Detecting minute damage in beam-like structures using the neutral axis location

The neutral axis is the curve within a cross-section of a beam under loading at which the normal stress and strain vanish. The neutral axis is correlated with the centroid of stiffness which depends on material and geometric properties of the beam cross-section. The centroid is sensitive to changes in area and stiffness of the cross-section, i.e. damage. The neutral axis is a universal parameter in beam-like structures, and since the majority of structural systems is based on beams, the neutral axis location can be used as a damage sensitive feature in a variety of structures. In this paper the sensitivity of the neutral axis location to minute damage is studied through a test structure with induced artificial damage. The structure was equipped with long-gauge fiber-optic strain sensors in three cross-sections, one with partial delamination in the concrete, one with a small crack, and one with no damage (healthy). The structure was monitored during lifting and transport, and dynamic tests were performed in close-to-real conditions. A detailed uncertainty analysis was performed both on the evaluations of the centroid location and the neutral axis measurements. A Z-score hypothesis test was used to set thresholds and determine whether the damage was detected or not. The results show that the neutral axis location is sensitive to minute damage, and that the latter can be detected within the uncertainty limits in on-site conditions.