Development of bi-axial tensile tester to investigate yield locus for aluminum film under multi-axial stresses

This paper describes novel biaxial tensile test technique for a film specimen to investigate material responses to uniaxial and biaxial tensile forces. We have developed biaxial tensile tester that consists of four sets of actuator, load cell, and displacement meter in order to apply an arbitrary amount of tensile force to a film specimen in each axis. With the uniaxial test specimen of sputtered Al film, average Young´s modulus of 35 GPa and yield strength of 115 MPa have been obtained. With the biaxial test specimen, evaluation of the yield locus for Al film has carried out by applying biaxial stresses with various strain rate ratios. The obtained yield stresses could be fitted by the yield criterion based on Logan-Hosford equation. This indicates that the flow stress under biaxial stress condition differ from that under uniaxial condition. Information about the yield locus would be useful for the structural design of MEMS including Al film structures subjected to biaxial stresses.