Some material property discussions that may or may not be of interest.
Mild Steel (major alloying element carbon) example: AISI 1018
density: 0.284 lb/in³ elastic modulus: 30,000,000 psi tensile strength: varies with heat treatment yield strength: varies with heat treatment
Chromoly Steel (major alloying elements chromium and molybdenum) example: AISI 4130
density: 0.284 lb/in³ elastic modulus: 30,000,000 psi tensile strength: varies with heat treatment yield strength: varies with heat treatment
Both steel alloys are more than 98% iron, so the density is virtually the same. A similar equality applies to something called the elastic modulus of these materials, which is the ratio of stress to strain.
Visualize a piece of sample (mild steel or chromoly steel) material 1.00" wide, 1.00" tall, and 100" long. If you pull on this piece longitudinally with 3,000 pounds, it will elongate (stretch). For these materials, it would elongate 0.010" so the new length (under load) would be 100.010".
Stress is force divided by area. Strain is the change in length divided by the original length. Therefore, in our example, the stress is 3,000 pounds divided by 1.0 square inch or 3,000 psi, and the strain is .010" divided by 100.000" or .0001. Since the elastic modulus is stress divided by strain, the elastic modulus is 3,000 psi/.0001, which is 30,000,000 psi.
Since these materials are linearly elastic, we can perform the experiment with 300 pounds or 30,000 pounds, but each time we would compute the same elastic modulus of 30,000,000 psi, assuming we stay below the yield strength. More on that next.
OK, now about strength. Two parameters are usually used to describe the strength of a material: tensile strength and yield strength. Tensile strength is the maximum stress the material can handle, and leads to failure. Yield strength is the maximum stress the material can handle and still return to it's initial shape when the load is removed. If you stress a material beyond it's yield strength, it will bend, and stay bent.
Values of tensile strength and yield strength for these two materials varies wildly, depending on the heat treatment used. Also, things like welding and forming can change these values. Some make the material weaker, some actually make it stronger.
I would be interested to hear what heat treatment is used on bicycle frame materials, both mild steel and chromoly steel.
