AFIT School of Engineering Contributions to Air Force Research and Technology. Calendar Year 1972

This report contains abstracts of Master of Science Theses, Doctoral Dissertations, and faculty publications completed during the 1972 calendar year at the School of Engineering, Air Force Institute of Technology, at Wright-Patterson Air Force Base, Ohio

Graduate School: Course Decriptions, 1972-73

Official publication of Cornell University V.64 1972/7

Advances in structural dynamics, aeroelasticity and material science

This submission for the degree of Doctor of Science includes all the publications by the author and a description of his research, covering the period 1969-2015. The main contributions to knowledge made by the author concern his new approaches to structural dynamics, aeroelasticity, material science and related problems. In particular, the major activities of his research relate to the (i) free vibration and buckling analysis of structures, (ii) dynamic stiffness formulation, (iii) response of metallic and composite structures to deterministic and random loads, (iv) aeroelasticity of metallic and composite aircraft, (v) a unified approach to flutter, dynamic stability and response of aircraft, (vi) aeroelastic optimisation and active control, (vii) application of symbolic computation in structural engineering research, (viii) development of software packages for computer aided structural analysis and design and (ix) thermal properties of polymer nanocomposites and hot ductility of steel. The free vibration analysis of structures is a research topic which has been an age old companion of the author ever since he was working for his Master’s degree in Mechanical Engineering in the early 1970s, when he chose a crankshaft vibration problem of the Indian Railways as the research topic for his Master’s thesis. With increasing maturity and experience, he provided solutions to vibration and buckling problems ranging from a simple single structural element to a high capacity transport airliner capable of carrying more than 500 passengers and a large space platform with a plan dimension of more than 30 metres. To provide these solutions, he resorted to an elegant, accurate, but efficient method, called the dynamic stiffness method, which uses the so-called dynamic stiffness matrix of a structural element as the basic building block in the analysis. The author has developed dynamic stiffness matrices of a large number of structural elements including beams, plates and shells with varying degrees of complexity, particularly including those made of composite materials. Recently he published the dynamic stiffness matrices of isotropic and anisotropic rectangular plates for the most general case when the plate boundaries are free at all edges. Computation of natural frequencies of isotropic and anisotropic plates and their assemblies for any boundary conditions in an exact sense has now become possible for the first time as a result of this development. This ground-breaking research has opened up the possibility of developing general purpose computer programs using the dynamic stiffness method for computer-aided structural analysis and design. Such computer programs will be vastly superior to existing computer programs based on the finite element method, both in terms to accuracy and computational efficiency. This is in line with the author’s earlier research on free vibration and buckling analysis of skeletal structures which led to the development of the computer program BUNVIS (Buckling or Natural Vibration of Space Frames) and BUNVIS-RG (Buckling or Natural Vibration of Space Frames with Repetitive Geometry) which received widespread attention. Numerous research papers emerged using BUNVIS and BUNVIS-RG as research tools. The author’s main contributions in the Aeronautical Engineering field are, however, related to the solutions of problems in aeroelasticity, initially for metallic aircraft and in later years for composite aircraft. He investigated the aeroelastic problems of tailless aircraft for the first time in his doctoral studies about 40 years ago. In this research, a unified method combining two major disciplines of aircraft design, namely that of stability and control, and that of flutter and response, was developed to study the interaction between the rigid body motions of an aircraft and its elastic modes of distortion. The computer program CALFUN (CALculation of Flutter speed Using Normal modes) was developed by the author for metallic aircraft and later extended to cover composite aircraft. The associated theories for composite aircraft were developed and the allied problems of dynamic response to both deterministic and random loads were solved. With the advent of advanced composite materials, the author’s research turned to aeroelasticity of composite aircraft and then to optimization studies. New, novel and accurate methods were developed and significant inroads were made. The author broke new ground by applying symbolic computation as an aid to the solution of his research problems. The computational efficiency of this new approach became evident as a by-product of his research. The development of software based on his theories has paved the way for industrial applications. His research works on dynamic stiffness modelling of composite structures using layer-wise and higher order shear deformation theory are significant developments in composites engineering. Such pioneering developments were necessitated by the fact that existing methodologies using classical lamination theory are not sufficiently accurate, particularly when the structural components made from composite materials are thick, e.g. the fuselage of a transport airliner. Given the close relationship between structural engineering and material science, the author’s research has broadened into polymers and nano-composites, functionally graded materials and hot ductility of steel. His research activities are continuing and expanding with further diversification of his interests

On the whole spectrum of Timoshenko beams. Part I: a theoretical revisitation

The problem of free vibrations of the Timoshenko beam model is here addressed. A careful analysis of the governing equations allows identifying that the vibration spectrum consists of two parts, separated by a transition frequency, which, depending on the applied boundary conditions, might be itself part of the spectrum. For both parts of the spectrum, the values of natural frequencies are computed and the expressions of eigenmodes are provided. This allows to acknowledge that the nature of vibration modes changes when moving across the transition frequency. Among all possible combination of end constraints which can be applied to single-span beams, the case of a simply supported beam is considered. These theoretical results can be used as benchmarks for assessing the correctness of the numerical values provided by several numerical techniques, e.g. traditional Lagrangian-based finite element models or the newly developed isogeometric approach