Stress describes the internal forces that materials experience when external loads are applied, distributing throughout a material’s internal structure. It is a fundamental concept in physics and engineering, providing insight into how materials respond to these applied forces. By understanding stress, engineers can predict a material’s behavior under various conditions and ensure the integrity of designs, from bridges to tiny electronic components.
Understanding Stress: Force and Area
Stress is defined as force applied over a material’s cross-sectional area. For instance, pushing a wall with your hand spreads the force, while a sharp pin concentrates it, creating a larger localized effect. The basic formula is Stress = Force / Area. This relationship means that for a constant force, a smaller area will result in higher stress, while a larger area will lead to lower stress.
Key Units of Stress
The primary units for measuring stress fall into two main systems: the International System of Units (SI) and the Imperial/US Customary System. In the SI system, the standard unit of stress is the Pascal (Pa). One Pascal is defined as one Newton of force applied over one square meter of area (1 Pa = 1 N/m²).
Because a single Pascal represents a very small amount of stress, larger multiples are commonly used in practical applications. These include the kilopascal (kPa), equivalent to 1,000 Pascals, the megapascal (MPa), equal to 1,000,000 Pascals, and the gigapascal (GPa), which is 1,000,000,000 Pascals. For instance, material scientists and engineers frequently use megapascals and gigapascals to describe the strength and stiffness of materials.
In countries that use the Imperial or US customary system, such as the United States, stress is often measured in Pounds per Square Inch (PSI). One PSI is defined as one pound-force (lbf) exerted over one square inch of area (1 PSI = 1 lbf/in²). PSI is commonly encountered in everyday contexts, such as measuring tire pressure or the pressure in water pipes.
Stress vs. Pressure
While both stress and pressure are quantified as force per unit area and share the same units, they describe distinct physical phenomena. Pressure typically refers to the force exerted by a fluid, such as a liquid or gas, on the surface of an object. This force acts uniformly in all directions on the submerged surface or perpendicular to a solid surface. Pressure gauges, for example, measure the external force exerted by fluids.
Stress, in contrast, refers to the internal forces that develop within a solid material in response to external loads. These internal forces resist deformation and can be either normal, acting perpendicular to a surface within the material, or shear, acting parallel to a surface within the material. Unlike pressure, which is an external application, stress represents the material’s internal resistance and its capacity to deform.