Stress

Tensors are field variables which characterise the internal state of a continuum and are, perhaps, the most difficult quantities to intuitively understand. Indeed, at least three of them have to be used in the following and these are the stress, strain and strain rate tensors.

Stress is the internal distribution and intensity of force acting at any point within a continuum in response to various internal and external loads applied to the continuum. Stress is defined as a force per unit area and we can easily ‘apprehend’ its effect by pressing two fingers against each other – equal force is applied from both sides and therefore nothing moves, but we have a feeling of pressure between the fingers, which is a sign of the presence of stress. This stress is directly proportional to the applied force – the stronger we press the stronger the feeling is. On the other hand, the stress is inversely proportional to the contact surface between the fingers – if we press one finger with the nail of the other the feeling is much stronger because the same force is applied to a much smaller area.