a thin - wall tube specimen is biaxially tested for stress analysis by applying compressive axial stress and either internal surface pressure or external surface pressure to the specimen . torsion is not required . the sample is positioned between platens which are assembled inside a pressure collet . axial compressive stress is applied through the platens to the specimen , and hydraulic pressure is applied through the assembly to the internal and external cylindrical surfaces of the specimen . rubber bladders may be employed when applying the surface pressures .