Nature of Bending Stress
If the beam is not loaded, it will not bend as shown in the figure-a. But if the beam is loaded, it will bend as shown in the figure-b, whatever may be nature of load and number of loads. Because of bending of the beam, the upper layers of it are compressed and the lower layers are stretched. So, longitudinal compressive stresses are induced in upper layers and longitudinal tensile stresses are induced in lower layers. These stresses are the bending stress. In the case of cantilevers, reverse will happen, that is, tensile stresses will be induced in upper layers and compressive stresses will be induced in lower layers.