No.QuarterThread size and stress concentration should be inverse relationships right? So an insert would result in a lower stress concentration than a normal screw.
Ya, you are totally right but in my experience that is not what happens in skis.
We have seen many broken skis at the location of the inserts. This even happens on skis that don't break otherwise. My belief is that it has more to do with removing a large amount of the composites on the top of the beam which are handling large compressive loads in bending. The neutral axis in skis is relatively low because the steel is soo stiff. a Low neutral axis means the top composite layers have to carry a lot of compressive stress. The different in stiffness between the insert, fiberglass layers and core induce additional stresses in that location where the amount of reinforcement is lower. Ultimately the failure we see it the top layers buckling away from the core at the location of the inserts.
That make sense?