  Buckled membranes become ever more important with further miniaturization and
development of ultra-thin film based systems. It is well established that the
bulge test method, generally considered the gold standard for characterizing
freestanding thin films, is unsuited to characterize buckled membranes, because
of compressive residual stresses and a negligible out-of-plane bending
stiffness. When pressurized, buckled membranes immediately start entering the
ripple regime, but they typically plastically deform or fracture before
reaching the cylindrical regime. In this paper the bulge test method is
extended to enable characterization of buckled freestanding ultra-thin
membranes in the ripple regime. In a combined experimental-numerical approach,
the advanced technique of digital height correlation was first extended towards
the sub-micron scale, to enable measurement of the highly varying local 3D
strain and curvature fields on top of a single ripple in a total region of
interest as small as approximately 25 microns. Subsequently, a finite element
(FE) model was set up to analyze the post-buckled membrane under pressure
loading. In the seemingly complex ripple configuration, a suitable combination
of local region of interest and pressure range was identified for which the
stress-strain state can be extracted from the local strain and curvature
fields. This enables the extraction of both the Young's modulus and Poisson's
ratio from a single bulge sample, contrary to the conventional bulge test
method. Virtual experiments demonstrate the feasibility of the approach, while
real proof of principle of the method was demonstrated for fragile specimens
with rather narrow ( approximately 25 microns) ripples.
