Determination of the yield stress in Al thin film by applying bulge test

In this work, we have applied an improved method to determine the yield stress and residual stress in a freestanding thin aluminum film by analysing experimental data obtained by the bulge test. The Al thin film was deposited by a magnetron sputtering technique. The film was cyclically loaded with increasing maximum gas pressure. The method to determine the plasticity parameters is based on the load-deflection relation that presents a linear behavior in the elastic regime when it is scaled with the displacement parameter. The plastic deformation induces nonlinear effects that allow determining the elastic limit of the film. At that point, the gas pressure value that generates the elastoplastic transition is identified on the scaled curve. For a bulged square film, the curvatures are computed from an approximated spatial deflection equation to calculate the stresses within the proposed model. The analysis enables construction of biaxial stress-strain curve for the studied film and determination of the yield stress (132 MPa for the studied film). The second important development of the paper is the methodology which permits the characterization of the residual stress from pressure-stress relation without additional numerical computations.