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Resistance of materials

the Science and technics
 
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Resistance of materials Once builders erected bearing designs at random that at times had catastrophic consequences. Today the knowledge of resistance of materials allows to build economic and reliable constructions.

At a skyscraper and the web weaved by a spider much in common. In both cases the skeleton from very strong materials, providing durability of all construction is created. The skyscraper skeleton consists of steel beams, and the web trudges from stronger material - silks. It means that the spidery thread can sustain bolshy weight, than a steel thread of the same thickness.


Loading

The cargo Suspended on a wire creates the force directed downwards concerning the area of its cross-section section. The size of the applied force divided into the area of its appendix, is called as loading.

Bolshy cargo creates the big force directed downwards, therefore loading on a wire of the same section will be more. The loading size also will be bolshej if equal cargo is suspended on more thin wire as force created by it operates on the smaller area of cross-section section. At comparison of resistance of materials it is important to know, they can sustain what loading till the moment of residual deformation or rupture.

There are three kinds of the pressure tested by materials. The stretching effort stretches a material and arises, for example, at suspension to it of cargo. Pressure of compression squeezes a material (table legs under weight of subjects being on it). Pressure of shift influences a material and bends it (a springboard for a diving under weight of the person standing on it).


Relative deformation

It is uniform deformation, or change of the sizes of a material under the influence of loading. We will assume that the wire in length of 4000 sm is stretched on 2 sm at suspension to it of cargo. In this case relative deformation represents proportional change of length and makes 2: 4000=0,0005.


Elasticity

If gradually to increase loading by a material, since zero in the beginning the arisen pressure grows proportionally. If to clean cargo, the material will return to the initial sizes. This phenomenon is called as elasticity. But if to continue to load a wire it, at achievement of certain size of loading, will not come back any more to the initial sizes. This size is called as a limit of elasticity of a wire. In this case the wire is treated to action of plastic deformation and now will be constant to be extended with loading increase.

The Further increase in loading finally will lead to material rupture. Plastic materials (for example, copper) before rupture are strongly deformed, and fragile (we will tell, pig-iron) at gradual increase in loading are at some instant torn absolutely suddenly.


Concrete

Not always materials equally well maintain different kinds of pressure. For example, concrete has the big durability on compression, but rather small durability on a stretching. Therefore concrete often reinforce steel cores for increase in its breaking strength. Preliminary intense concrete is the improved kind of ferro-concrete. In the beginning steel armaturnye cores subject to a stretching, and then fill in with concrete. After shvatyvanija concrete cores any more do not test stretching pressure and aspire to restore the initial length. But it is impossible, as they are strongly walled in concrete and cause in it huge compressing pressure. At use of the given material, any efforts, aspiring to break off concrete, should overcome at first forces of compression in steel cores. That is why preliminary intense concrete possesses the big breaking strength and compression.

To consolidation of some kinds of concrete apply a method of the subsequent pressure with an armature tension. Steel cores insert into apertures in monolithic concrete blocks and subject to their stretching. As well as in the previous case, cores create strong compressing pressure in concrete and give it the big breaking strength.


Composite materials

Such materials consist of two and more various materials, thus the end-product possesses the best properties, than any of its components. In many cases such way durability increase achieve. As an example of a composite material ferro-concrete can serve. At lower level, thin fibres of various materials with high breaking strength add to the materials, capable to maintain the big compressing loadings. Fiber glass in a combination to web silk possesses much more bolshej breaking strength, than the best grades of a steel. But, unfortunately, the slightest superficial scratch leads to crack formation even at rather small loadings. Therefore, to use advantages of fiber glass, it add to epoksidnoj and polyester pitch which protects fibres from scratches and keeps their high breaking strength.


Carbon and ceramics

For this purpose are used a fibre and other materials, including carbon (in the form of graphite), and also many kinds of strong, rigid ceramic materials, such as silicon carbide, carbide of a pine forest and okis aluminium. But not all materials such receive an artificial way. For example, wood - the natural composite material consisting of strong flexible fibres of cellulose, connected by firmer and fragile substance - ligninom.


Structure of materials

Durability of a material depends on its internal structure - arrangements in it of atoms or molecules. All firm metals and the majority of other firm materials have crystal structure in which atoms and molecules are located in a correct order. An arrangement of these particles and communication between them define durability of a material. For example, rubber consists of chains of molecules. In the course of vulcanization sulphur incorporates to chains of molecules of rubber, having their number with each other. Occurs so-called "§шьшёхёъюх ё°штрэшх" therefore rubber becomes stronger.


the Disposition

When metal is exposed to pressure without excess of its limit of elasticity, it is stretched, as atoms it is a little "Ёрчфтшур¦=ё ". At removal of pressure atoms of metal take the primary positions owing to what metal is compressed. If pressure above the elasticity limit, some metals keep the new form after its removal. It occurs because at structure of crystals of such metals there are numerous defects - dispositions. One crystals can have superfluous atom, and in others one atom can not suffice. Such dispositions are formed at cooling and crystallisation of the fused metal. They can be formed also when metal is exposed to mechanical pressure, and it is deformed at smaller pressure. Pressure above an elasticity limit force nuclear planes to slide atop each other. The more dispositions, the easier, without destroying metal, to give it the necessary form by kovki, proskating rinks or protjazhki (extensions). At deformation of metal of a disposition in it move along borders of planes of sliding atoms. If to obstruct to traffic of dispositions, metal becomes firmer and stronger.


Crystal structure

There are some ways of preventing to such moving. One of them consists in monitoring of process of hardening at manufacturing to receive the metal consisting of rather considerable quantity of small crystals. The more crystals, the there are more than section borders between them, dispositions interfering moving in metal. Other way of blocking of movement of dispositions is connected with addition of atoms of other metal. That is why the alloy from two and more metals is firmer and stronger, than separately taken metals of which it consists.

It is possible to create also by means of pressure such quantity of dispositions in metal at which they will prevent to move each other. Such mechanical consolidation (named also "эръыхя") it is possible to achieve and by means of the pressure arising in the course of usual metal working.


Weariness of metal

The Weariness of metal can arise, if a metal detail to subject to certain loading during the long period of time, or in case of numerous changes of force of pressure. Dispositions move to an intense zone and obstruct to traffic each other. As a result metal becomes fragile, and the crack which can extend on all thickness of a detail up to its final break is formed. In order to avoid accidents, and details of planes regularly check knots on weariness by means of so-called. Not destroying control.