There is an important quantity, called the strain rate or shear rate (given as a symbol the Greek letter gamma, γ, with a dot on top) that describes how the speed of the liquid moving along the x-axis (left to right) changes, layer by layer, in the y-direction. What does this picture have to do with viscosity?

Maximum Shear Stress Theory – This theory postulates that failure will occur if the magnitude of the maximum shear stress in the part exceeds the shear strength of the material determined from uniaxial testing.

The strain accompanying the shear stress \(\tau_{xy}\) is a shear strain denoted \(\gamma_{xy}\). This quantity is a deformation per unit length just as was the normal strain \(\epsilon\), but now the displacement is transverse to the length over which it is distributed (see Figure 5).

The shear stress is the stress acting tangent to the cross section, and it takes on an average value of: It's important to note that the stresses we have just described are average stresses . We have assumed that all of the external force has been evenly distributed over the cross sectional area of the structure – this is not always the case.