The non-linear material stress-strain curve of stainless steel (refer to Comparison of structural design in stainless steel and carbon steel) implies that the stiffness of a stainless steel component varies with the stress level, the stiffness decreasing as the stress increases. Consequently, deflections are greater than what you would expect with carbon steel. It is therefore necessary to use a reduced modulus to predict the deflection of stainless steel members in which high stresses occur. Using standard structural theory, but with the secant modulus corresponding to the highest level of stress in the member, is a conservative method of estimating deflections in stainless steel members.

Secant modulus
The secant modulus, Es, to be used in deflection calculations should be ascertained for the member with respect to the rolling direction. If the orientation is not known, or cannot be ensured, then the lesser value of Es should be assumed. The value of the secant modulus may be obtained as follows: