Acoustic condition monitoring

For a number of years railroad wheels were used as test objects in developing acoustic signature monitoring techniques. This work will be reviewed as well as later laboratory studies using beams to determine the sensitivity of the method. Cracks and other defects can cause shifting and splitting of resonance frequencies, so that these changes can be used to indicate the presence of a defect. The limitation of the vibration monitoring method is that other conditions, such as uncertainties in the geometry of the test object, its surface conditions, and loading can also affect the vibration response, and it is necessary to distinguish the effects due to harmful conditions from those due to harmless ones. The sensitivity of the method is thus determined by the need to make this distinction. The concepts will be illustrated with experimental results from a test fixture for the case of a slotted steel beam, assuming that a slot can be used as a model for a crack. Some experiments with concrete beams will also be viewed.