The cracking in the chassis rail is due to `work hardening`...this could be caused by vibration (frequency) over a long period of time...then the belt it obviously recieved at some time was enough to start it off......
more likely though to be down to having been worked (forged/rolled) `cold`...that is to say the material wasn`t hot enough when worked....
cracking like that is a typical sign of `cold forging`....iron is particularly susceptible to this....
iron (unlike steel) is fibreous...if you forge it cold it kind of `splits`....and you can see the strands seperate....its almost like looking at heavy cloth...
but anyway, thats what the crack in the frame is all about boys....