Significant Research Contribution to Structural Dynamics

Smart Structures Technology

Dr. Hanagud pioneered the work in the area of the use of smart structures for vibration control in 1983 with a first publication in 1985. He directed the first Ph.D. thesis in the area in1986. This work started as a small project to provide an active electronic damping to a cantilever beam by using piezoceramic wafer actuators and active control techniques. Since then, the field has grown significantly and has been a very active research field during the past two decades. In addition to many papers and Ph.D. theses directed by Dr. Hanagud, three practical problems of interest have also been solved at Georgia Tech. These include

  • Active Control of Buffet-induced vibrations in a high performance twin-tail aircraft at high angles of attack by the use of smart structure technology. An U.S. patent has been applied for the actuator used in this project.
  • Hovering an unmanned helicopter by using shape memory alloy-based primary controllers. An U.S. patent has been issued for this invention.
  • Development of a smart guitar to improve the quality of sound radiated by the guitar. This was achieved by using piezoceramic wafer sensors, piezoceramic wafer actuators and portable active controllers.

Active Structural Control for a Smart Guitar (1996-)

Prices of musical instruments like violins and guitars vary significantly. For example, a store bought violin may cost about $2500 while a Stradivarius may cost approximately 2 million dollars. However, we know that the quality of the sound radiated by a violin or a guitar depends on the musician and the structural dynamic characteristics of the guitar like the natural frequencies, modes and damping ratios. For example, first three significant modes of a guitar correspond to the mode associated with the air moving in and out of the rose, the first plate bending mode of the top plate and the second plate bending mode of the top plate. In a relatively inexpensive store bought guitar, the second plate-bending mode is heavily damped while the same plate-bending mode is very lightly damped in a good quality guitar. Similar differences in structural dynamic characteristics have been studied both for the guitar and the violin.
The objective of this study was to use smart structural technology to change the structural dynamic characteristics of a relatively inexpensive guitar or a violin to follow the structural dynamic characteristics of a good quality or expensive guitar or a violin. In other words, the objective was to explore the possibility of producing the legendary quality of the sound of a Stradivarius in a store bought guitar by using piezoelectric sensors, piezoelectric actuators and suitable active control technique. In this first stage of the program we have successfully developed a smart guitar. In this guitar, the damping in the second mode is actively reduced when a musician excites the strings to create music that has the second plate bending mode components. Technically, a partially destabilizing controller is developed by the use of piezoelectric sensors, piezoceramic actuators and positive position feedback controllers. Theses controllers have been implemented in a store bought guitar and demonstrated by playing a piece of music that includes the second plate-bending mode of the guitar.

Delamination Modes in a Composite Structure (1990-1999)

The dynamic response of a delaminated composite structure contains non-linear modes. In some of these non-linear modes the delaminated surfaces can open and close. Dr. Hanagud identified such modes, which are now called as “delamination modes”. The objectives of the studies are to understand these non-linear modes and explain the growth of the delaminations under different types of external loads. Special features that are observed in this study are super-harmonics in the dynamic response of delaminated structures when the structure is subjected to a harmonic loading. This is similar to those observed in a structure with a quadratic non-linearity in the system. This feature is being exploited to non-destructively identify delaminations in a structural system and de-bonding in space shuttle tiles.

Development of analysis tools for Structural Health Monitoring by the use of Structural Dynamic Response (1993-1999)

The structural health monitoring techniques need strong dynamic signals from small defects. Before this work changes in natural frequencies and sometimes changes in displacement modes of a structure due to defects like cracks and impact damage were used. In this work, it has been shown the monitoring of changes in curvature modes due to defects will result in a very strong signal that can used to monitor these defects. An integral equation model has been developed to relate the modes of an intact structure, modes of structure with defects, magnitude of these defects and changes in natural frequencies due to defects. The integral equation and experimental results can be used to identify the existence, location and the magnitude of the defects. A parallel neural network technique for structural health monitoring has also been developed.

Active Damage Arrest Techniques (1996-)

Dr. Hanagud and his colleagues have pioneered the research work in the area of Active Damage arrest techniques (1995-) by the use of Smart structure technology. Defects in a structure like cracks and delaminations can grow to cause catastrophic damage of the structure. Usually, a vehicle made of such a structural system should be repaired to eliminate these defects. In space structures, it is very difficult to repair such structures. After knowing the existence of such defects, we need to arrest the growth of these defects. This work is to develop active control techniques and smart structures technology to actively prevent the growth of these defects. (Journal of composite Structures 1997)

Scanning Lasers in Structural Dynamics System Identification (1984-91)

The dynamic response of a structural system is usually measured using point sensors like accelerometers and strain gages. Thus, we obtain the response as a function of time at a given location. Usually a given structural system of interest is continuous (a distributed parameter system). We need measurements as a function of both space and time. Dr. Hanagud and his graduate student P. Sriram developed a scanning laser-Doppler system to measure the dynamic (velocity) response as a function of space and time during the years 1987-1991.The work is published in a series of papers. The results of the research can also be used to measure response from systems that are not accessible and can not be easily instrumented by using conventional instruments.

