How are stress and strain defined

Stress is defined with respect to the force applied to an object and is measured as the force applied to a unit area of the specimen. The application of a stress results in a dimensional change, which is called the strain. 

For practical purposes, it is often adequate to ignore the continuous change in cross-sectional area that occurs when a force is applied. The stress so defined is called the engineering (or nominal) stress. 

The strain that is found on application of a load to a rod is equal to the elongation of the rod. The increment in tensile strain experienced, A e, when a rod is extended, is defined as the ratio of the increase in length, A I, to the total length  

Many materials are used under compression rather than tension. At low loads, the compressed material behaves in a similar way to materials tested under tension. In compression tests, the value of the force is taken as negative and hence we have negative values of stress and strain compared with those obtained in tension. 

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