A bulge test based methodology for characterizing ultra-thin buckled membranes

TL;DR

This paper extends the bulge test method to characterize ultra-thin buckled membranes in the ripple regime by combining digital height correlation and finite element modeling, enabling extraction of material properties from fragile specimens.

Contribution

The study introduces a novel combined experimental-numerical approach to measure material properties of buckled membranes in the ripple regime, overcoming limitations of traditional bulge testing.

Findings

Successful measurement of local strain and curvature fields on ripples.

Extraction of Young's modulus and Poisson's ratio from a single sample.

Feasibility demonstrated on fragile, narrow ripple membranes.

Abstract

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.