Research Article | Open Access

Volume 2011 |Article ID 686139 | https://doi.org/10.1155/2011/686139

Boudjemaa Teniou, Sabrina Benferdi, "Quasistatic Elastic Contact with Adhesion", International Journal of Mathematics and Mathematical Sciences, vol. 2011, Article ID 686139, 13 pages, 2011. https://doi.org/10.1155/2011/686139

# Quasistatic Elastic Contact with Adhesion

Accepted22 Aug 2011
Published31 Oct 2011

#### Abstract

The aim of this paper is the variational study of the contact with adhesion between an elastic material and a rigid foundation in the quasistatic process where the deformations are supposed to be small. The behavior of this material is modelled by a nonlinear elastic law and the contact is modelled with Signorini's conditions and adhesion. The evolution of bonding field is described by a nonlinear differential equation. We derive a variational formulation of the mechanical problem, and we prove the existence and uniqueness of the weak solution using a theorem on variational inequalities, the theorem of Cauchy-Lipschitz, a lemma of Gronwall, as well as the fixed point of Banach.

#### 1. Introduction

The phenomena of contact with or without friction between deformable bodies or between deformable and rigid bodies abound in industry and everyday life. The contact of the tires with the roads, the shoe with disc of break, pistons with skirts are current examples. Because of the importance of contact process in structural and mechanical systems, a considerable effort has been made in its mathematical modeling, mathematical analysis, and numerical simulations.

Process of adhesion is important in many industrial settings where parts, nonmetallic, are glued together. Recently, composite materials reached prominence, since they are very strong and light, and therefore, of considerable importance in aviation, space exploration, and in the automotive industry. Composite materials may undergo delamination under stress, a process in which different layers debond and move relatively to each other. To model the process when bonding is not permanent and debonding may take place, there is a need to add adhesion to the description of the contact process. For these reasons, adhesive contact between bodies when a glue is added to prevent the surfaces from relative motion, has recently received increased attention in the mathematical literature. In this paper we introduce an internal variable of surface, known as bonding field and denoted in this paper by , which describes the fractional density of active bonds on the contact surface. When at a point of contact surface, the adhesion is complete and all the bonds are active; when all the bonds are inactive, severed, and there is no adhesion; when the adhesion is partial and only a fraction of the bonds is active. The problems of contact with adhesion were studied by several authors. Significant results on these problems can be found in  and references therein.

Here, the novelty consists in the introduction of the bonding field into the contact between a material elastic and a rigid foundation where the process is quasistatic and the material is modelled by a nonlinear elastic law. The main contribution of this study lies in the proof of existence and uniqueness of the weak solution of the mechanical problem.

This work is organized as follows. In Section 2 we present some notations and preliminaries. In Section 3 we state the mechanical models of elastic contact with adhesion, list the assumptions on the data of the mechanical problem and deduce its variational formulation. In Section 4 we state and prove the existence of a unique weak solution to the mechanical problem; the proof is based on arguments of evolutionary equations and Banach fixed point.

#### 2. Notations and Preliminaries

In this section, we specify the standard notations used and we remind of some definitions and necessary results for the study of this mechanical problem.

We denote by the space of second order symmetric tensors on () while “” and represent the inner product and the Euclidean norm on and , respectively. Thus, for every , and we have

Here and below, the indices , run between 1, and the summation convention over repeated indices is adopted.

Let be a bounded domain with a Lipschitz boundary and let denote the unit outer normal on . Moreover, we use also the spaces where , are the deformation and the divergence operators, respectively, defined by

The spaces , , , and are real Hilbert spaces endowed with the canonical inner products given by

The associated norms are denoted by , , , and , respectively.

Since the boundary is Lipschitz continuous, the unit outward normal vector on the boundary is defined almost everywhere for every vector field , we also use the notation for the trace of on and we denote by and the normal and tangential components of on the boundary given by

For a regular (say ) stress field , the application of its trace on the boundary to is the Cauchy stress vector . We define, similarly, the normal and tangential components of the stress on the boundary by

And we recall that the following Green's formula holds

Let be a measurable part of such that meas and let be the closed subset of defined by Since meas, then Korn's inequality holds, and thus there exists a constant , depending only on and such that

A proof of Korn's inequality may be found in [8, page 79].

