Naked eye observations of microstructurally short fatigue cracks

An experimental methodology is described whereby interactions between cracks and microstructural barriers, and the consequent non-uniform propagation rates are observed without the assistance of any microscopy technique. This experimental procedure consists in increasing the grain size of Al1050 and Al1100 aluminum alloys specimens until the centimeter scale by applying a series of mechanical and heat treatments. By properly adjusting the strains, temperatures and furnace times of both stages a very precise control of the microstructural size is achieved. Once the thermomechanical treatment is completed and the sought microstructural size is obtained, a small circular notch is machined on each specimen in order to initiate the cracks at the desired location, and the samples are subjected to mode I fatigue loading. The fluctuating crack growth rate, the twist and tilt angles of the crack-plane at grain boundaries and crack arrest and branching can be easily observed with the naked eye. Production of secondary crack branches caused by roughness induced closure has also been observed. Tests were performed varying grain size and notch diameter and it was observed that the distance between successive minima in crack growth rate correlates well with the grain size of the specimens. .