Structural Dynamic System Identification and Structural Control Techniques (1981-86)

This research work was motivated by the fact that the finite element models for helicopters were not able to predict the experimentally measured response or experimentally identified structural dynamic characteristics. One of the key contributions of this work was the perturbation method to identify non-linear structural dynamic systems. The other key contribution is the development of smart structure technology that resulted in the five previous contributions.

Crashworthy Design of Structures (1968-1989)

When this work was started, automobile rollover accidents were frequent, especially in rural driving conditions. These accidents caused significant amount of injuries to the occupants of the vehicle. Causes of these injuries were attributed to a non-crashworthy design of the roof and it’s supporting structures including excessive deformations of the roof structure. There were some techniques available to correct some of the design by using roll cage techniques and was being used in racecars. However, these techniques were not aesthetically appealing for use in a passenger vehicle. Dr. Hanagud selected a commercially available passenger vehicle and modified the design to with stand crash loads. He validated his design by using inverted drop tests on full-scale vehicles with and without design modifications. The design was recognized and the James Lincoln Arc Welding Foundation (1980) gave an award.
This work was later extended for application to different types of crashworthy design of light aircraft and helicopters as a part of the research work of the Rotorcraft Center at Georgia Tech. A senior level design course was developed in the field of crashworthy design at Georgia Tech.

Vulnerability of Satellites (1978-80)

During late seventies, satellites became essential tools for weather reports, communication and warning. Thus, the problem of the vulnerability of satellites became an important issue. The need for hardening the satellites to different types of attacks became important. As a pioneering study in this area Georgia Tech Research Institute and Georgia Tech participated in a project on the study of the vulnerability of satellites to kinetic energy impacts. Dr. Hanagud studied the effects of kinetic energy projectiles on satellites with Mr. Hilsen and Mr. Minardi of GTRI. Because of the nature of the work, this unclassified work was not published externally.

Fracture Mechanics in Aircraft Structural Design and Maintenance (1973-78)

This work was done starting from 1973. In this work, cracks and crack growth pattern in C-130 center box wing was studied. The objective was to investigate the feasibility and benefits of the use of fracture mechanics and dynamic fatigue crack growth in the design and maintenance of aircraft structures. This was a pioneering work that was supported by NASA. Today, this type of design and maintenance is a routine practice and is known by terms like ASIP (Aircraft Structural Integrity Program) and (HSIP Helicopter Structural Integrity Program)

Cavity Expansion Theory for Perforation and Penetration of Solids

This work is published in a series of reports and AIAA Journal 1971. This study began as a research project, at Stanford Research Institute. The objective of the project was to protect space vehicles from meteorite impact. The work was completed and published in the AIAA Journal after Dr. Hanagud joined the faculty of the School of Aerospace Engineering at Georgia Tech. The work has been extended to penetration of sea-ice and many different type of metals and soils. The results of this work and extensions of the approach are still being used extensively in the field of civil engineering. Dr. Hanagud has also used the results of this work in a Georgia Tech Project to study the vulnerability of satellites to ballistic projectiles. The field of cavity expansion theory has resulted in the development of many Ph.D. Theses and is described in a book by Professor W. Goldsmith of the University of California at Berkley.

Exact Solutions to the St. Venant Torsion Problem for Rod with the Cross Section in the Form of a Parallelogram or a Diamond shaped Airfoil J. of Applied Sci. Res., 1963.

This problem was studied to obtain solutions to the aeroelastic problems at supersonic speeds. During these years idealized diamond shaped airfoils were studied analytically and experimentally. A natural choice for aeroelastic studies was to use this shape. However, prior to this solution only numerical solution to this problem was available. In studying aeroelastic instabilities analytical solutions are preferable. Thus, this work was done while Dr. Hanagud was a graduate student at the Indian Institute of Science during the years 1957-58. The paper was presented at the second national congress of the India Society of Theoretical and Applied Mechanics in the year 1957. Even to date; there are very few exact solutions to the St. Venant’s Torsion Problem. Besides this work, only exact solutions that are available are for rods of cross sections of circle, triangle, ellipse and square.