UC San Diego

In-situ stress determination in structures under environmental variability without a reference value is a challenging experimental mechanics task. One potential application of this task is the management of longitudinal loads in railroad structures: the absence of expansion joints in Continuous Welded Rail (CWR) has created the need for the railroad industry to determine the in-situ thermal stress levels for rail buckling and breakage prevention. This dissertation examines the potentials of a nondestructive method, namely Electro-Mechanical Impedance (EMI) method, and a semi-destructive method, the hole-drilling test procedure, to provide an estimation of axial stress in bar-like structures.

The EMI method is completely non-destructive, as it simply involves bonding a piezoelectric element on the host structure and measuring the electrical admittance signature of the PZT-structure assembly in selected frequency bands. This non-invasive method features the easiness of implementation and interpretation, while it is notoriously known for being vulnerable to environmental variability. A comprehensive analytical model is proposed to relate the measured electric admittance signatures to uniaxial applied stress and temperature, respectively, as functions of relevant parameters of the EMI monitoring system. The model results compare favorably to the experimental ones, where the sensitivities of features extracted from the admittance signatures to the varying stress levels and temperatures are determined. The temperature compensation algorithms are proposed, and the final results illustrate that the frameworks are capable to eliminate the temperature effect and highlight the ones from thermal stress. On the other hand, the semi-destructive hole-drilling method is explored as a possible solution for thermal stress measurement. A new set of calibration coefficients to compute the stress field relieved by fine hole depth increments required by the high strength steel was determined. The new calibration coefficients were experimentally validated on an aluminum plate subjected to a known uniaxial load. The thermal stress levels of constrained rails were estimated after compensation for the residual stress components, based on statistical relationships developed experimentally between the longitudinal and the vertical residual stresses. The results showed that the hole-drilling procedure, with appropriate calibration coefficients and residual stress compensation, can estimate the in-situ rail thermal stresses with an expected accuracy that is within the industry acceptable levels.