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Equilibrium of thin shells under large strains without through-the-thickness shear and self-penetration of matter

  • We consider elastic thin shells without through-the-thickness shear and depict them as Gauss graphs of parametric surfaces. (We use the term shells to include plates and thin films therein.) We consider an energy depending on the first derivative of the Gauss map (so, it involves curvatures) and its second-rank minors. For it we prove existence of minimizers in terms of currents carried by Gauss graphs. In the limiting process we adopt sequences of competitors that satisfy a condition that prevents self-penetration of matter.

    Citation: Paolo Maria Mariano, Domenico Mucci. Equilibrium of thin shells under large strains without through-the-thickness shear and self-penetration of matter[J]. Mathematics in Engineering, 2023, 5(6): 1-21. doi: 10.3934/mine.2023092

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  • We consider elastic thin shells without through-the-thickness shear and depict them as Gauss graphs of parametric surfaces. (We use the term shells to include plates and thin films therein.) We consider an energy depending on the first derivative of the Gauss map (so, it involves curvatures) and its second-rank minors. For it we prove existence of minimizers in terms of currents carried by Gauss graphs. In the limiting process we adopt sequences of competitors that satisfy a condition that prevents self-penetration of matter.



    Shells are three-dimensional bodies with one dimension that is largely smaller than the others. This geometric class of bodies includes plates, the relaxed shape of which is flat, and thin films, characterized by vanishing thickness. Such geometric features suggest to represent shells by looking at their middle surface only, with the proviso of assigning to each point of such a surface information on the through-the-thickness behavior. Every point of the middle surface is thus endowed with degrees of freedom that are additional to those natural for a point in 3D space; they summarize the through-the-thickness behavior in a way rendered precise in each specific model. The variability of possible choices generated a number of models, each implying peculiar analytical problems. However, pertinent analyses appear in a sense more manageable than those based on looking at shells as genuine three-dimensional bodies. A related question concerns the rigorous justification of such approximate models. In the elastic case, non trivial relevant results are available. For them, the energy scaling with the thickness plays a crucial role (pertinent analyses are in reference [10]; see also [9,16,21] on this matter).

    In 1958 [8], Jerald LaVerne Ericksen and Clifford Ambrose Truesdell III resumed the 1909 theory by Eugène and François Cosserat, noticing that Cosserat's surfaces were an appropriate ground for models of beams and shells. Such surfaces are endowed with out-of-surface vector field. Every element of this field is in principle free to rotate independently of its neighbors, along any deformation, but constrained to avoid falling within the tangent plane to the surface at each point. In this way, looking at shells, one considers at each point on the middle surface any out-of-middle-surface material fiber as a rigid body. Of course the scheme can be further enriched but also reduced, as, for example, when we consider such an additional vector field to coincide with the fields of normal to the middle surface that we consider to be smooth.

    This last choice is the one that we adopt in the analyses presented here. With it we avoid considering through-the-thickness shear. We have however the advantage to look at the deformed shell as the Gauss graph of a smooth surface. Such a graph is the set of pairs (y,ν(y)), where yR3 is a point over the deformed surface and ν(y) the pertinent normal to the surface itself. Of course, with u a deformation from Ω, a bounded domain in R2, into R3, we have a map Φu assigning to each xΩ the pair (u(x),νu(x))=(y,ν(y)). In such a setting, we consider an elastic shell with an energy given by

    Fq(u):=Ω(|DΦu(x)|q+f(|adj2u(x)|))dx+E(u), (1.1)

    where adj2u indicates the second-rank minors of u, E(u) is a potential of external body actions, f a convex function, and |DΦu(x)|2=|u(x)|2+|νu(x)|2 (in (1.1) unitary dimensional constants are left understood, for the sake of simplicity). With this energy we account for bending and in-tangent-plane stretching and shear. We leave a part out-of-tangent-plane shear and thickness stretching, the latter due to Poisson's effect.

    We prove here existence of minimizers for such an energy in terms of currents carried by Gauss graphs. For this reason we speak of our result as describing weak deformations for shells. In the proof we adopt minimizing sequences of deformed shells with non-vanishing thickness. Physics suggests the existence of at least subsequences of this type. Roughly speaking, under elongation in the middle surface tangent plane, Poisson's effect does not allow a reduction of shell thickness below the one of a single atomic layer.

    With this type of sequences, we are able to assure that each their element avoids self-penetration of matter. And such a property is stable in the limit process.

    First, we collect some basic aspects concerning currents associated with graphs of approximately differentiable maps. Then, we connect them with Gauss graphs of surfaces in the Euclidean space. Eventually, we recall some structure properties about weak limits of currents carried by Gauss graphs.

    The theory of currents is systematically discussed in the two-volume treatise [13,14] (see also [15]). Here, we limit ourselves to maps u:ΩR3, where Ω is a bounded domain in R2. We indicate by y points in R3 while with x those in R2.

    For uL1(Ω,R3) an almost everywhere (a.e.) approximately differentiable map, we denote by u its approximate gradient. Measurable functions into topological spaces with a countable basis can be approximated by continuous functions on arbitrarily large portions of their domain (this is Lusin's theorem). Such continuous functions are what we call the Lusin representatives of the original functions. So, the map u has a Lusin representative on the subset ˜Ω of Lebesgue points pertaining to both u and u. Also, we have L2(Ω˜Ω)=0. We shall thus denote by adj2FR3 the 3-vector given by the 2×2 minors of a matrix FM3×2.

    Definition 2.1. We say that u belongs to A1(Ω,R3) if uL1(Ω,M3×2) and adj2uL1(Ω,R3).

