Finite strain theory explained
In continuum mechanics, the finite strain theory—also called large strain theory, or large deformation theory—deals with deformations in which strains and/or rotations are large enough to invalidate assumptions inherent in infinitesimal strain theory. In this case, the undeformed and deformed configurations of the continuum are significantly different, requiring a clear distinction between them. This is commonly the case with elastomers, plastically deforming materials and other fluids and biological soft tissue.
Deformation gradient tensor
The deformation gradient tensor
is related to both the reference and current configuration, as seen by the unit vectors
and
, therefore it is a
two-point tensor.Two types of deformation gradient tensor may be defined.
Due to the assumption of continuity of
,
has the inverse
, where
is the
spatial deformation gradient tensor. Then, by the
implicit function theorem,
[1] the
Jacobian determinant
must be nonsingular, i.e.
The material deformation gradient tensor
is a second-order tensor that represents the gradient of the mapping function or functional relation
, which describes the
motion of a continuum. The material deformation gradient tensor characterizes the local deformation at a material point with position vector
, i.e., deformation at neighbouring points, by transforming (
linear transformation) a material line element emanating from that point from the reference configuration to the current or deformed configuration, assuming continuity in the mapping function
, i.e.
differentiable function of
and time
, which implies that
cracks and voids do not open or close during the deformation. Thus we have,
Relative displacement vector
with position vector
in the undeformed configuration (Figure 2). After a displacement of the body, the new position of the particle indicated by
in the new configuration is given by the vector position
. The coordinate systems for the undeformed and deformed configuration can be superimposed for convenience.
Consider now a material point
neighboring
, with position vector
X+\DeltaX=(XI+\DeltaXI)II
. In the deformed configuration this particle has a new position
given by the position vector
. Assuming that the line segments
and
joining the particles
and
in both the undeformed and deformed configuration, respectively, to be very small, then we can express them as
and
. Thus from Figure 2 we have
where
is the
relative displacement vector, which represents the relative displacement of
with respect to
in the deformed configuration.
Taylor approximation
For an infinitesimal element
, and assuming continuity on the displacement field, it is possible to use a
Taylor series expansion around point
, neglecting higher-order terms, to approximate the components of the relative displacement vector for the neighboring particle
as
Thus, the previous equation
can be written as
Time-derivative of the deformation gradient
Calculations that involve the time-dependent deformation of a body often require a time derivative of the deformation gradient to be calculated. A geometrically consistent definition of such a derivative requires an excursion into differential geometry[2] but we avoid those issues in this article.
The time derivative of
is
where
is the (material) velocity. The derivative on the right hand side represents a
material velocity gradient. It is common to convert that into a spatial gradient by applying the chain rule for derivatives, i.e.,
where
\boldsymbol{l}=(\nablaxv)T
is the
spatial velocity gradient and where
is the spatial (Eulerian) velocity at
. If the spatial velocity gradient is constant in time, the above equation can be solved exactly to give
assuming
at
. There are several methods of computing the
exponential above.
Related quantities often used in continuum mechanics are the rate of deformation tensor and the spin tensor defined, respectively, as:The rate of deformation tensor gives the rate of stretching of line elements while the spin tensor indicates the rate of rotation or vorticity of the motion.
The material time derivative of the inverse of the deformation gradient (keeping the reference configuration fixed) is often required in analyses that involve finite strains. This derivative isThe above relation can be verified by taking the material time derivative of
and noting that
.
Polar decomposition of the deformation gradient tensor
The deformation gradient
, like any invertible second-order tensor, can be decomposed, using the
polar decomposition theorem, into a product of two second-order tensors (Truesdell and Noll, 1965): an orthogonal tensor and a positive definite symmetric tensor, i.e.,
where the tensor
is a proper orthogonal tensor, i.e.,
and
, representing a rotation; the tensor
is the
right stretch tensor; and
the
left stretch tensor. The terms
right and
left means that they are to the right and left of the rotation tensor
, respectively.
and
are both
positive definite, i.e.
and
for all non-zero
, and
symmetric tensors, i.e.
and
, of second order.
