Wölbtest

Wölbtest, in English known as the bulge test, is a mechanical characterization method for thin-film and membrane materials. The technique measures the in-plane mechanical response by applying a controlled pressure to a freestanding film that is clamped at its edges, causing a central bulge. A differential pressure is applied across the membrane, generating a bulging height at the center. The pressure–deflection data are collected using optical profilometry, laser triangulation, interferometry, or confocal microscopy. The resulting pressure–deflection curve is analyzed with plate- or membrane-theory models to extract material properties such as Young's modulus, residual stress, and, in some formulations, the biaxial modulus or yield behavior.

Two main variants exist: bulge tests on freestanding membranes and bulge tests integrated into MEMS-like structures where the membrane is supported by a frame. The technique is widely used for ultrathin films (e.g., silicon nitride, oxide films), polymeric membranes, and two-dimensional materials such as graphene, as well as for MEMS devices.

Advantages include the ability to apply biaxial stress in a single test, suitability for very thin samples, and non-destructive measurement. Limitations involve the need for accurate knowledge of boundary conditions, precise pressure and deflection measurements, and appropriate constitutive models, as results depend on geometry and boundary constraints.

Origin: the bulge test has been used since the mid-20th century as a simple method to characterize thin membranes.