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# A finite element formulation for piezoelectric shell structures considering geometrical and material non-linearities

12 Aug 2011--Vol. 87, Iss: 6, pp 491-520
TL;DR: In this paper, an electro-mechanical coupled shell element is developed considering geometrically and materially non-linear behavior of ferroelectric ceramics, and the mixed formulation includes the independent fields of displacements, electric potential, strains, electric field, stresses, and dielectric displacements.
Abstract: In this paper, we present a non-linear finite element formulation for piezoelectric shell structures. Based on a mixed multi-field variational formulation, an electro-mechanical coupled shell element is developed considering geometrically and materially non-linear behavior of ferroelectric ceramics. The mixed formulation includes the independent fields of displacements, electric potential, strains, electric field, stresses, and dielectric displacements. Besides the mechanical degrees of freedom, the shell counts only one electrical degree of freedom. This is the difference in the electric potential in the thickness direction of the shell. Incorporating non-linear kinematic assumptions, structures with large deformations and stability problems can be analyzed. According to a Reissner–Mindlin theory, the shell element accounts for constant transversal shear strains. The formulation incorporates a three-dimensional transversal isotropic material law, thus the kinematic in the thickness direction of the shell is considered. The normal zero stress condition and the normal zero dielectric displacement condition of shells are enforced by the independent resultant stress and the resultant dielectric displacement fields. Accounting for material non-linearities, the ferroelectric hysteresis phenomena are considered using the Preisach model. As a special aspect, the formulation includes temperature-dependent effects and thus the change of the piezoelectric material parameters due to the temperature. This enables the element to describe temperature-dependent hysteresis curves. Copyright © 2011 John Wiley & Sons, Ltd.

## Summary (5 min read)

### 1 Introduction

• Piezoelectric material plays an important role for sensor and actuator devices.
• In order to consider the nonlinear material behavior in a classical shell formulation, the strain and the electric field in thickness direction have to be comprised.
• Thus, temperature can influence the performance of piezoelectric shell structures due to a change of the temperature dependent material parameters.
• Here the change of the saturation parameters of the polarization and the strain due to temperature is phenomenologically included, thus temperature-dependent hysteresis curves can be determined.

### 2 Kinematics

• The authors obtain the corresponding inextensible director vector d of the current configuration with the rotation tensor R by the orthogonal transformation d = RD.
• Commas denote a partial differentiation with respect to the coordinates ξα.
• Due to the shell geometry, the authors assume that the piezoelectric material is poled in thickness direction and the electrodes are arranged at the lower and upper surface.

### 3 Constitutive equations

• The authors introduce linear constitutive equations with the Green-Lagrangean strain E, the Lagrangean electric field E, the second Piola-Kirchhoff stresses S, and the dielectric displacements D. Focusing on their material model, here they neglect thermal stresses and pyroelectric effects.
• (8) The strains and the electric fields are summarized in the vector ε.
• The three dimensional elasticity matrix , the permittivity matrix , and the piezoelectric coupling modulus are arranged in ̄.
• In , the authors assume transversal isotropic material behavior with isotropy in the 23-plane, which can be specified with five independent parameters, see [44].
• The stress and dielectric displacement in thickness direction are defined as zero, thus the authors fulfill the normal zero stress condition of shells.

### 4.1 Nonlinear constitutive equations

• Ferroelectric ceramics show strong nonlinear behavior under high electric fields.
• The imprinted initial polarization changes its direction under high electric loading and shows the dielectric hysteresis.
• Thus, the linear constitutive behavior according to (8) has to be set up under consideration of the current state of polarization.
• P i,rel characterizes the part of the piezoelectric material that shows a macroscopic polarization.
• Since the switching effects of the ferroelectric domains can be treated as a volume conserving process, the irreversible strain Ei can be determined as proposed by Reference [25].

### 4.3 Interpretation for ferroelectric materials

• To interpret the Preisach model for ferroelectric hysteresis phenomena, the input and output variables have to be identified.
• The loading parameter for piezoelectric devices is the electric field E. With the parameter of the material specific saturation value of the electric field Esat the normalized value Erel is chosen as x(t) = Erel = ‖ E‖ Esat . (24) The corresponding output quantity is chosen as the normalized polarization P i,rel y(t) =.
• As a phenomenological model, the Preisach concept adjusts the final hysteresis form by means of an experimental determined function.
• For a detailed description and a discussion regarding the choice of the Preisach function see e.g. References [49, 50, 47, 51].
• It is remarked that the polarization output value denotes the normalized irreversible polarization.