    The graph of a map uA1(Ω,R3) is defined by

    Gu:={(x,y)Ω×R3x˜Ω, y=˜u(x)},

    where ˜u(x) is the Lebesgue value of u. It turns out that Gu is a countably 2-rectifiable set of Ω×R3, with H2(Gu)<, where Hk denotes the k-dimensional Hausdorff measure. The approximate tangent plane at (x,u(x)) is generated by the vectors t1(x)=(1,0,1u(x)) and t2=(0,1,2u(x)), where the partial derivatives are the column vectors of the gradient u, and we take u(x) as the Lebesgue value of u at x˜Ω. Therefore, the 2-vector

    ξ(x):=t1(x)t2(x)|t1(x)t2(x)|

    provides an orientation to the graph Gu.

    Integration of compactly supported smooth 2-forms ω in D2(Ω×R3) on Gu defines the current Gu carried by the graph of u, namely

    Gu,ω:=Guω,ξdH2.

    Gu is called an integer multiplicity (in short i.m.) rectifiable current in R2(Ω×R3), with mass M(Gu) equal to the area H2(Gu) pertaining to the graph of u, so a finite mass. (We write R2 instead of the common (natural) notation D2 to recall the rectifiable nature of the current.) Since the Jacobian of the graph map x(x,u(x)) is equal to |t1(x)t2(x)|, by the area formula we get

    M(Gu)=H2(Gu)=Ω1+|u|2+|adj2u|2dx< .

    By duality, the boundary of Gu is the 1-current Gu acting on D1(Ω×R3), the space of compactly supported smooth 1-forms η in Ω×R3, as

    Gu,η:=Gu,dη,ηD1(Ω×R3),

    where dη is the differential of η. By Stokes theorem we get

    Gu=0onD1(Ω×R3) (2.1)

    if u is of class C1. However, in general, the boundary Gu does not vanish and may not have finite mass in Ω×R3. On the other hand, if Gu has finite mass, the boundary rectifiability theorem states that Gu is an i.m. rectifiable current in R1(Ω×R3).

    Example 2.1. If u is a Sobolev map in W1,q(Ω,R3), with q1, the approximate gradient agrees with the density of the weak derivative of u. Moreover, since |adj2u|c|u|2, we get W1,2(Ω,R3)A1(Ω,R3).

    The null-boundary condition (2.1) holds true for Sobolev maps uW1,2(Ω,R3), by approximation. In fact, if {uh}C1(Ω,R3) is such that uhu strongly in W1,2, by dominated convergence we infer that Guh converges to Gu weakly as currents, i.e., Guh,ωGu,ω for each ωD2(Ω×R3). So, condition (2.1) is preserved in the weak limit process.

    Take Ω=B2, with B2 the unit disk centered at the origin O, and u:B2R3 as the zero-homogeneous map

    u(x)=φ(x|x|),xO

    for some Lipschitz-continuous map φ:B2R3. Then, uW1,q(B2,R3) for each q<2, and adj2u=0, by the area formula, whence uA1(B2,R3). However (compare Example 2 in reference [13, Sec. 3.2.2]), we compute

    (Gu)B2×R3=δO×φ#[[B2]], (2.2)

    where δO is the Dirac mass at the origin and φ#[[B2]] is the image through the map φ of the 1-current [[B2]] associated with the naturally oriented unit circle B2. Therefore, condition (2.1) fails to hold whenever φ#[[B2]] is non-trivial (see Examples 7.1 and 7.2 below).

    We summarize in this section some issues concerning Gauss graphs of surfaces with codimension one. Our main reference is [5] (see also [4]). Such notions are essential for the rest of this work because we refer to functionals depending on curvatures.

    Given a smooth (say C2), bounded, and oriented surface MR3, with smooth boundary M, the Gauss map ν:MS2 associates to each point y in M the unit normal ν(y)S2, where

    S2:={zR3:|z|=1}.

    The graph of such a map, or Gauss graph, is the 2-dimensional surface in R6 given by

    GM:={(y,ν(y))yM}M×S2R3×R3zR6,

    where R3z is the isomorphic copy of R3 in which we consider embedded the sphere S2.

    Therefore, we shall denote by (ε1,ε2) and (e1,e2,e3) the canonical bases of R2 and R3y, respectively, and by (ε1,ε2) and (e1,e2,e3) the natural dual bases defined by εiεj=δji and ehek=δkh, where the interposed dot means dual pairing. In particular, we will identify the dual bases with (dx1,dx2) and (dy1,dy2,dy3), which are naturally dual to (x1,x2) and (y1,y2,y3). We also correspondingly denote by (ϵ1,ϵ2,ϵ3) the canonical basis of R3z, and by (dz1,dz2,dz3) the dual basis, after adopting the previous identification.

    The Hodge star, applied to the normal vector ν(y), defines a 2-form over the tangent space at y. We then have a field, that we call a tangent 2-vector field τ:M2TM2R3y with values τ(y)=ν(y). Denoting by Φ:MR3y×R3z the graph map Φ(y):=(y,ν(y)), a continuous tangent 2-vector field ξ:GM2(R3y×R3z) is given by ξ(y,ν(y)):=2dΦ(τ(y)). Since |ξ|1 on GM, the normalized 2-vector field ζ:=ξ/|ξ| determines an orientation to GM. Therefore, the corresponding i.m. rectifiable 2-current [[GM]] in R2(R3y×R3z) carried by the Gauss graph has multiplicity one and support contained in ¯M×S2. Its action on compactly supported smooth 2-forms ω in R3y×R3z is (by integration) given by

    [[GM]],ω=GMω(y,z),ζ(y,z)dH2,ωD2(R3y×R3z).

    By Stokes' theorem, the boundary current [[GM]] acts by integration of 1-forms on the naturally oriented boundary of GM, so that [[GM]]=0 if M is a closed smooth surface.

