Tensile, Compressive and Shear Stresses

We have observed that forces generate elongations or contractions on solids based on how they act. The forces that generate elongation on solids are called as tensile forces & the stresses induced by them are called as tensile stresses. Tensile forces & tensile stresses are both let positive. Likewise, the forces which cause shortening of length are called as compressive forces and the stresses induced by them are called as compressive stresses. Compressive forces & compressive stresses both are considered negative.

We might also observe that both compressive stresses and tensile stresses act normal to the surface on which they act. For this cause, they are both classified as normal stresses. Though, there are a lot of instances of load applications while the stresses are induced in directions other than that of the normal to the surface. A small element is illustrated enlarged in scale. In addition the element, the coordinate system is also specified. The plane BEFC is normal to the x axis and is, therefore, defined as x plane. Likewise, the planes DCFG & ABCD are described as y and z planes respectively.

You might observe that the stress on x plane is normal (tensile), stress on y plane is inclined and that on z plane is parallel to the plane itself. The stresses which are acting parallel to the plane on which they are applied are called as shear stresses. On further consideration we might resolve the stress on the y plane into components parallel & normal to the surface. Therefore, on any plane, there might be normal stresses, shear stresses or both. Furthermore, while the direction of normal to a plane is uniquely described, there are infinitely large numbers of directions in which shear stresses might be applied on a plane.

Whatever might be the direction of shear stress, it might be further resolved into components in two mutually perpendicular directions, and with done this and you would have totally defined the stresses working on a plane. Defining such states of stress on all the planes shall furnish the complete state of stress on the small element (or at a point).