To al you techy geeks out there, With the correct proportions and geometry, pvc can be stiff as this morning's wood or compliant as 9 inches of Mr. Limpy whipped across your face. Steel also has an unlimited fatigue life that is determined by the net shape of the specimen in question and given material stress levels that are fluctuated between.
Fatigue life is the amount of stress cycles a specimen can undergo before breaking.
I hope you are still with me. Aluminum is best used in large diameter, thin walled sections and is best utilized in the main triangle. Aluminum is not used in spokes, it would make sense to because of its high tensile strength to density ratio, but due to the limited fatigue life of Al it will break under fluctuating loads. It is much more likely for an aluminum frame to crack near the welds than a ferrous-based frame, and bike manufactures must take this into consideration. The way bike manufactures get around Al's limited fatigue life is large diameter tubing.