This decomposition implies that the deformation of a line element
in the undeformed configuration onto
in the deformed configuration, i.e.,
, may be obtained either by first stretching the element by
, i.e.
, followed by a rotation
, i.e.,
; or equivalently, by applying a rigid rotation
first, i.e.,
, followed later by a stretching
, i.e.,
(See Figure 3).
Due to the orthogonality of
so that
and
have the same
eigenvalues or
principal stretches, but different
eigenvectors or
principal directions
and
, respectively. The principal directions are related by
This polar decomposition, which is unique as
is invertible with a positive determinant, is a corollary of the
singular-value decomposition.
Transformation of a surface and volume element
To transform quantities that are defined with respect to areas in a deformed configuration to those relative to areas in a reference configuration, and vice versa, we use Nanson's relation, expressed as where
is an area of a region in the deformed configuration,
is the same area in the reference configuration, and
is the outward normal to the area element in the current configuration while
is the outward normal in the reference configuration,
is the deformation gradient, and
.
The corresponding formula for the transformation of the volume element is
Fundamental strain tensors
A strain tensor is defined by the IUPAC as:
"A symmetric tensor that results when a deformation gradient tensor is factorized into a rotation tensor followed or preceded by a symmetric tensor".
Since a pure rotation should not induce any strains in a deformable body, it is often convenient to use rotation-independent measures of deformation in continuum mechanics. As a rotation followed by its inverse rotation leads to no change (
) we can exclude the rotation by multiplying the deformation gradient tensor
by its
transpose.
Several rotation-independent deformation gradient tensors (or "deformation tensors", for short) are used in mechanics. In solid mechanics, the most popular of these are the right and left Cauchy–Green deformation tensors.
Cauchy strain tensor (right Cauchy–Green deformation tensor)
In 1839, George Green introduced a deformation tensor known as the right Cauchy–Green deformation tensor or Green's deformation tensor (the IUPAC recommends that this tensor be called the Cauchy strain tensor),[3] defined as:
Physically, the Cauchy–Green tensor gives us the square of local change in distances due to deformation, i.e.
Invariants of
are often used in the expressions for
strain energy density functions. The most commonly used
invariants are
where
is the determinant of the deformation gradient
and
are stretch ratios for the unit fibers that are initially oriented along the eigenvector directions of the right (reference) stretch tensor (these are not generally aligned with the three axis of the coordinate systems).
Finger strain tensor
The IUPAC recommends[3] that the inverse of the right Cauchy–Green deformation tensor (called the Cauchy strain tensor in that document), i. e.,
, be called the
Finger strain tensor. However, that nomenclature is not universally accepted in applied mechanics.
Green strain tensor (left Cauchy–Green deformation tensor)
Reversing the order of multiplication in the formula for the right Green–Cauchy deformation tensor leads to the left Cauchy–Green deformation tensor which is defined as:
The left Cauchy–Green deformation tensor is often called the Finger deformation tensor, named after Josef Finger (1894).[4]
The IUPAC recommends that this tensor be called the Green strain tensor.[3]
Invariants of
are also used in the expressions for
strain energy density functions. The conventional invariants are defined as
where
is the determinant of the deformation gradient.
For compressible materials, a slightly different set of invariants is used:
Piola strain tensor (Cauchy deformation tensor)
Earlier in 1828,[5] Augustin-Louis Cauchy introduced a deformation tensor defined as the inverse of the left Cauchy–Green deformation tensor,
. This tensor has also been called the
Piola strain tensor by the IUPAC and the
Finger tensor[6] in the rheology and fluid dynamics literature.
Spectral representation
If there are three distinct principal stretches
, the
spectral decompositions of
and
is given by
Furthermore,
Observe thatTherefore, the uniqueness of the spectral decomposition also implies that
. The left stretch (
) is also called the
spatial stretch tensor while the right stretch (
) is called the
material stretch tensor.