We define the inner product over the space by

It follows from Korn's inequality that and are equivalent norms on . Therefore is a real Hilbert space. Moreover, by the Sobolev trace theorem, there exists a positive constant , depending only on , and such that

Finally, we shall use the notation for the set defined by

We end this preliminary with the following version of the classical theorem of Cauchy-Lipschitz which can be found in [9, page 60].

Theorem 2.1. Assume that is a real Banach space. Let be an operator defined almost everywhere on , satisfying the following conditions:(1) a.e. , for some ,(2), and some .Then, for every , there exists a unique function such that

This theorem will be used in Section 4 to prove the theorem of existence and uniqueness of weak solution of the mechanical problem.

#### 3. Problem Statement and Variational Formulation

We consider an elastic material which occupies a bounded domain () and assume that its boundary is regular and partitioned into three disjoint measurable parts , , and such that meas . Let be the time interval of interest, where . The material is clamped on and therefore the displacement field vanishes there. We also assume that a volume force of density acts in and that a surface traction of density acts on . On the material may come in contact with arigid foundation. Moreover, the process is quasistatic and the evolution of the bonding field is described by a nonlinear differential equation.

Under these conditions, the formulation of the mechanical problem is the following.

Problem P. Find a displacement field , a stress field and a bonding field such that

Here (3.1) is the nonlinear elastic constitutive law. Equation (3.2) represents the equilibrium (3.3) and (3.4) are the displacement-traction boundary conditions, respectively. Conditions (3.5) represent the Signorini conditions with adhesion where is a given adhesion coefficient and is the truncation operator defined by Here being the characteristic length of the bond, beyond which it does not offer additional traction. The introduction of the operator , defined below, is motivated by the mathematical argument but is not restrictive in terms of application, since no restriction on the size of the parameter is made in the sequel. Thus, by choosing very large, we can assume that and, therefore, from (3.5) we recover the contact conditions:

it follows from (3.5) that there is no penetration between the material and the foundation, since during the process. Also, note that when the bonding vanishes, then the contact conditions (3.5) become the classical Signironi contact conditions with zero gap function, that is,

Condition (3.6) represents the frictionless contact and shows that the tangential stress vanishes on the contact surface during the process. Equation (3.7) describes the evolution of the bonding field with given material parameters and . Also, the data in (3.8) is the given initial bonding field.

Assumptions. For the variational study of the mechanical problem, we assume that the operator satisfies the following conditions:

We suppose that the adhesion coefficients satisfy And the body forces and surface traction have the regularity: The initial data satisfy We use the convex subset of admissible displacements defined by It follows from (3.14) and Riesz-Frechet representation theorem that there exists a unique function such that: Moreover, we note that the conditions (3.14) imply Finally, we define the adhesion functional by

##### 3.1. Variational Formulation

By applying Green's formula, and using the equilibrium equation and the boundary conditions, we easily deduce the following variational formulation of the mechanical Problem P.

Problem PV. Find a displacement , and a bonding field such that:

Note that the variational Problem PV is formulated in terms of displacement and bonding field, since the stress field was eliminated. However, if the solution of these variational problems is known, the corresponding stress field can be easily obtained by using the nonlinear elastic law (3.1).

Below in this subsection , , and denote various elements of , and , , , and represent elements of , and represent constants which may depend on , , , , and . Note that the adhesion functional is linear with respect to the last argument and therefore Next using (3.19), as well as the properties (3.9) of the truncation operator , we find

And by the Sobolev theorem trace we obtain We now take in (3.25) to deduce Similar computations, based on the Lipschitz continuity of , show that the following inequality also holds We take and in (3.26) then we use equality and (3.23) to obtain The inequalities (3.25)–(3.28) will be used in various places to prove the theorem of existence and uniqueness of the weak solution.