    The tangential Jacobian JMΦ of the graph map is given by

    JMΦ(y)=(1+(k12+k22)+(k1k2)2)1/2,yM

    where k1=k1(y) and k2=k2(y) are the principal curvatures at yM. In fact, we have JMΦ(y)=|ξ(y,ν(y))|. Moreover, denoting by τ1 and τ2 the principal directions, and considering the natural homomorphism v˜v from R3y onto R3z, we get

    ξ(y,ν(y))=τ1τ2+(k2τ1˜τ2k1τ2˜τ1)+k1k2˜τ1˜τ2. (2.3)

    Also, with the mean curvature and the Gauss curvature given by

    H:=12(k1+k2),K:=k1k2,

    so that k1,2=H±H2K, we may equivalently write

    (JMΦ)2=1+(2H)22K+K2=4H2+(1K)2.

    Therefore, by the area formula, area of the Gauss graph is

    H2(GM)=M(1+(k12+k22)+(k1k2)2)1/2dH2=M1+(4H22K)+K2dH2. (2.4)

    It agrees with the mass M([[GM]]) of the current [[GM]].

    The curvature functional M of a smooth surface MR3y is defined in reference [5] to be

    M:=H2(M)+Mk21+k22dH2+M|k1k2|dH2. (2.5)

    It can be equivalently written as

    M:=M(1+4H22K+|K|)dH2

    so that, by formula (2.4), we obtain bounds such as

    12MH2(GM)M,H2(GM)=M([[GM]]).

    Also, two real measures on R3y×R3z are naturally associated with mean and Gauss curvatures [5]:

    χM1:=Φ#(HH2M),χM2:=Φ#(KH2M). (2.6)

    Consequently, for any ψC0(R3y×R3z) we have

    χM1,ψ=MH(y)ψ(y,ν(y))dH2(y),

    and

    χM2,ψ=MK(y)ψ(y,ν(y))dH2(y).

    In terms of the current [[GM]], such curvature measures read

    χM,ψ=(1)[[GM]],ψΘ,ψCc(R3y×R3z),=1,2.

    Θ=Θ(y,z) are 2-forms in R3y×R3z, defined in [5]. For two-dimensional surfaces they are explicitly given by

    Θ1:=12(z1(dy2dz3dy3dz2)+z2(dy3dz1dy1dz3)+z3(dy1dz2dy2dz1)),Θ2:=z1dz2dz3+z2dz3dz1+z3dz1dz2. (2.7)

    Remark 2.1. The dependence of mean curvature from the sign of principal curvatures motivates the introduction of a factor 1 in order to recover, for parametric surfaces, standard notations (see expression (3.13) below).

    Weak limits Σ of sequences of currents carried by Gauss graphs of smooth surfaces {Mh} are analyzed in reference [5]. Assuming for simplicity that each Mh is closed, supported in a given compact set KR3, and suphMh<, it turns out that possibly passing to a subsequence the currents [[GMh]] weakly converge in D2(R3y×R3z) to an i.m. rectifiable current Σ in R2(R3y×R3z), with null boundary, Σ=0, and with support contained in K×S2.

    We thus have

    Σ,ω=Rθω,ηdH2,ωD2(R3y×R3z),

    for some 2-rectifiable subset R of K×S2, some positive and integer-valued H2R-measurable multiplicity function θ:RN+, and some H2R-measurable function η:R2(R3y×R3z) such that η(y,z) is a unit 2-vector orienting the approximate tangent 2-space T(y,z)R to R at (y,z), for H2-a.e. (y,z)R. In that case, one usually writes Σ=[[R,θ,η]]. We also denote by

    p:R3y×R3zR3y (2.8)

    the orthogonal projection onto the first factor. Therefore, P:=p(R) is a 2-rectifiable set. Finally, φ and φ denote the canonical 1-form and 2-form, respectively given by

    φ(y,z):=z1dy1+z2dy2+z3dy3,φ(y,z):=z1dy2dy3+z2dy3dy1+z3dy1dy2.

    Theorem 2.1. ([5]) With the previous notation, the following statements hold:

    1) Σ,ηφ=0 for each ηD1(R3y×R3z)\,;

    2) Σ,ψφ0 for each ψC(R3y×R3z) such that ψ0\,;

    3) for H2-a.e. yP

    p|R1({x}){(x,ν(x)),(x,ν(x))},

    where ν:PS2 is an H2P-measurable map with ν(y) orthogonal to the approximate tangent 2-space TyP, for H2-a.e. yP.

    Remark 2.2. We finally recall from reference [5] that property (1) is equivalent to the orthogonality condition

    v(z,0R3z)=0vT(y,z)R

    for H2-a.e. (y,z)R, where denotes the scalar product in R6. As in reference [5], the stratification

    η=η(0)+η(1)+η(2)

    holds, where according to the number of ϵj-entries we have set

    η(0)=1i<j3ηijeiej,η(1)=3i,j=1ηijeiϵj,η(2)=1i<j3ηijϵiϵj.

    Then, it turns out that properties (1) and (2) are equivalent to

    Σ,ψφ=Rψ|η(0)|θdH2,ψC(R3y×R3z).

    As already recalled in the Introduction, shells (we include plates and thin films in the nomenclature, as special cases) are bodies for which one dimension is largely smaller with respect to the others. This geometric peculiarity suggests approximate representations of shells as two-dimensional bodies, each point of which is endowed with additional information on what happens in the (real) thickness (essential references on the matter are [2,8,22,23]; see also [3,17,19,20]).

