Most Ecologically Benign Common Bike Frame Material?
did you ever wear tank tops and flares? 
At university we had some springs made out of cement. Micro defect free cement, I think it was called.
Also dropping light bulbs and watching them bounce off a steel bench. On the second colision with the bench they exploded because of the scratches from the first impact.
Perfect glass is tough.
Any mild steel is tough no matter how old, imperfect etc because it has natural mechanisms within itself to negate stress concentrations.
Aluminium fatigues because it doesnt have these mechanisms.
All of the above are gross oversimplifications but broadly true.
Also dropping light bulbs and watching them bounce off a steel bench. On the second colision with the bench they exploded because of the scratches from the first impact.
Perfect glass is tough.
Any mild steel is tough no matter how old, imperfect etc because it has natural mechanisms within itself to negate stress concentrations.
Aluminium fatigues because it doesnt have these mechanisms.
All of the above are gross oversimplifications but broadly true.
Yma o Hyd
Has to be steel I'm afraid because of its durability and can be melted and re used over and over again. There are 100 year old steel bikes still in use. Wood and plastic and all the other things mentioned still have a long way to go to make a viable cycling product and therefore aren't ecologically very benign.
