Stress is the intensity at a point in a body of the internal forces or components of force that act on a given plane through the point. Stress is expressed in force per unit of area (pounds per square inch, kilograms per square millimeter, etc.).

There are three kinds of stress: tensile, compressive, and shearing.

Flexure involves a combination of tensile and compressive stress. Torsion involves shearing stress. It is customary to compute stress on the basis of the original dimensions of the cross section of the body, though “true stress” in tension or compression is sometimes calculated from the area of the time a given stress exists rather than from the original area.

Strain is a measure of the change, due to a force, in the size or shape of a body referred to its original size or shape. Strain is a nondimensional quantity but is frequently expressed in inches per inch, etc.

Under tensile or compressive stress, strain is measured along the dimension under consideration. Shear strain is defined as the tangent of the angular change between two lines originally perpendicular to each other.

Stress-Strain Diagram.
A stress-strain diagram is a diagram plotted with values of stress as ordinates and values of strain as abscissas. Diagrams plotted with values of applied load, moment, or torque as ordinates and with values of deformation, deflection, or angle of twist as abscissas are sometimes referred to as stress-strain diagrams but are more correctly called load-deformation diagrams.

The stress-strain diagram for some materials is affected by the rate of application of the load, by cycles of previous loading, and again by the time during which the load is held constant at specified values; for precise testing, these conditions should be stated definitely in order that the complete significance of any particular diagram may be clearly understood.

Modulus of Elasticity.
The modulus of elasticity is the ratio of stress to corresponding strain below the proportional limit. For many materials, the stress-strain diagram is approximately a straight line below a more or less well-defined stress known as the proportional limit.

Since there are three kinds of stress, there are three moduli of elasticity for a material, that is, the modulus in tension, the modulus in compression, and the modulus in shear.

The value in tension is practically the same, for most ductile metals, as the modulus in compression; the modulus in shear is only about 0.36 to 0.42 of the modulus in tension.

The modulus is expressed in pounds per square inch (or kilograms per square millimeter) and measures the elastic stiffness (the ability to resist elastic deformation under stress) of the material.

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