#### 4. Existence and Uniqueness of Weak Solution

Theorem 4.1. Assume that (3.12a)–(3.15) hold. Then there exists a unique solution to Problem PV and it satisfies

A triple which satisfies (3.1) and (3.20)–(3.22) is called weak solution of the mechanical Problem P. We conclude that under the stated assumptions, the mechanical problem has a unique weak solution. The regularity of the weak solution in terms of stress is given by Indeed, taking in (3.20) and using (3.1), (3.17) we find: Now (3.14) and (4.3) imply that , which in its turn implies .

The proof of Theorem 4.1 will be carried out in several steps. In the first step we consider the following problem in which , is given.

Problem 1. Find a displacement such that We have the following result.

Lemma 4.2. There exists a unique solution to Problem 1 which satisfies

Proof. Let . We consider the operator defined by
Let . We have
We use (3.26), (3.12a), Korn's inequality and the equivalence of the two norms and to show that
The operator is, therefore, strongly monotone.
Let . We have
Using Cauchy-Schwartz's inequality, (3.12b) and (3.27), one obtains
Therefore, is a continuous Lipschitz operator. Since is a nonempty convex closed subset of , it follows from the standard results on elliptic variational inequalities that there exists a unique element , such that
Using (4.6) we get
We now show that .
Let , . Denote , , and by , and (for ), respectively. Then we have
Using (3.12a) we obtain
Using Korn's inequality and the fact that , are equivalent norms on , Cauchy-Schwartz's inequality and (3.25) we get
From the previous inequality, the fact that and the regularity of the function given by (3.18), it follows that .

In the second step we use the displacement field , obtained in Lemma 4.2 and we consider the following auxiliary problem.

Problem 2. Find a bonding field such that We have the following result.

Lemma 4.3. There exists a unique solution to Problem 2 which satisfies

Proof. Consider the mapping defined by
It follows from the properties of the truncation operator , that is Lipschitz continuous with respect to the second argument, uniformly in time. Moreover, for any , the mapping
belongs to . Then from Theorem 2.1, we deduce the existence of a unique function , which satisfies (4.16)-(4.17). The regularity , follows from (4.16)-(4.17) and assumption a.e. on . Indeed, (4.16) implies that for a.e. , the function is decreasing and its derivative vanishes when . Combining these properties with the inequality we deduce that , for all , a.e. on , which shows that .

In the third step we denote by the solution of Problem 1 and the solution of Problem 2, for every . Moreover, we define the operator by

Lemma 4.4. The operator has a unique fixed point .

Proof. We show that for a positive integer the mapping is a contraction on . For this, suppose that are two functions of and we denote by , the functions obtained in Lemmas 4.2 and 4.3, respectively, for , ().
Let . We use (4.4) and (3.25) to deduce that
Using the fact that is a strongly monotone, Korn's inequality and , are equivalent norms on , we get which implies
On the other hand, it follows from (4.16) and (4.17) that
And then
Using the definition (3.9) and writing , we get
By Gronwall's inequality and the Sobolev trace theorem, it follows that:
Using (4.20) and (4.27), we obtain
We now combine (4.23) and (4.28) to see that
And reiterating this inequality times, yields
Recall that is a nonempty closed set in the Banach space and note that inequality (4.30) shows that for sufficiently large , then has a unique fixed point .

Now, the proof of the Theorem 4.1, is a consequence of the previous lemmas.

Proof. Existence. Let be the fixed point of and be the solution of Problem 1 for , that is, . Arguments similar to those used in proof of (4.22) lead to
Since it follows from Lemma 4.3 that and therefore (4.31) implies that . From (4.4), (4.16) and (4.17) we conclude that is a solution of the Problem PV which satisfies (4.1).
Uniqueness. The uniqueness of the solution is a consequence of the fixed point of the operator defined by (4.20). Indeed, let be a solution of Problem PV which satisfies (4.1). Since , it follows from (3.20) that is solution to Problem 1, but the Lemma 4.2 implies that this problem has a unique solution denoted , we get
We put in (3.21) and using the initial condition (3.22) then we can see that is a solution to Problem 2, but the Lemma 4.3 implies that this last problem has a unique solution denoted , we get
We use now (4.20) and (4.33) to see that , that is, is a fixed point of the operator . It follows now from Lemma 4.4 that
The uniqueness of the solution is now a consequence of (4.32) and (4.34).

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