    In this view, we consider a planar reference configuration for the shell middle surface, i.e., we refer in essence to something that can be a plate in some configuration. So, we select a two-dimensional smooth domain Ω in R2, where Cartesian coordinates x=(x1,x2) are fixed. A differentiable map u:ΩR3, say u=(u1,u2,u3), represents a deformation. It is taken to determine an immersion of Ω into R3. The tangent plane to the deformed shape is assumed not to degenerate. Formally, it is tantamount to impose |adj2u(x)|>0 for any xΩ. In other words, if iu denotes a column vector of the gradient matrix u, the previous condition is equivalent to say that the vector product 1u×2u does not vanish at every point. Normal to the deformed configuration

    Mu=u(Ω)

    is the unit vector

    νu(x):=1u(x)×2u(x)|1u(x)×2u(x)|. (3.1)

    It can be considered as a descriptor of out-of-middle-surface shell behavior. However, such an information can be carried out by an S2-valued vector field xζ(x) defined over Ω and constrained to be at every xΩ such that

    (1u(x)×2u(x))ζ(x)>0, (3.2)

    where is the scalar product in R3. In both cases we are representing a shell as a Cosserat surface (see [8,22,23]). By choosing ζ we allow the description of through-the-thickness shear, while with νu we exclude it. We adopt here this last choice so that smooth deformed shells are represented by parametric surfaces.

    In the present setting, the Gauss graph of the surface Mu=u(Ω) is described by

    GMu={Φu(x)xΩ}, (3.3)

    where Φu:ΩR3y×R3z is the smooth map

    Φu(x):=(u(x),νu(x)). (3.4)

    The first fundamental form of Mu is given by the symmetric matrix

    I=(EFFG):=(|1u|21u2u1u2u|2u|2),

    with determinant

    g=gu:=EGF2=|1u×2u|2=|adj2u|2. (3.5)

    Second derivatives 2i,ju of the deformation map u, where 22,1u=21,2u, determine the second fundamental form given by the symmetric matrix

    II=(mmn):=(21,1uνu21,2uνu22,1uνu22,2uνu)=1gu(21,1un21,2un22,1un22,2un),

    where, for short-hand notation, we have denoted n:=1u×2u, so that gu=|n|2. Therefore, the mean curvature at u(x) becomes

    Hu=12g(En+G2Fm)=12gu3/2(|1u|222,2u+|2u|221,1u2(1u2u)21,2u)n (3.6)

    and the Gauss curvature is then

    Ku=nm2EGF2=1gu2((21,1un)(22,2un)(21,2un)2). (3.7)

    The tangent space at a point (y,z) in the Gauss graph GMu is oriented by the wedge product

    ξu(x):=1Φu(x)2Φu(x),xΩ,(y,z)=(u(x),νu(x)).

    We have

    αΦu=(αu1,αu2,αu3,ανu1,ανu2,ανu3),α=1,2,

    and hence, according to the number of ϵj-entries, we can write as before the stratification

    ξu=ξ(0)u+ξ(1)u+ξ(2)u,

    where

    ξ(0)u=1i<j3det(1ui2ui1uj2uj)eiej,ξ(1)u=3i,j=1det(1ui2ui1νuj2νuj)eiϵj,ξ(2)u=1i<j3det(1νui2νui1νuj2νuj)ϵiϵj. (3.8)

    As a consequence of the expression (2.4), by the area formula we can write the Gauss graph area H2(GMu) as

    Ωgu1+(4H2u2Ku)+K2udx=Mu1+(4H2u2Ku)+K2udH2=Ω|ξu|dx, (3.9)

    which yields a formula for the Jacobian JΦu of Φu:

    JΦu=|ξu|=gu1+(4H2u2Ku)+K2u. (3.10)

    Finally, by the expression (2.3) we infer

    |ξ(0)u|2=gu,|ξ(1)u|2=gu(4H2u2Ku),|ξ(2)u|2=guK2u. (3.11)

    Let u:ΩR3 a smooth function with 1u×2u0 everywhere. Assume uK<. The current [[GMu]] in R2(R6) is defined as above, with Mu=u(Ω). The Gauss graph surface GMu is equipped with the natural orientation induced by the function u, so that

    ζu(y,z):=ξu|ξu|(x),(y,z)=(u(x),νu(x))GMu.

    We shall restrict to the action of compactly supported forms in R3×S2. By the definition (3.3) we thus equivalently have [[GMu]]=Φu#[[Ω]], that is

    [[GMu]],ω=ΩΦu#ω,ωD2(R3×S2).

    Moreover, recalling the notation (2.8), we have (see also formula (5.1) below)

    p#[[GMu]]=p#(Φu#[[Ω]])=(pΦu)#[[Ω]]=u#[[Ω]].

    A property that we call the absence of cancellations plays a role here. So far we left a part direct references to the shell thickness, considering it implicitly very thin. Take it to be vanishing, i.e., infinitesimal. Take also circumstances in which two pieces of the deformed shell Mu=u(Ω) are in contact, with the same or opposite orientation. More precisely, we have two pairwise disjoint and connected open sets A,BΩ such that both the restrictions u|A and u|B are injective, but u(A)=u(B). Then, for each xA there exists a unique point ˜x=˜x(x)B such that u(x)=u(˜x), and the map x˜x(x) is a diffeomorphism from A to B. Moreover, notice that a unit 2-vector field orienting the tangent plane to Mu at y=u(x) is

    τu:=νu=ξ(0)u|ξ(0)u|=1|1u×2u|1i<j3det(1ui2ui1uj2uj)eiej.

    Therefore, one of the two following alternatives holds: either τu(x)=τu(˜x(x)) for all xA, or τu(x)=τu(˜x(x)) for all xA, according to the fact that the diffeomorphism from A to B preserves or reverses the natural orientation. In both cases, the tangent plane to u(Ω) at u(x) agrees with the one at u(˜x(x)). In the first case, u#[[A]]=u#[[B]] and the image current u#[[AB]] has multiplicity two, whereas in the second one u#[[A]]=u#[[B]], whence u#[[AB]]=0, a cancellation occurs and actually

    M(u#[[Ω]])<Ω|ξ(0)u|dx,|ξ(0)u|=|adj2u|.

