Journal of Aerospace Engineering

A technique to identify structural damage in real time using limited instrumentation is presented. Contrast maximization is used to find the excitation forces that create maximum differences in the response of the damaged structure and the analytical response of the undamaged structure. The optimal excitations for the damage structure are then matched against a database of optimal excitations to locate the damage. To increase the reliability of the approach under modeling and measurement errors, the contrast maximization approach is combined with an approach based on changes in frequency signature. The detectability of any particular damage with the proposed technique depends on the ratio of the magnitude of damage and the magnitude of errors in the measurements, as well as on how much the damage influences the measurements. A damage detectability prediction measure, that incorporates these effects, is developed. The technique is first tested numerically on a 36 degree-of-freedom space truss. To simulate experimental conditions, an extensive study is carried out in the presence of noise. A similar truss is then built and the finite-element method (FEM) model of the structure is corrected using experimental data. The technique is applied to locate the damage in several members. The experimental results indicate that this technique can robustly identify the damaged member with limited measurements and real-time computation. The effectiveness of the damage detection measure is also demonstrated.

The results of a detailed study of the nonlinear response of curved sandwich panels with composite face sheets, subjected to a temperature gradient through the thickness combined with mechanical loadings, are presented. The analysis is based on a first-order shear-deformation Sanders-Budiansky-type theory, including the effects of large displacements, moderate rotations, transverse shear deformation, and laminated anisotropic material behavior. A mixed formulation is used with the fundamental unknowns consisting of the generalized displacements and the stress resultants of the panel. The nonlinear displacements, strain energy, principal strains, transverse shear stresses, transverse shear strain energy density, and their hierarchical sensitivity coefficients are evaluated. The hierarchical sensitivity coefficients measure the sensitivity of the nonlinear response to variations in the panel parameters, the effective properties of the face sheet layers and the core, and the micromechanical parameters. Numerical results are presented for cylindrical panels subjected to combined pressure loading, edge shortening or extension, edge shear, and a temperature gradient through the thickness. The results show the effects of variations in the loading and the panel aspect ratio, on the nonlinear response, and its sensitivity to changes in the various panel, effective layer, and micromechanical parameters.

In this paper, some analysis techniques of nonlinear dynamics are applied to physical systems which may be modeled by the Duffing nonlinear differential equation. The response of the Duffing oscillator to both deterministic sinusoidal and stochastic loadings is investigated and distinct regimes of the response motion are discerned and discussed. The stochastic input to the system is low-pass Gaussian white noise. The efficacy of studying the variation in time of the probability density of one or more of the system output states to determine the type of motion of the system is examined. Attractors in phase space are defined via Poincaré mapping and bounds on motion which serve as signatures for particular types of motion (e.g., chaotic, periodic) are identified by a hypervolume measurement technique. An accepted method for adapting one measured output state into a higher dimensional space by using time-delayed coordinates is used in conjunction with correlation dimension calculation to supply a lower-bound estimate of the fractal dimension and insight into the character of the motion of a nonlinear dynamic system.

An approach for the analysis of systems comprising multiple components subjected to dynamic loading is presented. It allows for an efficient treatment, stepwise in time, of linear and nonlinear connections between components. The constraint forces at the junctions of the components are computed directly without the synthesis of component modes of the determination of system modes. This is accomplished by expressing the displacements at the junction coordinates of the components in terms of the retained unconstrained normal modes and the residual flexibility of the unretained modes, in conjunction with a Newmark algorithm representation of nodal kinematics within a time step. This leads to a set of junction-sized equations, similar in form to that of the flexibility formulation in statics, in terms of the unknown junction forces. For the linear problem, the connection forces are solved for directly. For the nonlinear problem, the connection forces are determined in an iterative manner. The approach is applied to a problem involving the dynamic response of a Mini-Pressurized Logistic Module (MPLM) rack in a Space Shuttle liftoff event. The results of the proposed approach are compared with pertinent results derived by relying on component-mode synthesis.

A recent trend towards greater automation of earthmoving machines, such as backhoes, loaders, and dozers, reflects a larger movement in the construction industry to improve productivity, efficiency, and safety. This paper reviews related work in various disciplines drawn upon by researchers—soil mechanics, computer graphics, kinematic and dynamical modeling, optimization, control, and decision theory. A taxonomy is suggested into which various automated systems reported in the literature, can be classified.