Elementary Definitions of Stress, Strain and Material Properties

Elementary Definitions of Stress, Strain and Material Properties 

This section will describe the most elementary definitions of stress and strain typically found in undergraduate strength of materials texts. These definitions will serve to describe some basic test methods used to determine elastic material properties. A later section will revisit stress and strain, defining them in a more rigorous manner. 

The engineering (average) stress can be calculated by dividing the applied tensile force, P, (normal to the cross section) by the area of the original cross sectional area Aq as follows, 

The term X in the above equation is called the extension ratio and is sometimes used for large deformations such as those which may occur with low modulus rubbery polymers. 

True stress and strain are calculated using the instantaneous (deformed at a particular load) values of the cross-sectional area. A, and the length of the rectangle, L, 

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