    However, since in the second case νu(x)=νu(˜x(x)) for each xA, by the expressions (3.8) we infer

    ξ(0)u(x)=αξ(0)u(˜x(x)),ξ(2)u(x)=βξ(2)u(˜x(x)),

    for some α,β>0, whence ξu(x)±ξu(˜x(x)), and definitely no cancellation occurs in the Gauss graph current [[GMu]]. In conclusion, even in the presence of cancellations for the projected current u#[[Ω]]=p#[[GMu]], we always have

    M([[GMu]])=H2(GMu)=Ω|ξu|dx<. (3.12)

    Compare Example 7.2 below for a deformation with folding.

    For the sake of completeness, we compute the action of a current [[GMu]] over the Gauss graph and explicit formulas for the curvature measures χMu.

    Since for i,j=1,2,3

    Φu#dyi=1uidx1+2uidx2,Φu#dzj=1νujdx1+2νujdx2

    the pull-back of the basis of 2-forms in R3y×R3z gives the fifteen formulas

    Φu#(dyidyj)=det(1ui2ui1uj2uj)dx1dx2,1i<j3,Φu#(dyidzj)=det(1ui2ui1νuj2νuj)dx1dx2,i,j=1,2,3,Φu#(dzidzj)=det(1νui2νui1νuj2νuj)dx1dx2,1i<j3.

    Therefore, for each compactly supported smooth function ψCc(R3×S2) we obtain

    [[GMu]],ψdyidyj=Ωψ(Φu)det(1ui2ui1uj2uj)dx,1i<j3,[[GMu]],ψdyidzj=Ωψ(Φu)det(1ui2ui1νuj2νuj)dx,i,j=1,2,3,[[GMu]],ψdzidzj=Ωψ(Φu)det(1νui2νui1νuj2νuj)dx,1i<j3.

    By referring to the expressions (2.6), if M=Mu for some smooth and injective function u:ΩR3 as above, we may write

    χMu1:=Φu#(HuH2Mu),χMu2:=Φu#(KuH2Mu),

    where Hu and Ku are given by (3.6) and (3.7). In terms of the Gauss graph current [[GMu]], on account of definitions (2.7) we thus get

    χMu,ψ=(1)[[GMu]],ψΘ),ψCc(R3×S2),=1,2.

    In fact, recalling that [[GMu]]=Φu#[[Ω]] we obtain the following formulas:

    Φu#Θ1=guHudx1dx2,Φu#Θ2=guKudx1dx2,

    where gu is given by (3.5). Therefore, for any ψCc(R3×S2) we have

    χMu1(ψ)=Ωψ(Φu)guHudx,χMu2(ψ)=Ωψ(Φu)guKudx. (3.13)

    We omit any further detail and address to reference [18] for a similar computation involving Gauss graphs of smooth Cartesian surfaces.

    If we consider only pure bending, the energy refers to the curvature functional GMu defined by the expression (2.5). By the area formula, it agrees with the energy functional E(u) defined by

    E(u):=Ωgu(1+4H2u2Ku+|Ku|)dx.

    On account of the expressions (3.9), (3.10), and (3.11), we get

    E(u)=Ω(|ξ(0)u|+|ξ(1)u|+|ξ(2)u|)dx, (4.1)

    where |ξu|2=|ξ(0)u|2+|ξ(1)u|2+|ξ(2)u|2 and, by (3.8),

    |ξ(0)u|2=gu=|1u×2u|2=|adj2u|2,|ξ(1)u|2=gu(4H2u2Ku)=3i,j=1(1ui2νuj2ui1νuj)2,|ξ(2)u|2=guK2u=1i<j3(1νui2νuj2νui1νuj)2. (4.2)

    On account of the expression (3.12), we have

    12E(u)M([[GMu]])E(u).

    We aim at a bound for the total variation of the function u and of the Gauss map νu. Also, we wish to preserve condition 1u×2u0, where |1u×2u|=|adj2u|. These two issues suggest us to work with an expression of the energy given by

    Fq(u):=Ω(|DΦu(x)|q+f(|adj2u(x)|))dx+E(u) (4.3)

    for some real exponent q>1. The map Φu is given by formula (3.4), so that

    |DΦu(x)|2=|u(x)|2+|νu(x)|2,xΩ, (4.4)

    and f:]0,+[R+ is a positive and convex function with linear growth at + and such that f(ρ)+ as ρ0+, as e.g., f(ρ)=ρ1ρ.

    Here, the presence of u implies considering middle surface stretch and shear. We leave a part the consequent thickness stretching (due to the Poisson's effect), considering it not prominent from an energetic point of view as the thickness is thin and possibly vanishing in the minimizing sequences.

    Theorem 5.1. For q>1 and K>0, let {uh}C2(Ω) a sequence satisfying suphFq(uh)< and suphuhK. There exists a map uW1,q(Ω,R3), with uK, and a (not relabeled) subsequence such that

    1) uhu weakly in W1,q(Ω,R3y),

    2) for L2-a.e. xΩ, one has 1u×2u0, and |adj2u|L1(Ω), and

    3) νuhνu weakly in W1,q(Ω,R3z), where the approximate normal νu(x) is defined for L2-a.e. xΩ by (3.1), but in terms of the approximate gradient u.

    Proof. We repeatedly pass to not relabeled subsequences. The first assertion is trivial. Since

    M(Guh)Ω(1+|uh|+|adj2uh|)dx,

    by formulas (4.1) and (4.2) we get suphM(Guh)<. Therefore, by the closure theorem for Cartesian currents (see [13,14]) there exists a current T such that

    T=Gu+ST

    with GuhT weakly in D2(Ω×R3) and STD1(Ω×R3), which we write in short Tcart(Ω×R3). This implies |adj2u|L1(Ω). Moreover, since the integrand f is a convex function, by L2-a.e. convergence of the gradients uh to the approximate gradient u, which follows from Lq-convergence, we get

    Ωf(|adj2u|)dxlim infhΩf(|adj2uh|)dx<.

