Any material that is subjected to an increasing or constant or continuous change of external force will eventually be destroyed beyond a certain limit. There are many kinds of external forces, such as tensile, pressure, shearing force, torsion and so on. Yield strength and tensile strength of these two strengths, just for the tension. These two strengths are obtained through tensile tests, through tensile testing machines (generally universal testing machines, with various tensile and compressive and bending tests), with a constant loading rate (that is, the increase in the amount of tension within the unit time), continuous stretching of the material until breaking or reaching the specified degree of damage (for example, some butt weld strength tests can not be broken), the ultimate damage to the material, which is the tensile strength of the material. The tensile limit load is a force expression, the Unit is Newton (n), because Newton is a very small unit, so, in most cases, more than thousand cows (kn). The tensile limit load is difficult to judge the strength of the material because of its different sizes. Therefore, the tensile limit load of the unit area is obtained by dividing the cutting area of the experimental material. The force of the unit area, which is the expression of a strength, the unit is Pascal (PA), similarly, Pascal is a very small unit, generally using MPA to describe.