Originally Posted by
cyccommute
All of my experience with broken frames (4) and broken parts have been with aluminum parts tearing more than fracturing. Both of the aluminum frames I've broken complained before breaking and then gently broke. The same has occurred with aluminum rims and crank arms...lots of noise than then they slowly tear apart.
Both steel frames I've broken have gone 'Plink!' and fractured like most people would expect aluminum to do. Pedal spindles have done the same thing as have all the spokes and hub axles I've ever broken. None of them have given any kind of warning like aluminum does. This make sense if you think about the material. Steel is really stiffer and springier than aluminum. It should fracture rather than tear.
Now Geosammy's frame is broken. It probably makes a lot of noise...creaks and groans...while he pedals. At some point, the crack is going to reach a point where the weld will separate but I doubt that it will be a sudden release. The lower tube will probably buckle and bend...although it could be cracked too...but I doubt that it will be connected one minute and completely disconnected the next. If the frame were steel and the weld had failed like it has, I'd suspect that he would have been dragging butt on the rear wheel long ago.
As for the titanium, I'd suspect something else occurred. The frame may have not been welded properly. This could include not properly flooding the area of the weld with inert gas so as to keep from oxidizing the titanium. But that could happen with any weld on any material.
Your explanation of why steel fails more quickly than aluminum is erroneous for two reasons - (1) steel does not fail more quickly - it usually deforms or bends before failure, and (2) the stiffness or 'springiness' of steel has nothing to do with failure.
1. Causes of failures of bike frames
In my experience, there are two main reasons why bike frames fail - either from a crash, accident, cased landing from a jump, etc. that exceeds the strength of the frame, or from a mistake in manufacturing that causes a joint or frame component to be weaker or more brittle. In the first scenario, steel has the advantage of bending considerably before breaking, and many accidents will result in a bent or mangled frame that may still be somewhat rideable - if only to get you home or to safety. Aluminum has far less ductility (the amount it can deform without actually snapping) so if you crash an aluminum frame hard enough to exceed the material's strength it will usually simply snap. Watch people riding bikes though any large city and you will see many who have had crashes on their bikes hard enough to bend the frame, but keep riding them anyways - usually the headtube is bent backwards, the front wheel is a little too close to the pedals, and there are 'wrinkles' on the bottom of the top- and down-tubes. I don't think I have ever seen an aluminum bike with similar damage - when you crash hard enough to bend the frame you almost always break the frame.
2. Material Properties
The other problem with your argument is that, while steel is 'stiffer' (higher modulus of elasticity than aluminum), aluminum frames almost always have much larger tubes and much thicker walls to make the frame acceptably stiff and strong. THe result of the larger tubes with thicker walls is that the actually deform less under load - not more... Aluminum bikes are often 'stiffer' than steel bikes, and they get this way by each of the individual frame components being made stiffer.