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Research article Special Issues

On some non-local approximation of nonisotropic Griffith-type functionals

  • Received: 13 May 2021 Accepted: 31 August 2021 Published: 16 September 2021
  • The approximation in the sense of Γ-convergence of nonisotropic Griffith-type functionals, with pgrowth (p>1) in the symmetrized gradient, by means of a suitable sequence of non-local convolution type functionals defined on Sobolev spaces, is analysed.

    Citation: Fernando Farroni, Giovanni Scilla, Francesco Solombrino. On some non-local approximation of nonisotropic Griffith-type functionals[J]. Mathematics in Engineering, 2022, 4(4): 1-22. doi: 10.3934/mine.2022031

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  • The approximation in the sense of Γ-convergence of nonisotropic Griffith-type functionals, with pgrowth (p>1) in the symmetrized gradient, by means of a suitable sequence of non-local convolution type functionals defined on Sobolev spaces, is analysed.



    The scope of this paper is to provide a generalization of recent results, obtained in [23], concerning the approximation of brittle fracture energies for linearly elastic materials, by means of nonlocal functionals defined on Sobolev spaces, which are easier to handle also from a computational point of view.

    In [23] an approach originally devised by Braides and Dal Maso [6] for the approximation of the Mumford-Shah functional has been generalized to the linearly elastic context. Namely, it was shown that, for a given bounded increasing function f:R+R+ the energies

    Fε(u):=1εΩf(Bε(x)ΩW(Eu(y))dy)dx

    Γ-converge to the functional

    αΩW(Eu(x))dx+2βHn1(Ju),

    with α=f(0) and β=limt+f(t), in the L1(Ω)-topology. Above, W(Eu(y)) is a convex elastic energy depending on the linearized strain Eu, given by the symmetrized gradient of a vector-valued displacement u, whose jump set Ju represents the cracked part of a material. The energy space of the limit functional is the one of generalized functions with bounded deformation, introduced in [17].

    It is noteworthy that the above result allowed one for a general (convex) bulk energy W having p-growth for p>1. The proof strategy must then avoid, at least when estimating the bulk part, any slicing procedure. This latter is instead successful in the special case* W(ξ)=|ξ|p, considered for instance in [22]. There, non-local convolution-type energies of the form

    * We remark that this particular case is however not the most relevant one from a mechanical point of view, as even for an isotropic material additional terms in the bulk energy are expected to appear.

    1εΩf(εRn|Eu(y)|pρε(xy)dy)dx (1.1)

    are considered, where ρ is a convolution kernel whose support is a convex bounded domain and ρε(z) is the usual sequence of convolution kernels ρ(z/ε)/εn. The Γ-limit of (1.1) with respect to the L1 convergence is given by

    Ω|Eu(x)|pdx+Juϕρ(ν)dHn1,

    where the anisotropic surface density ϕρ depends on the geometry and on the size of suppρ. A similar effort of generalizing the results of [6] to Mumford-Shah type energies with non-isotropic surface part has been previously performed in [13].

    In this paper, we extend the focus of [22,23] by showing that general Griffith-type functionals of the form

    αΩW(Eu(x))dx+2βJuϕ(ν)dHn1, (1.2)

    where ϕ is any norm on Rn, can be obtained as variational limit of non-local convolution-type functionals

    1εΩf(εRnW(Eu(y))ρε(xy)dy)dx.

    Above, f is again a bounded nondecreasing function with α=f(0) and β=limt+f(t), and the unscaled kernel ρ has the bounded convex symmetric domain ¯S:={ξRn:ϕ(ξ)1} as its support. This is the analogue, in the linear elastic setting, of the results in [13].

    The proof strategy we devise is based on a localization method and involves nontrivial adaptions to the method used in [23], in particular when estimating the bulk term in the Γ-liminf inequality (Proposition 4.2). There, we have to impose (and this is the only point in the paper) an additional restriction on the convolution kernel ρ, namely of being nonincreasing with respect to the given norm ϕ (see Assumption (N2) below). This is namely needed in order to be able to estimate from below the size of the nonlocal approximations of the bulk term in an anisotropic tubular neighborhood of the set where they exceed the threshold βα, which heuristically corresponds to the breaking of the elastic bonds. With this, a set Kε with small area and bounded perimeter, where the fracture energy concentrates can be explicitly constructed. This yields an estimate of the Γ-liminf which has an optimal constant in front of the bulk term, although being non-optimal for the surface energy.

    Another non-optimal estimate for the Γ-liminf, but with an optimal constant for the surface energy can be instead obtained by a slicing procedure, involving a comparison argument and the convexity of the open set S (Proposition 4.3). As bulk and surface energy in (1.2) are mutually singular as measures, a localization procedure entails then the Γ-liminf inequality (Proposition 4.6). Finally, the Γ-limsup inequality (Proposition 5.1) can be obtained by a direct construction for a regular class of competitors having a "nice" jump set, and which are dense in energy. Notice indeed that such an approximation (see Theorem 2.3 for a precise statement) is possibile also with respect to an anisotropic norm ϕ, combining the recent results in [8] with the ones in [12].

    As a final remark, it would be desirable to get rid of the structural assumption (N2) on the convolution kernels, which is used only in Proposition 4.2. It is our opinion that this is going to require quite a delicate abstract analysis of the Γ-limit of nonlocal functionals which approximate free-discontinuity problems in GSBD, possibly including also finite-difference models which are well suited to numerical approximations (see [15] for a recent discrete finite-difference approximation of some Griffith-type functionals in GSBD). A similar analysis for the SBV setting has been performed in [11], where integral representation formulas for the limit energy have been provided. Furthermore, nontrivial sufficient conditions have been given under which the bulk part of the energy can be recovered by only considering weakly compact sequences in Sobolev spaces. We plan to defer this abstract analysis to a forthcoming contribution. For the asymptotic analysis via Γ-convergence of local free-discontinuity functionals in linear elasticity and the related issues, we refer the reader to the very recent papers [7,14,20].

