WAIT!! What? Yes, mild steel has a yield point, then drops a bit, then another before it finally breaks. Remember this for later. Very important.
Fracture Toughness: (work of fracture): (http://en.wikipedia.org/wiki/Fracture_toughness)
"In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. Fracture toughness is a quantitative way of expressing a materials resistance to brittle fracture when a crack is present. If a material has a large value of fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with a low fracture toughness value."
Toughness: (http://en.wikipedia.org/wiki/Toughness)
"In materials science and metallurgy, toughness is the resistance to fracture of a material when stressed. It is defined as the amount of energy that a material can absorb before rupturing, and can be found by taking the area (i.e., by taking the integral) underneath the stress-strain curve. The ability to with stand sudden loading. Toughness is measured in units of joules per cubic meter (J/m?) In the SI system and pound-force per square inch (sometimes expressed as in-lbf/in?), in US customary units."
So Fracture Toughness is a big one? You bet.
"Thus the energy which is needed to cause fracture in wrought iron or mild steel may be about a million times as high as that needed to break the equivalent cross-section of glass or pottery, although the static tensile strengths of these materials are not very different." (J.E. Gordon, Structures, or why things don't fall down. ref. page 95). This is an important extended example which should clarify why one would not build a race car out of glass tubing, even if strong enough in tensile strength.
The work of fracture of mild steel can be 10 to 100 times that of high tensile steel, even though the tensile yield is much less. Anyone who has snapped a knife blade in half or broken a piece of Rc 60 tool steel knows how easy it is compared to a piece of 4130N. The 4130N is just real tough.
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Charpy Impact Test: (Wiki again; http://en.wikipedia.org/wiki/Charpy_impact_test)
"The Charpy impact test is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given materials toughness and acts as a tool to study brittle-ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. But a major disadvantage is that all results are only comparative. The qualitative results of the fracture may be used to determine the toughness of the material. Also, this test may be done with the material at various temperatures to determine the brittle-ductile transition temperature."
The Charpy Impact Test is very similar to failure in a crash situation. Search the web for this test for a better understanding and videos.
Heat treatment: (http://en.wikipedia.org/wiki/Heat_treatment)
"Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure. In carbon and low alloy steels, fast rates of cooling result in a high degree of hardness."
Normalization: (http://en.wikipedia.org/wiki/Normalization_%28metallurgy%29)
"Normalization is an annealing process in which a metal is cooled in air-cool to room temperature after heating. This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum softness achieved by annealing."
Normalizing is a heat treatment to complete the transformation to austenite, followed by cooling in air. This refines the grain structure resulting in an increase of strength of about 20%. Most structural steels are supplied in the normalized condition.
Full anneal: (http://en.wikipedia.org/wiki/Normalization_%28metallurgy%29)
"A full anneal typically results in the softest state a metal can assume. To perform a full anneal, a metal is heated to its annealing point, and the furnace is turned off. The metal is allowed to cool in the furnace, causing grain growth and resulting in a ductile metal with a lowered yield point."
Carbon Steel: (http://en.wikipedia.org/wiki/Plain-carbon_steel)
Some Important Material Properties of 4130N:
Tensile Strength, Ultimate: 97,200 psi
Tensile Strength, Yield: 63,100 psi
Elongation at break: 25.5%
Wow, that is quite a lot of elongation isn't it? It takes a lot of energy to stretch 4130N. It work hardens and increases strength when stretched into the plastic region also.
The alloy 4130 has .28 to .33 carbon and when normalized, it is slightly harder than in it's annealed state.
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=M4130R