Most practical situations involve multi-axial loading. For example, any pressure vessel will see biaxial loading from the axial and hoop stress components. A planar biaxial test system consists of four servo-actuators positioned at right angles to each other on a free-standing self-reacting load frame. Planar biaxial test systems differ from axial-torsion systems. They involve the use of cruciform shaped specimens.
Planar biaxial testing poses a number of challenges in that the two axes of loading should be amenable to precise independent control in the desired mode (Stroke, Load or local (axial) strain response) and with desired distribution of load and phase lag between the two axes. Another challenge is to restrict loading to strictly biaxial, without any shear component whatsoever that may not only distort the desired stress state, but also threaten unintended damage to the specimen outside the gauge area.
The conventional approach to meeting this requirement is to drive opposing actuators in so called ‘Master-Slave’ mode, whereby the Slave tracks the position of the Master actuator, resulting in unchanged position of the perceived specimen center.