    Outline of the paper: The paper is structured as follows. In Section 2 we fix the basic notation and results on the function spaces we will deal with (Section 2.2), together with some technical lemmas (Section 2.3) which will be useful throughout the paper. In Section 3 we list the main assumptions, introduce our model (Eq (3.4)), and state the main results of the paper, provided {in} Theorem 3.1 and Theorem 3.2. Section 4 is devoted to the proof of the compactness statements in the main Theorems (Proposition 4.1), and to the Γ-liminf inequality, which is proved in Section 4.3 combining the estimates in Sections 4.1 and 4.2. The proof of the upper bound is given in Section 5.

    The symbol || denotes the Euclidean norm in any dimension, while , stands for the scalar product in Rn. We will always denote by Ω an open, bounded subset of Rn {with Lipschitz boundary}, and by Sn1 the (n1)-dimensional unit sphere. The Lebesgue measure in Rn and the s-dimensional Hausdorff measure are written as Ln and Hs, respectively. A(Ω) stands for the family of the open subsets of Ω.

    Let S be a bounded, open, convex and symmetrical set, i.e., S=S. For η>0, we denote by ηS the η-dilation of S and we will often use the shorthand {S(x,η)} in place of x+ηS. We consider ||S the norm induced by S, defined as

    |x|S:=inf{η>0:xηS}, (2.1)

    whose unit ball {|x|S<1} coincides with S, and, correspondingly, we introduce the distance to a closed bounded set KRn; namely,

    distS(x,K):=minyK|xy|S,xRn. (2.2)

    In this section we recall some basic definitions and results on generalized functions with bounded deformation, as introduced in [17]. Throughout the paper we will use standard notations for the spaces (G)SBV and (G)SBD, referring the reader to [2] and [1,3,24], respectively, for a detailed treatment on the topics.

    Let ξRn{0} and Πξ={yRn:ξ,y=0}. If ΩRn and yΠξ we set Ωξ,y:={tR:y+tξΩ} and Ωξ:={yΠξ:Ωξ,y}. Given u:ΩRn, n2, we define uξ,y:Ωξ,yR by

    uξ,y(t):=u(y+tξ),ξ, (2.3)

    while if v:ΩR, the symbol vξ,y will denote the restriction of v to the set Ωξ,y; namely,

    vξ,y(t):=v(y+tξ). (2.4)

    Let ξSn1. For any xRn we denote by xξ and yξ the projections onto the subspaces Ξ:={tξ:tR} and Πξ, respectively. For σ,r>0 and xRn we define the cylinders

    Cξσ,r(0):={xRn:|xξ|<σ,|yξ|<r},Cξσ,r(x):=x+Cξσ,r(0).

    Note that Cξσ,r(x)=(xξσ,xξ+σ)×Bn1r(yξ), where Bn1 denotes a ball in the (n1)-dimensional space Πξ.

    Definition 2.1. An Ln-measurable function u:ΩRn belongs to GBD(Ω) if there exists a positive bounded Radon measure λu such that, for all τC1(Rn) with 12τ12 and 0τ1, and all ξSn1, the distributional derivative Dξ(τ(u,ξ)) is a bounded Radon measure on Ω whose total variation satisfies

    |Dξ(τ(u,ξ))|(B)λu(B)

    for every Borel subset B of Ω.

    If uGBD(Ω) and ξRn{0} then, in view of [17,Theorem 9.1,Theorem 8.1], the following properties hold:

    (a) ˙uξ,y(t)=Eu(y+tξ)ξ,ξ for a.e. tΩξ,y;

    (b) Juξ,y=(Jξu)ξ,y for Hn1-a.e. yΠξ, where

    Jξu:={xJu:u+(x)u(x),ξ0}.

    Definition 2.2. A function uGBD(Ω) belongs to the subset GSBD(Ω) of special functions of bounded deformation if, in addition, for every ξSn1 and Hn1-a.e. yΠξ, it holds that uξ,ySBVloc(Ωξ,y).

    The inclusions BD(Ω)GBD(Ω) and SBD(Ω)GSBD(Ω) hold (see [17,Remark 4.5]). Although they are, in general, strict, relevant properties of BD functions are retained also in this weak setting. In particular, GBD-functions have an approximate symmetric differential Eu(x) at Ln-a.e. xΩ. Furthermore the jump set Ju of a GBD-function is Hn1-rectifiable (this is proven in [17,Theorem 6.2 and Theorem 9.1], but it has been recently shown that this property is actually a general one for measurable functions [18]).

    Let p>1. The space GSBDp(Ω) is defined as

    GSBDp(Ω):={uGSBD(Ω):EuLp(Ω;Rn×nsym),Hn1(Ju)<+}.

    Every function in GSBDp(Ω) can be approximated with the so-called "piecewise smooth" SBV-functions, denoted W(Ω;Rn), characterized by the three properties

    {uSBV(Ω;Rn)Wm,(ΩJu;Rn) for every mN,Hn1(ˉJuJu)=0,ˉJu is the intersection of Ω with a finite union of (n1)-dimensional simplexes . (2.5)

    This is stated by the following result, which combines [8,Theorem 1.1] with [12,Theorem 3.9].

