Uniaxial tension

We consider an element in a tensile test-piece in uniaxial deformation and follow the process from an initial small change in shape. Up to the maximum load, the deformation is uniform and the element chosen can be large and, in Figure 2.1, we consider the whole gauge section. During deformation, the faces of the element will remain perpendicular to each other as it is, by inspection, a principal element, i.e. there is no shear strain associated with the principal directions, 1, 2 and 3, along the axis, across the width and through the thickness, respectively.