Quote:
Originally Posted by Mark McM
Except that the brittleness and flexibility are not directly related. You can harden steel and make it brittle, or anneal steel and make it soft, but it doesn't change its modulus (flexibility).
|
Correct. The measure of ductility/brittleness is elongation. Most steel bicycle frame tubes typically have post treatment elongation of 10-15% (in comparison, carbon fiber composite elongation is typically much lower - more brittle - at around 3-5%). Brittle materials tend to break rather than yield when they fail. The modulus of elasticity (also known as Young's modulus) which is a measure of stiffness remains constant at ~200 GPa for all steels.
Many who should know better equate yield strength with stiffness. The bottom line is that for a given length, diameter, wall thickness and butting profile, a tube made of 1020 gaspipe with a yield strength of 205 MPa will have the same "stiffness" as a Reynolds 953 tube with a yield strength of 1450 MPa.
Tubes made from stronger steel alloys can be drawn with thinner walls than weaker steels to make them lighter, but if they have the same diameter the stronger tube will be flexier because of the thinner walls. There are basically two ways to make a steel tube stiffer: 1) increase the wall thickness, 2) increase the tube diameter, or both. Jan Heine does a passable job of explaining the difference between stiffness and yield strength in
THIS BLOG POST.