    Theorem 2.3. Let ϕ be a norm on Rn. Let ΩRn be a bounded open Lipschitz set, and let uGSBDp(Ω;Rn). Then there exists a sequence (uj) such that ujW(Ω;Rn) and

    ujuin  measure  on  Ω, (2.6)
    EujEu  in  Lp(Ω;Rn×nsym), (2.7)
    Jujϕ(νuj)Hn1Juϕ(νu)Hn1. (2.8)

    Moreover, if Ωψ(|u|)dx is finite for ψ:[0,+)[0,+) continuous, increasing, with

    ψ(0)=0,ψ(s+t)C(ψ(s)+ψ(t)),ψ(s)C(1+sp),lims+ψ(s)=+

    then

    limj+Ωψ(|uju|)dx=0. (2.9)

    As observed in [9,Remark 4.3], we may even approximate through functions u such that, besides (2.5), have a closed jump set strictly contained in Ω made of pairwise disjoint (n1)-dimensional simplexes, with JuΠiΠl= for any two different hyperplanes Πi, Πl.

    We recall the following general GSBDp compactness result from [10], which generalizes [17,Theorem 11.3].

    Theorem 2.4 (GSBDp compactness). Let ΩRn be an open, bounded set, and let (uj)jGSBDp(Ω) be a sequence satisfying

    supjN(EujLp(Ω)+Hn1(Juj))<+.

    Then there exists a subsequence, still denoted by (uj), such that the set A:={xΩ:|uj(x)|+} has finite perimeter, and there exists uGSBDp(Ω) such that

    (i)  uju    in  measure  on  ΩA,(ii)  EujEu   in  Lp(ΩA;Rn×nsym),(iii)  lim infjHn1(Juj)Hn1(Ju(AΩ)), (2.10)

    where denotes the essential boundary of a set with finite perimeter.

    Remark 2.5. If in the statement above one additionally assumes that

    supjNΩψ(|uj|)dx<+

    for a positive, continuous and increasing function ψ with lims+ψ(s)=+, then A=, so that |u| is finite a.e., and (i) holds on Ω. Moreover, if ψ is superlinear at infinity, that is

    lims+ψ(s)s=+,

    by the Vitali dominated convergence theorem one gets that uL1(Ω) and (i) holds with respect to the L1-convergence in Ω.

    We recall here (without adding the standard proofs) some properties of integral convolutions in the setting of Sobolev spaces.

    Proposition 2.6. Let wW1,p(Ω;Rn) and ρL(Rn) be a convolution kernel, with suppρ¯S for some bounded, open and convex set SΩ. Set ρθ(x):=1θnρ(xθ). Then the following holds:

    (i) let Ω⊂⊂Ω and 0θdistS(Ω,Ω). The convolution

    φθ(x):=Ωw(y)ρθ(yx)dy

    belongs to W1,p(Ω;Rn). Moreover, it holds that

    φθ(x)=Ωw(y)ρθ(yx)dya.e.  on  Ω. (2.11)

    (ii) assume that wεw in L1(Ω;Rn) and let θε be any sequence with θε0 when ε0. Then the sequence

    ˆwε(x):=Ωwε(y)ρθε(yx)dy

    satisfies ˆwεcw in L1(Ω;Rn), where c=Rnρ(x)dx.

    We also recall the following convergence property of one-dimensional sections of averaged functions (see, e.g., [23,Lemma 2.7(ii)]).

    Lemma 2.7. Assume that wεw in L1(Ω;Rn) and let ηε be any sequence with ηε0 when ε0. Then for all ξSn1 and a.e. yΠξ, the sequence

    ˆwξ,yε(t):=Bn1ηε(y)wε(z+tξ)dz

    satisfies ˆwξ,yεwξ,y in L1(Ωξ,y;Rn), where wξ,y(t):=w(y+tξ).

    We will also make use of the following localization result, dealing with the supremum of a family of measures (see, e.g., [4,Proposition 1.16]).

    Lemma 2.8. Let μ:A(Ω)[0,+) be a superadditive function on disjoint open sets, let λ be a positive measure on Ω and let φh:Ω[0,+] be a countable family of Borel functions such that μ(A)Aφhdλ for every AA(Ω). Then, setting φ:=suphNφh, it holds that

    μ(A)Aφdλ

    for every AA(Ω).

    Lower semicontinuous increasing functions can be approximated from below with truncated affine functions. We refer the reader to [23,Lemma 2.10] for a proof of the following result.

    Lemma 2.9. Consider a lower semicontinuous increasing function f:[0,+)[0,+) such that there exist α,β>0 with

    limt0+f(t)t=α,limt+f(t)=β.

    Then there exist two positive sequences (ai)iN, (bi)iN with

    supiai=α,supibi=β

    and min{ait,bi}f(t) for all iN and tR.

    Let (X,d) be a metric space. We recall here the definition of Γ-convergence for families of functionals Fε:X[,+] depending on a real parameter ε (see, e.g., [5,16]).

    For all uX, we define the lower Γ-limit of (Fε) as ε0+ by

    F(u):=inf{liminfj+Fεj(uj):εj0+,uju}, (2.12)

    and the upper Γ-limit of (Fε) as ε0+ by

    F"(u):=inf{limsupj+Fεj(uj):εj0+,uju}. (2.13)

    We then say that (Fε) Γ-converges to F:X[,+] as ε0+ iff

    F(u)=F(u)=F"(u),for all  uX.

    The following one-dimensional Γ-convergence result will be useful in the proof of the lower bound for the surface term. In the statement below, functions in L1(I) with IR are extended by 0 outside I, so that the functionals Hε are well-defined (actually, the result is not affected by the considered extension).

    Theorem 2.10. Let p>1, let IR be a bounded interval and consider a lower semicontinuous, increasing function f:[0,+)[0,+) complying with

    limt0+f(t)t=α,limt+f(t)=β

    for some α,β>0. Let Hε:L1(I)[0,+] be defined by

    Hε(u):=1εIf(12x+εxε|u(y)|pdy)dx,

    where it is understood that

    f(12x+εxε|u(y)|pdy)=β

    if uW1,p(xε,x+ε). Then the functionals (Hε) Γ-converge as ε0+ to the functional

    H(u):={αI|u|pdt+2β#(Ju),if  uSBV(I),+,otherwise

    in L1(I).

