S. Hanagud

Dynamics of delaminated beams

In this paper, we present a new model for composite beams with through-width delaminations. Shear effect and rotary inertia terms, as well as bending-extension coupling, are taken into account in the governing equations of vibration. Nonlinear interaction, due piecewise linear spring models between the delaminated sublaminates, is also included. Based on this model, eigensolutions for vibrations of intact and delaminated beams are found analytically. Dynamic behavior predicted by this model is then compared with previously reported experimental results. Better agreements with the experimental results are found. Discrepancies among previously proposed models are explained without difficulty.

Curvature Mode Shape-based Damage Assessment of Carbon/Epoxy Composite Beams

In this article, a combined analytical and experimental damage assessment method using curvature mode shapes is developed. The curvature mode is selected due to its sensitivity to the presence of the damage and the localized nature of the changes. An analytical relationship between the damaged and the healthy beams is formulated, for which the effect of damage in the form of stiffness loss is accounted. This relationship is later used to estimate the extent of damage from the experimentally identified changes in structural dynamic characteristics. Surface-bonded piezoelectric sensors are used to directly acquire the curvature modes of composite structures, which simplify the identification procedure. The specimens are made of carbon/epoxy laminated composite beams. Several different types of damages are introduced in the beams (i.e., delamination, impact, and saw-cut damages) to simulate possible damage scenarios. Several limitations and remarks of the proposed experimental and damage identification approaches are discussed. The study shows that the present technique using curvature mode shapes and piezoelectric materials can be used effectively to locate the damage in the laminated composite structures.

Nonlinear vibration of buckled beams : some exact solutions

In this paper, exact solutions are obtained for the dynamics of buckled beams with different types of end conditions. The differential equation for the vibrations of buckled beams, with restrictions on axial stretch, is nonlinear. Using the modes of the corresponding linear problem, which readily satisfy the boundary conditions, the natural frequencies are obtained for the nonlinear problem by using Jacobi elliptic functions. Analytical solutions for the dynamics of buckled beams, with various rotational restraints, are presented.

An integral equation for changes in the structural dynamics characteristics of damaged structures

This paper presents a relationship between the structural dynamic characteristics of damaged and intact structures. Such a relationship is derived in the form of an integral equation and is used to develop a method of flaw identification. The resulting flaw identification procedure uses the experimental data and relates the flaw to the observed structural dynamic response. The algorithm enables one to detect the damage locations and the corresponding damage magnitude simultaneously. Numerical simulations and experiments are conducted to validate the theory.

Chaotic Vibrations of Beams: Numerical Solution of Partial Differential Equations

A simply supported buckled beam governed by a Euler Bernoulli beam equation with transverse loading and non linear membrane stretching effect has been studied. By using a stable, explicit finite difference scheme to solve the governing partial differential equation it has been demonstrated that various problems are easily handled

Computational Techniques for Structural Health Monitoring

The increased level of activity on structural health monitoring (SHM) in various universities and research labs has resulted in the development of new methodologies for both identifying the existing damage in structures and predicting the onset of damage that may occur during service. Designers often have to consult a variety of textbooks, journal papers and reports, because many of these methodologies require advanced knowledge of mechanics, dynamics, wave propagation, and material science. Computational Techniques for Structural Health Monitoring gives a one-volume, in-depth introduction to the different computational methodologies available for rapid detection of flaws in structures. Techniques, algorithms and results are presented in a way that allows their direct application. A number of case studies are included to highlight further the practical aspects of the selected topics. Computational Techniques for Structural Health Monitoring also provides the reader with numerical simulation tools that are essential to the development of novel algorithms for the interpretation of experimental measurements, and for the identification of damage and its characterization. Upon reading Computational Techniques for Structural Health Monitoring, graduate students will be able to begin research-level work in the area of structural health monitoring. The level of detail in the description of formulation and implementation also allows engineers to apply the concepts directly in their research.

Energy Absorption Behavior of Graphite Epoxy Composite Sine Webs

The static energy absorption behavior of graphite epoxy composite cor rugated (sine wave) webs loaded in axial compression is reported in this paper. Tests have been conducted to study the effects of various geometric parameters of the web specimen including width and gross thickness. The importance of the failure initiator and its effect on energy absorption are described along with other observed energy absorption trends. Comparisons are made with published tube specimen behavior where appropriate. The existence of a stability boundary within which efficient crushing occurs is shown in the case of gross thickness variation of the web.

