I have conducted a Load vs deflection test and the graph shows that there is an initial decrease (concave curve) and then an increase in the stiffness of the beam. Can anybody explain this phenomenon and factors causing it.
If it is initially decreasing, the reason might be grip slippage or some other issue during testing. If it was otherwise (increase first, then decrease, then increase) we could explain it from mechanics of materials.
Others may weigh in. It would be best if you could include the graph. It will be more accurate to answer after observing the graph.