Laser vibrometry technique for measurement of contained stress in railroad rail

Based on the known sensitivity of a beam's effective flexural rigidity to its static axial load, we experimentally investigate whether measurements of lateral bending wavenumber, at a specified imposed frequency, can be used to nondestructively determine that load. Initial estimates are that at an imposed frequency in the vicinity of 200 Hz, where wavelengths are of the order of 2 m, the sensitivity should be adequate if the wavenumber can be extracted with sufficient precision. Scanned laser vibrometry, followed by digital lock-in, and referencing with a fixed accelerometer, is found to accurately measure steady-state vibration distributions. Nonlinear least-square fits to theoretical forms consisting of a sum of guided vibration modes then give a best-fit value for that wavenumber, a value that correlates well with the known levels of load in the experiments. The proposed technique appears viable.