Mindlin–Reissner plate theory

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Deformation of a plate highlighting the displacement, the mid-surface (red) and the normal to the mid-surface (blue)

The Uflyand-Mindlin theory of vibrating plates is an extension of Kirchhoff–Love plate theory that takes into account shear deformations through-the-thickness of a plate. The theory was proposed in 1948 by Yakov Solomonovich Uflyand[1] (1916-1991) and in 1951 by Raymond Mindlin[2] with Mindlin making reference to Uflyand's work. Hence, this theory has to be referred to us Uflyand-Mindlin plate theory, as is done in the handbook by Elishakoff,[3] and in papers by Andronov,[4] Elishakoff, Hache and Challamel,[5] Loktev,[6] Rossikhin and Shitikova[7] and Wojnar.[8] In 1994, Elishakoff[9] suggested to neglect the fourth-order time derivative in Uflyand-Mindlin equations. A similar, but not identical, theory in static setting, had been proposed earlier by Eric Reissner in 1945.[10] Both theories are intended for thick plates in which the normal to the mid-surface remains straight but not necessarily perpendicular to the mid-surface. The Uflyand-Mindlin theory is used to calculate the deformations and stresses in a plate whose thickness is of the order of one tenth the planar dimensions while the Kirchhoff–Love theory is applicable to thinner plates.

The form of Uflyand-Mindlin plate theory that is most commonly used is actually due to Mindlin. The Reissner theory is slightly different and is a static counterpart of the Uflyand-Mindlin theory. Both theories include in-plane shear strains and both are extensions of Kirchhoff–Love plate theory incorporating first-order shear effects.

Uflyand-Mindlin's theory assumes that there is a linear variation of displacement across the plate thickness but that the plate thickness does not change during deformation. An additional assumption is that the normal stress through the thickness is ignored; an assumption which is also called the plane stress condition. On the other hand, Reissner's static theory assumes that the bending stress is linear while the shear stress is quadratic through the thickness of the plate. This leads to a situation where the displacement through-the-thickness is not necessarily linear and where the plate thickness may change during deformation. Therefore, Reissner's static theory does not invoke the plane stress condition.

The Uflyand-Mindlin theory is often called the first-order shear deformation theory of plates. Since a first-order shear deformation theory implies a linear displacement variation through the thickness, it is incompatible with Reissner's static plate theory.

Mindlin theory[edit]

Mindlin's theory was originally derived for isotropic plates using equilibrium considerations by Uflyand.[1] A more general version of the theory based on energy considerations is discussed here.[11]

Assumed displacement field[edit]

The Mindlin hypothesis implies that the displacements in the plate have the form

where and are the Cartesian coordinates on the mid-surface of the undeformed plate and is the coordinate for the thickness direction, are the in-plane displacements of the mid-surface, is the displacement of the mid-surface in the direction, and designate the angles which the normal to the mid-surface makes with the axis. Unlike Kirchhoff–Love plate theory where are directly related to , Mindlin's theory does not require that and .

Displacement of the mid-surface (left) and of a normal (right)

Strain-displacement relations[edit]

Depending on the amount of rotation of the plate normals two different approximations for the strains can be derived from the basic kinematic assumptions.

For small strains and small rotations the strain–displacement relations for Mindlin–Reissner plates are

The shear strain, and hence the shear stress, across the thickness of the plate is not neglected in this theory. However, the shear strain is constant across the thickness of the plate. This cannot be accurate since the shear stress is known to be parabolic even for simple plate geometries. To account for the inaccuracy in the shear strain, a shear correction factor () is applied so that the correct amount of internal energy is predicted by the theory. Then

Equilibrium equations[edit]

The equilibrium equations of a Mindlin–Reissner plate for small strains and small rotations have the form

where is an applied out-of-plane load, the in-plane stress resultants are defined as

the moment resultants are defined as

and the shear resultants are defined as

Bending moments and normal stresses
Torques and shear stresses
Shear resultant and shear stresses

Boundary conditions[edit]

The boundary conditions are indicated by the boundary terms in the principle of virtual work.

If the only external force is a vertical force on the top surface of the plate, the boundary conditions are

Stress–strain relations[edit]

The stress–strain relations for a linear elastic Mindlin–Reissner plate are given by

Since does not appear in the equilibrium equations it is implicitly assumed that it does not have any effect on the momentum balance and is neglected. This assumption is also called the plane stress assumption. The remaining stress–strain relations for an orthotropic material, in matrix form, can be written as

Then

and

For the shear terms

The extensional stiffnesses are the quantities

The bending stiffnesses are the quantities

Mindlin theory for isotropic plates[edit]

For uniformly thick, homogeneous, and isotropic plates, the stress–strain relations in the plane of the plate are

where is the Young's modulus, is the Poisson's ratio, and are the in-plane strains. The through-the-thickness shear stresses and strains are related by

where is the shear modulus.