    Proof. The proof can be found, e.g., in [4,Theorem 3.30].

    In this section we list our assumptions and introduce the main results of the paper. Let ΩRn be an open set with Lipschitz boundary, let 1<p<+ and f:[0,+)[0,+) a lower semicontinuous, increasing function satisfying

    limt0+f(t)t=α>0,limt+f(t)=β>0. (3.1)

    Let ρL(Rn;[0,+)) be a convolution kernel. The minimal assumption is that

    (N1) ρ is Riemann integrable with ρ1=1 and S=Sρ:={xRn:ρ(x)0} is a bounded, open, convex and symmetrical set.

    As every Riemann integrable function is continuous at almost every point, we may also suppose, up to a modification on a null set, that ρ is lower semicontinuous. Also notice that, by a simple scaling argument, one can always consider the case of kernels with unit mass, up to modifying the constant α in (3.1).

    A sequence (ρε)ε>0 of convolution nuclei is then obtained by setting, for every xRn and ε>0,

    ρε(x):=1εnρ(xε).

    For every vRn we define

     ϕρ(v):=2sup{|y,v|:yS}. (3.2)

    Under the previous assumptions on S, the function ϕρ turns out to be a norm on Rn.

    To obtain our main result, we will have to couple (N1) with the additional assumption that the convolution kernel is a non-increasing function with respect to the norm ||S, that is

    (N2) |x|S|y|Sρ(x)ρ(y) for all x, yRn.

    Equivalently, we require that it exists a non-increasing function ϱ:R+R+ such that ρ(x)=ϱ(|x|S). Notice that, in the case S=B1, every non-increasing radial function ρ complies with (N2).

    Let W:Rn×nR be a convex positive function on the subspace Mn×nsym of symmetric matrices, such that

    W(0)=0,c|M|pW(M)C(1+|M|p). (3.3)

    For every ε>0 we consider the functional Fε:L1(Ω;Rn)[0,+] defined as

    Fε(u)={1εΩf(εΩW(Eu(y))ρε(xy)dy)dx, if uW1,p(Ω;Rn),+, otherwise on  L1(Ω;Rn). (3.4)

    We will deal with a localized version of the energies (3.4). Namely, for every AA(Ω), we will denote by Fε(u,A) the same functional as in (3.4) with the set A in place of Ω. When A=Ω, we simply write Fε(u) in place of Fε(u,Ω).

    The following theorem is the first main result of this paper. We notice that the additional assumption (N2) on the structure of the convolution kernel is required in (ii) below only to obtain the optimal lower bound for the bulk term of the energy.

    Theorem 3.1. Let ρL(Rn;[0,+)) be a convolution kernel as in (N1), and let Fε be defined as in (3.4). Under assumptions (3.1) and (3.3), it holds that

    (i) there exists a constant c0 independent of ε such that, for all (uε)Lp(Ω;Rn) satisfying Fε(uε)C for every ε>0, one can find a sequence ¯uεGSBVp(Ω;Rn) with

    ¯uεuε0  in  measure  on  ΩFε(uε)c0(ΩW(E¯uε)dx+2Hn1(J¯uεΩ)).

    (ii) If, in addition, ρ complies with (N2), then the functionals (Fε) Γ-converge, as ε0, to the functional

    F(u)={αΩW(Eu)dx+βJuϕρ(ν)dHn1,if  uGSBDp(Ω)L1(Ω;Rn),+,otherwise  on  L1(Ω;Rn), (3.5)

    with respect to the L1 convergence in Ω.

    The L1-convergence on the whole Ω can be enforced with the addition of a lower order fidelity term, as we have discussed in Remark 2.5. This motivates the statement below, {where} we consider a continuous increasing function ψ:[0,+)[0,+) such that

    ψ(0)=0,ψ(s+t)C(ψ(s)+ψ(t)),ψ(s)C(1+sp),lims+ψ(s)s=+ (3.6)

    and we set for every AA(Ω)

    Gε(u,A)={Fε(u,A)+Aψ(|u|)dx, if uW1,p(A;Rn),+, otherwise on  L1(A;Rn). (3.7)

    As before, we simply write Gε(u) in place of Gε(u,Ω).

    Then we have the following result.

    Theorem 3.2. Under assumptions (3.1), (N1), (3.3), and (3.6) it holds that

    (i) If (uε)Lp(Ω;Rn) is such that Gε(uε)C for every ε>0, then (uε) is compact in L1(Ω;Rn).

    (ii) If, in addition, (N2) holds, the {functionals} (Gε) Γ-converge, as ε0, to the functional

    G(u)={F(u)+Ωψ(|u|)dx,if  uGSBDp(Ω)L1(Ω;Rn),+,otherwise  on  L1(Ω;Rn),

    with respect to the L1 convergence in Ω.

    With the following proposition, we prove the compactness statements in Theorem 3.1(i), and Theorem 3.2(i), respectively. These results can be easily inferred by a comparison with non-local integral energies whose densities are averages of the gradient on balls with small radii, for which a compactness result has been provided in [23,Proposition 4.1]. In order to do that, we will only require assumption (N1) on the convolution kernel ρ.

    Proposition 4.1. Let AA(Ω), and let Fε, Gε be defined as in (3.4), and (3.7), respectively, where ρL(Rn;[0,+)) satisfies (N1). Then:

    (i) Assume (3.1), (3.3). If (uε)Lp(Ω;Rn) is such that Fε(uε,A)C for every ε>0, one can find a sequence ¯uεGSBVp(A;Rn) with

    ¯uεuε0  in  measure  on  AFε(uε,A)c0(AW(E¯uε)dx+2Hn1(J¯uεA))

    for some c0>0.

