Fiberglass has internal stresses built up inside of the sheet. When a sheet is cut, it relieves that stress. As a result, the piece that is cut, especially for thinner sheets,
warp and creep may occur in the center area of the sheet.
Fiberglass has a balanced stress state with respect to the shape centerline. When Cutting to size, more material is removed from one side of the centerline than the other,
the material will “dish up” towards the side with more material removed. The amount of residual stress locked into the material determines how much the material will move
once the outer material is removed.
The same condition exists in metals. However, the lower stiffness of fiberglass makes the magnitude of this more significant.