When referring to stress as used in applied mechanics, the value is usually expressed in:

Stress in applied mechanics is defined as the force applied per unit area within materials and is a measure of the internal forces that develop in response to external loads. The standard unit of measurement for stress is the pascal (Pa), which is equivalent to one newton per square meter. Therefore, when expressed in kilopascals (kPa), the value is one thousand pascals.

The correct answer highlights that stress is most commonly expressed in kilopascals because it provides a convenient scale for engineering applications, where stresses often exceed the simpler pascal unit due to the magnitude of forces typically handled in engineering contexts.

The other options do not appropriately represent the units of stress. Kilowatts pertains to power, joules refer to energy, and square meters are a unit of area. These units are essential in their respective contexts but do not directly relate to the measure of stress in mechanics. Understanding stress in the context of applied forces on materials is crucial for engineers when designing and analyzing structural components.