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Material considerations for welded steel tubular structures It's 4130N welded with an ER70S-2 rod in case you wish to read no further. But in case one needs to know why I will attempt, in my usual way, to "Drain Loch Ness". Welded steel tubular structures are typically used in many applications such as aircraft, but, per the request of some friends, their use in Top Fuel Dragsters and Funny Cars are what we are to target in this paper. The recent change in material from 4130N, to 4130 with some level of additional heat treating to increase it's yield, is undoubtedly in the wrong direction. Relax, and let us see why. Historically, welded steel tubular structures for aircraft and race cars have used 4130 in the normalized condition for their structure. This is simply because it is the best material for that application, all things considered. Let us first brush up on some definitions so we are all on the same page and then take a look at the properties of this material. Tensile Strength: (from Wikipedia; http://en.wikipedia.org/wiki/Tensile_strength "The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. This is an important concept in engineering, especially in the fields of material science, mechanical engineering and structural engineering." This is an important property and typically the one that gets the most attention but it sometimes is confusing. The general feeling is, "the stronger the better" but this is very misleading. Click on the above Wikipedia link and read it. So now we understand that there is tensile strength and there is tensile strength. Good. Yield Strength: (from Wikipedia; http://en.wikipedia.org/wiki/Yield_(engineering) "The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to plastically deform. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible." Go ahead and click the link and read this too. Stress: "Stress is a measure of force per unit area within a body. It is a body's internal distribution of force per area that reacts to external applied loads. Stress is often broken down into its shear and normal components as these have unique physical significance. In short, stress is to force as strain is to elongation." Right from Wiki and spot on. Strain: "Strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape." Proportionality Limit: The point at which the stress–strain curve deviates from Hooke's law, i.e., becomes nonlinear. The top of the material's straight line portion of the Stress-Strain diagram. Elastic Limit: The lowest stress at which permanent deformation can be measured. For most materials, this is very close to the Proportionality Limit. Offset Yield Point: This is the most widely used strength measure of metals, and is found from the stress-strain curve as shown in the figure to the right. A plastic strain of 0.2% is usually used to define the offset yield stress, although other values may be used depending on the material and the application. Although somewhat arbitrary, this method does allow for a consistent comparison of materials. The reason one can't use the Elastic limit is because it is hard to measure. But .2% can be measured. Upper yield point and lower yield point: Some metals, such as mild steel, reaches an upper yield point before it drops rapidly to a lower yield point. The material response is linear up until the upper yield point, but the lower yield point is used in structural engineering as a conservative value. 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. 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." NOTE: !!!! "The purpose of heat treating plain-carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, and impact resistance. Note that the electrical and thermal conductivity are slightly altered. And, as with most strengthening techniques for steel, the FLEXIBILITY, (Young's modulus of elasticity), is never affected by heat treating." NEVER AFFECTED !!!! (ref. http://en.wikipedia.org/wiki/Plain-carbon_steel) Normalization: http://en.wikipedia.org/wiki/Normalization_(metallurgy) "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_(metallurgy) "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#Mild_and_low_carbon_steel This Wiki artlicle on Carbon Steel is a good read for a basic education on 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 By just going through the definitions it appears that 4130 in it's normalized state is an ideal material for these structures. But as stated earlier, stronger is often considered better. But in reading all of the definitions, it may appear that toughness is just as important. Well, actually it is more important in this application as we shall see. But first, let's see where one would use a high tensile strength material. Piano wire, spring steel, drawn steel wire for suspension bridges, submarine hulls, etc are all applications that benefit from higher tensile strength at the expense of toughness. But in structures that have to be damage tolerant, toughness, with an adequate yield strength, are desirable properties. The energy needed to fracture a mild steel part is very large compared to that of high carbon heat treated part. Look at the area under the stress-strain curve in the following diagram. Even though the tensile yield is higher, the Fracture Toughness is lower. Again notice the fact that heat treatment does not affect the modulus, (flexibility), of the material. Heat treat that part all you want, the flexibility remains the same. It is just the yield point that changes. |
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There we have it. If you want a structure that can take the beating and not fall apart, it's mild steel and not high tensile steel. But we must ask why 4130N. Well, 4130N has properties that maximize both strength AND toughness. This relationship is well proportioned for use in vibrating, crash tolerant structures. It also has the added benefit of slow crack propagation which is important in aircraft welded steel tube structures. Other very important benefits are price, weldability, ease of forming, etc. But it is possible that a material with a lower yield and higher toughness than 4130N be better suited for this application? Possibly yes. But with 4130N being a very common controlled material because of its applications in aerospace, it may just be the ideal material. Although generally not done or required, slight pre-heating of 4130N might be prudent, especially if welding in a cold environment. For some very interesting information on welding 4130, and why mild steel filler material is used such as ER70S-2, visit these interesting web sites. http://www.netwelding.com/Welding%204130.htm http://www.lincolnelectric.com/en-us/support/welding-how-to/pages/chrome-moly-detail.aspx Here is an interesting web site on the use of Mild Steel for chassis material. It has a lower yield but higher toughness. This single article presents very compelling reasons to not use high yield / low toughness material in the chassis. http://www.netwelding.com/Welding_Race_Cars.htm While on the subject, we should briefly touch on design. Many have commented on the past and recent catastrophic failures of Top Fuel Dragsters as if the "tremendous loads" were to blame. Also mentioned was that the chassis is "designed to break apart in a crash". This of course is all non-sense. The loads on a Top Fuel Dragster chassis, while higher than an average race car, are by no means tremendous. And in a crash, containment is always preferred to non-containment. The reasons for many of the failures are due to poor, or lack of, proper structural design. Steel structures, if designed so the maximum stress is less than .5 of the tensile yield of the material used, should not only take the load easily, but have an infinite fatigue life. But another issue which is apparent on Top Fuel Dragsters is the long unsupported lengths of tubing on the lower compression rail. This is a buckling situation in the waiting. Any damage, bending, or misalignment due to clamping on brackets, will greatly increase the likelyhood of buckling. The reason the sport has been getting away with a poorly designed chassis, is due to the outstanding properties of 4130N. Deviations from this ideal material will allow this poor design to show in spectacular ways. By searching around and finding the correct information and presenting it in proper form, the reason for using 4130N becomes clear. Joe LaCour, 9-25-2007 & added to as needed. 8-13-2008. 360-406-4019 West Coast Printed References: Wikipedia Lincoln technical references John Schwaner, Sky Ranch Engineering Manual J.E. Gordon, Structures, or why things don't fall down Marks' Standard Handbook for Mechanical Engineers
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