Health monitoring of structures - Multiple delamination dynamics in composite beams

In this paper, we discuss the dynamics of beams with multiple delaminations. The objective of the study is to use the dynamic response of the delaminated beams in health monitoring of composite structures. As a first step, we present experimental results on the vibrations of a cantilever beam with multiple delaminations. These experiments are conducted by exciting the beam with piezoelectric actuators. The response is detected by using a high-speed video camera to study the existence of delamination opening modes in a beam with multiple delaminations. Then, an analysis of natural frequencies and mode shapes, for a delaminated beam, is presented by using analytical techniques. The measured dynamic responses of the beam with multiple delaminations are also presented. The experimentally detected nonlinear response is discussed. Introduction Delaminations or interlayer cracks and impact damages are typical defects observed in composite structures. The presence of these defects changes the dynamic characteristics of the structure . Some of the specific changes in the observed dynamic characteristics include changes in natural frequencies, changes in the mode shapes and changes in damping ratios. These changes can be used to identify the existence, location and magnitude of a delamination or an impact damage, before they can grow to their critical sizes. Studies, to date, are concerned with the dynamics of beams and plates with a single delamination. In this paper, we specifically discuss the structural dynamics of a beam with multiple delaminations. This information can be used to develop health-monitoring techniques for composite beams with multiple delaminations. In an earlier paper, Luo and Hanagud studied the effects of defects, like saw-cuts, impact damage and delamination, on the structural dynamic characteristics of composite beams. They have shown that perturbations in natural frequencies and modes of a beam, due to defects, can be used for health monitoring of structures. In particular, it has been shown that an integral equation that considers perturbations in curvature modes and natural frequencies can be very effectively used to detect local defects. To distinguish delaminations from other types of monitored (identified) defects Luo and Hanagud have discussed nonlinear effects like superharmonic response in the dynamic response of a harmonically excited delaminated beam. They considered only a single delamination. In this paper we study beams with multiple delaminations. More specifically, we discuss dynamic response and the changes in natural frequencies, mode shapes due to multiple delaminations in a composite beam. One of the earliest models for vibration analysis of composite beams with delaminations was proposed by Ramkumar et al. This model simply used four Timoshenko beams connected at the delamination edges to model a composite beam with one throughwidth delamination. The predicted frequencies, based on this model, were consistently lower than the results of experimental measurements. Wang et al. improved the analytical solution by including coupling between flexural and axial vibrations of the delaminated sublaminates. Using an isotropic beam with splits and the classical beam model, they found that the calculated natural frequencies were closer to experimental results. With similar considerations, Nagesh Gummadi and Hanagud formulated a finite element solution for arbitrary composite beams. In the finite element models, they considered a classical composite beam model as well as the beam model with high order shear deformations. Mujumdar and Suryanarayan have proposed a model which imposed a constraint between the delaminated sublaminates to force them to have the same flexural displacement. This model was unable to * Ph.D. Student, School of Aerospace Engineering † Professor, School of Aerospace Engineering, AIAA Member Copyright

Tail buffet alleviation of high-performance twin-tail aircraft using piezostack actuators

The phenomenon of the tail buffet is usually associated with high-angle-of-attack operations of a twin-tail aircraft. The unsteady pressures that result from the tail buffet, along with the aeroelastic coupling of the vertical tail structural assembly, cause vibrations of the empennage structural assembly. Such buffet-induced vibrations can shorten the fatigue life the empennage structure and limit the flight envelope of the aircraft due to the large amplitudes of the fin vibrations. Methods of alleviating the tail buffet-induced vibrations, by the use of smart structure concepts, is presented. Theoretical and experimental investigations were conducted to estimate and enhance the required control authority for tail buffet alleviation using piezoceramic-stack-based actuators. This was accomplished by designing an effective piezoelectric ceramic actuator assembly, developing procedures to place the sensors and actuators optimally on the vertical tail subassembly, designing controllers using acceleration feedback control concepts, and developing procedures to implement these controllers by using digital signal processor-based systems

Scanning laser Doppler Technique for velocity profile sensing on a moving surface.

A scanning laser Doppler technique based on Chebyshev demodulation has been developed for the rapid measurement of spatially distributed velocity profiles. Scan frequencies up to 100 Hz can be used over scan lengths up to 270 mm. The Doppler signals are processed in the conventional manner using a frequency counter. The analog velocity output from the counter is post-processed to obtain the velocity profile. The Chebyshev demodulation post-processing technique for processing the velocity signals from solid surfaces has been introduced. The data processing technique directly yields the spatial velocity distribution in approximate functional form through frequency domain analysis of the scanning LDV velocity output. Results from a rotating disk setup are presented to illustrate the concept.