### 4.4 Temperature-dependent hysteresis

• The influence of the temperature on the saturation parameter of the polarization P sat and the electric coercitive field Ec has been experimentally studied by Reference [37] for Pb(Zn1/3Nb2/3).
• O3− PbT iO3 single crystals and by Reference [38] for PZT ceramics.
• Following the experimental investigations in Reference [39] the authors assume a linear relation between the natural logarithms of the temperature and the saturation polarization.
• As there do not exist sufficient test results, this influence has been neglected.

### 5 Variational formulation

• The generalized stress tensor σ containing the second Piola-Kirchhoff stresses reads σ = 2∂CŴ (C).
• The virtual quantities of v and the independently assumed strains, electric field, stresses, and dielectric displace- ments summarized in the generalized electromechanic fields ε̂(Ê, ̂E) and σ̂(Ŝ, ̂D) arrive to δv(δu, δω, δΔϕ), δε̂(δÊ, δ̂E) and δσ̂(δŜ, δ ̂D).

### 6 Finite element approximation

• The finite element formulation models the shell structure by a reference surface.
• The nodal position vector XI and the local cartesian coordinate system [A1I ,A2I ,A3I ] are generated with the mesh input.
• Here, t3 represents the normal vector in the midpoint of the element.
• The authors assume that the shell structure only counts for an electric potential in thickness direction of the shell by means of electrodes on the upper and lower surface of the shell.
• The element has to fulfill the patch tests.

### 6.1 Interpolation of the assumed strains and electric field

• The independent fields of the strains and the electric field are interpolated by ˆ̄ε =.
• Here ˆ̄ε characterizes the complete vector of the assumed strains and the assumed electric fields, whereas ε̂ specifies the reduced vector without the components in thickness direction.
• The area element dA = j dξdη is given with j(ξ, η) = |Xh,ξ ×Xh,η |. The matrix Neas contains parameters that are set orthogonal to the interpolations of the stresses, which is similar to the enhanced strain formulation given by Reference [54].

### 6.3 Approximation of the weak form and linearization

• The authors incorporate the interpolations of the strains, the electric fields, the stress resultants, and the dielectric displacements in equation (58) and formulate the approximation of the variational formulation on element level as G(θ, δθ) = δvTe ∫ Ωe [ BTNσ β − fa ].
• Considering nonlinear structural and material behavior, this formulation has to be linearized.
• The authors simplify the formulation and define the following element matrices kg[18×18] = ∫.

### 6.4 Actuator formulation

• In order to deal with the actuator use of piezoelectric shell structures, the authors postulate a linear distribution of the electric potential in thickness direction.
• The authors write the corresponding electric field in thickness direction as the average value for every element.
• According to the gradient relation, see equation (4), the electric potential is divided through the thickness.

### 7.1 Patchtests

• The basic benchmark test of a finite element formulation is the well known patch test.
• The test is passed if the formulation is able to display a state of constant stresses and constant dielectric displacements along with constant strains and a constant electric field for distributed element geometry.
• The geometry of the quadratic patch with distorted elements inside the patch is shown in Figure 5.
• For the shear test the same nodes are subjected to load F3 in 3-direction.
• The loadings and the boundary conditions are depicted in Figure 5.

### 7.2 Piezoelectric bimorph

• The piezoelectric bimorph is a well known piezoelectric benchmark test in order to proof the numerical formulation to the general applicability for sensor and actuator systems.
• For the discretisation, five elements are chosen, which correspond to five pairs of electrodes that are put along the length of the cantilever.
• Due to the deflection, an electric potential arises.
• The element ”H8D” has independent variables for the displacement, the electric potential and the dielectric displacements, in ”H8DS” also the stresses are included.
• The present shell element does not show any shear locking, thus the results fit the analytical solution even for strong distorted meshes.