    (ii) Assume (3.1), (3.3), and (3.6). If (uε)Lp(Ω;Rn) is such that Gε(uε,A)C for every ε>0, then (uε) is compact in L1(A;Rn).

    Proof. Let η(0,1) be fixed such that Bη(0)⊂⊂S, and denote by mη the minimum of ρ on ¯Bη, which is strictly positive as we are assuming that ρ>0 on S. Setting ˜f(t):=f(mηωnηnt) and for any ε>0, we consider the energies

    ˜Fε(u)={1εΩ˜f(εBηε(x)ΩW(Eu(y))dy)dx, if uW1,p(Ω;Rn),+, otherwise on   L1(Ω;Rn). (4.1)

    Since Bη(0)S and ρmη on Bη(0), a simple computation shows that

    ˜Fε(u,A)Fε(u,A) (4.2)

    for every uW1,p(Ω;Rn) and AΩ open set. By virtue of (4.2), to obtain (i) it will suffice to apply the argument of [23,Proposition 4.1] to the sequence ˜Fε in (4.1). We then omit the details.

    We now come to (ii). If additionally Gε(uε,A)C, following the argument for [23,Proposition 4.1(ii)], it can be shown that the sequence (ˉuε) constructed in (i) complies with

    Aψ(|¯uε(x)|)dx+A|E¯uε(x)|pdx+Hn1(J¯uεA)C<+

    for all ε. Therefore, in view of the growth assumption (3.6) on ψ, Theorem 2.4 and Remark 2.5 apply, and this provides the compactness of the sequence (¯uε) in L1(A;Rn). Then, since ¯uεuε0 in measure on A, with the Vitali dominated convergence Theorem we infer that (uε) is compact in L1(A;Rn) as well. This concludes the proof of (ii).

    Now, we turn to provide a first estimate of the Γ-liminf of the functionals Fε. This estimate is optimal, up to a small error, only for the bulk part of the energy, and this is the only very point where we need to require the additional assumptions (N2) on the convolution kernels (see Section 4.1). The proof of an optimal estimate for the surface term, instead, will be derived separately by means of a slicing argument (see Proposition 4.3 below) for more general kernels complying only with (N1) providing the comparison estimate (4.2). As the two parts of the energy are mutually singular, the localization method of Lemma 2.8 will eventually allow us to get the Γ-liminf inequality.

    We begin by giving the announced estimate for the bulk term.

    Proposition 4.2. Let AA(Ω) with A⊂⊂Ω, and consider a sequence uεW1,p(Ω;Rn) converging to u in L1(Ω;Rn). Assume (3.1) and (3.3), let η(0,1) be fixed and let ρ comply with (N1)–(N2). Suppose that

    supε>0Fε(uε,A)C. (4.3)

    Then, for every fixed 0<δ<1, there exist a constant Mδ,η only depending on f, δ and η, a constant ση depending on ρ,η such that ση0 as η0, and a sequence of functions (vδ,ηε)GSBVp(A;Rn) such that

    (i) α(1ση)2(1δ)2n+1AW(Evδ,ηε(x))dxFε(uε,A);

    (ii) Hn1(Jvδ,ηε)Mδ,ηFε(uε,A);

    (iii) vδ,ηεu in L1(A;Rn) as ε0.

    Proof. We first consider the case f(t)=min{at,b}, with a,b>0. For given η, we introduce the truncated kernel ρη(x):=11σηρ(x)(1χηS(x)), where the constant ση is given by

    ση:=ηSρ(x)dx,

    and ση0 as η0. Notice that with this choice of ση one has Rnρη(x)dx=1.

    For fixed δ(0,1), we then define

    Cδ,η:=1(1δ)n(1ση), (4.4)

    and the functions

    ψη,δε(x):=εΩW(Euε(y))ρηε(1δ)(yx)dyψε(x):=εΩW(Euε(y))ρε(yx)dy.

    Observe that, since W0, by the definition of ρη and assumption (N2), we get

    ψη,δε(x)Cδ,ηψε(x) (4.5)

    for all xA. Define now the following sets, depending on δ,η and S:

    Kε:={xA:ψη,δε(x)Cδ,ηba}, (4.6)
    Kε:={xA:distS(x,Kε)δηε}. (4.7)

    We prove the inclusion

    Kε{xA:ψε(x)ba}. (4.8)

    For this, if xKε then there exists zKε such that |xz|Sδηε. Now, by the triangle inequality, for every yΩ it holds that

    |xy|Sεδη+|zy|Sε,

    whence

    |xy|Sε|zy|S(1δ)ε

    if and only if |zy|S(1δ)ηε. In this case, since ρ is non-increasing with respect to ||S, we have

    ρ(yxε)(1ση)ρη(yz(1δ)ε).

    Notice that this inequality holds true also if |zy|S<(1δ)ηε. In this case, indeed, one has yz(1δ)εηS and hence ρη(yz(1δ)ε)=0 by definition of ρη. Rescaling the kernels and using (4.4) we get

    ρηε(1δ)(yz)Cδ,ηρε(yx),

    so that

    ψε(x)ψη,δε(z)Cδ,ηba

    and the proof of (4.8) is concluded.

    Now, from the inclusion (4.8) and the fact that f(t)=b for tba, we deduce that

    Ln(Kε)εbFε(uε,A). (4.9)

    Applying the coarea formula (see for instance [19,Theorem 3.14]) to the 1-Lipschitz function g(x):=distS(x,Kε) in the open set {0<g(x)<ηδε}Kε we get

    εbFε(uε,A)Ln(Kε)ηδε0Hn1({g=t})dt.

