Your right that the stresses are different, but you are missing my point. The spine of an arrow is a function of a number of things, but mostly derives from slight variations in wall thickness caused by uneven distribution of the graphite/epoxy matrix. This does not and can not change. As to arrows getting different stess then fishing rods- you are certainly right. Fishing rods flex across a much wider arc- often greater than 90 degrees- which one would expect would be very hard on the spine. But it isn' t- rods get flexed more than 90 degrees on testing machines for up to a million repetitions with no loss of structural integrity. It' s not the flexing thats the problem

The wear you are measuring is no surprise, and is at the heart of why graphite arrows must be replaced after a period of time. You have the insert being forced into the arrow every time it hits a target, which can cause the shaft to expand slightly or even break at the tip. You have a little of the graphite/epoxy matrix lost in the target every time it impacts too, making it smaller. Over time, this forces you to replace your arrows.

Graphite arrows will wear out, there is no question about that- but it isn' t " spine degradation" that' s the problem- it' s wear and tear on the tip of the arrow.