### 7.3 90◦ cylindrical shell

• The system, see Figure 9, consists of four graphite epoxy layers, for which the authors account the orientation angles ϕF with [0/90/90/0] referring to the x axis.
• The geometry and the material parameters for the graphite epoxy and PZT layers according to Balamurugan and Narayanan [64] are given in Table 2.
• The radial system displacement w is measured along the centerline at b/2.
• The authors compare the present shell formulation with data from Balamurugan and Narayanan [64] for a degenerated nine-node quadrilateral shell element with quadratic approach for the electric potential in thickness direction and with Saravanos [65] who provides a laminated eight-node shell element with lineare electric thickness potential.
• The good accordance of the results shows the reliable applicability of the present laminated four node formulation for layered piezo-mechanical structures.

### 7.4 Steering of an antenna

• The authors show two versions of an antenna that can be manipulated via piezoelectric devices.
• Two piezoelectric patches with the width b and the length l are arranged with the distance a from the small hole of 2◦ in the middle of the antenna shell.
• The authors compare those results to the experimental data and a numerical calculation with a reduced eight-node element by Gupta et al. [67].
• Figure 12(b) shows the displacement curve dependent on the angle around the middle axis of the antenna.
• Here, the authors distinguish between the following loading cases 1.

### 7.5 Test of the Preisach model for ferroelectric hysteresis

• In order to validate the results of the temperature-dependent Preisach model for the ferro- electric hysteresis effects, a simple material cube made of soft PZT with an edge length a, see Figure 16, is chosen.
• Besides experimental results also a micro-mechanical model is introduced in [36].
• For the micro-mechanical calculation, the material paramters in [36] are derived from the single crystal parameters of barium titanate and are modified via a correction parameter to get the material parameter of soft PZT single crystal.
• Figure 17 presents the dielectric hysteresis and the butterfly hysteresis for a temperature of 25◦C.
• However above the saturation values the reversible part is underestimated.

### 7.6 Piezoelectric ceramic disc

• A thin piezoelectric PZT ceramic disc is introduced by Yimnirun et al. [38] who gives experimental results for the temperature-dependent polarization behavior, see Figure 19.
• For the numerical calculation the authors choose the three representative temperatures 298K, 373K and 453K.
• To simplify the calculation, the authors additionally introduce a small hole at the disc center characterized by the diameter d2, which lets the polarization change unaffected and thus does not influence the dielectric hysteresis curve.
• The missing parameters of the permittivity are added from experienced data.
• The reversible polarization is underestimated compared to the experiment and shows a nonlinear behavior.

### 7.7 Telescopic cylinder

• The quite small displacements of piezoelectric structures can be enlarged by special architectures.
• With several nested cylinders, which are alternately connected at the top and the bottom, actuators with much higher displacements and be composed.
• The radii of the cylinders numerated from the inner to the outer tube are r1, r2, r3, r4 and r5 and correspond to the middle surface of every cylinder.
• With respect to the material nonlinear behavior, it results a hysteresis curve for the maximal displacement w, which is displayed in Figure 22 together with the experimental data from [69].
• An open question is how the experimental results reach the initial value of the remanent displacement again after −1200V for the first subhysteresis ±300V .

### 8 Conclusions

• The authors have presented a piezoelectric finite shell element.
• The mixed hybrid formulation includes independent thickness strains, which allows a consideration of three-dimensional nonlinear constitutive equations.
• By means of a temperature-dependent Preisach model the authors consider the actual polarization state and thus they incorporate ferroelectric hysteresis phenomena.
• With only one electrical degree of freedom, the formulation simulates the behavior of both piezoelectric sensor and actuator systems appropriately.
• The presented examples show the influence of the temperature for the ferroelectric nonlinear behavior.

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Karlsruher Institut f
¨
ur Technologie
Institut f
¨
ur Baustatik
A Finite element formulation for
piezoelectric shell structures considering
geometrical and material nonlinearities
K. Schulz, S. Klinkel, W. Wagner
Mitteilung 3(2010)
BAUSTATIK

Karlsruher Institut f
¨
ur Technologie
Institut f
¨
ur Baustatik
A Finite element formulation for
piezoelectric shell structures considering
geometrical and material nonlinearities
K. Schulz, S. Klinkel, W. Wagner
Mitteilung 3(2010)
BAUSTATIK
c
Prof. Dr.–Ing. W. Wagner Telefon: (0721) 608–2280
Institut f
¨
ur Baustatik Telefax: (0721) 608–6015
Karlsruher Institut f
¨
ur Technologie E–mail: info@ibs.kit.edu
Kaiserstr. 12 Internet: http://www.ibs.kit.edu
76131 Karlsruhe

