Guided-wave-based acousto-ultrasound structural health monitoring (SHM) methods have attracted the interest of the SHM community as guided waves can travel long distances without significant dissipation and are capable of detecting small damage sizes of several types. However, when subject to changing environmental and operational conditions (EOC), guided-wave-based methods may give false indications of damage as they exhibit increased sensitivity to varying EOC. In order to improve the reliability and enable the large-scale applicability of these methods, and to build a robust SHM system, it is necessary to quantify the uncertainty in guided wave propagation due to changing EOC. In this paper, a rigorous investigation on the uncertainty involved in the propagation of Lamb waves due to the variation in temperature and material properties of nominally-identical structures has been performed both numerically and experimentally. A high fidelity finite element model is established to study the effect of small temperature perturbation on the S_0 and A_0 modes of Lamb waves and the associated uncertainty is quantified. Then experiments are performed under ambient laboratory temperature variations during an eleven day period. The experimental results have indicated that temperature variations as small as 0.5 °C may result variations in the amplitude of Lamb waves and affect the damage index. Then uncertainty due to the variation in material properties has been considered by taking into account the statistical Gamma distributed dependency between Young's modulus and Poisson ratio jointly and the associated variation in the damage index is also investigated.
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