    Hence, condition 1u×2u0 holds a.e. in Ω. We finally obtain weak W1,q and L2-a.e. convergence of νuh to νu.

    If u is the limit function in Theorem 5.1, we can express the energy E(u) by equation (4.1), where the three terms |ξ()u|, =0,1,2, are defined L2-a.e. on Ω by the right-hand side of the system (4.2), now in terms of the approximate gradient of u and νu.

    We impose Dirichlet-type boundary conditions as follows:

    (H) there exists an open set ˜ΩR2, with ¯Ω˜Ω, and an injective function ˜uC2(˜Ω) such that each uh can be smoothly extended to ˜Ω so that uh=˜u on ˜ΩΩ.

    Recalling the notation (2.8), we also denote by ˜π:R2×R3yR3y the orthogonal projection onto the second factor.

    Theorem 5.2. Let {uh} as in Theorem 5.1. Assume that condition (H) holds. Then, passing to a (not relabeled) subsequence, the Gauss graphs [[Muh]] weakly converge to a current ΣR2(R3×S2) satisfying the properties listed in Theorem 2.1. Moreover,

    p#Σ=˜π#T=u#[[Ω]]+˜π#ST, (5.1)

    where T=Gu+ST is the current in cart(Ω×R3) obtained in Theorem 5.1. In addition, the estimates ΣE(u) and E(u)lim infhE(uh) hold. Eventually, Fq(u)lim infhFq(uh).

    Proof. The boundary condition (H) yields a uniform bound on mass and curvatures of the boundary currents uh#[[Ω]]=uh#[[Ω]]. Therefore, we can apply the closure theorem on Gauss graphs proven in reference [5], obtaining the current Σ that satisfies the properties listed in Theorem 2.1. Since

    p#[[Muh]]=uh#[[Ω]]=˜π#Guh,

    by the weak convergence GuhT in the proof of Theorem 5.1, we get the projection formula (5.1).

    Even if a cancellation may occur in the projected current p#Σ, the argument exploited in Section 3.3 (see relations (3.12)) allows us to conclude that

    M(Σ)=M(Φu#[[Ω]])+M(ΣΦu#[[Ω]]).

    As a consequence, by lower semicontinuity we get

    Φu#[[Ω]]Σlim infh[[Muh]],

    where Φu#[[Ω]]=E(u) and [[Muh]]=E(uh) for each h. The lower semicontinuity inequalities readily follow.

    Under large bending, distant portions of shells may touch. Physics, however, suggests that an appropriate model should avoid solutions describing self-penetration of the matter. A constraint excluding such a possibility is then required in the setting represented here. Evidently, for it the shell thickness – otherwise not exploited directly so far – should play a role. We propose a way to obtain such a necessary constraint on the basis of what is done for fully three-dimensional bodies.

    Let u:¯ΩR3 be a map of class C2(¯Ω), with 1u×2u0 on ¯Ω. For s>0 let vu,s:UR3, where U:=Ω×(h2,h2), h the constant shell thickness, be given by

    vu,s(x1,x2,x3):=u(x1,x2)+sx3νu(x1,x2). (6.1)

    We compute

    detvu,s=s|1u×2u|+s2x3λu(x)+s3x23μu(x)

    on U, where

    λu(x):=ν1u(1u22νu32u21νu3+1νu22u32νu21u3)+ν2u(1u32νu12u31νu1+1νu32u12νu31u1)+ν3u(1u12νu22u11νu2+1νu12u22νu11u2)

    and

    μu(x):=νu1(1νu22νu32νu21νu3)+νu2(1νu32νu12νu31νu1)+νu3(1νu12νu22νu11νu2).

    By the equiboundedness of λu(x) and μu(x), we infer that s1detvu,s|1u×2u| as s0+ uniformly on U. Hence, we get

    detvu,s(x1,x2,x3)>0,(x1,x2,x3)U,

    if s>0 is sufficiently small. On account of the relations (4.2) and (4.4), we also estimate

    |vu,s|+|adj2vu,s|+|detvu,s|c(|DΦuh|+|ξ(0)u|+|ξ(1)u|+|ξ(2)u|) (6.2)

    for some absolute constant c>0 not depending on u.

    Moreover, since the principal curvatures of the deformed surface u(Ω) are bounded, there exists s=s(u)>0 such that for each (x1,x2)Ω we can find a neighborhood V of (x1,x2) in Ω such that the restriction of vu,s to V×(h2,h2) is injective.

    In order to obtain global injectivity, one may e.g., impose the well-known Ciarlet-Nečas condition [6]. In reference [7], under suitable additional hypotheses, the inequality

    Ω|adj2u|dxH2(u(Ω))

    is proposed. However, it is violated by a folding deformation. Then, what is suggested in reference [7] is to adopt a notion of approximately injective deformations. It requires (in a sense not specified here) that the parametric surface u:ΩR3 can be approximated by sequences of functions uh such that each uh can be seen as a parameterization of the middle surface of a thick shell satisfying the Ciarlet-Nečas condition, and we can take the thickness so that it gradually disappears. This property yields to a more reasonable scenario from a physical point of view.

    We go along the path summarized above to avoid self-penetration of matter. We start by a condition introduced in 1989 by M. Giaquinta, G. Modica, and J. Souček [11,12] (see also [14, Sec. 2.3.2]). It is equivalent to Ciarlet-Nečas' one.

    For a sufficiently smooth map v:UR3 with detv>0 a.e. in U, a fit region in R3, by setting w=(x1,x2,x3), the global invertibility condition above mentioned reads

    Uf(w,v(w))detv(w)dwR3supwUf(w,y)dy (6.3)

    for every compactly supported smooth function f:U×R3[0,+). Weak convergence in terms of currents preserves such condition. It implies L3-a.e. injectivity.

