| Abstract: |
Impact and dynamic contact problems have received a wide attention due to their importance in many engineering fields such as aircraft industry, machine shop design and during the life of a structure. Impact by external objects can be expected to occur during manufacturing, service, and maintenance operations. An example of in-service impact occurs during aircraft takeoffs and landings, when stones and other small debris from the runway are propelled at high velocities by the tires. During the manufacturing process or during maintenance, impact on the structures can be occurred. In this case, the impact is known as a low velocity impact.In recent years, composite materials have been developed extensively for use in structural components. A composite material typically consists of a load carrying material phase, such as fibers, held together by a binder or matrix material, often an organic polymer. Composite materials used for many structure applications requiring strength and stiffness are known as advanced composite. These composite materials have advantage of high strength-to-weight and stiffness-to-weight ratios compared to metals.Since advanced composite materials are being used in aerospace structural components, emphasis continues on understanding damage and failure mechanisms in composite under the low velocity impacts. The first step to detect the damage, which is the main objective of the thesis, is the investigation of the response of viscoelastic composite structure under the low velocity impact events. Events such as tool DROP and runway kick-up are examples of impact on aircraft structures.To describe the damping behavior of the structures, two damping models are adopted. One model, Rayleigh model, is selected to describe the source of external damping or the simple memory material damping. The other model, the Wiechert model, is proposed to simulate the internal damping or the fading memory material damping.A general and efficient computational model capable of analyzing the nonlinear response of the viscoelastic composite structures under the low velocity impact is presented.Since the impact problem is a dynamic one, Newmark constant average acceleration method is used to time integrate the governing equations and transfer the dynamic problem to a static one for each increment. The integration of governing are performed step by step using recurrence relationships for approximate calculations of strain derivatives. The incremental convex programming method, which has been proved to be efficient, is modified to accommodate viscoelastic dynamic contact problems. The Lagrange multiplier method is used to incorporate the contact conditions into global equilibrium equations. The first-order shear deformation (Mindlin) theory of plates is proposed in the plate analysis.Verification studies are conducted to evaluate the accuracy of the present numerical algorithm, and they show that the numerical solutions of the proposed models are in favorable agreement with the analytical solutions developed for special simple cases. Parametric studies such as the effects of material characteristics, initial conditions, plate thickness, fiber orientations and lamination schemes are studied in case of a fading memory materials.Finally, the proposed numerical model is exploited to estimate the coefficient of restitution between to bodies numerically. This coefficient represents the loss of Kinetic energy during impact process, and the relation between relative velocities before and after im
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