Compliance Ball

The compliance of the tester (or the reciprocal of the stiffness) is the deformation observed in the tester overall structure for a certain level of force. All mechanical testers present a certain level of compliance that the manufacturer tries to minimize and that is generally negligible in regards to the compliance of the sample. For multiaxial tester, ensuring low compliance on all axis is more challenging. This compliance ball is made of hard metal, present a high degree of sphericity and is screwed onto the testing chamber base at the location of the sample. After setting the tester in the same configuration (actuators stacking and position on the frame), installation of the multiaxial load cell and the desired spherical indenter, the ball is mapped at various positions with the normal indentation function. Assuming that this rigid ball have a known and limited deformation for the indentation force applied, the outcome of the normal indentation can be used to map the 3D compliance of the tester. The outcome of this testing can be used to compensate 3D stiffness mapping of stiff sample.