The effect of
acting on
is to stretch the vector by
and to rotate it to the new orientation
, i.e.,
In a similar vein,
Examples
- Uniaxial extension of an incompressible material
This is the case where a specimen is stretched in 1-direction with a stretch ratio of
. If the volume remains constant, the contraction in the other two directions is such that
or
. Then:
- Simple shear
- Rigid body rotation
Derivatives of stretch
Derivatives of the stretch with respect to the right Cauchy–Green deformation tensor are used to derive the stress-strain relations of many solids, particularly hyperelastic materials. These derivatives areand follow from the observations that
Physical interpretation of deformation tensors
Let
be a Cartesian coordinate system defined on the undeformed body and let
be another system defined on the deformed body. Let a curve
in the undeformed body be parametrized using
. Its image in the deformed body is
.The undeformed length of the curve is given byAfter deformation, the length becomesNote that the right Cauchy–Green deformation tensor is defined asHence,which indicates that changes in length are characterized by
.Finite strain tensors
The concept of strain is used to evaluate how much a given displacement differs locally from a rigid body displacement.[1] [7] [8] One of such strains for large deformations is the Lagrangian finite strain tensor, also called the Green-Lagrangian strain tensor or Green–St-Venant strain tensor, defined as
or as a function of the displacement gradient tensoror
The Green-Lagrangian strain tensor is a measure of how much
differs from
.The Eulerian finite strain tensor, or Eulerian-Almansi finite strain tensor, referenced to the deformed configuration (i.e. Eulerian description) is defined as
or as a function of the displacement gradients we have
Seth–Hill family of generalized strain tensors
B. R. Seth from the Indian Institute of Technology Kharagpur was the first to show that the Green and Almansi strain tensors are special cases of a more general strain measure. The idea was further expanded upon by Rodney Hill in 1968. The Seth–Hill family of strain measures (also called Doyle-Ericksen tensors)[9] can be expressed as
For different values of
we have:- Green-Lagrangian strain tensor
- Biot strain tensor
- Logarithmic strain, Natural strain, True strain, or Hencky strain
- Almansi strain
The second-order approximation of these tensors iswhere
is the infinitesimal strain tensor.Many other different definitions of tensors
are admissible, provided that they all satisfy the conditions that:[10]
vanishes for all rigid-body motions
on the displacement gradient tensor
is continuous, continuously differentiable and monotonic
reduces to the infinitesimal strain tensor
as the norm
An example is the set of tensorswhich do not belong to the Seth–Hill class, but have the same 2nd-order approximation as the Seth–Hill measures at
for any value of
.[11] Physical interpretation of the finite strain tensor
The diagonal components
of the Lagrangian finite strain tensor are related to the normal strain, e.g.
where
is the normal strain or engineering strain in the direction
.The off-diagonal components
of the Lagrangian finite strain tensor are related to shear strain, e.g.
where
is the change in the angle between two line elements that were originally perpendicular with directions
and
, respectively.Under certain circumstances, i.e. small displacements and small displacement rates, the components of the Lagrangian finite strain tensor may be approximated by the components of the infinitesimal strain tensor
Compatibility conditions
See main article: Compatibility (mechanics). The problem of compatibility in continuum mechanics involves the determination of allowable single-valued continuous fields on bodies. These allowable conditions leave the body without unphysical gaps or overlaps after a deformation. Most such conditions apply to simply-connected bodies. Additional conditions are required for the internal boundaries of multiply connected bodies.
Compatibility of the deformation gradient
The necessary and sufficient conditions for the existence of a compatible
field over a simply connected body areCompatibility of the right Cauchy–Green deformation tensor
The necessary and sufficient conditions for the existence of a compatible
field over a simply connected body areWe can show these are the mixed components of the Riemann–Christoffel curvature tensor. Therefore, the necessary conditions for
-compatibility are that the Riemann–Christoffel curvature of the deformation is zero.Compatibility of the left Cauchy–Green deformation tensor
General sufficiency conditions for the left Cauchy–Green deformation tensor in three-dimensions were derived by Amit Acharya.[12] Compatibility conditions for two-dimensional
fields were found by Janet Blume.[13] See also
Further reading
- Book: Dill, Ellis Harold
. Continuum Mechanics: Elasticity, Plasticity, Viscoelasticity. CRC Press. 2006. Germany. 0-8493-9779-0.
