what is difference between pressure and stress.?

PRESSURE :

Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.

Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.

STRESS :

stress is a measure of the internal distribution of force per unit area within a body that balances and reacts to the loads applied to it. Stress is a tensor quantity with nine terms, but can be described fully by six terms due to symmetry. Simplifying assumptions are often used to represent stress as a vector for engineering calculations.

Since stresses are defined in relation to the plane that passes through the point under consideration, and the number of such planes is infinite, there appear an infinite set of stresses at a point P. Fortunately, it can be proven by equilibrium that the stresses on any plane can be computed from the stresses on three orthogonal planes passing through the point. The three planes are normally chosen to be the x-, y-, and z-planes. As each plane has three stresses, the stress tensor has nine stress components, which completely describe the state of stress at a point. By using Mohr's circle method or stress tensor transformation, the stresses on an arbitrary plane through P can be computed from the stress tensor at P.

Stress can occur in liquids, gases, and solids. Liquids and gases support normal stress (pressure), but flow under shear stress (see viscosity). Solids support both shear and normal stress, with brittle materials failing under normal stress, and plastic or ductile materials failing under shear stress.

Hope that now you got the clear understanding about the difference between Pressure and Stress.

Pressure and stress mean the same thing. When you deal with fluids mechanics specially air, then you study the analytic forces that results from the atmospheric pressure in three dimensions. These forces are called tensors. Tensors are like vectors except it has 9 different derivatives of forces: 3 in the x-dimension, 3 in the y and 3 in the z. When you study these forces, then you start using the terms stress and strain instead of pressure. The units of pressure, stress and strain are the same, namely, PSI in the English system of measurement.

pressure is when you are made/or feel you have to do something whether you want to or not, or if you have to do any number of things in a timed amount of time. Stress is prolonged pressure once you have had pressure for a couple of weeks or days (depending on the person)

Both stress and pressure is force per unit area.

In the case of fluids (gas and liquid) when a pressure is increased at a point, the pressure is equally distributed through out its volume.

In the case of solids we don’t call the ‘force per unit area’ as pressure instead we call it as stress; because the increase in stress is not distributed through out its volume; the stress is increased only in the direction of increase of stress.

Thus to distinguish them, for solids we call the force per unit area as stress; for liquids and gases we call the force ( thrust) per unit area as pressure.

stress is the reaction to pressure

and to compliment that, pressure is the force per unit area.