    Theorem 6.1. Let {uh} satisfy assumptions in Theorems 5.1 and 5.2. Assume that, for every h, the function vh=vuh,sh given by expression (6.1) satisfies the global invertibility condition (6.3), for some sh(0,1). The weak limit deformation map u is the trace on x3=0 of a function v:UR3 satisfying |v|,|adj2v|,detvL1(U), detv0 a.e. in U, and condition (6.3).

    Proof. For every h, we have already seen that by taking sh(0,1) sufficiently small, the function vh=vuh,sh also satisfies detvh>0 on U. Furthermore, on account of the estimate (6.2), we infer that the current Gvh carried by the graph of vh has finite mass, which satisfies the bound

    M(Gvh)c(Ω(1+|Φuh|)dx+E(uh)),h.

    Therefore, we get suphM(Gvh)<, whereas, by the smoothness hypothesis, Gvh=0 on U×R3. As a consequence, by the closure theorem for Cartesian currents, it turns out that a (not relabeled) subsequence satisfies Gvh˜T weakly in D3(U×R3) for some Cartesian current ˜Tcart(U×R3). Let v denote the underlying function to ˜T, so that ˜T=Gv+S˜T. We find that v:UR3 is approximately differentiable a.e. in U. Also, all the minors of its approximate gradient v are in L1(U).

    By the weak convergence Gvh˜T, it turns out that the function v satisfies the global invertibility condition (6.3), too, whereas detv0 a.e. in U.

    In addition, if we look at Gvh as an i.m. rectifiable current in R3w×R3y, slicing theory implies that the restriction of Gvh to (Ω×{0})×R3 agrees with the graph current Guh. Therefore, by the weak convergence of Gvh to ˜T and of Guh to T, where, we recall, ˜T=Gv+S˜T and T=Gu+ST, it turns out that the restriction of ˜T to (Ω×{0})×R3 agrees with the current T. This property indicates us that u is the trace of v on Ω×{0}. So, the proof is complete.

    Remark 6.1. A sort of thickness condition holds if we choose a size s0>0 and assume that vh=vuh,sh satisfies the constraint (6.3) and detvh>0 on U for some shs0. In this case the function v is given L3-a.e. by (6.1), where ss0 and the unit normal νu is defined L2-a.e. in Ω by the formula (3.1), now in terms of the approximate gradient u of the limit deformation u.

    Moreover, in order to preserve the local orientation, namely detv>0 a.e. on U, we may add to the energy functional (4.3) a term of the type

    U|detvu,s|rdx,r>0.

    Fix q>1, K>0, and ˜u:˜ΩR3, together with the boundary condition (H). On account of the above results, we may say that a function u:ΩR3 represents a weak deformation for a shell, and we write for short uA=A(q,K,˜u), provided that the following properties hold:

    1)uW1,q(Ω,R3) with uK, 1u×2u0 for L2-a.e. xΩ, and |adj2u|L1(Ω) (see Theorem 5.1); moreover, the approximate normal νu belongs to W1,q(Ω,S2) (see definition (3.1));

    2)u=˜u on ˜Ω¯Ω (see Theorem 5.2); moreover, there exists a current ΣR2(R3×S2) satisfying Theorem 2.1, such that the projection formula (5.1) holds, where T=Gu+STcart(Ω×R3), and ΣE(u);

    3)u is the trace on x3=0 of a function v:UR3 satisfying |v|,|adj2v|, detvL1(U), detv0 a.e. in U, and the global invertibility condition (6.3) (see Theorem 6.1).

    For future use, we also denote by ˜A=˜A(q,K,˜u) the class of maps u that are generated by a weak limit process as in Theorems 5.1, 5.2, and 6.1. It may be called the class representing smoothly accessible weak deformations for shells.

    Therefore, ˜AA, and we expect that equality holds: if u represents a weak deformed shell. Then, we can find a sequence {uh} as in the above mentioned theorems such that uhu weakly in W1,q(Ω,R3).

    The smooth density property for a generic weak deformation u of a shell is an open question. However, due to the Dirichlet-type boundary condition (H), and to the equiboundedness hypothesis, all the involved weak convergences as currents, say GuhT, [[GMuh]]Σ, and Gvh˜T are metrizable. Therefore, we can apply a diagonal argument and prove the following existence result:

    Theorem 6.2. For q>1, K>0, and ˜u as in (H), let ˜A denote the class of smoothly accessible weak deformation of shells. Then, the problem

    inf{Fq(u)u˜A}

    for the energy defined by formula (4.3) has a solution in the class ˜A.

    Proof. Let {uk}˜A a minimizing sequence. Then, possibly passing to a (not relabeled) subsequence uku weakly in W1,q to some uW1,q(Ω,R3), and νukνu weakly in W1,q where, we recall, νuk is defined L2-a.e. in Ω as in formula (3.1), for each k¯N:=N{}. Moreover, for each kN we can find a sequence {u(k)h} that satisfies Theorems 5.1, 5.2, and 6.1, such that u(k)huk weakly in W1,q, as h. Due to the metrizable character of the (weak) convergences involved, by a diagonal argument we infer that actually u˜A. Moreover, arguing as in the proof of Theorem 5.2, by the weak convergences uku and νukνu in W1,q, where q>1, we obtain

    Φu#[[Ω]]lim infkΦuk#[[Ω]],

    where Φuk#[[Ω]]=E(uk) for each k¯N. We thus get

    Fq(u)lim infkFq(uk)=inf{Fq(u)u˜A},

    and the proof is complete.

    If u represents a weak deformation for a shell, as in the previous section, where q=2, the membership of u to the Sobolev class W1,2(Ω,R3) yields that the graph current Gu satisfies the null-boundary condition (2.1). On account of the Dirichlet-type condition (H), this implies that u#[[Ω]]=u#[[Ω]]. Therefore, when q2, we are actually avoiding the formation of fractures.

