Adaptive structure

Thus we have studied two examples of the intergrowth structure. The concept of the intergrowth structure plays an important role in inorganic structural chemistry. 

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  

Within certain composition limits, every possible composition can attain a unique, fully ordered structure, without defects arising from solid solution effects and with no biphasic coexistence ranges between successive structures. 

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. 

These compounds show two sets of reflections in their diffraction patterns  

Embedded sensors are relatively inaccessible for inspection but better protected and an interconnection with other sensors can be implemented easier. Integrated piezo sensors must have a Yoimgs modulus comparable to the base structure to avoid structural discontinuities. The Curie temperature should be higher than the curing temperature of the base component. Furthermore piezoelectric sensors should be electrically isolated from the base structure. The isolation must not reduce the force-lock between the sensors and the structure. 

To embed electronic circuits into the structure they have to be isolated electrically, cooled if necessary and isolated mechanically from the load paths. 

For the optimal performance of a multilayer structure (e. g. FRP) it is important to minimize the number of ply interruptions. Presently applied piezo ceramics allow a maximum strain of about 1000 ppm and exhibit a stiffness in the range of 50... 90 GPa. In order to design an adaptive structure with integrated sensors comprehensive tests are necessary to cover operating conditions and service strengths such as mechanical stresses, strains, temperatures and voltages. 

The manufacturing of adaptive structures requires new production methods that need a lot of experience and expertise for the fabrication of complex systems with embedded or applied actuators and sensors. Patch-like sensors such as 1-3 piezo fiber composites are quite appropriate to be attached to the surface or to be integrated into lightweight multilayer constructions. The bond layer between the piezo element and the base structure determines the transfer behaviour of strain, vibrations and acoustic waves from the actuator to the structure and from the structure to the sensor. Local stress distributions can strongly be influenced by the bonding technique. 

Up to now piezoelectric composite transducers (e. g. AFC, MFC and PFC) are not widespread in adaptive structures. There exist many publications about the application of piezoelectric ceramic plates or wafers in structures but there is rarely something said about realistic application conditions or the load capacity of such devices. With regard to that piezo composite transducers promise to be robust alternatives to bulk ceramic devices. 

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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). 

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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