THIN-WALLED STRUCTURES

RESEARCH AND DEVELOPMENT

ELSEVIER

Copyright © 1998 Elsevier Science Ltd.
All right reserved.

ISBN: 978-0-08-055205-7

Contents

Preface......................................................................v
Author Index.................................................................817
Keyword Index................................................................821


Chapter One

SMART THIN-WALLED STRUCTURES

K.P. Chong and B. K. Wada

ABSTRACT

Research and development in smart thin-walled structures and materials have shown great potential for enhancing the functionality, serviceability and increased life span of the aerospace, civil and mechanical infrastructure systems and as a result, could contribute significantly to the improvement of every nation's productivity and quality of life. The intelligent renewal of aging and deteriorating aerospace, civil and mechanical infrastructure systems as well as the manufacturing or construction of new ones, includes efficient and innovative use of high performance sensors, actuators, materials, mechanical and structural systems. In this paper some examples of NSF funded projects and NASA projects, as well as some research needs are presented.

KEYWORDS

thin-walled structures, smart structures, structural control, solid mechanics, smart materials, composites, adaptive structures, space structures, inflatable structures.

INTRODUCTION

Spacecraft enhances opportunity to observe earth, planets and solar system and to provide communication from various locations on earth to other locations. Various types of observations and communications are limited by the size and precision of very large structures. The size of structure may range upwards of 100 meters with submicron precision requirements. Other demands on the structure include a very efficient lightweight system with the capability of being stowed under a launch vehicle shroud and the deployed/erected to its final dimensions in space. Thin walled structures are essential to space programs. Beginning in the early 1980's, Smart Structures technology focused on simpler approaches to add damping to large precision space structures to attenuate the small vibrations of these large space structures by using new developments in actuators and sensors. Smart Structures is a multidisciplinary activity that integrates materials, actuators, sensors, controls, composites, structures, concepts and dynamics. One of many definitions is "a structural system whose geometric and inherent structural characteristics can be beneficially changed to meet mission requirements either through remote commands or automatically in response to adverse external stimulation" [Wada, 1990]. The approach provides solutions to a broad range of space structures challenges including: ground validation tests, on-orbit deployment, system identification, static adjustment and reliability. It results in a robust structure [Wada, 1994]; a robust structure meets the mission requirements by reducing the influence of uncontrollable, uncertain or costly design parameters and processes. The full benefits of smart structures are the realization that it enables many of the future space missions and can reduce overall mission cost while increasing overall reliability.

Thin-walled composites have been candidates for use in high tech applications for some time (Chong, et al, 1994). However, for use in the civil infrastructure systems and construction type applications, in the clean car and even in civilian aircraft, substantial total lifetime cost improvements must be realized. Thus the durability and lifecycle performance as well as maintenance and repair are very important considerations. Study on the collapse of highway bridges in the January 1994 Northridge Earthquake indicates that the massive inertia due to the heavy reinforced concrete in earlier designs is a major cause. A composite thin-walled bridge should be about 3 to 5 times lighter. In the next section a futuristic smart composite bridge will be presented.

In recent years, researchers from diverse disciplines have been drawn into vigorous efforts to develop smart or intelligent structures that can monitor their own condition, detect impending failure, control damage, and adapt to changing environments (Rogers and Rogers, 1992; Chong, 1998). The applications of such smart materials/systems are abundant — ranging from design of smart aircraft skin embedded with fiber optic sensors to detect structural flaws; thin-walled bridges with sensoring/actuating dements to counter violent vibrations; flying micro-electrical-mechanical systems (MEMS) with remote control for surveying and rescue missions; and stealth submarine vehicles with swimming muscles made of special polymers. Often times these structures are thin-walled structures due to structural efficiency and/or payload constraints. Such a multidisciplinary research front (Chong, et al, 1993), represented by material scientists, physicists, chemists, biologists, and engineers of diverse fields—mechanical, electrical, civil, control, computer, aeronautical, etc.—has collectively created a new entity defined by the interface of these research elements. Smart structures/materials are generally created through synthesis by combining sensoring, processing, and actuating elements integrated with conventional structural materials such as steel, aluminum, concrete, or composites. Some of these structures/materials currently being researched or in use are (Chong, et al, 1994; Liu et al, 1994):

• piezoelectric composites, which convert electric current to (or from) mechanical forces;

• shape memory alloys, which can generate force through changing the temperature across a transition state;

• electro-rheological (ER) fluids, which can change from liquid to solid (or the reverse) in an electric field, altering basic material properties dramatically. Since ER fluids are really suspended solutions, the settlement of solids under long-term inactivity is a problem.

