Influence of damage versus temperature on modal strains and neutral axis positions of beam-like structures

Vibration-based Structural Health Monitoring is a non-destructive condition assessment method that exploits damage-related changes in the dynamic characteristics of a structure. The sensitivity of some commonly employed dynamic characteristics such as natural frequencies, to local damage of moderate severity might be lower than their sensitivity to environmental influences such as temperature. Modal strains are more sensitive to local damage, but the accurate dynamic monitoring of the very low strain levels that occur in civil engineering structures under ambient excitation became possible only recently. In this work, the influence of both temperature and damage on modal strains is experimentally investigated for a prestressed concreted beam in controlled laboratory conditions. Not only the modal strains themselves are considered, but also the neutral axis position under bending deformation, which relates directly to the bending stiffness. It is found that the induced temperature changes in the concrete beam do not have a measurable influence on the modal strains and neutral axis positions. The cracks that are induced in the beam by external loading in a progressive damage test do have a clear and local influence on the strain mode shapes and neutral axis positions, even at low damage levels.