In case of a simply supported rectangular beam of span L and loaded with a central load W, the length of elasto-plastic zone of the plastic hinge, is A. $$\frac{{\text{L}}}{2}$$ B. $$\frac{{\text{L}}}{3}$$ C. $$\frac{{\text{L}}}{4}$$ D. $$\frac{{\text{L}}}{5}$$

The correct answer is $\boxed{\frac{{\text{L}}}{2}}$.

A plastic hinge is a region in a structure where the material has yielded and is undergoing plastic deformation. In a simply supported rectangular beam of span $L$ and loaded with a central load $W$, the plastic hinge will form at the point of maximum bending moment, which is at the center of the beam. The length of the plastic hinge is equal to the distance from the center of the beam to the point where the bending moment becomes zero. This distance is equal to $\frac{{\text{L}}}{2}$.

Option A is incorrect because it is the length of the beam. Option B is incorrect because it is the length of the beam divided by three. Option C is incorrect because it is the length of the beam divided by four. Option D is incorrect because it is the length of the beam divided by five.