Each material has a set of properties that determine its further characteristics. One of these qualities is resistance to mechanical stress, which is called ultimate stress. By this concept is meant not only the destruction of the material at the break point, but also the appearance of permanent deformation. In other words, it is a counteraction to external forces that lead to a weakening of strength. The article talks about what such a voltage is, how it is calculated and how it is determined.
What is this indicator?
The ultimate stress of a material is the maximum tensile strength that must be applied to its cross-sectional area, which it can resist until it is completely destroyed or fractured. A simple calculation formula looks like this: stress is equal to force divided by area. It can be seen from it that the larger the area, the less force must be applied. The same is true and vice versa. The smaller the cross section of the workpiece, the more effort will be required to break it.
However, the stiffness indices for different materials are not the same. Some are fragile, others are elastic. The maximum allowable voltage for each is determined by mechanical testing. The result is considered achieved when external signs of integrity loss appear on the surface of the sample. They can be expressed as destruction or kink. For the latter, the term “yield strength” is used. The first speaks of fragility, the second - of plasticity.
Both concepts are associated with the ultimate stress at which the strength of the material is violated. Let us consider in more detail how these two concepts are distinguished.
Stress and fluidity
The rigidity of materials can be divided into two concepts such as fragility and ductility:
- The first involves the destruction of the structure of the sample even at low acting forces. Elastic materials resist external influences, leaving only permanent deformation in the form of a fracture. It follows that for plastic elements the criterion of fragility is bending, since it occurs earlier than complete destruction.
- In order to bend the sample, you need to make less effort than to break. Therefore, for plastic parts, the ultimate stress is the yield strength. Fragile products also have fluidity, but for them this figure is too small.
The stress that occurs in the cross section of the sample is called calculated. Further we will consider it in more detail.
Formulas for calculating stress
The calculation of ultimate stresses is performed according to the following formula:
Where:
- s is the normal stress directed perpendicular to the surface of the product;
- s (prev.) - ultimate stress, which leads to complete destruction of the sample or to its deformation, and for plastic (soft) materials, the value implies the yield strength, and for brittle elements - the ultimate strength;
- n is the normalized safety factor, which is necessary to compensate for temporary overloads on the working structures made of this material.
To calculate the tangential loads, use the formula:
In it:
- t is the shear stress;
- v is the Poisson's ratio, which is applied to a particular material of construction.
Conclusion
The voltage index is an important parameter for calculating the strength of the working structure. It is used in the design of load-bearing elements. Helps determine the degree to which a part fulfills its function and its service life.