15.1 Introduction

Through evolution, biological structures have become highly optimized both in their mechanical properties (e.g., strength, stiffness, and toughness), and in their ability to sense and respond to the environment to sense threats, heal damage, and respond to external demands (e.g., adaptive shape and other functions such as stealth).

The active/sensory "smart" behavior of these materials involves the following systems:

• Sensor

  • nerve: develops signals from external stimuli: senses overstress, physical damage, temperature, chemical attack

• Actuator

  • muscle: provides response to a signal; provides force for movement or shape change

• Processor

  • brain: monitors sensor, processes data, provides signal to actuator

Smart behavior of biological structures includes:

• Adaptive shape

  • e.g., for aerodynamic control

• Adaptive stiffness

  • to optimize for loading conditions

• Adaptive strengthening

  • reinforcement deposition to optimize strength and stiffness

• Health monitoring

  • indication of damage or overstress

• Self repair

  • heals damage by local deposition of material

  • re-grows damaged component

• Reversible adhesion

  • ability to form and break strong adhesive bonds at will

• Stealth

  • change in form, colour, and/or pattern

Fiber composites simulate natural materials, such as wood and bone, in forming lightweight, stiff, tough structures. Now there is a rapidly growing interest in simulating selected aspects of this intelligent or "smart" behavior, particularly in aerospace composite structures.

15.2 Engineering Approaches

Based on the discussion of the proceding section, smart structures can be defined as structures that are "aware" of their state and have the ability to respond to changes in the operating environment or to other stimuli in an intelligent way. This ability may be...