Blister test has known a huge development in the 90's to study thin films. Indeed, this test allows, in one single experiment, to measure different mechanical properties of thin film on substrate: elastic modulus, residual stress and adhesion to the substrate.
In its principle, the test consists in applying a fluid pressure on one side of a freestanding film window through a hole in the substrate. As the name indicates, the bulge-and-blister test is divided in two steps:
during the first part of the experiment (the bulge test), the fluid is progressively introduced through the hole in the substrate to cause an outward deflection of the freestanding membrane (like inflating a balloon!). While injection proceeds, the resulting pressure p and the membrane deflection h follow an increasing curve, which can be analyzed in terms of mechanical properties of the film.
at a critical pressure value, delamination starts with an interfacial crack front propagation from the initial outline of the freestanding membrane. As soon as this blister step begins the height and volume of the blister increases rapidly, thus causing a pressure decreases. When the film behaves elastically, the stationary debonding rate is controlled by the injection rate and the pressure p versus height h curve follows a decreasing hyperbolic laws depending on the adhesion energy Gc of the film on the substrate.
Our set-up consists of a pressurizing block filled with an incompressible fluid (DI water). A micrometric screw piston and a syringe pump were used to inject water towards the specimen, while the internal pressure was measured thanks to a piezoelectric sensor. Two systems are used to measure the blister geometry:
a stereo-digital image correlation system: Based on the principle of digital image correlation (DIC) for 2D displacement measurement at the surface of a material, stereo-DIC introduces the out of plane displacement thanks to a second camera. This system allows to acquire data more rapidly than the fringe projection system and gives not only the shape of the blister but also its displacement field.
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