DEFORMATION PROPERTIES OF THE OPTIMAL STRUCTURE SUIT
From an artificial building conglomerate working in the bearing structures of buildings and structures, it is required that sufficient mechanical strength be combined with deformation resistance, i.e. with its ability to resist the occurrence and development of irreversible deformations (plastic, creep) or the appearance and growth of microcracks . Deformation resistance manifests itself in the decaying character of the deformation formation process, in the relaxation capacity of the material, with the increase of which stresses arising under the influence of internal and external factors - operational loads and own weight of the structure, thermal and shrinkage phenomena - are more intensively removed. There are numerous examples where the conglomerate material, having sufficient strength, verified by design loads, is prematurely destroyed due to insufficient deformation stability, the appearance and development of irreversible deformations. Excessively long (or a period) of relaxation, exceeding by several decimal orders the periods of observation or the action of the load, affects the increase in the brittleness of the material with the possible formation of cracks.
The most deformation-resistant are those conglomerates that are characterized by high values of elastic and elastic-elastic deformations in the region of a certain temperature range and their real difference in the structure. Elastic-elastic materials are characterized by indices of elasticity-the percentage of the decreasing deformation for a certain period of time (for example, 5, 10, 30, 100 min or more) after their unloading from the load P, equal to the limiting shear stress , or any other power load. The elasticity index is expressed as:
where eo is the deformation of the shear (or other nature) that has arisen over time under the load P; t - the deformation remaining after elastic-elastic recovery for the selected period of time ті when the load was removed (Р = 0).
According to the value of the elasticity index, ISC is conventionally divided into highly elastic, when the deformation deformation occurs quickly, and is low-elastic, with a slow decrease of the deformation that appeared under load and after its deletion. Obviously, if the material in the structure is subject to cyclic loading and the period between the loads (rest period ) will be commensurable with the duration of the low-elastic deformation, a part of the deformation remains until the new cycle. With the new load application, the amount of the non-fallen part of the elastic deformation accumulates, and it can gradually develop into an irreversible one, which creates the prerequisites for destruction of the material and destruction of the structure. In this sense, any deformation, including elastic, entails a predisposition of the structure to damage, development of defects and even microcracks in it, to a long process of destruction. Germination of microcracks is accelerated with increasing deformation from repeated repetition (especially under vibration loading) of deformation with transition at the final stage into a dangerous crack and destruction.
The most elastic-elastic part of the deformations is possessed by binding substances with phase relations equal to c */φ, that is, when their structure is optimal. On the graph in the coordinate system Єy & gt; (c/f), as for the strength dependence, a maximum is observed (Fig. 3.12). To the left and to the right of the maximum is *, astringents of those compositions are placed at which elastic-elastic deformations have a smaller value.
The astringent substance with c */φ has the least value of irreversible deformations, and those astringents are located to the left and right of the minimum for which irreversible deformations increase rapidly, for example, using organic binding materials.
Decrease in elastic-elastic and increase in irreversible deformations in the left branches of the curve at values of c/φ & lt; c7f is due to the increase in the porosity and discreteness of the film of the medium on the particles of the solid phase of the binder. Irreversible deformations of ICS, in which the compositions are placed to the left of the extremum, are quasiplastic.
Decrease in elastic-elastic and increase in irreversible deformations in the right branches of the curve for values of c/φ & gt; c */f occurs under the influence of an increasing amount of free medium (c) in the conglomerate.
Fig. 3.12. Scheme of the change in the deformation properties of a binder and a conglomerate with a change in the ratio c/φ:
/-/- the line of plastic deformations & pound; M ; // - // - the line of elastic-elastic strains Єy & gt ;; III-III - the line of elastic moduli E, MPa;/- binder; 2 - ISK with an increasing amount of fillerWith gradual addition of the aggregate (active or inactive) to the astringent aggregate of the deformations, as well as strength indices, they have an extreme character - a minimum in irreversible and maximum elastic-elastic properties with a phase ratio c/f at which the strength index was the maximum (see Figure 3.12). The more aggregate in the conglomerate is contained, the smaller the value of the elastic-elastic properties and the more irreversible deformations. If the corresponding extrema are connected by an envelope curve, then, by analogy with the strength curve, we get a line, most often in the form of a tangent to the points of maxima or minima. Consequently, all points of the envelope curve are due to the optimal composition of the conglomerates. Each point of the curve corresponds to a maximum of strength and elastic-elastic deformations, but a minimum of plastic deformations. This combination of mechanical properties is always the most desirable in relation to ICS used in building structures of buildings and structures. The points on the right and left branches of the curves do not possess such a favorable combination of strength and deformation properties, and the compositions at these points are not optimal. The structure contains the discreteness of the medium or increased porosity of a secondary nature, for example, under the influence of evaporation of a part of the medium.
The envelope curve of the optimal compositions, under which the maximum elasto-elastic properties are ensured under the conditions of the adopted technology, can be described with sufficient accuracy for practical purposes by an equation of the hyperbolic type analogous to equation (3.3):
In the formula, the exponent s is defined similarly to the exponent n in formula (3.3).
If the value of the stress corresponding to the strength of the ICS is divided by the relative elastic deformation, then the obtained values of the elastic moduli can be applied to the general graph in the system E ( c/φ). With an increase in the amount of the filling part in the conglomerate, and accordingly with an increase in the phase ratio of the binder, the modulus of elasticity decreases in it, ie, the conglomerate becomes less rigid.
The deformability and modulus of elasticity, on which the total deformation resistance of the ICS depends, are directly related, like the strength, to the structure of the material. Moreover, the more the structure of the binder is coagulated, the more typical are the irreversible deformations, lower than the strength index and the modulus of elasticity. With an increase in
Fig. 3.13. The graph of the correspondence between the quality indicators of ICS in the sections /- I and II-II to the specified level III-III of the crystallizing phase increases the proportion of elastic or elastic- elastic deformation.
With a constant structure, the nature of the deformation is determined by the magnitude of the stress and the duration of the stress state, the relaxation capacity of the conglomerate. The latter, in turn, depends on the phase relationships, the content of the binding agent and the filler, its variety, that is, on the structure and individual structural elements.
Generalizing the formulas (3.3) and (3.15), we can formulate a general regularity of the mechanical properties of ICS.
The general and objective laws of optimal structures are not isolated from each other, but are interconnected in a single system, and they are commonly used in aggregate, for example, when designing mix compositions or when developing new conglomerate materials and manufacturing technologies. It is important that the resulting design compositions ensure the optimal structure for this technology, and the technical properties strictly correspond only to the level of the specified indicators, but also to their extreme values (Figure 3.13), i.e., not the section abc and the extremum d. At points 5 The excess of the quality index should be justified by economic calculation and operational data.
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