A Finite element formulation for piezoelectric shell structures
considering geometrical and material nonlinearities
K. Schulz
[1]
,S.Klinkel
[2]
and W. Wagner
[1]
[1 ]Karlsruhe Institute of Technology(KIT), Institute for Structural Analysis, Kaiserstr.12, 76131 Karl-
sruhe, Germany
[2 ]Statik u. Dynamik d. Tragwerke, Technische Universit¨at Kaiserslautern, Paul-Ehrlich-Str. 14, D-
67663 Kaiserslautern, Germany
Abstract In this paper we present a nonlinear ﬁnite element formulation for piezoelectric
shell structures. Based on a mixed multi ﬁeld variational formulation, an electro-mechanical
coupled shell element is developed considering geometrically and materially nonlinear behavior
of ferroelectric ceramics. The mixed formulation includes the independent ﬁelds of displace-
ments, electric potential, strains, electric ﬁeld, stresses, and dielectric displacements. Besides
the mechanical degrees of freedom, the shell counts only one electrical degree of freedom.
This is the diﬀerence of the electric potential in thickness direction of the shell. Incorporating
nonlinear kinematic assumptions, structures with large deformations and stability problems
can be analyzed. According to a Reissner-Mindlin theory, the shell element accounts for con-
stant transversal shear strains. The formulation incorporates a three-dimensional transversal
isotropic material law, thus the kinematic in thickness direction of the shell is considered.
The normal zero stress condition and the normal zero dielectric displacement condition of
shells are enforced by the independent resultant stress and resultant dielectric displacement
ﬁelds. Accounting for material nonlinearities, the ferroelectric hysteresis phenomena are con-
sidered using the Preisach model. As a special aspect, the formulation includes temperature-
dependent eﬀects and thus the change of the piezoelectric material parameters due to the
temperature. This enables the element to describe temperature dependent hysteresis curves.
1 Introduction
Piezoelectric material plays an important role for sensor and actuator devices. In the course
of optimization of systems, shell structures have become more and more interesting. In recent
years, several piezoelectric shell formulations based on the ﬁnite element method have been
introduced. One can distinguish between solid shell elements, see e.g. References [1, 2, 3],
and classical formulations, which model the shell by a reference surface, see e.g. References
[4, 5, 6, 7, 8, 9, 10, 11]. Some of these element formulations are restricted to shallow shell
structures, [5, 6, 9, 11], where the initial shell curvature is assumed to be small. In order to
consider laminated structures, the above mentioned formulations include a more or less sophis-
ticated laminate theory. References [5, 10, 12, 13, 14] point out that geometrically nonlinear
characteristics can signiﬁcantly inﬂuence the performance of piezoelectric systems, especially
for the sensor usage. A geometrically nonlinear theory that incorporates large rotations is
presented in References [1, 5, 6, 12]. A common assumption in piezoelectric models is that
the electric eld is constant through the thickness. This is not correct for bending dominated
problems. According to Reference [15], a quadratic approach for the electric potential through
the thickness is necessary to fulﬁll the electric charge conservation law. Shell formulations
incorporating a quadratic thickness potential can be found in References [16, 8, 7]. Mixed
1

