Structural adaptation

Smart or adaptive structures are a class of advanced structures with integrated sensors, actuators, and controls which allow it to adaptively change or respond to external conditions (Figure 10.1). Examples are buildings, bridges, and roadways that can sensor, mitigate, and control damage, e.g., aircraft that can actively minimize a stmcture-bome noise in the interior. 

DeSanctis, G. c Poole, M. S. 1994. Capturing the complexity in advanced technology use adaptive structuration theory. Organization Science, 5 121-147. 

Organic polymers are responsible for the very life—both plant and animal—that exists. Their complexity allows for the variety that is necessary for life to occur, reproduce, and adapt. Structures of largely linear natural and synthetic polymers can be divided into primary structures, which are used to describe the particular sequence of (approximate) repeat units secondary structures, which are used to describe the molecular shape or conformation of the polymer tertiary structures, which describe the shaping or folding of macromolecules and quaternary structures, which give the overall shape to groups of tertiary-structured macromolecules. The two basic secondary structures are the helix and the sheet. 

The design of smart materials and adaptive structures has required the development of constitutive equations that describe the temperature, stress, strain, and percentage of martensite volume transformation of a shape-memory alloy. These equations can be integrated with similar constitutive equations for composite materials to make possible the quantitative design of structures having embedded sensors and actuators for vibration control. The constitutive equations for one-dimensional systems as well as a three-dimensional representation have been developed (7). 

The design of shape-memory devices is quite different from that of conventional alloys. These materials are nonlinear, have properties that are very temperature-dependent, including an elastic modulus that not only increases with increasing temperature, but can change by a laige factor over a small temperature span. This difficulty in design has been addressed as a result of the demands made in the design of complicated smart and adaptive structures. Informative references on all aspects of SMAs are available (7—9). 

In the various intergrowth systems examined (see Table 5.3) there is no evidence for the presence of point defects. The origin of long-range periodicity in the complex recurrent intergrowth systems is, however, intriguing. The importance of elastic forces in the formation of polytypes, shear structures and infinitely adaptive structures was... 

Thus we have studied the structure of the Y O iF +2 and Ba-Fe-S systems as typical examples of the vernier structure. In the vernier structure, the deviation from stoichiometry originates from the coexistence of different unit dimensions of the subcells, i.e. Aj and Aj sheets for the Y-O-F system, and Fe2S4 and Ba subcells for the Ba-Fe-S system. The structural principle of the vernier structure can be extended to a more general one, called adaptive structure , in which independent, though mutually related, structures for all the compositions can exist in a limited composition range (see Section 2.6). 

In the above sections, we have described the four types of non-stoichiometric compounds derived from extended defects based on the difference of structural characteristics. The concept of adaptive structure, which was proposed by the late Professor J. S. Anderson in 1973, is a more general concept which explains some of the examples mentioned in the preceding sections. The compounds which have the adaptive structure are defined by Anderson as ... 

For most, but not all, of the infinitely adaptive structures there may be a multiplicity of discrete, related, fully ordered structures for any one composition and, for certain compositions, the number of possible structures is potentially infinite. 

Adaptive structures have a common structural principle. A large unit cell of the adaptive structure is built up from the ordered repetition of a set of basic sub-units, derived from the mother structure by systematic changes in site occupancy (see Section 1.4.11) or in stacking sequence, or by a shear operation. For example, the homologous compounds V O2 i (n = 3-9, see Section 2.2), which are derived from the mother compounds (rutile) by the shear operation (121)j[0il], are able to be a set of basic sub-units. 

Fig. 2.109 Composition (O/M) versus k curves. In this figure, fe stands for the inverse of the mean distance of distortion planes. Each line corresponds to a fixed value of m. White circles on each line correspond to the compositions derived from DP operations. Closed circles are the compositions observed in this system. This figure clearly indicates the characteristics of the adaptive structure.

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