    It follows that we can choose 0<δε<ηδε such that, for

    Kε:={xA:distS(x,Kε)δε}, (4.10)

    it holds

    Hn1(Kε)=Hn1({xA:distS(x,Kε)=δε})1ηδbFε(uε,A). (4.11)

    We define a sequence (vδ,ηε) of functions in GSBVp(A;Rn) as

    vδ,ηε(x):={Ωuε(y)ρη(1δ)ε(yx)dy if xAKε,0 otherwise.  (4.12)

    Since ρη1=1, by Proposition 2.6(ii) (applied for θε=ε(1δ)) and the fact that, by construction and (4.9), it holds Ln(Kε)0 when ε0, we have that vδ,ηεu in L1(A;Rn) as ε0. We also have Hn1(Jvδ,ηε)Hn1(Kε), so that with (4.11) we deduce (ii) for Mδ,η=1ηδb.

    Now, since KεKε and A⊂⊂Ω, it holds ψη,δε(x)<Cδ,ηba for all xKε. As f(t)=min{at,b}, this gives

    f(ψη,δε(x))aCδ,ηψη,δε(x) (4.13)

    for all xAKε. Now, since the function f is concave and f(0)=0, it holds f(λt)λf(t) for all λ[0,1]. Combining with the monotonicity of f and (4.5), we have

    f(ψε(x))1Cδ,ηf(ψη,δε(x)) (4.14)

    for all xA. With this, using (4.13), the Jensen's inequality, (2.11), (4.12), and since W(0)=0, we get

    Fε(uε,A)1εAKεf(ψε(x))dx1εCδ,ηAKεf(ψη,δε(x))dxaεC2δ,ηAKεψη,δε(x)dxaC2δ,ηAKεW(ΩEuε(y)ρη(1δ)ε(yx)dy)dx=aC2δ,ηAKεW(Evδ,ηε(x))dx=a(1ση)2(1δ)2nAW(Evδ,ηε(x))dx.

    For a general f complying with (3.1), use Lemma 2.9 to find aδ,bδ>0 with aδα(1δ) and f(t)min{aδt,bδ} for all tR, and perform the same construction as in the previous step. This gives (iii), (ii) (with Mδ:=1δηbδ) and

    Fε(uε,A)aδ(1ση)2(1δ)2nAW(Evδ,ηε(x))dxα(1ση)2(1δ)2n+1AW(Evδ,ηε(x))dx,

    that is (i).

    In this section we derive by slicing a lower bound for the surface term in the energy. It is worth mentioning that, by virtue of (4.17), the desired estimate could be probably also obtained by adapting to the GSBD-setting the semi-discrete approach of [22,Proposition 6.4]. Nonetheless, that argument is quite delicate for our purposes, and more complicated than we need. It indeed aimed to provide an optimal lower bound for both the bulk and the surface terms in a unique proof by means of a slicing procedure. In our case, the general form of the bulk energy we are considering does not comply with slicing arguments. Therefore, on the one hand, the two terms have to be estimated separately. On the other hand, an independent and simpler strategy can be followed to provide a lower bound with optimal constant in front of the surface energy.

    We set

    τξ:=H1({xS:x=tξ for tR}), (4.15)

    for every ξSn1.

    Proposition 4.3. Let ρL(Rn;[0,+)) be a convolution kernel complying with (N1), and let Fε be defined as in (3.4). Assume (3.1) and (3.3). Let δ(0,1) be fixed, and consider a sequence εj0. Let AA(Ω) and ujW1,p(A;Rn) converging to u in L1(A;Rn). Assume that

    liminfj+Fεj(uj,A)<+.

    Then uGSBDp(A) and

    liminfj+Fεj(uj,A)β(1δ)JξuAτξ|ν,ξ|dHn1 (4.16)

    for every ξSn1.

    Proof. It follows from Proposition 4.1 and Theorem 2.4 that uGSBDp(A). To prove (4.16), we first note that, by virtue of the growth assumption (3.3), we have

    W(Eu)c|Eu|pc|(Eu)ξ,ξ|p,

    for every ξSn1. Thus, for every fixed ξ, since f is non-decreasing, it will be sufficient to provide a lower estimate for the energies

    Fξεj(uj,A):=1εjAf(cεjSεj(x)|(Euj(z))ξ,ξ|pρεj(zx)dz)dx. (4.17)

    We proceed by a slicing argument, and for each xA we denote by xξ and yξ the projections of x onto Ξ and Πξ, respectively. Since S is open and convex, for every fixed ξSn1 we can find a radius r=r(δ,S)>0 such that the cylinder

    Cξ(1δ),r=(λξ,δ,λξ,δ)×Bn1r(0)⊂⊂S, (4.18)

    where λξ,δ:=τξ2(1δ) and τξ is the length of the section Sξ. Indeed, since S is open, some η>0 can be found such that ¯Bη(0) is contained in S. Now, if t=(1δ)s for some sSξ and yξ with |y|η, then tξ+δyS from the convexity of S. Thus, it will suffice to choose r:=δη.

    If we denote by mC the minimum of ρ on ¯Cξ(1δ),r, we then have

    Fξεj(uj,A)=ΠξdHn1(yξ)(1εjAξ,yξf(cεjSεj(x)|(Euj(z))ξ,ξ|pρεj(zx)dz)dxξ)ΠξdHn1(yξ)(1εjAξ,yξ˜f(1εn1jCξ(1δ)εj,rεj(x)|(Euj(z))ξ,ξ|pdz)dxξ), (4.19)

    where ˜f(t):=f(cmCt). Note that ˜f(t)β as t+.

    We now set

    Fξ,yξεj(uj,Aξ,yξ):=1εjAξ,yξ˜f(1εn1jCξ(1δ)εj,rεj(x)|(Euj(z))ξ,ξ|pdz)dxξ.

    We denote (with a slight abuse of notation) still with z the (n1)-dimensional variable in Bn1rεj(yξ). Set wξ,yξj(t):=Bn1rεj(yξ)uj(z+tξ)),ξdz.

