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Active materials and structures: efficiency and application
S.B. Teodorovich
Siberian State Industrial University, Novokuznetsk, 654080, Russia
The questions related to the problems of development of technical systems, materials and structures with the properties being controlled while in service are considered. The property of “activity” inherent in similar systems is analyzed in view of principles of functioning of “alive” systems and use of the thermodynamic approach, by the example of active technical system (ATS) with flow process. The generalized functional structure and the criteria of energy efficiency of ATS are proposed and specified for an active structure operating on the basis of shape memory effect; the manifestation of activity is discussed for a number of other examples.
1. Introduction
In the report there is considered a number of methodology propositions of active technical systems (ATS) or active materials, designs, media, structures (AMS) in realization of which not only materials science rules, but also cybernetic aspects play an essential role. As a review of the appropriate works of domestic and foreign authors shows the statement of the given task is in itself new, in despite of huge attention shown in last 10-15 years in the world to active, “smart”, and intelligent structure problems. The experience shows that active or smart properties of a technical system (TS), material structure not always are the result of carrying sensors, actuators from the outside into their structural basis and formation of microprocessor equipment of their abilities to self-control (as it is frequently treated by foreign researchers [1, 2]). Many unique physical effects and effects of a more complex nature can make necessary internal control elements at various structural levels of TS. Such an approach brings in a new quality to the majority of traditional lines of search designing, development of the combined technical structures and mechanisms, to methods of solution of various technological tasks.
Physical and cybernetic aspects of AMS idea have something in common with a scientific direction considering material adaptation to external influences [3, 4]. The activity and adaptation can be observed in components made of Hadfield steel, in which high wear resistance is achieved
by austenite strengthening (hardening) owing to dynamic external influence on a superficial layer during operation with martensite formation; in ZrO2-based powder materials where under action of stresses in a crack tip modifying transformation accompanied with volume increase, absorption of energy, “lock-out” of a growing crack proceeds [5]. In heterogeneous superficial structures of a wear-resistant alloy under the influence of mechanical and thermal pulses, and also of diffusive processes between a body, rider, grease, atmosphere or other medium, there is a reversible reorganization of one structure into the other, favourable to each of the stages of work on friction [4]. The efficiency of layer combination with various hardness in multilayer coatings is known. Under vibrations and shock loadings the material layers with reduced hardness prevent from hard layer crumbling, allowing (and providing) its deflection due to their deformation.
In all considered cases the maintenance of working capacity of the TS is realized while in service due to operation of the self-regulation mechanism previously organized on material structural levels, this mechanism roughly can be considered as an analogue of an automatic regulator. Based on the laws of TS development, it is easy to conclude that, as a rule, integral solution of technological, power and administrative functions realization is characteristic for active technical systems and AMS [6, 7]. Adaptive and smart structures illustrate different degrees of activity in a such
© S.B. Teodorovich, 2004
way. So, the smart structures precede intelligent ones in this hierarchy; and the presence of a closed loop of control is inherent in the former ones, while the latter, in addition to properties of smart structures, are characterized by properties more peculiar to the second and, partly, even the third levels of an alive organism control system. It is also necessary to take into account that already at the first level the major feature of alive systems originates. Though in physics, engineering there are many examples of direct use of the term “activity”, the specification of this concept is expedient by comparison with the standard as which active systems successfully functioning in an alive nature can serve.
2. Factors of active structure efficiency
Physical and chemical basis of life is the condition of the steady nonequilibrium inherent to alive systems [8]. From the point of view of open system thermodynamics for free energy F of the system it is possible to write [9]
= -st-X Akak-Xm (-N) - m, (1)
dt k i
where n is the entropy production inside the system having internal energy U and being subjected to the influence of varied external parameters ak and also having varied chemical composition described with numbers of system particles Ni (m is the chemical potential of component i; the points above symbols in expression (1) designate derivative on time t). Tending of free energy F to its maximum value, which can be steadily maintained by the system, is explained by the conditions of substance exchange with an environment, at which X MN > Tn. However further increase of
this energy (or energy U) is limited according to kinetic equations containing both linear and nonlinear terms.
It is important that the significant part of the TS (structures, materials) with a selective mechanism functions according to the rules that submit to their treatment in the frameworks of quasi- or local-equilibrium thermodynamics, and, in view of hierarchical character (both structural and temporary) of development of separate stages ofprocess, macrothermodynamics [10]. We carried out the analysis of elementary ATS functioning with a flow process, in which the activity is characterized by selectivity of consumption, accumulation and distribution of energy (and generally substance and information). The special attention were paid to fulfillment by the active system of internal work on reorganization of own structure [11]. In such context expression for internal work wdr, made in a unit of time by driving parts, has a structure varying with the time. For the simplified variant of active structure with a flow process
wdr = X Xiji = wa + wp-y + WY + W + ws> (2)
where wa is the internal work on formation of an output flow (response of system), w^-y + wy= w^ is the internal work on maintenance of ATS steady nonequilibrium, the term wy energetically provides a working condition of the reception converter, ws is the contribution responsible for entropy production inside system. The rates of structure parameter variation j and generalized forces Xt attended by themare periodic functions of time. For stationary conditions it concerns to a lesser degree the last term in (2) and to a greater extent — contribution w5, responsible for selective consumption of an input flow and for initiation of structure formation steps of a flow leaving system. ATS with flow processes realization of the internal work mechanism and required level of organization are due to first of all the initial transitive stage. At this stage the structure of active system modifies into a more ordered state, acquiring internal quasi-equilibrium, not possessing essentially nonequilibrum in relation to an environment.
