Comments on Previous Chapters
The new Figures 6.1-1 and 6.1-2 serve to further clarify the concepts of pressure and stress without resorting to mathematical descriptions. On each figure is shown at the centre bottom a small plate (sensor plate) mounted on a gimbal system. This little plate is capable of recording normal pressure acting across the plate (σ) as well as shear stresses acting parallel to the plate (τ). In the case of Figure 6.1-1 the little plate is embedded in a fluid and while it assumes all possible orientations, by activating the gimbal system, the output is measured. One finds that the normal stress is constant and the shear stress zero regardless of the orientation. This physical quantity is referred to as PRESSURE. It is a scalar, fully described by a single number.
In Figure 6.1-2 the plate is embedded in a solid. The same experiment is repeated (mentally only !) and one finds that the normal stress varies from a minimum to a maximum value depending on orientation. The shear stress also varies and vanishes for three particular directions, two of which coincide with the minimum and maximum values for the normal stress. Quite clearly the situation in a solid is much more complex. The condition is one of STRESS, mathematically speaking a tensor and physically described by a three-axial ellipsoid as shown in Figure 1.2-1. In the lectures these two new illustrations have been found helpful for clarifying the concepts of pressure and stress.