Title page for ETD etd-03092007-132508

Type of Document Dissertation
Author Seresta, Omprakash
Author's Email Address oseresta@vt.edu
URN etd-03092007-132508
Title Buckling, Flutter, and Postbuckling Optimization of Composite Structures
Degree PhD
Department Aerospace and Ocean Engineering
Advisory Committee
Advisor Name Title
Gürdal, Zafer Committee Chair
Hyer, Michael W. Committee Member
Librescu, Liviu Committee Member
Lindner, Douglas K. Committee Member
Patil, Mayuresh J. Committee Member
  • Postbuckling
  • Flutter
  • Discrete Optimization
  • Genetic Algorithms
  • Global/Local Methodology
  • Composites
  • Buckling
  • Blending
Date of Defense 2007-02-27
Availability unrestricted
This research work deals with the design and optimization of a large composite structure. In

design of large structural systems, it is customary to divide the problem into many smaller

independent/semi-independent local design problems. For example, the wing structure design

problem is decomposed into several local panel design problem. The use of composite

necessitates the inclusion of ply angles as design variables. These design variables are discrete

in nature because of manufacturing constraint. The multilevel approach results into

a nonblended solution with no continuity of laminate layups across the panels. The nonblended

solution is not desirable because of two reasons. First, the structural integrity of

the whole system is questionable. Second, even if there is continuity to some extent, the

manufacturing process ends up being costlier.

In this work, we develop a global local design methodology to design blended composite

laminates across the whole structural system. The blending constraint is imposed via a guide

based approach within the genetic algorithm optimization scheme. Two different blending

schemes are investigated, outer and inner blending. The global local approach is implemented

for a complex composite wing structure design problem, which is known to have a strong

global local coupling. To reduce the computational cost, the originally proposed local one

dimensional search is replaced by an intuitive local improvement operator. The local panels

design problem arises in global/local wing structure design has a straight edge boundary

condition. A postbuckling analysis module is developed for such panels with applied edge

displacements. A parametric study of the effects of flexural and inplane stiffnesses on the

design of composite laminates for optimal postbuckling performance is done. The design

optimization of composite laminates for postbuckling strength is properly formulated with

stacking sequence as design variables.

Next, we formulate the stacking sequence design (fiber orientation angle of the layers) of

laminated composite flat panels for maximum supersonic flutter speed and maximum thermal

buckling capacity. The design is constrained so that the behavior of the panel in the vicinity

of the flutter boundary should be limited to stable limit cycle oscillation. A parametric study

is carried out to investigate the tradeoff between designs for thermal buckling and flutter.

In an effort to include the postbuckling constraint into the multilevel design optimization

of large composite structure, an alternative cheap methodology for predicting load paths

in postbuckled structure is presented. This approach being computationally less expensive

compared to full scale nonlinear analysis can be used in conjunction with an optimizer for

preliminary design of large composite structure with postbuckling constraint. This approach

assumes that the postbuckled stiffness of the structure, though reduced considerably, remains

linear. The analytical expressions for postbuckled stiffness are given in a form that can

be used with any commercially available linear finite element solver. Using the developed

approximate load path prediction scheme, a global local design approach is developed to

design large composite structure with blending and local postbuckling constraints. The

methodology is demonstrated via a composite wing box design with blended laminates.

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