A study of cracks in riveted joints using acoustic, electron and optical microscopy

The scanning acoustic microscope (SAM) was used quantitatively investigate subsurface fatigue cracks that are not visible on an outside surface. In combination with optical and electron microscopic examination of an outside surface and with fractography of fractured specimens, the formation and growth of subsurface cracks were characterized in detail. Observations were made in multi-riveted lap-joint Al alloy panel specimens fatigued in tension. Plots of subsurface crack length vs, number of cycles show slow linear crack growth. However, when the crack becomes a surface breaking crack, i.e. a through crack, the crack growth rate increases rapidly. The length of the crack on the hidden back surface of the specimen remains longer than the length of the same crack on the front surface. Variation in the maximum nominal stress changes the fatigue crack mode.