Material Considerations for Welded
Steel Tubular Structures
| Ed Note I received this paper by e-mail today and after reading it decided that for most of us this at least gave us a reasonable explanation as to why a lot of people are questioning the requirement of heat treated 4130N tubing in Top Fuel and Funny Car chassis. We would welcome anyone to send a similar document that supporting the heat-treating of the spec tubing to build Top fuel, and Fuel Funny cars so that we may present that information to our readers. Jeff Burk |
It's 4130N welded with an ER70S-2 rod in case you wish to read no further.
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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 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_%28engineering%29)
"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.