These two concepts begin with how materials are destroyed. Any material will eventually exceed a certain limit and be destroyed under the action of increasing or constant or alternating external forces. There are many kinds of external forces that cause damage to materials, such as tension, pressure, shear, torsion, and so on. The two strengths, yield strength and tensile strength, are for tensile forces only.

These two strengths are obtained by tensile testing using a tensile testing machine (generally a universal testing machine, which can perform various tensile and compressive tests as well as bending tests) , the material is maintained in tension at a constant loading rate (that is, the increase in tension per unit time) until it breaks or reaches a specified level of damage (for example, some Butt Weld strength tests can be performed without breaking) , the force that causes the ultimate failure of the material is the ultimate tensile load of the material. The tensile limit load is a force expression in Newton (N) , and since Newton is a very small unit, KN is used more often than not. It is difficult to judge the strength of materials by the ultimate tensile load because of the different sizes of materials. Therefore, the ultimate tensile load per unit area is obtained by dividing the ultimate tensile load by the cross-sectional area of the material. Force per unit area, which is a statement of strength, in the form of a Pascal (PA) , and again, Pascal is a very small unit, usually expressed in MPA.

So, the ratio of the ultimate tensile load to the cross-sectional area of the material is the tensile strength. Tensile strength is the limit of a material per unit area to withstand external forces. Beyond this limit, the material will be dissociatively destroyed.

What is yield strength? Yield strength is only for elastic materials, inelastic materials have no yield strength. For example, all kinds of metal materials, plastic, rubber, and so on, are elastic, have yield strength. And Glass, ceramics, masonry and so on, generally no elasticity, this kind of material even if elastic, also very little, so, there is no yield strength.

An elastic material is subjected to a constant and increasing external force until it breaks. What has changed?

First, the material under the external force, elastic deformation, in accordance with Hug's law. What is elastic deformation? When the force is removed, the material reverts to its original size and shape. When the external force continues to increase to a certain value, the material will enter the plastic deformation period. Material once into plastic deformation, when the external force, the material's original size and shape can not be restored! And the strength of the critical point that causes the two deformations is the yield strength of the material! The value of this critical point in terms of the applied tension is called the yield point. From a crystalline point of view, only when the tensile force exceeds the yield point, the crystalline bonding of the material begins to break down! The failure of the material begins at the point of yield, not at the moment of fracture!

To figure out where these two strengths come from, so it's true that a material with high yield strength can withstand a lot of damage.

But I would like to say that no matter which strength, only one case, can not say whether this material is safe or strong or not!

Let's talk about steel here and nothing else. There is another parameter about yield strength and tensile strength, which may not be known much, and which may not be known much. This parameter is the yield-to-strength ratio! The yield-strength ratio is the ratio of yield strength to tensile strength. The Range is between 0 and 1. Yield ratio is one of the indexes to measure the Brittleness of steel. The bigger the yield-strength ratio, the smaller the difference between yield strength and tensile strength, and the worse the plasticity and Brittleness of the steel!

Why do you say that? We need to introduce a new indicator here -- elongation. In Layman's terms, it's how much longer the steel is when it's pulled apart. This is an important index to check the plasticity of steel. The greater the value, the better the ductility of the steel. As I said above, when the steel is stretched beyond the yield point, it is impossible for the steel to return to its original size until it breaks. The larger the yield-to-strength ratio, the smaller the difference between yield strength and tensile strength, the shorter the stretching time and the lower the elongation of the steel with the constant loading rate.

According to the conservation of energy, energy can only be transferred or transferred. When steel is stretched, it comes down to energy transfer and absorption. Before the yield point, the steel is in the elastic deformation period, the external tension is almost completely offset by the elastic force (converted into the elastic potential energy) , the external energy is not much absorbed or converted, only a small amount into heat energy. After the yield point, the external force is partly offset by the elastic force (converted into the elastic potential energy) , and partly converted into heat energy!

As I mentioned above, material damage begins at the yield point. The lower the yield-to-strength ratio, the longer the time from the beginning of failure to fracture, the higher the yield-to-strength ratio, the shorter the time from the beginning of failure to fracture. Energy is largely converted into heat energy between the yield point and the fracture point.

Therefore, simply saying that the yield strength is high or tensile strength is high, then the material must be good or safer. NOT NECESSARILY! Only the yield strength is high, at the same time yield strength than the low steel, just a little safer! Unfortunately, such steel is too expensive to be used in civilian vehicles.

Now Steel in addition to strength, there is an important index is toughness! So far, I have not seen that a car enterprise on the use of steel toughness how to do a description! It's all about the strength of the steel! On the contrary, in most cases, increasing the strength of steel tends to reduce the toughness of steel! To reduce toughness is to increase Brittleness! The toughness of steel is an important index related to the safety of steel

There is one indicator that may be intentionally or unintentionally forgotten by vehicle manufacturers-impact toughness or impact work. With the same force, push you or Meng hit you, which hurts you more? The answer is obvious! The impact resistance of steel is an important factor in relation to safety! Never seen an accident where you slowly add power until you break it, right? It's all in an instant! If you can't handle the force in an instant, what good is your tensile strength?

From now until the steel, any strength greater than 1000 MPA, mostly tensile strength, yield strength more than 800 MPA is not a difficult thing, such as 40Cr, a common "universal steel" (basically anything) , general modulation process yield strength can also be close to 800 MPA, tensile strength above 900 MPA.

But all three, high yield, high extension, a good impact resistance is more difficult! Almost all steel has the same problem, that is, while improving the strength of steel, reduce the IMPACT RESISTANCE OF STEEL! For example, 10.9 grade high-strength Bolt, tensile strength in 1040-1240MPa for qualified, yield strength more than 940MPA, elongation more than 10% , impact toughness 59J/cm2; and the same material 8.8 grade high-strength Bolt (the lower level) , tensile strength in 830-1030MPa for qualified, the yield strength is more than 660 MPA, the elongation is more than 12% , and the impact toughness is 78 j/CM2. Therefore, for most metal materials, while improving the performance of some technical indicators, it is at the cost of reducing some technical indicators to achieve. You can't do both. The Iron and steel industry is one of the most mature industrial technology, there is no too many secrets. Each technical index of iron and steel material is not the higher the better, or the lower the better, but according to the need, adjust each index to a can take into account of the scope. For people in our industry, steel in addition to structural problems (refers to the product defects) , there is no distinction between the technical indicators, depending on where you use. There's only the wrong thing, not the wrong thing. 

Source: Fast Information Quality Inspection Alliance