shell formulations including independent ﬁelds for the electric potential, the electric ﬁeld, the
dielectric displacement as well as the mechanical ﬁelds of the displacement, the strain, and
the stress can be found in [19].
Accounting for nonlinear material behavior, the polarization state has to be analyzed. Due
to high electric ﬁelds, ferroelectric switching processes take place and the material proper-
ties change. To consider the resultant ferroelectric hysteresis phenomena, one can refer to
microscopic models, see References [20, 21], and macroscopic models, see e.g. References
[22, 23, 24, 25, 26, 27, 28]. Microscopic analysis looks at single crystals and models the
switching process via an energy criterion. A macroscopic approach using a switching criterion
based on thermodynamic energy function is presented in [22, 23, 24, 25, 26, 27]. Macro-
scopic phenomenological hysteresis models can be found in Reference [28]. A well known
phenomenological model is the Preisach model, see Reference [47]. Several piezo-mechanical
coupled formulations have used the Preisach model to display piezoelectric material behavior,
see References [30, 31, 32]. A shell element that accounts for material nonlinearities is [33]; it
is based on a phenomenological switching function. In order to consider the nonlinear material
behavior in a classical shell formulation, the strain and the electric ﬁeld in thickness direction
have to be comprised. A classical mechanical shell element incorporating three dimensional
constitutive equations is e.g. proposed by Reference [34]. The inﬂuence of the temperature on
piezoelectric material behavior has been studied experimentally in References [36, 37, 38, 39].
[39] shows that the ferroelectric hysteresis curves become smaller with increasing temperature.
A micropolar model to consider the temperature-dependent relation between the electric ﬁeld
and the polarization is introduced by [40]. Here, the inﬂuence of mechanical stress is neglected
and there is no remark on the strain behavior. A simple one-dimensional phenomenological
model considering the strain is presented in Reference [41]. The authors assume a volume
part of paraelectric phase for every hysteresis. [35] presents a piezoelectric plate element
considering the deformation of a linear temperature gradient through the thickness, and gives
the results depending on the piezoelectric material parameters for diﬀerent temperature lev-
els. Thus, temperature can inﬂuence the performance of piezoelectric shell structures due to
a change of the temperature dependent material parameters. The essentiell aspects of the
piezoelectric shell formulation presented in this paper are the following:
(i) The bilinear, four-node shell element is based on a six ﬁeld variational functional. Be-
sides the six mechanical degrees of freedom, three displacements and three rotations, the
only electrical degree of freedom is the diﬀerence of the electric potential in thickness
direction;
(ii) The element includes nonlinear kinematic assumptions, thus a geometric nonlinear anal-
ysis becomes feasible;
(iii) The formulation incorporates three dimensional transversal isotropic constitutive equa-
tions. In the shell formulation a linear approach for both the strain and the electric
ﬁeld in thickness direction is considered;
(iv) Using the Preisach model, the material nonlinear ferroelectric hysteresis eﬀects are in-
corporated. Here the change of the saturation parameters of the polarization and the
strain due to temperature is phenomenologically included, thus temperature-dependent
hysteresis curves can be determined.
2

2 Kinematics
We model the shell by a reference surface Ω with the boundary Γ. Every point of Ω is part
of the Euklidean space B. In order to display the geometry of the structure in B,wedenote
a convected coordinate system of the body ξ
i
andanoriginO with the global cartesian
coordinate system e
i
. The initial thickness of the shell in the reference conﬁguration is given
as h, thus we deﬁne the arbitrary reference surface by the thickness coordinate ξ
3
=0with
h
ξ
3
h
+
. X(ξ
1
2
)andx(ξ
1
2
) denote the position vectors of the shell surface Ω by
means of the convective coordinates in the reference and the current conﬁguration respectively.
The covariant tangent vectors for the reference and the current conﬁguration, A
i
and a
i
,are
given as
A
i
=
X
∂ξ
i
, a
i
=
x
∂ξ
i
,i=1, 2, 3 . (1)
The contravariant basis A
i
is deﬁned by the orthogonality δ
j
i
= A
i
· A
j
. The director vector
D(ξ
1
2
)with|D(ξ
1
2
)| = 1 is given perpendicular to Ω. It holds D = A
3
. We obtain the
corresponding inextensible director vector d of the current conﬁguration with the rotation
tensor R by the orthogonal transformation d = RD. In the following, we refer to the notation
that Latin indices range from 1 to 3 and Greek indices range from 1 to 2, whereas we use
the summation convention for repeated indices. Commas denote a partial diﬀerentiation with
respect to the coordinates ξ
α
. A displacement u can be determined by the diﬀerence of the
current and the initial position vectors u = x X. Including a Reissner-Mindlin kinematic,
we consider transverse shear strain, thus it holds d · x,
α
= 0. For the geometric in-plane and
thickness strains we assume
E
αβ
= ε
αβ
+ ξ
3
κ
αβ
2E
α3
= γ
α
E
33
=0 .
(2)
We write the membrane strains ε
αβ
, the curvatures κ
αβ
, and the shear strains γ
α
of the shell,
see Reference [42], as
ε
αβ
=
1
2
(x,
α
·x,
β
X,
α
·X,
β
)
κ
αβ
=
1
2
(x,
α
·d,
β
+x,
β
·d,
α
X,
α
·D,
β
X,
β
·D,
α
)
γ
α
= x,
α
·d X,
α
·D .
(3)
The electric ﬁeld
E =
E
1
E
2
E
3
T
is given as the gradient ﬁeld of the electric potential
ϕ. Due to the shell geometry, we assume that the piezoelectric material is poled in thickness
direction and the electrodes are arranged at the lower and upper surface. Therefore, we only
consider the diﬀerence of the electric potential in thickness direction of the shell Δϕ and write
the geometric electric ﬁeld
E
g
=
E
1
E
2
0
T
as
E
g
=
Δϕ
∂ξ
i
A
i
. (4)
We summarize the strains and the electric ﬁeld of the shell in a generalized geometric strain
vector ε
g
(v)
ε
g
(v)=[ε
11
22
, 2ε
12
11
22
, 2κ
12
1
2
,
E
1
,
E
2
]
T
. (5)
3