Current research activities aim at understanding, synthesizing, and processing material systems, which behave like biological systems. Smart structures/materials basically possess their own sensors (nervous system), processor (brain system), and actuators (muscular systems)—thus mimicking biological systems (Rogers and Rogers, 1992). Sensors used in smart structures/materials include optical fibers, corrosion sensors, and other environmental sensors and sensing particles. Examples of actuators include shape memory alloys that would return to their original shape when heated, hydraulic systems, and piezoelectric ceramic polymer composites. The processor or control aspects of smart structures/materials are based on microchip, computer software and hardware systems. In the past, engineers and material scientists have been involved extensively with the characterization of given materials. With the availability of advanced computing and new developments in material sciences, researchers can now characterize processes, design and manufacture materials with desirable performance and properties. One of the challenges is to model short-term micro-scale material behavior, through meso-scale and, macro-scale behavior into long term structural systems performance (cf. Fig. 1). Accelerated tests (Chong et al, 1998) to simulate various environmental forces and impacts are needed. Supercomputers and/or workstations used in parallel are useful tools to solve this scaling problem by taking into account the large number of variables and unknowns to project micro-behavior into infrastructure systems performance, and to model or extrapolate short term test results into long term lifecycle behavior.

RESEARCH NEEDS IN SMART STRUCTURES AND MATERIALS

Recent developments in precision actuators, sensors, and miniature electronics with frequency response from static to kilo-hertz capable of direct integration into the structure itself to sense and actuate structural strains enables Smart Structures. Fortunately, the availability of commercial piezoelectric materials for actuators suitable (space compatible and low load carrying requirements) for spacecraft applications allowed rapid transition from laboratory experiments to space applications. The traditional approach to the design of structures is to increase the wall thickness as necessary to carry the loads generated by external loads or to maintain the necessary geometric shape. For smart structures, a thin wall structure is preferable to provide transfer of strain from the smart material to the structure with smallerforces. The strain induced by the smart materials will then be used to reduce the loads in the structure and to provide the desired geometric shape. Until recently, space structures were not capable of changing its initial characteristics. The capability to design structures with changing characteristics during its operational life excites many engineers by providing many more options and opportunities. To date, the rapid worldwide growth in research and applications in Smart Structures is almost exponential.

One of the challenges is to achieve optimal performance of the total system rather than just in the individual components. Among the topics requiring study is energy - absorbing and variable dampening properties as well as those having a stiffness that varies with changes in stress, temperature or acceleration. The National Materials Advisory Board (NAS, 1993) has published a good perspective on the materials problems associated with a high performance car and a civilian aircraft which develops the "values" associated with these applications. Among the characteristics sought in smart structures/materials are self healing when cracks develop and in-situ repair of damage to structures such as bridges and water systems in order that their useful life can be significantly extended. There is the associated problem of simply being able to detect (predict) when repair is needed and when it has been satisfactorily accomplished. The use of smart materials as sensors may make future improvements possible in this area. The concept of adaptive behavior has been an underlying theme of active control of structures that are subjected to earthquake and other environmental type loads. Through feedback control and using the measured structural response, the structure adapts its dynamic characteristics for performance instantaneously.

Fig. 1 is a sketch of a futuristic smart composite bridge system, illustrating some new concepts: including advanced composite materials with protective coating to mitigate ultraviolet damage, wireless sensors, optical fiber sensors, data acquisition and processing systems, advanced composite materials, structural controls, dampers, and geothermal energy bridge deck de-icing. Since a composite bridge is very much lighter (up to five times lighter) than a conventional concrete or steel bridge, excessive vibration/deformation as well as stability and the resulting fatigue damages can be minimized and durability maximized by elaborate structural control systems (Liu, Chong and Singh, 1994; Rogers and Rogers, 1992). An artist rendition of this bridge appeared in USA Today, 3/3/97.

The recent research activities in infrastructure (Chong, et al, 1993) for example call for efforts in:

• deterioration science

• assessment technologies

• renewal engineering

• institutional effectiveness and productivity at the system level.

These are in addition to the implied needs in:

• Stability, large deformation and buckling

• Reliable accelerated tests for long term durability behavior

• improved computers, microprocessors and networking

• more accurate/complete modeling of lifetime predictions

• new sensors and control systems; NDT; new materials

• electro-rheological fluids (e.g. settlement of solids over time in the suspended solution)

• shape memory alloys, etc.

• understanding corrosion better at the detail level

• life-cycle performance and costs.

This list is meant to illustrate rather than be complete.

SOME BASIC PRINCIPLES

Traditional Approach: External Force Control

The basic fundamental principles using Smart Structures is almost the direct opposite to the historically traditional approach of forcing the distorted shape to the desired accuracy by applying a set of external vector forces to caned the distortion. The application of external vector forces becomes more difficult in space because a "ground" does not exist to react the forces. Fig. 3 illustrates a conceptual structure in space using the traditional approach to change the shape of the structure. Challenges with the external force control approach are:

• The external vector forces Fe are applied by expelling mass or by proof-mass actuators (PM) that applies forces through reaction against a mass.

• The magnitude, phase and direction of FE. must be accurately coordinated with the distortions of the structure. This requires very accurate knowledge of the structural distortion through a multitude of inertial sensors. Thus the actuators and sensors are at locations of maximum distortion. To visualize the problem, imagine trying to push a balloon inwards with thrusters. No matter in how many places you push, it seems impossible to make the sphere shrink uniformly and prevent rigid body motion.

(Continues...)



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