Stress cracks in doors

[boghog]

Before I go and start working on my doors, I would like to know what you guys have done to repair and improve the stress crack that forms down inside the the top of the door, allowing the inner and outer to spread apart. The original u shaped piece seems to be too thin,or allowed to flex to much. Has anybody come up with a good way to reinforce this area so the crack will not reform or cause the next weakest link to show its self?
Its been awhile sine I looked at the doors but if my memory serves me well still, I wonder if a epoxy mix stuffed into either side of this u shaped piece or bridge would help? I know it might be tough to do, and it most likely would have to be kept with in the width of the metal piece but it sounds like a possibility?
Any other ideas?