- Book: Dimitrienko, Yuriy
. Nonlinear Continuum Mechanics and Large Inelastic Deformations. Springer. 2011. Germany. 978-94-007-0033-8.
- Book: Hutter, Kolumban
. Klaus Jöhnk. Continuum Methods of Physical Modeling. Springer. 2004. Germany. 3-540-20619-1.
- Book: Lubarda
, Vlado A.
. Elastoplasticity Theory. CRC Press. 2001. 0-8493-1138-1 .
- Book: Macosko
, C. W.
. Rheology: principles, measurement and applications. VCH Publishers. 1994. 1-56081-579-5.
- Book: Mase
, George E.
. Continuum Mechanics. McGraw-Hill Professional. 1970. 0-07-040663-4 .
- Book: Mase
, G. Thomas
. George E. Mase. Continuum Mechanics for Engineers. CRC Press. 1999. Second. 0-8493-1855-6 .
- Book: Nemat-Nasser
, Sia
. Plasticity: A Treatise on Finite Deformation of Heterogeneous Inelastic Materials. Cambridge University Press. 2006. Cambridge. 0-521-83979-3.
- Book: Rees, David
. Basic Engineering Plasticity – An Introduction with Engineering and Manufacturing Applications. Butterworth-Heinemann. 2006. 0-7506-8025-3.
External links
Notes and References
- Book: Lubliner
, Jacob
. Plasticity Theory . Dover Publications . 2008 . 978-0-486-46290-5 . dead . https://web.archive.org/web/20100331022415/http://www.ce.berkeley.edu/~coby/plas/pdf/book.pdf . 2010-03-31. Revised .
- A. Yavari, J.E. Marsden, and M. Ortiz, On spatial and material covariant balance laws in elasticity, Journal of Mathematical Physics, 47, 2006, 042903; pp. 1–53.
- A. Kaye, R. F. T. Stepto, W. J. Work, J. V. Aleman (Spain), A. Ya. Malkin. 1998. Definition of terms relating to the non-ultimate mechanical properties of polymers. Pure Appl. Chem.. 70. 3. 701–754. 10.1351/pac199870030701. free.
- Eduardo N. Dvorkin, Marcela B. Goldschmit, 2006 Nonlinear Continua, p. 25, Springer .
- Jirásek,Milan; Bažant, Z. P. (2002) Inelastic analysis of structures, Wiley, p. 463
- J. N. Reddy, David K. Gartling (2000) The finite element method in heat transfer and fluid dynamics, p. 317, CRC Press .
- Book: Belytschko. Ted. Liu. Wing Kam. Moran. Brian. Nonlinear Finite Elements for Continua and Structures. John Wiley & Sons Ltd.. 2000. reprint with corrections, 2006. 978-0-471-98773-4. 92–94.
- Zeidi. Mahdi. Kim. Chun IL. 2018. Mechanics of an elastic solid reinforced with bidirectional fiber in finite plane elastostatics: complete analysis. Continuum Mechanics and Thermodynamics. 30. 3. 573–592. 10.1007/s00161-018-0623-0. 2018CMT....30..573Z . 253674037 . 1432-0959.
- T.C. Doyle and J.L. Eriksen (1956). "Non-linear elasticity." Advances in Applied Mechanics 4, 53–115.
- Z.P. Bažant and L. Cedolin (1991). Stability of Structures. Elastic, Inelastic, Fracture and Damage Theories. Oxford Univ. Press, New York (2nd ed. Dover Publ., New York 2003; 3rd ed., World Scientific 2010).
- Z.P. Bažant (1998). "Easy-to-compute tensors with symmetric inverse approximating Hencky finite strain and its rate." Journal of Materials of Technology ASME, 120 (April), 131–136.
- Acharya, A.. On Compatibility Conditions for the Left Cauchy–Green Deformation Field in Three Dimensions. 1999. Journal of Elasticity. 56 . 2 . 95–105. 10.1023/A:1007653400249. 116767781.
- Blume, J. A.. 1989. Compatibility conditions for a left Cauchy–Green strain field. 10.1007/BF00045780. Journal of Elasticity. 21. 3. 271–308. 54889553.