Bearing Strength of Composites

I have a question concerning the bearing strength in FRP. The ASTM D953 standard defines the bearing strength taken as the force at 4% hole deformation divided by the bearing area (hole diameter x thickness).

What is the reason for choosing 4% hole deformation as the strength limit instead of the ultimate load?

For example: I’ve seen some design guidelines which state disregard this provision completely and to simply take the ultimate failure load as the bearing strength.



ASTM D5961 considers both the bearing offset strength and the ultimate strength.

From ASTM D5961, “Discussion—Two types of bearing strengths are commonly identified, and noted by an additional superscript: offset strength and ultimate strength.”

Also, from “Practical Analysis of Aircraft Composites”:
“The bearing strength design value may also be limited by program requirements, fatigue requirements, or other limitations. For some structures,such as those that require fuel seals, excessive hole elongation may be unacceptable.”

Above is a snippet.