Pipes, pressure vessels, steam boilers etc. are considered as thin-walled vessels during design. The principal stresses are key variables in calculating and designing these vessels. The stresses and strains occurring in a vessel are not directly measured but are
determined by measuring the strains on the surface using strain gauges.

The FL 130 experimental unit is used to investigate stresses and strains in a thin-walled vessel subjected to internal pressure. The oil-filled vessel is closed with a lid at one end and with a movable piston at the other end. A hand wheel with a threaded spindle is used to move the piston. Two load cases are presented: biaxial stress state of a closed vessel such as a boiler tank and uniaxial stress state of an open vessel such as a pipe. Internal pressure is generated inside the vessel by a hydraulic pump. A manometer indicates the internal pressure. Strain gauges are attached to the surface of the vessel to record the strains. The FL 152 measuring amplifier displays the measured values. The measured values can be imported into the application software for visualisation to assist in the evaluation of the experiment.

Mohr’s stress circle is used to graphically represent the conversion of the strain and to determine the principal strains. The principal stresses are calculated from the principal strains by applying the law of elasticity.