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[...]

Journal ArticleDOI
TL;DR: In this paper, a three-field mixed formulation in terms of displacements, stresses and an enhanced strain field is presented which encompasses, as a particular case, the classical method of incompatible modes.
Abstract: A three-field mixed formulation in terms of displacements, stresses and an enhanced strain field is presented which encompasses, as a particular case, the classical method of incompatible modes. Within this frame-work, incompatible elements arise as particular ‘compatible’ mixed approximations of the enhanced strain field. The conditions that the stress interpolation contain piece-wise constant functions and be L2-ortho-gonal to the enhanced strain interpolation, ensure satisfaction of the patch test and allow the elimination of the stress field from the formulation. The preceding conditions are formulated in a form particularly convenient for element design. As an illustration of the methodology three new elements are developed and shown to exhibit good performance: a plane 3D elastic/plastic QUAD, an axisymmetric element and a thick plate bending QUAD. The formulation described herein is suitable for non-linear analysis.

1,559 citations

### Additional excerpts

• ...The matrix Neas contains parameters that are set orthogonal to the interpolations of the stresses, which is similar to the enhanced strain formulation given by Reference [54]....

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Journal ArticleDOI
TL;DR: In this article, a general quadrilateral shell element for geometric and material nonlinear analysis is presented, which is formulated using three-dimensional continuum mechanics theory and it is applicable to the analysis of thin and thick shells.
Abstract: A new four‐node (non‐flat) general quadrilateral shell element for geometric and material non‐linear analysis is presented. The element is formulated using three‐dimensional continuum mechanics theory and it is applicable to the analysis of thin and thick shells. The formulation of the element and the solutions to various test and demonstrative example problems are presented and discussed.

1,187 citations

Journal ArticleDOI
TL;DR: In this paper, a new method for the formulation of hybrid elements by the Hellinger-Reissner principle is established by expanding the essential terms of the assumed stresses as complete polynomials in the natural coordinates of the element.
Abstract: A new method for the formulation of hybrid elements by the Hellinger-Reissner principle is established by expanding the essential terms of the assumed stresses as complete polynomials in the natural coordinates of the element. The equilibrium conditions are imposed in a variational sense through the internal displacements which are also expanded in the natural co-ordinates. The resulting element possesses all the ideal qualities, i.e. it is invariant, it is less sensitive to geometric distortion, it contains a minimum number of stress parameters and it provides accurate stress calculations. For the formulation of a 4-node plane stress element, a small perturbation method is used to determine the equilibrium constraint equations. The element has been proved to be always rank sufficient.

736 citations

### Additional excerpts

• ...For ̄= ̄=0, the plane strain problem according to Reference [54] is solved....

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Book
01 Jan 2003
TL;DR: The classical Preisach model of hysteresis, Generalized scalar preisach models of hystresis (GSPH), Vector PREISACH models of HSTs, Stochastic aspects of HS, Superconducting HS, Eddy current HSTS, core losses as mentioned in this paper.
Abstract: The classical Preisach model of hysteresis, Generalized scalar Preisach models of hysteresis, Vector Preisach models of hysteresis, Stochastic aspects of hysteresis, Superconducting hysteresis, Eddy current hysteresis. Core losses.

733 citations

### "A finite element formulation for pi..." refers methods in this paper

• ...A well-known phenomenological model is the Preisach model, see Reference [27]....

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