| Beam-like structural elements with length much greater than the characteristic length of the cross-section are vulnerable to large forces induced either by constrained thermal expansions and contractions, or by the weights and constraints from neighboring elements in the structure. The often violent consequences of the resulting buckling and breakage continue to emphasize the need for cost-effective and accurate methods for in situ measurement of axial stress in slender elements, especially in the circumstances where only a small span of the total length is free from the loads and supports. A new method for in situ contained stress measurements, based on the change of dynamic flexural rigidity under the influence of compressive load, is proposed in this thesis. Scanned laser vibrometry measurements of vibration fields at a prescribed frequency, followed by a comparison with guided wave theory, allow extraction of the wavenumbers associated with all the propagating and evanescent modes in the body. The wavenumber associated with the bending mode is in turn related to contained stress and to the intrinsic rigidity of the beam. For the particular and technologically pressing case of railroad rail, a priori estimates for the needed precisions, followed by the numerical simulations and laboratory measurements, have established that the technique is viable. It is sensitive to contained load and, most importantly, it does not require modeling of supports, it is insensitive to residual stress and microstructure and it does not require zero-state referencing. The measurement precisions sufficient to make it practical have been achieved successfully in the lab. |
