1.Introduction This lab report will explore the transformation of strain theory as well as thin walled cylindrical pressure vessel theory, mainly focusing on the calculation aspects of these theories. Using a thin walled cylinder undergoing biaxial stress, the application of yield criteria to predict onset of yield will be demonstrated. 2.Equipment A mild steel hydraulically pressurised cylinder is used as a thin walled cylinder. This apparatus simulates both open and closed end conditions through a thrust knob, situated on the front of the cylinder. Open end condition is achieved when the thrust knob is “wound in”, the external frame reacts to the force placed on the piston and there is no axial load on the cylinder walls. Closed end condition is achieved when the thrust knob is “wound out”, the pressure from the piston on the cylinder develops an axial stress in the cylinder walls.