Thus, selective ATS behavior is meant as its abilities to leave from balance, making useful work at the expense of internal transformation of energy information flows. The given concept allows constructing a number of ATS, each subsequent representative of that is developed by construction improvements of the previous one.
Let’s define the active TS as follows:
ATS (AMS) is a technical system, in which the realization of transformation functions of the material (energy and information flows and (or) stored free energy and information and also structural internal transformations) is provided by construction, information and energy conditions. The realization of these functions is actually equivalent to the action in the TS structure of a control subsystem with own aim function and is capable to result in properties considerably distinguished from those of the base system (i.e. the TS identical with assignment ATS, with the only difference that the realization of the specified functions in it is not provided).
As an ATS example we shall consider the anchor device (Fig. 1(a)) with force elements from an alloy with shape memory effect and its generalized functional structure (Fig. 1(b)). The base system for given ATS serves an anchor with the mechanical lock having self-blocking property. In the process of easing with time of the force of rock massif reaction on the pressing elements of the lock, deformation of active elements and the rendering of force influence by them does not stop, supporting thus a sufficient level of fixing and self-regulation of a anchor tension [12]. The functions shown in Fig. 1(b) correspond to functioning of an active design immediately after its installation in a borehole (when on the input of system a thermal flow comes), and also to subsequent period of ATS operation. For an arbitrary ATS it is meant that each of the basic groups of transformations given in Fig. 1 generally can be represented
External
ean\/irnnmeant
/ / / / / /\ Control from outside'
Fig. 1. An active structure of anchor: a — general view; b — chart of the generalized functional structure; 1 — bar; 2 — lock; 3 — thrust half-muffs; 4 — active elements (AE) with shape memory effect (SME); 5 — back stop; 6 — control washer; 7 — rock massif
by the whole complex of operations both monoenergy, and dual types, according to a physical variety of tasks solved by the concrete ATS (AMS).
Complex estimation S of ATS efficiency is obtained on the basis of several indices Aj, using, for example, additive form:
H-2* j Л j,
j=1
(3)
where Xj are the weight factors. As one of basic ATS efficiency indices we shall obtain
Л w =1 -
we
(4)
where w^x and are the average specific works made in unit of time against external forces by an active and base systems, accordingly. When a useful output of ATS is informational (or other) product, the treatment of quantities w^x and w^ varies accordingly (for example, volume of the output useful information or other is estimated).
The second parameter of ATS efficiency characterizes its advantages over ideal variant (which differs by complete absence of energy losses in the active system),
Лп = 1 - —
D + L
Wl + DD + L
(5)
where D is the work carried out by the driving parts of the active system, L is the not utilized losses in ATS. We also use an index reflecting efficiency of system internal function realization (for averaged characteristics). The value of this index should not differ from zero if w^x = 0. One of variants of the criterion of an internal ATS efficiency we shall present
as
Л
Wa
Y
vwa+ WPA
W1 + DD + L
(6)
At last, we shall include in the structure of a complex estimation S an index characterizing the degree of functional-structural integration in the system:
Л= 1-
where I(s) is determined according to the expression into which enter the low bound of the number of functional transformations at the chosen scheme of information and energy flows and the number of modules used at its structural realization [13]. For the low bound of the third level of integration I = 1.33, thus A j = 0.735 (that is typical for modules from an alloy with SME).
3. Discussion of certain applications
For an active anchor from results of model experiments we obtain values of S in the range from 0.37 to 0.48 (if
X j = 0.25). Let’s note that with an expert way it is possible to establish a limit of complex criterion values adequate to the transition of the active TS in a higher rank — smart system, equal Ssm = 0.33. However for final specification of Ssm value the additional analysis of an actual material is obviously necessary.
Consecutive definition S should be constructed on the basis of the well-known physical principle of action and a mathematical model of ATS. At the same time, for a number of tasks the apriori information on functional features of a complex parameter can be obtained from the response of particular parameters to change of the input flow, knowing a subclass to which the system concerns. Besides, to expand the semantic contents of the complex criterion it is required to take into account characteristics of ATS reliability. So, in a number of active structures constructed on the basis of smart materials, known composite materials, in operational conditions the mechanism of structural self-restoration or “self-repair” works. To take into account the efficiency of this mechanism during formation of a complex estimation S it is possible with the help of an index, which is estimated through made ATS specific works done equivalent accordingly: to its energy losses owing to occurrence of failures, internal work on restoration of serviceability of elements, work of serviceable parts on partial indemnification of losses caused by failures.
The stated positions can be useful during treatment of the concrete technical applications. So, we experimentally testify the development of undesirable vibrating process in a rail at high-speed rolling of a wheel. Owing to weak attenuation of these fluctuations there are grounds to believe, that even at small loadings on a wheel the action of the dangerous factor (raising the stiffness of dynamic influence and almost not taken into account in the traditional approaches to rail testing) takes place. It is manifested in a significant growth of a number of loading cycles, activiza-tion of contact fatigue process [14]. The individual methodical aspects for application of one of the active methods of acoustic control of materials, products and structure elements — the measurement of frequency dependences of attenuation factor, in the variant of its realization excluding dependence of results of registration from physical distinctions of measurement channels, are considered; the design of the appropriate receiving converter is developed. The laboratory estimations of frequency dependences of transverse and surface ultrasonic waves damping in plates subjec-
ted alternating bending in the area of low-cycle fatigue, which confirmed prospects of a residual resource prognosis [15] have been carried out.
4. Conclusion
The success of formation of active structures will be defined by search of the technical solutions adequate to the maximum values of a complex estimation S. The latter is provided after completion of the choice of physical action principle from alternatives and establishment of relations of individual functional components with the solution of the appropriate task of optimization.
References
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