    Things change when the growth exponent q is lower than two.

    Let Ω=B2 be the unit disk at the origin O. Let u:B2R3 be given by

    u(x)=(|x|+1)φ(x|x|),xO,

    for some Lipschitz-continuous map φ:B2R3. We have uW1,q(B2,R3) and adj2uLq(B2,R3) for each q<2, whence uA1(B2,R3).

    Consequently, we are presuming the existence of a hole at the origin in B2, and the deformed hole has a boundary that is described by the map φ. Moreover, formula (2.2) continues to hold. More precisely, viewing the graph Gu as a current in R2(R2×R3), one gets

    Gu=Ψ[φ]#[[B2]]δO×φ#[[B2]],

    where Ψ[φ]:B2R2×R3 is the Lipschitz map Ψ[φ](x):=(x,2φ(x)) parameterizing the exterior boundary of the graph associated with u.

    Example 7.1. We can stretch the hole (without bending) by taking, in polar coordinates, φ(cosθ,sinθ):=(cosθ,sinθ,0), so that

    u(x)=(x+x|x|,0) .

    Denoting by [[D2]] the 2-current given by integration of 2-forms on the positively oriented disk

    D2:={(y1,y2)R2y21+y22<1},

    we get

    φ#[[B2]]=[[D2]]×δ0,Ψ[φ](x)=(x,(2x,0)).

    Example 7.2. We can stretch, bend, and fold the hole by letting

    φ(cosθ,sinθ):={(cos4θ,sin4θ,1)if0θπ/2,(1,0,4θ/π3)ifπ/2θπ,(cos4θ,sin4θ,1)ifπθ3π/2,(1,0,74θ/π)if3π/2θ<2π.

    In this case, we get

    φ#[[S1]]=[[D2]]×(δ1δ1)

    and a cancellation occurs. We have the same phenomenon in the image current u#[[B2]]. In fact, let us denote by Ωk, for k=1,,4, the four quarters of the holed unit disk B2{O}. We have

    Ωk:={(ρcosθ,ρsinθ)0<ρ<1,(k1)π/2<θ<kπ/2},

    so that it turns out that u(Ω2)=u(Ω4). In other words, folding without self-penetration of matter occurs but u#[[Ω2]]=u#[[Ω4]] and, actually,

    u#[[Ω]]=u#[[Ω1]]+u#[[Ω3]].

    More precisely, coming back to Remark 3.3, the map

    ˜x(ρcosθ,ρsinθ):=(ρcos(π/2θ),ρsin(π/2θ))

    is an orientation reversing diffeomorphism from Ω2 to Ω4 such that

    u(x)=u(˜x(x)),τu(x)=τu(˜x(x)),xΩ2.

    Thus, by formulas (3.8) we have ξ(0)u(x)=ξ(0)u(˜x(x)) and ξ(2)u(x)=ξ(2)u(˜x(x)) for each xΩ2, whence no cancellation occurs in the Gauss graph current [[GMu]].

    In our results above collected, when q<2 we do not consider an energy term accounting for the occurrence of holes. For it, a physically reasonable choice could be given by the hole boundary size in the deformed surface u(Ω). In Example 7.1, the latter contribution might be represented by the graph current boundary mass

    H(u):=M((Gu)Ω×R3).

    In fact, sequences of currents Guh carried by the graph of functions uhW1,q(Ω,R3), which satisfy

    suph(Fq(uh)+H(uh))<

    for some q>1, sub-converge (in the sense of currents) to an element Gu in the same class. Two drawbacks appear.

    In general, the term H(u) does not have a satisfactory physical significance. The map in Example 7.2, e.g., satisfies H(u)=M(φ#[[S1]])=4π, but the curve φ:B2R3y describing a hole in the deformed configuration has length equal to 4(π+1).

    Also, an essential difficulty in this framework is that we are not allowed to apply the closure theorem on Gauss graphs (see results in [5]). In fact, since equality u#[[Ω]]=u#[[Ω]] fails to hold, the boundary condition (H) does not guarantee a uniform bound on curvatures of the boundary terms produced by the holes.

    Similar problems appear if one tries to analyze fractured shells. In that case, a natural ambient is given by suitable deformations u:ΩR3 that belong to the class of special functions of bounded variation. Referring to the treatise [1] for an accurate analysis of SBV functions, we only observe here that the jump set S(u) describes a crack path in the reference configuration, whereas fractures in the deformed shape may be described again by the term H(u). In fact, a map uA1(Ω,R3) satisfying H(u)<, actually belongs to the class SBV(Ω,R3) (compare [15, Prop. 3.3.1]).

    For those reasons, in order to apply the closure theorem on Gauss graphs by reference [5] (that in Theorem 5.2 – where we worked with Cartesian currents – led to the validity of Theorem 2.1), one has to introduce a further energy term that bounds the curvature of the Gauss graph boundary of the deformed surface u(Ω). For these issues, future work is necessary.

    The analyses presented here can be extended variously:

    ● We could consider through-the-thickness shear by opting for an out-of-tangent-plane vector field ζ in the deformed shape.

    ● We could look at multi-layer shells, with consequent need of refining the scheme adopted here.

    ● Different modeling needs would emerge if we would consider a through-the-thickness microstructure. An example is a thin film of a smectic liquid crystal.

    All the previous issues involve both modeling and analytical questions. Looking back to what we have discussed here, a purely analytical open problem emerges: the regularity of minimizers assured by Theorem 6.2. This is probably a nontrivial matter of future specialist work.

    The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.

    This work has been developed within the activities of the research group in "Theoretical Mechanics" of the "Centro di Ricerca Matematica Ennio De Giorgi" of the Scuola Normale Superiore in Pisa. We acknowledge also the support of GNFM-INDAM (to PMM) and GNAMPA-INDAM (to DM).

    The authors declare no conflict of interest.



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