Force-based Finite Element for Large Displacement Inelastic Analysis of Frames

Flutter and limit-cycle oscillations(LCO) of a delta-wing model are studied theoretically and correlated with results from an earlier experiment and an earlier simpler theoretical model. The present theoretical model uses a high-fidelity nonlinear structural model and a linear vortex lattice aerodynamic model. The commercial finite element package ANSYS is selected to model the structure and is coupled to the vortex lattice aerodynamic model using a subiteration procedure to carry out time simulations. The delta-wing model is studied for five angles of attack (0, 1, 2, 3, and 4 deg) and for various flow speeds. Theoretical results are calculated for two different root-chord boundary conditions, that is, fully fixed and also another that allows some in-plane movement at the root chord by attaching stiff in-plane springs that connect the structure to the root boundary. The results obtained using the high-fidelity structural model are compared to earlier results computed using a lower-fidelity von Karman plate theory. For all angles of attack studied here, the correlation between theory and experiment is better for the aeroelastic model, which uses the high-fidelity (ANSYS) structural model. Both flutter velocity and frequency as well as the LCO amplitudes and frequencies that are predicted using the higher-fidelity stuctural model correlate well with experiment. In particular the flutter and LCO results predicted using the high-fidelity structural model are similar, both qualitatively and quantitatively, for the two different in-plane boundary conditions. However the results obtained from the von Karman model differ substantially for the two different in-plane boundary conditions.

Modeling Delta Wing Limit-Cycle Oscillations Using a High-Fidelity Structural Model

Gives a bibliographical review of the finite element methods (FEMs) applied for the linear and nonlinear, static and dynamic analyses of basic structural elements from the theoretical as well as practical points of view. The range of applications of FEMs in this area is wide and cannot be presented in a single paper; therefore aims to give the reader an encyclopaedic view on the subject. The bibliography at the end of the paper contains 2,025 references to papers, conference proceedings and theses/dissertations dealing with the analysis of beams, columns, rods, bars, cables, discs, blades, shafts, membranes, plates and shells that were published in 1992‐1995.

Finite element linear and nonlinear, static and dynamic analysis of structural elements: a bibliography (1992‐1995)

Quaternion-based dynamics of geometrically nonlinear spatial beams using the Runge-Kutta method

Mitigating Crack Propagation in a Highly Maneuverable Flight Vehicle Using Life Extending Control Logic Mohamed Mostafa Yousef Bassyouny Elshabasy Old Dominion University, May 2009 Director: Dr. Brett A. Newman In this research, life extending control logic is proposed to reduce the cost of treating the aging problem of military aircraft structures and to avoid catastrophic failures and fatal accidents due to undetected cracks in the airframe components. The life extending control logic is based on load tailoring to facilitate a desired stress sequence that prolongs the structural life of the cracked airframe components by exploiting certain nonlinear crack retardation phenomena. The load is tailored to include infrequent injections of a single-cycle overload or a single-cycle overload and underload. These irregular loadings have an anti-intuitive but beneficial effect, which has been experimentally validated, on the extension of the operational structural life of the aircraft. A rigid six-degree-of freedom dynamic model of a highly maneuverable air vehicle coupled with an elastic dynamic wing model is used to generate the stress history at the lower skin of the wing. A three-dimensional equivalent plate finite element model is used to calculate the stress in the cracked skin. The plate is chosen to be of uniform chord-wise and span-wise thickness where the mechanical properties are assigned using an ad-hoc approach to mimic the full scale wing model. An in-extensional 3-node triangular element is used as the gridding finite element while the aerodynamic load is calculated using the vortexlattice method where each lattice is laid upon two triangular finite elements with common hypotenuse. The aerodynamic loads, along with the base-excitation which is due to the motion of the rigid aircraft model, are the driving forces acting on the wing finite element model. An aerodynamic control surface is modulated based on the proposed life extending control logic within an existing flight control system without requiring major modification. One of the main goals of life extending control logic is to enhance the aircraft's service life, without incurring significant loss of vehicle dynamic performance. The value of the control-surface deflection angle is modulated so that the created overstress is sufficiently below the yield stress of the panel material. The results show that extension in crack length was reduced by 40% to 75% with an absence of damage mitigation logic. Moreover, the desired structural integrity is satisfied without affecting the air vehicle dynamic performance. Members of Advisory Committee: Dr. Osama A. Kandil Dr. Due T. Nguyen Dr. Keejoo Lee

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Mitigating crack propagation in a highly maneuverable flight vehicle using life extending control logic

The consistent co-rotational beam formulation has been modified to include inelastic deformation using a sublayer material model. An explicit tangent stiffness matrix for a non-conservative beam was consistently derived from the assumed beam kinematics. Several example problems were solved to verify the formulation and, subsequently, a comparative study was carried out to examine the validity of the von Karman theory in large deflection analysis. It is found that the presence of beam axial constraint plays an important role on the performance of the von Karman approximation. In general, the von Karman formulation can provide reasonable solutions to large deformation problems of extensional structures, but its performance becomes unsatisfactory for the inextensional case even under relatively small deflections.

J. Jiang

Papers

Large elastic-plastic deformations of slender beams: Co-rotational theory vs. von Karman theory