Design of a Test Machine for Biaxial Testing of Composite-Laminate Cylinders.

A design is presented for a facility to test composite laminate tubes under generalized plane stress loading conditions. The system is designed around multiple closed-loop servo-controlled hydraulic load actuators to provide independent, programmable axial, circumferential and torsional stress to tubular specimens. The unique feature of the design is an actively controlled, hydraulic, collet-type grip. The grip is designed to provide radial displacement of the collet to compensate for deformation in the specimen during loading. This radial compensation is shown to significantly reduce the grip induced constraint stresses which previously have been a serious problem in the testing of composite tubes.