Simultaneous determination of an unknown bending moment and shear force in the Euler Bernoulli cantilever beam from measured boundary defection and slope

Abstract

It is known that the study of vibration characteristics at the tip of the micro-cantilever and its relationship to the sample plays a very important role in improving the resolution of an Atomic Force Microscopy (AFM). In this paper, within the Euler Bernoulli beam model, a mathematical model, defined as a model with two unknown inputs and two measured outputs, is considered for the simultaneous determination of the unknown bending moment and the shear force at the tip of the micro-cantilever from two feasible measured outputs at the same tip: the deflection and the slope. This model leads to the following inverse problem: find $M(t)$ and $g(t)$ in $ρ_A(x)u_{tt}+μ(x) u_{t}+(r(x)u_{xx})_{xx}=0$, $(x,t)\in Ω_T:=(0,\ell)\times (0,T)$ subject to the boundary conditions $u(0,t)=u_{x}(0,t)=0$, $\left(r(x)u_{xx}\right)_{x=\ell}=M(t)$, $\left((r(x)u_{xx})_x\right)_{x=\ell}=g(t)$, and the homogenous initial conditions, from the measured outputs $w_{\ell}(t):=u(\ell,t)$ and $θ_{\ell}(t):=u_x(\ell,t)$. It is proved that the vector-form input-output map $\mathcal{P}:=\left (Φ, Ψ\right )$, with $\left (Φq \right )(t):=u(0,t;q)$ and $\left (Ψq\right )(t):=u_x(0,t;q)$, where $q(t):=\left (M(t),g(t)\right )$, corresponding to the inverse problem, is compact and Lipschitz continuous. This result allows us to prove the existence of a solution of the minimization problem for the Tikhonov functional. As a consequence, the existence of a quasi-solution to the inverse problem is established. Furthermore, a vector form expression for the Frechet gradient of the Tikhonov functional is derived, and the Lipschitz continuity of the Frechet gradient is rigorously proven. This crucial property ensures the monotonic behavior of iterative gradient-based numerical methods.