The problem is based on the equation

which was proposed by Woinowsky-krieger [1], as a model for vibrating beams with hinged ends. One of the first mathematical analysis for the equation

was done by Ball [2], which was later extended to an abstract setting by defining a linear operator A by Medeiros [3]. In [4], Tucsnak considered the above beam equation which clamped boundary and obtained the exponential decay of the energy when a damping of the type a(x)u_t is effective near the boundary. In the same direction, Kouemon Patchen [5] obtained the exponential decay of the energy for above-equation when a nonlinear damping g(u_t) was effective in Ω. To [6] considered the above kirchhoff-type beam equation under non-linear boundary conditions

which the model is clamped at x = 0 and is supported x = l. He proved the existence and decay rates of the solutions. A rather general kirchhoff-type beam equation

was set up by Ball [7], who presented the existence and uniqueness of solution under linear boundary conditions. However the global solution and exponential decay for the more general beam equation is open under nonlinear boundary conditions. In the present work, we are concerned with the existence and uniqueness of solutions and the exponential decay property of energy on the nonlinear beam equation with external load

with nonlinear boundary conditions

and initial conditions

and (4)

In this paper, our analysis is based on the Sobolev spaces

,

espectively equipped with the norm and. We assume that f, g:R → R are continuously differentiable functions such that

and (5)

where and

and (6)

for some ρ > 0.

Assume that the functions are non-negative functions and respectively satisfy

and (7)

and (8)

Now we come to the following conclusions of the existence and uniqueness of global solutions.

Theorem 1. Assume that the assumptions of (5)-(8) and hold. Then for any satisfying the compatibility condition

There exists a function u satisfying (1)-(4) such that

Proof. Let us solve the variational problem associated with (1)-(4), which is given by: find such that

for all. Let be a complete orthogonal system of W. For each, let us put

.

We search for a function

where is a unknown function such that for any, and it satisfies the approximating equation

with the initial conditions

and       (12)

Thus (11) and (12) are equivalent to the Cauchy problem of ODES in the variable t, which is known to have a local solution u^m(t) in an interval [0, t_m) (t_m < T) for any given T > 0.

Estimate 1. By integration of (11) over [0, t] (t < t_m) with, we see that

where    and.

Considering that

,

and the initial conditions, we get

.

Using Gronwall inequality, we have. Then there exists a constant M₁ depending only on T such that

for any and for all.

In this paper, C is a constant independent of m, t and denotes different value in different mathematical expression.

Estimate 2. Integrating by parts (11) with and t = 0, and considering the compatibility condition (3) we get

Thus there exists a positive constant M₂ such that

Estimate 3. Let us fix t, ξ > 0 such that ξ < T – t. Taking the difference of (11) with t = t + ξ and t = t, and replacing ω by, we get

where

.

Let us estimate. Since

we have

Noting that ΔM₁ = M(z(t + ξ) – M(z(t)) and ΔM₂ = N(z_t(t + ξ)) – N(z_t(t)), then integrating by parts we have

Since, by the Mean value theorem, from estimates 1 and (16) we have

where η₁ is between and.

By the Mean value theorem, we also have

Considering that M(z(t + ξ) ≤ C and N(z(t + ξ) ≤ C, we conclude that there exists constants k₁ > 0 and k₂ > 0 such that

A argument for f yields

where k₃ > 0 is a constant. Putting

and taking into account of (17)-(18) and the assumptions of g, we deduce from (15) that

where k₄ = max{k₁ + k₃, k₂}. Therefore

Dividing the above inequality by ξ² and letting ξ → 0 gives

.

From estimate 2 we find a constant M₃ > 0 such that

.

With the estimates 1 - 3 we can use Lions-Aubin Lemma to get the necessary compactness in order to pass (11) to the limit. Then it is a matter of routine to conclude the existence of the global solution in [0, T].

Theorem 2. The solution u(t) of theorem 1 is unique.

Proof. Let u, v be two solutions of (1)-(4) with the same initial data. Then writing p = u – v, putting ω = p_t in (10) and using mean value theorem, chauchy-schwarz inequality and Gronwall inequality, we may get p = 0. Thus u = v.

In order to establish our decay result, we define the energy of the system by

where. We have Theorem 3. Let u(t) be the solution given by theorem 1 as q(x, t) = 0 and g = 0. And assume that N(s)s ≥ 0 and f(s)s ≥ 0. Then there exist constants λ₂, λ₄ > 0 and λ₃ < 0 such that.

To prove Theorem 3, we firstly introduce two lemmas.

Let us define    .

Then we have the following lemmas.

Lemma 1. Let E_ε(t) = μE(t) + εψ(t). Then there exists a constant k₅ > 0 such that

.

Proof. By, and there exists k₅ > 0 such that

where.

Lemma 2. There exist constants λ₀ > 0 and λ₁ such that

.

Proof. Taking the inner product of (1) with u_t and considering that N(s)s ≥ 0, we have

.

Taking the inner product of (1) with u, we have

Thus

.

Set .

Since, we have. Therefore

.

From the Mean value theorem, there exists a constant λ₁ such that

On writing, we have

The proof of theorem 3. From lemma 1, we have

From Lemma 2, we have

Therefore

.

By Gronwall inequality and combing (21), we have

.

Hence, for sufficiently small ε > 0

.

On writing

and we have .

The proof of theorem 3 is now completed.