Constitutive relations[edit]

The relations between the stress resultants and the generalized deformations are,

and

The bending rigidity is defined as the quantity

For a plate of thickness ( of the following all indicates thickness), the bending rigidity has the form

Governing equations[edit]

If we ignore the in-plane extension of the plate, the governing equations are

In terms of the generalized deformations, these equations can be written as

The boundary conditions along the edges of a rectangular plate are

Relationship to Reissner's static theory[edit]

The canonical constitutive relations for shear deformation theories of isotropic plates can be expressed as[12][13]

Note that the plate thickness is (and not ) in the above equations and . If we define a Marcus moment,

we can express the shear resultants as

These relations and the governing equations of equilibrium, when combined, lead to the following canonical equilibrium equations in terms of the generalized displacements.

where

In Mindlin's theory, is the transverse displacement of the mid-surface of the plate and the quantities and are the rotations of the mid-surface normal about the and -axes, respectively. The canonical parameters for this theory are and . The shear correction factor usually has the value .

On the other hand, in Reissner's theory, is the weighted average transverse deflection while and are equivalent rotations which are not identical to those in Mindlin's theory.

Relationship to Kirchhoff–Love theory[edit]

If we define the moment sum for Kirchhoff–Love theory as

we can show that [12]

where is a biharmonic function such that . We can also show that, if is the displacement predicted for a Kirchhoff–Love plate,

where is a function that satisfies the Laplace equation, . The rotations of the normal are related to the displacements of a Kirchhoff–Love plate by

where

References[edit]

  1. ^ a b Uflyand, Ya. S.,1948, Wave Propagation by Transverse Vibrations of Beams and Plates, PMM: Journal of Applied Mathematics and Mechanics, Vol. 12, 287-300 (in Russian)
  2. ^ R. D. Mindlin, 1951, Influence of rotatory inertia and shear on flexural motions of isotropic, elastic plates, ASME Journal of Applied Mechanics, Vol. 18 pp. 31–38.
  3. ^ Elishakoff ,I.,2020, Handbook on Timoshenko-Ehrenfest Beam and Uflyand-Mindlin Plate Theories, World Scientific, Singapore, ISBN 978-981-3236-51-6
  4. ^ Andronov, I.V.,2007, The Analytic Properties and Uniqueness of the Solutions To Problems of Scattering by Compact Obstacles in an Infinite Plate Described by the Uflyand-Mindlin Model, Acoustical Physics, Vol. 53(6), 653-659
  5. ^ Elishakoff, I., Hache, F. , Challamel N., 2017, Vibrations of Asymptotically and Variationally Based Uflyand-Mindlin Plate Models, International Journal of Engineering Science, Vol. 116, 58-73
  6. ^ Loktev, A.A.,2011, Dynamic Contact of a Spherical Center and Prestressed Orthtropic Uflyand-Mindlin Plate, Acta Mechanica, Vol. 222(1-2), 17-25
  7. ^ Rossikhin Y.A. and Shitikova M.V., Problem of the Impact Interaction of an Elastic Rod With a Uflyand-Mindlin Plate, International Applied Mechanics, Vol. 29(2), 118-125, 1993
  8. ^ Wojnar, R.,1979, Stress Equations of Motion for Uflyand-Mindlin Plate, Bulletin de l’ Academie Polonaise des Sciences – Serie des Sciences Techniques, Vol. 27(8-9), 731-740
  9. ^ Elishakoff, I, 1994, “Generalization of the Bolotin's dynamic edge effect method for vibration analysis of Mindlin plates,” Proceedings, the 1994 National Conference on Noise Control Engineering, (J. M. Cuschieri, S.A.L. Glegg and D.M. Yeager, eds.), New York, pp. 911 916
  10. ^ E. Reissner, 1945, The effect of transverse shear deformation on the bending of elastic plates, ASME Journal of Applied Mechanics, Vol. 12, pp. A68–77.
  11. ^ Reddy, J. N., 1999, Theory and analysis of elastic plates, Taylor and Francis, Philadelphia.
  12. ^ a b Lim, G. T. and Reddy, J. N., 2003, On canonical bending relationships for plates, International Journal of Solids and Structures, vol. 40, pp. 3039–3067.
  13. ^ These equations use a slightly different sign convention than the preceding discussion.

See also[edit]