Limit strength is based on the theory that any material able indefinitely to withstand a static load of any power, if it forms a voltage quantity which does not exceed the tensile strength. If the title is resistance, which is equal to the temporary voltage, the destruction of the prototype will happen in an undefined finite time interval.
For measuring limit strength also apply the concepts of yield, proportionality, endurance, etc. the value of the temporary fracture resistance in tension of the material and its compression for different substances vary significantly. For brittle materials, which include ceramics, limit strength in compression is greater than its value in tension, for composite materials characterized by the opposite situation, and plastics and metals usually show the same limit of strength in both directions.
To calculate the limit of strength, it is necessary to know the force that occurs in the body during the deformation of the object, and the exposure area on the subject of outside forces. Mechanical stress at a point is equal to the ratio of internal force in Newtons per unit area at a particular point of cross section in m2. I.e. the external influence aimed at changing the position of particles relative to each other, and the tension that occurs in a substance at the same time, prevents this change in location and limit its spread. Distinguish normal and tangential stress, which differ in the direction of application of force.
In the formula σB is expressed as Q = FS, where S is the surface area, and F is the force deformation, resulting in the body.The maximum possible number of mechanical stresses for a particular substance and is the limit of his strength. So the limit for steel is 24000 MPa, and the yield stress for nylon - 500 MPa.
Advice 2: How to determine the yield stress
The limit of fluidity is the magnitude of the mechanical load (stress) at which the material irreversible deformation, changes its size and shape. The amount of yield today is used for determining the qualitative characteristics of metals, steels. It affects the strength of metal structures, fasteners, and mechanisms.
For hard steels which do not have a site of fluidity, define the conditional limit of fluidity, which is determined when reaching 0.2% of residual deformation. There are special tables that the values for the limits of yield strength for different materials and steels. Values are defined by the relevant Standards adopted in Russia.
To determine the limit of yield stress using two methods: analytical and graphical.An analytical method. In accordance with the recommendations of GOST, select the rebar sample (material) for testing and determine its initial cross-section by measuring or determining the appropriate formula. Place the sample in the tensometer, corresponding to GOST 18957-73, spend a couple of tests, measuring the magnitude of mechanical stresses and cross-sectional area, the magnitude of the elongation up to rupture of the material. Determine the limit of fluidity of the sample by the formula, where it is equal to the ratio of the applied (measured) mechanical stress to the initial cross-sectional area of the sample. Measured in MPa (kgf/mm2).
The graphical method is plotting the "tension-elongation" of the sample in the tensometer. Take graph paper and construct a coordinate system in which the axis of ordinate (y) defer the load applied to the material during the test, and the horizontal axis (x), and the amount of deformation (elongation) of the sample until its break. The force corresponding the limit of yield strength of the sample is determined at the point of intersection of a straight line corresponding to the load acting on the sample at the time of testing, with the tension diagram.
The definition of conditional limit of fluidity is allowed in accordance with GOST 1497-84, engine diagram, subject to periodic check tests of samples using strain gauge, what should be recorded in the normative-technical documentation for manufactured products.
The physical quantity is the limit of fluidity is now used for international use under the name of most grades, since most fully determines its structural characteristics.