3.7 The diagram on the next page shows a pressure vessel subjected to internal pressure p. The circumferential stress (hoop stress) is given by σ = pr/t and the CHAPTER 3 The Elastic Moduli longitudinal (axial) stress is given by σ = pr/2t. Write out the principal stress tensor for this stress state. Explain why the 1, 2 and 3 axes are the principal directions. 2 3