the neutral axis of the cross-section of a beam is that axis at which the bending stress is Zero.

Whenever there is some increase or decrease in the temperature of a body, it causes the body to expand or contract. A little consideration will show that if the body is allowed to expand or contract freely, with the rise or fall of the temperature, no stresses are induced in the body. But, if the deformation of the body is prevented, some stresses are induced in the body. Such stresses are known as thermal stresses or temperature stresses and the corresponding strains are called thermal strains or temperature strains.

since the body is free to expand no stresses are induced in the body.

A Newtonian fluid is one where there is a linear relationship between stress and strain-rate. The ratio of stress to strain-rate is the Viscosity of the fluid. A fluid where there is no linear relationship between stress and strain-rate is called a Non-Newtonian fluid.