Combined optical–acoustic microscopy for investigating short fatigue crack propagation

Abstract

The acoustic microscope has the advantage of being able to detect microstructurally short cracks and to locate crack tips exactly. However, due to scattering of the sound waves and fringe interference effects, the surface texture relationship with the crack tip is not so clear. The optical polarising microscope on the other hand can clearly exhibit surface texture following a specimen surface treatment such as anodising of an Al–Li alloy. However anodising produces a thin layer of a brittle oxide on the surface of the specimen which will reduce the accuracy of the acoustic microscope in exactly determining the crack tip position. Whereas the acoustic microscope does not need any treatment of the specimen surface, and so does not affect the material properties, some materials such as carbon steels and anodised aluminium alloys can be very sensitive to corrosion pitting due to the lens water couplant. It follows that both of these microscopes have complementary advantages and disadvantages and therefore combining both microscopes in the same facility can permit more data to be gathered on the behaviour of very small fatigue cracks and their interactions with microstructural barriers. The present paper reports on these developments using the SIRIUS acoustic microscope facility.

MST/2048