Is stress scalar or vector quantity?

Stress is a tensor quantity. It is not simply a scalar or vector quantity. This means that it has both magnitude and direction, and it also has a shearing component.

To understand why stress is a tensor quantity, consider the following. When you apply a force to an object, the object experiences a deformation. The amount of deformation depends on the magnitude of the force, the direction of the force, and the material properties of the object. Stress is a measure of the internal forces that resist this deformation. These internal forces can be either compressive or tensile, so the stress can be either positive or negative.

Stress is a tensor quantity because it has nine components: three normal components and six shear components. The normal components are the stresses that act perpendicular to the surface of an object, and the shear components are the stresses that act parallel to the surface of an object.

In summary, stress is a tensor quantity because it has both magnitude and direction, and it also has a shearing component. This means that it is more complex than a scalar or vector quantity, and it requires a more sophisticated mathematical framework to describe it.