    By virtue of Lemma 2.7(ii), applied with θεj=rεj, we have that wξ,yξj converges to uξ,yξ in L1(Aξ,yξ) for a.e. yξ. Furthermore, setting g(t):=˜f(ωn1rn1t), Fubini's Theorem, Jensen's inequality and the monotonicity of ˜f entail that

    Fξ,yξεj(uj,Aξ,yξ)=1εjAξ,yξ˜f(1εn1jBn1rεj(yξ)dzxξ+λξ,δεjxξλξ,δεj|(Euj(z+tξ))ξ,ξ|pdt)dxξ=1εjAξ,yξ˜f(1εn1jxξ+λξ,δεjxξλξ,δεj(Bn1rεj(yξ)|(Euj(z+tξ))ξ,ξ|pdz)dt)dxξ1εjAξ,yξ˜f(ωn1rn1xξ+λξ,δεjxξλξ,δεj(Bn1rεj(yξ)(Euj(z+tξ))ξ,ξdz)pdt)dxξ=1εjAξ,yξg(xξ+λξ,δεjxξλξ,δεj|˙wξ,yξj(t)|pdt)dxξ=λξ,δ1λξ,δεjAξ,yξg(xξ+λξ,δεjxξλξ,δεj|˙wξ,yξj(t)|pdt)dxξ, (4.20)

    Now, for the function tg(t) it still holds g(t)β when t+. Hence, applying Theorem 2.10 to the one-dimensional energies

    ˜Fξ,yξεj(wξ,yξj,Aξ,yξ):=1λξ,δεjAξ,yξg(xξ+λξ,δεjxξλξ,δεj|˙wξ,yξj(t)|pdt)dxξ

    we obtain the lower bound

    liminfj+˜Fξ,yξεj(wξ,yξj,Aξ,yξ)2β#(Juξ,yξAξ,yξ). (4.21)

    Therefore, using (4.20) and (4.21) we deduce

    liminfj+Fξ,yξεj(uj,Aξ,yξ)λξ,δliminfj+˜Fξ,yξεj(wξ,yξj,Aξ,yξ)βτξ(1δ)#(Juξ,yξAξ,yξ).

    With (4.19) and Fatou's Lemma we finally have

    liminfj+Fεj(uj,A)liminfj+ΠξFξ,yξεj(uj,Aξ,yξ)dHn1(yξ)Πξ(liminfj+Fξ,yξεj(uj,Aξ,yξ))dHn1(yξ)βτξ(1δ)Πξ#(Juξ,yξAξ,yξ)dHn1(yξ)=βτξ(1δ)JξuA|νu,ξ|dHn1,

    where in the last equality we used the Area Formula. This concludes the proof of (4.16).

    For any AA(Ω), we denote by F(u,A) and G(u,A) the lower Γ-limits of Fε(u,A) and Gε(u,A), respectively, as defined in (2.12). It holds that G(u,A)F(u,A) for each AA(Ω) and uL1(A;Rn) (see, e.g., [16,Proposition 6.7]). The results of the previous subsection lead to the following estimate.

    Proposition 4.4. Assume (3.1), (3.3), and (3.6). Let Fε and Gε be defined as in (3.4) and (3.7), respectively, and let ρ comply with (N1) -(N2). Let uL1(Ω;Rn), AA(Ω), and define F(u,A) and G(u,A) by (2.12) in correspondence of Fε and Gε, respectively. If F(u,A)<+, then uGSBDp(A) and

    (i) F(u,A)αAW(Eu)dx

    (ii) G(u,A)F(u,A)βJξuAτξ|νu,ξ|dHn1

    for every ξSn1. If in addition G(u,A)<+ holds, then one also has

    (iii) G(u,A)αAW(Eu)dx+Aψ(|u|)dx.

    Proof. With (2.12) and a diagonal argument, one may find (not relabeled) subsequences (uj) and (˜uj)} converging to u in L1(A;Rn) such that

    F(u,A)=liminfj+Fεj(uj,A),G(u,A)=liminfj+Gεj(˜uj,A).

    With the first equality and Proposition 4.3 we have that, if F(u,A)<+, then uGSBDp(A). By the second one, the superadditivity of the liminf, Fatou's lemma and (2.12), we have

    G(u,A)=liminfj+Gεj(˜uj,A)liminfj+Fεj(˜uj,A)+liminfj+Aψ(|˜uj|)dxF(u,A)+Aψ(|u|)dx.

    Hence, (iii) will follow once we have proved (i).

    We therefore only have to check (i) and (ii). To this aim, let η,δ(0,1) be fixed. Then, by applying Proposition 4.2 to the sequence (uj), we can find a sequence (vδ,ηj)GSBVp(A;Rn) such that vδ,ηju in L1(A) as εj0 and

    (a) α(1ση)2(1δ)2n+1AW(Evδ,ηj(x))dxFεj(uj,A);

    (b) Hn1(Jvδ,ηjA)Mδ,ηFεj(uj,A).

    Now, the equiboundedness of Fεj(uj,A) combined with the bounds (a) and (b) allows to apply the lower semicontinuity part of Theorem 2.4 to the sequence (vδ,ηj). Taking into account that A= because uL1(A;Rn), by the convexity of W and (2.10), (ii), we have

    α(1ση)2(1δ)2n+1AW(Eu(x))dxliminfj+AW(Evδ,ηj(x))dxliminfj+Fεj(uj,A)=F(u,A).

    We then obtain (i) by letting δ0 and η0 above.

    For what concerns (ii), from (4.16) of Proposition 4.3 we get

    β(1δ)JξuAτξ|νu,ξ|dHn1liminfj+Fεj(uj,A)=F(u,A)

    for every ξSn1, so that (ii) follows by taking the limit as δ0 again.

