indenter in contact with an elastic half-space
A
rigid flat ended cylindrical punch with radius a is pushed into the surface of an elastic half-space with
Young’s modulus E and Poisson’s
ratio
The load is related to the displacement of the punch by
The solution can be generated from Papkovich-Neuber potentials
where
A symbolic manipulation program can handle the complex arithmetic in these formulas without difficulty. If you wish to find analytical formulas for the displacement or stress, the following expressions are helpful
where
Important features of these results include: 1. Contact pressure: The pressure exerted by the indenter on the elastic solid follows as
2. Surface displacement: The vertical displacement of the surface is
3. Contact
stiffness: the stiffness of a
contact is defined as the ratio of the force acting on the indenter to its
displacement
5.4.10 Frictionless contact between two elastic spheres
This solution is known as the `Hertz contact problem’ after its
author. The figure illustrates the
problem to be solved. Two elastic
spheres, with radii
The solution is conveniently expressed in terms of an effective modulus and radius for the contact pair:
Relations between
Contact pressure: The two solids are subjected
to a repulsive pressure
Contact stiffness: the stiffness of a contact is defined as the ratio
of the force acting on the indenter to its displacement
Stress field The two spheres are subjected to the same contact pressure, and are both assumed to deform like a half-space (with a flat surface). Consequently, the stress field is identical inside both spheres, and can be calculated from formulas derived by Hamilton, Proc I. Mech E. 197C (1983) p. 53.
where
The stresses on r=0 must be computed using a limiting process, with the result
Conditions to initiate yield: The material under the contact yields when the
maximum von-Mises effective stress
5.4.11 Contact area, pressure, stiffness and elastic limit for general non-conformal contacts
A non-conformal contact has the following properties: (i) the two contacting solids initially touch at a point or along a line; (ii) both contacting solids are smooth in the neighborhood of the contact, so that their local geometry can be approximated as ellipsoids, (iii) the size of the contact patch between the two solids is much smaller than either solid.
Complete solutions for such contacts can be found in Bryant and Keer Journal of Applied Mechanics, 49, p. 345 (1982) or Sackfield and Hills Journal of Strain Analysis 18 p107, p. 195 (1983). These papers also account for the effects of friction under sliding contacts. The results are lengthy. Here, we give formulas that predict the most important features of frictionless nonconformal contacts.
Contact Geometry: The geometry of the contacting solids is characterized as follows:
Elastic constants: The two contacting solids are isotropic, with
Young’s modulus
Contact area: The area of contact between the two solids is
elliptical, with semi-axes
where
(The
limit
Contact pressure: The contact pressure distribution is ellipsoidal, with the form
Approach of contacting solids: Points distant from the contact in the two solids approach one another by a displacement
Contact stiffness: The contact stiffness is defined as
Elastic Limit: The stresses in both solids are identical, and therefore yield occurs first in the solid with the lower yield stress. The figure shows the critical load required to cause yield in a solid with Von-Mises yield criterion, and uniaxial tensile yield stress Y, based on tabular values on page 99 of Johnson “Contact Mechanics” CUP (1985)
5.4.12 Load-displacement-contact area relations for arbitrarily shaped axisymmetric contacts
The most important properties of general frictionless axisymmetric contacts can be calculated from simple formulas, even when full expressions for the stress and displacement fields cannot be calculated.
Assume that:
These quantities are related by the following formulas: 1.
Approach as a function
of contact radius 2.
Applied force as a
function of contact radius 3. Contact stiffness
4. Contact pressure distribution
Once these formulas have been evaluated for a given contact geometry the results can be combined to determine other relationships, such as contact radius or stiffness as a function of load or approach h.
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(c) A.F. Bower, 2008 |
