A typical value ranges between 0.30 - 0.35 for connector spring alloys. The ratio is slightly sensitive to processing since high levels of cold work provide preferred crystallographic texturing. It follows that there is a line or region of zero stress between the two surfaces, called the neutral axis. The material of the beam is linearly elastic, homogeneous and isotropic. The proportional limit is not exceeded.
All deflections are small, so that planar cross-sections remain planar before and after bending.
The reported flexural modulus is usually the initial modulus from the stress-strain curve in tension. When available, take the value from a 4-point bend test. One of the methods to reduce maximum stress is to keep the strain energy in the beam constant while changing the beam profile. Additional beam profiles are trapezoidal, tapered and torsion. The Von Mises yield criterion states that yielding occurs when the Von Mises stress, exceeds the yield strength in tension.