E-Modulus of FRP Tube

What was the equation you used to calculate the stress?

M/I = ∑/ Y

Ok, what equation did you use to calculate 'I'?

I = (∏ / 4)*(R⁴ – Ri⁴)

Did you make a typo? Or is that the source of your problem?

I = (pi/64)(D^4-d^4)

.... also, how did you measure the strain? You aren't intermixing displacement with strain are you?

it is one of the same

We have measured the strain by strain gauge and deflection by LVDT.

There is no difference between your formula and that used by Mr. Bhagwat. Here is the reconciliation: I = (pi/64)[(2R)^4 -(2Ri)^4)] = (pi/64)(2X2X2X2R^4-2X2X2X2Ri^4) =(pi/64)(16R^4-16Ri^4) =(pi/4)(R^4-Ri^4) So that is not the problem

The bending formula is only a derived equation of a homogenous material fitted to the initial portion of the load versus deflection curve. All bets are off with a composite material where local plastic deformations at the load points might be a factor or if its modulus is different in compression than tension. The bending test should give you a good estimate of the flexural strength when the first surface layers begin to yield (or fracture depending on the composite's characteristics).