Structural health monitoring of ungraded timber beams using long-gauge fiber optic sensors

[EN] This study investigates the use of long-gauge fiber optic sensors (FOS) for the structural health monitoring of a sawn fir timber beam under bending. The 3 m long beam was made from ungraded timber-i.e., wood not classified by standardized criteria. FOS were installed near a support and at mid-span to measure strains, from which stresses, neutral axis positions, deflections, and the beam's global modulus of elasticity were estimated. Experimental stresses were compared with theoretical values using two approaches: one assuming the neutral axis coincides with the geometric centroid, and another using its experimentally determined position. Mean absolute errors (MAE) between theoretical and FOS-based stress values remained below 1.3 MPa, demonstrating excellent agreement. Three sensor configurations were then tested to estimate the beam's deflections and global modulus of elasticity: FOS at mid-span only, FOS near-support only, and combined (FOS at mid-span and near supports). Deflections were calculated from strainderived curvatures using closed-form expressions and compared with LVDT measurements, yielding MAE values below 0.8 mm (span/3750). The combined configuration performed best, with a deflection MAE of 0.24 mm and a + 6.0 % deviation in estimated global modulus of elasticity relative to LVDT-based values. By validating stress estimates and demonstrating accurate, equation-based deflection evaluation and the indirect determination of stiffness through the global modulus of elasticity, the study confirms the potential of long-gauge FOS as the basis for long-term, multi-parameter, strain-based SHM strategies that can be extended beyond ungraded timber to other structural materials and applications.