The use of material in construction is impossible without taking into account their characteristics. One of the most important is tensile strength. If this indicator is not taken into account, the erected building will collapse, since the structural elements will not be able to withstand the load. It is not enough to know about the property of a building material, you need to be able to apply it in practice.
Tensile strength is the maximum value of stress experienced by a material before it begins to fail. Its physical meaning determines the tensile force that must be applied to a rod-shaped specimen of a certain section in order to break it.
Strength limit – mechanical stress, above which material destruction occurs. In other words, this is a threshold value, exceeding which mechanical stress will destroy a certain body made of a specific material. A distinction should be made between static and dynamic ultimate strength. Compressive and tensile strengths are also distinguished.
Material strength value
The tensile strength of a material is abbreviated as PP. It is also allowed to use the expression “temporary resistance”. To designate the ultimate strength, the letters R or σ B (sigma) are used. The unit of measurement is megapascal. Indicator means the allowable amount of force that can act on an object before it begins to collapse. We are talking about mechanical action, but it should be borne in mind that chemical factors can change the initial properties of the material, including affecting the PP. Non-mechanical loads include the following:
- the heating;
- weather conditions (wind, precipitation, humidity);
- aggressive environment.
Tensile strength of the material
The formula for the tensile strength is written as follows: R = 0.64 (P / F), where F is the surface area of the object’s fracture, and P is the breaking load. When designing, you cannot rely on extreme values, so engineers leave allowances for various factors, as well as for the period of operation. This means that during construction, a material is used whose PP exceeds the design stress.
Initially, the ability of an element to withstand loads was determined empirically. The material was used without knowing how it would behave during operation, and after a breakdown it was replaced with a more durable one. Over time, we moved on to experiments and tests, and still the most accurate way to find the tensile strength and tensile strength remains empirical.
Research is carried out in laboratory conditions using precision technology. The devices record the characteristics of the material and how they change under loads of different magnitudes. As a rule, strength is measured as follows: an object is rigidly fixed and exerted on it.
How is strength measured?
First, the anchored element is stretched. It becomes longer, while an isthmus forms in one place, and it is here that the workpiece will break. Not all materials behave this way, but only viscous ones. Cast iron, steel and other brittle alloys stretch slightly. With increasing load, they crack and collapse along inclined planes. No necks are formed.
The force applied at each moment is measured to the nearest thousandths of a newton. At the same time, the size and nature of the deformation are determined. Data are checked against tables.
The second way is mathematical analysis. It lies in the fact that strength is determined using complex calculations. However, without testing, the data obtained by calculation cannot be considered complete. The fact is that in practice, a substance can behave differently.
The material has a temporary resistance in response to influences of a different nature, therefore, the characteristic is classified into several groups. The forces to which the workpiece or structural element is subjected:
- Stretching. The product is pulled by the edges using a special machine.
- Torsion. The object is placed in the conditions under which the torque shaft works.
- Bend. The workpiece is bent and unbent in several directions.
- Compression. The material is pressed alternately from different sides.
Material tensile strength
PP may differ for the same material. Steel is an example. It is used more often than other alloys because steel structures have proven to be the strongest, most durable and resistant to adverse factors. Moreover, they are reliable and do not emit harmful substances into the atmosphere.
There are several grades of steel. They are produced using different technologies, and depending on this, the characteristics of blanks and structures differ. For conventional grades, the PP is 300 MPa. As the carbon content increases, the strength increases. The hardest grades have an indicator of 900 MPa. Factors on which the strength characteristics depend:
- the amount of useful and undesirable impurities;
- method of heat treatment (cryoprocessing, quenching, annealing).
Temporary resistance and fatigue
There is a direct connection between the PCB and the temporary resistance of various loads. The second indicator in the documentation and technical literature is designated by the symbol T. It shows how long the deformation of the sample lasts when a constant load is applied to it. When the temporary resistance ceases, the crystal lattice of the substance is rebuilt. This is typical for hard materials. As a result, the substance becomes more durable than it was before. This phenomenon is called self-hardening.
Another important characteristic is metal fatigue. When speaking of steel, the expression “endurance limit” is used. The symbol R is used for designation. This characteristic shows the effect of what force the material can carry constantly, and not one-time. During the experiment, the sample is pressured with a predetermined force. The number of impacts is 107. During the tests, the material should not deform or lose its original characteristics.
Definition of characteristics
Materials have not only a strength limit, but also other characteristics. In the case of steel, this is hardness and shock absorption. The tests are carried out as follows: a diamond cone or ball is pressed into the workpiece. Diamond is the standard of hardness. The size of the footprint depends on how strong the test piece is. The softer it is, the larger the print, and vice versa.
The impact strength is calculated as follows: a cut is made on the sample, then it is struck. The results show the performance for the site that is most vulnerable. Other mechanical properties for which data are obtained empirically:
Plastic. It shows to what extent the sample can change shape while retaining the original structure.
Fatigue. This category reflects how long the material does not lose its properties under continuous stress.
Impact strength. Characteristic refers to the degree to which a substance is capable of resisting impact.
By strength, substances are divided into classes. They differ in one or more characteristics. So, for two classes, the PP indices can be the same, and the values of the relative elongation or yield strength can be different.
Dynamic tensile strength
Dynamic tensile strength is a threshold value of an alternating mechanical stress, exceeding which an alternating mechanical stress will destroy a body made of a particular material. In the case of a dynamic action on this body, the time of its loading often does not exceed several seconds from the beginning of loading to the moment of destruction. In such a situation, the corresponding characteristic is also called conditionally instantaneous tensile strength, or brittle-short-term tensile strength.
Compressive strength is a threshold value of constant or, respectively, variable mechanical stress, exceeding which mechanical stress will compress a body made of a specific material as a result – the body will collapse or unacceptably deform.
Ultimate tensile strength
Ultimate tensile strength is a threshold value of constant or, respectively, variable mechanical stress, exceeding which mechanical stress will tear a body from a particular material as a result.