Structural Stiffness Parameters Research Articles

Statistical damage identification of structures with frequency changes

Model updating methods based on structural vibration data have being rapidly developed and applied to detect structural damage in civil engineering. But uncertainties existing in the structural model and measured vibration data might lead to unreliable damage detection. In this paper a statistical damage identification algorithm based on frequency changes is developed to account for the effects of random noise in both the vibration data and finite element model. The structural stiffness parameters in the intact state and damaged state are, respectively, derived with a two-stage model updating process. The statistics of the parameters are estimated by the perturbation method and verified by Monte Carlo technique. The probability of damage existence is then estimated based on the probability density functions of the parameters in the two states. A higher probability statistically implies a more likelihood of damage occurrence. The presented technique is applied to detect damages in a numerical cantilever beam and a laboratory tested steel cantilever plate. The effects of using different number of modal frequencies, noise level and damage level on damage identification results are also discussed.

AN ASSESSMENT OF HYDROELASTICITY FOR VERY LARGE HINGED VESSELS

A computational analysis is presented for the effects of waves on very large hinged vessels consisting of several modules, connected by simple hinges. Two generic types of modules are considered: rectangular barges similar to the Megafloat prototype and semi-submersibles similar to those proposed for mobile offshore bases. Most of the computations are for configurations with five modules, each of length 300 m. The results include vertical motions, structural deflections, and hinge shear forces in head and oblique waves. A range of structural stiffness parameters is considered, to permit a quantitative assessment of the importance of hydroelasticity.

Structural Damage Detection Using Linear Matrix Inequality Methods

In this work, linear matrix inequality (LMI) methods are proposed for computationally efficient solution of damage detection problems in structures. The structural damage detection problem that is considered consists of estimating the existence, location, and extent of stiffness reduction in structures using experimental modal data. This problem is formulated as a convex optimization problem involving LMI constraints on the unknown structural stiffness parameters. LMI optimization problems have low computational complexity and can be solved efficiently using recently developed interior-point methods. Both a matrix update and a parameter update formulation of the damage detection is provided in terms of LMIs. The presence of noise in the experimental data is taken explicitly into account in these formulations. The proposed techniques are applied to detect damage in simulation examples and in a cantilevered beam test-bed using experimental data obtained from modal tests. [S0739-3717(00)00104-5]

An Attachment Theory for Microsphere Adhesion

A theory of the mechanics of adhesion between a microsphere and substrate is presented. When a force is applied to an elastic body, the deformation depends not only on the magnitude of the force but also its location and distribution. Molecular adhesion between bodies is a surface force localized to the contact area. In contrast, applied forces such as from gravity, flow fields, inertia, etc., are distributed over the volume (body forces) and/or surface areas. Effects of different types of force systems on deformation, particularly when these forces are combined, can influence adhesion. The Hertzian structural stiffness parameter K does not reflect the effects of differently distributed multiple forces. A theory is developed that takes into account simultaneous application of the adhesion force and applied forces through the development of a reduced stiffness, KR . The paper also develops an equivalent Hertzian process for the condition of adhesion forces alone so that the mechanics of adhesion can be modeled completely by Hertzian theory. Illustrations of how adhesion alone is handled and how the reduced stiffness behaves are provided using experimental data from compressed, crossed rods and from hard particles in static equilibrium with both relatively hard and soft substrates.

Mutual Residual Energy Method for Parameter Estimation in Structures

In this work we describe an approach to parameter estimation of complex linear structures that we call the mutual residual energy approach. We have endeavored to develop a unified approach to the discrete inverse problems describing static equilibrium and free, undamped vibration, with a particular view toward evolving methods that are amenable to large-scale computation. The mutual residual energy method is based on the assumption that the topology and geometry of the structure are known, and that the system matrices can be linearly parameterized in terms of kernel matrices that have a solid physical basis and are easy to assemble. Measured motions of the structure are used (in conjunction with measured loads for the static case) to make estimates of the constitutive parameters. The method is based on a particular statement of the principle of virtual work and yields equations for estimating stiffness and mass parameters of linear structures. A condensation procedure is presented to deal with the case of incompletely measured systems. The quantity and quality of response measurements required, the consequences of noisy data, and the choice of load form are among the issues important to the success of our parameter estimation scheme. A numerical simulation is presented to demonstrate the features of the method.

Sensitivity derivatives of flutter characteristics and stability margins for aeroservoelastic design

Analytical derivatives of flutter dynamic pressure, flutter frequency, gain margins, and phase margins with respect to various aeroservoelastic design variables are developed. The formulation is based on a first-order time-domain aeroservoelastic mathematical model. The structure is represented in the model by vibration modes, the unsteady aerodymanics by minimum-state rational approximation functions, and the control system is coupled with the aeroelastic system through motion sensors and control surfaces. The sensitivity derivatives are expressed as exact functions of the stability boundary eigenvectors and of the real-valued system matrix derivatives with respect to arbitrary design variables. These expressions may be applied to any aeroservoelastic design variable with respect to which the system matrix derivative is available. System matrix derivatives with respect to structural stiffness and mass parameters, control gains, actuator parameters, and sensor locations are presented. The new sensitivity derivatives facilitate efficient gradient-based aeroservoelastic algorithms, which directly address flutter and stability margin requirements. A numerical example utilizing the mathematical model of the Active Flexible Wing wind-tunnel model is given. A practical control design problem with roll maneuver constraints is used to demonstrate the accuracy and usage of the derivatives.

Approximate Modal Analysis of Bilinear MDF Systems

This paper describes two approximate response spectrum/modal analysis methods for dynamic response of hysteretic multi-degree-of-freedon lumped mass structural models subjected to earthquakes. The methods utilize either: (1)Elastic response spectra; or (2)inelastic response spectra. Both procedures are iterative and recognize changes in mode shapes as a function of inelastic deformation in the structural mumbers. Comparison of exact and approximate responses are made for 3 degree-of-freedom and 10 degree-of-freedom shear beam structures with bilinear hysteresis. Input excitation and several structural stiffness and strength parameters are varied.