Stress exceeds ultimate tensile strength

[Shel8]

I'm doing an analysis for the addition of a bracket to a GA aircraft, related to passenger seating (FAR 23.561). In a stress analysis for bending of the bracket, under the ultimate load case the stress is above the ultimate tensile strength (UTS).

Now the bracket is designed such that if it were to bend the contact point of the force would move and the moment arm for the bending moment would reduce. Therefore, theoretically as it bends the stress would decrease to within the UTS and after additional bending stress would fall below yield strength.

It has been suggested to me that it is valid to assume that although the bracket will initially experience a stress higher than the UTS it will then deform leading to the stress falling below the yield stress.

Does this represent a suitable/accurate design justification?

What happens when the load is applied instantaneously and the initial stress in the material is above UTS? Does it fail immediately or does it still bend?

 

[rb1957]

check the wording in FAR23 ... it permits plastic deformation at ultimate load; it prohibits excessive plastic deformation at limit.

you're doing a linear bending stress calc. the extreme fiber can yield without causing the section to fail, read up on "plastic bending" and "cozzone". the margin (or RF) is obviously pretty small and you might want to test to verify your load redistibution.