    For the proof of the Γ-liminf inequality, we need the following lemma, which can be found in [22,Lemma 4.5].

    Lemma 4.5. Let SRn be a bounded, convex and symmetrical set, and let ϕρ and τξ be defined as in (3.2) and (4.15), respectively. Then

    ϕρ(v)=supξSn1τξ|v,ξ|. (4.22)

    We are now in a position to prove the Γ-liminf inequality.

    Proposition 4.6. Let ρL(Rn;[0,+)) be a convolution kernel satisfying (N1)-(N2). Assume (3.1), (3.3), and (3.6). Consider Fε, and Gε given by (3.4), and (3.7), respectively. Let uL1(Ω;Rn) and let AA(Ω), and define F(u,A) and G(u,A) by (2.12) in correspondence of Fε and Gε, respectively. If F(u,A)<+, then uGSBDp(A) and

    F(u,A)αAW(Eu)dx+βJuAϕρ(ν)dHn1.

    If it additionally holds G(u,A)<+, then

    G(u,A)αAW(Eu)dx+βJuAϕρ(ν)dHn1+Aψ(|u|)dx.

    Proof. The proof can be obtained by a standard localization method based on Lemma 2.8. In order to prove, e.g., the second assertion containing an additional term, we can apply Lemma 2.8 to the set function μ(A):=G(u,A), which is superadditive on disjoint open sets since Gε(u,) is superadditive as a set function:

    G(u,A1A2)G(u,A1)+G(u,A2) whenever A1,A2A(Ω) with A1A2=.

    Then, we consider the positive measure λ(A):=Ln(A)+Hn1(JuA) and the sequence (φh)h0 of λ-measurable functions on A defined as

    φ0(x):={αW(Eu(x))+ψ(|u(x)|), if xAJu,0, if xAJu,
    φh(x):={0, if xAJu,βϕξh(x), if xAJu,

    where

    ϕξh(x)={τξh|νu(x),ξh|, if xJξhuA,0, otherwise in JuA,

    for (ξh)h1 a dense sequence in Sn1.

    Now, by virtue of Proposition 4.4 it holds that

    μ(A)Aφhdλ

    for every h=0,1,, so that all the assumptions of Lemma 2.8 are satisfied. The assertion then follows once we notice that, taking into account Lemma 4.5, it holds

    suph0φh(x)=φ(x):={αW(Eu(x))+ψ(|u(x)|), if xAJu,βϕρ(νu(x)), if xAJu,

    for λ-a.e. xA.

    We denote by F" and G" the upper Γ-limits of (Fε) and (Gε), respectively, as defined in (2.13).

    Proposition 5.1. Let uGSBDp(Ω)L1(Ω;Rn). Then

    F"(u)αΩW(Eu)dx+βJuϕρ(ν)dHn1. (5.1)

    If, in addition, it holds that Ωψ(|u|)dx<+, then

    G"(u)αΩW(Eu)dx+βJuϕρ(ν)dHn1+Ωψ(|u|)dx. (5.2)

    Proof. We only prove (5.1) by using the density result of Theorem 2.3, as (5.2) follows by an analogous construction with the additional property (2.9).

    In view of Theorem 2.3 and remarks below, since we perform a local costruction and by a diagonal argument it is not restrictive to assume that uW(Ω;Rn) and that Ju is a closed simplex contained in any of the coordinate hyperplanes, that we denote by K.

    For every h>0, let Kh:=xKS(x,h) be the anisotropic h-neighborhood of K. As K is compact and (n1)-rectifiable, it holds (see for instance [21,Theorem 3.7])

    limh01hLn(Kh)=Kϕρ(ν)dHn1 (5.3)

    (observe that a factor 2 is already contained in our definition (3.2) of ϕρ). Let γε>0 be a sequence such that γε/ε0 as ε0. Notice that, for ε small,

    KKγε⊂⊂Kγε+ε⊂⊂Ω,

    recalling that KΩ. Let ϕε be a smooth cut-off function between Kγε and Kγε+ε, and set

    uε(x):=u(x)(1ϕε(x)).

    Since uW1,(ΩJu;Rn) we have uεW1,(Ω;Rn). Note also that, by the Lebesgue Dominated Convergence Theorem, uεu in L1(Ω;Rn). Moreover, since uε=u on S(x,ε)Ω if xKγε+ε, we have

    Fε(uε)1εΩf(εS(x,ε)ΩW(Eu(y))ρε(yx)dy)dx+βLn(Kγε+ε)ε. (5.4)

    Setting

    wε(x):=S(x,ε)ΩW(Eu(y))ρε(yx)dy,

    we have that wε(x) converges to w(x):=W(Eu(x)) in L1loc(Ω) as ε0. Since f complies with (3.1) and it is increasing, there exists ˜α>α such that f(t)˜αt for every t0.

    This gives

    1εf(εwε(x))˜αwε(x) for every xΩ and every ε>0,

    and, taking into account that limt0+f(t)t=α, we also infer that

    1εf(εwε(x))αw(x) for a.e. xΩ.

    Thus, by Lebesgue's Dominated Convergence Theorem,

    limε01εΩf(εS(x,ε)ΩW(Eu(y))ρε(yx)dy)dx=αΩW(Eu)dx.

    As γε+εε1 as ε0, from (5.3), (5.4), the subadditivity of the limsup and (2.13) we get (5.1).

    Proof of Theorems 3.1 and 3.2. The two results follow by combining Propositions 4.1, 4.6, and 5.1

    The authors are members of Gruppo Nazionale per l'Analisi Matematica, la Probabilità e le loro Applicazioni (GNAMPA) of INdAM. G. Scilla and F. Solombrino have been supported by the Italian Ministry of Education, University and Research through the Project "Variational methods for stationary and evolution problems with singularities and interfaces" (PRIN 2017).

    The authors declare no conflict of interest.



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