- The stress that will cause a specified small, permanent extension of a tensile test piece. Commonly the stress to produce 0.2% extension is quoted in N/mm2 for steel. This value approximates to the yield stress in materials not exhibiting a definite yield point.
- The load per unit area that a structure can withstand without being permanently deformed by more than a specified amount. Stress that will cause a specified permanent deformation.
- Proof stress can be found by referring to the stress/strain curve at the point where strain is = 0.2% original volume (the material has grown 0.2% in volume) proof stress will be given as a measurement of energy (MPa,KPa etc.) as it specifically refers to the amount of energy required to stress the material to 0.2% its original volume.
- Proof stress, by definition, is the stress to cause some level of permanent deformation.
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.