Localization of transversal cracks in sandwich beams and evaluation of their severity

An algorithm to assess transversal cracks in composite structures based on natural frequency changes due to damage is proposed. The damage assessment is performed in two steps; first the crack location is found, and afterwards an evaluation of its severity is performed. The technique is based on a mathematical relation that provides the exact solution for the frequency changes of bending vibration modes, considering two terms. The first term is related to the strain energy stored in the beam, while the second term considers the increase of flexibility due to damage. Thus, it is possible to separate the problems of localization and severity assessment, which makes the localization process independent of the beams cross-section shape and boundary conditions. In fact, the process consists of comparing vectors representing the measured frequency shifts with patterns constructed using the mode shape curvatures of the undamaged beam. Once the damage is localized, the evaluation of its severity is made taking into account the global rigidity reduction. The damage identification algorithm was validated by experiments performed on numerous sandwich panel specimens

The influence of transversal crack geometry on the frequency changes of beams

Global non-destructive testing methods use changes in modal data as damage indicators. Depending on their geometry, damages influence the modal parameters in different way. While breathing cracks influence the natural frequencies due to only stiffness change, open cracks affect supplementary these frequencies due to loss of mass. This paper describes a study devoted to clarify the influence of the damage geometry on the natural frequencies of beams considering the two above mentioned aspects. First, analytical and numerical analyses are performed, in order to highlight the effect of mass changes on the frequency changes. Afterwards, the study focuses on breathing cracks in open and closing stages as well as on open cracks and emphasizes the influence of the geometrical discontinuity. It was demonstrated that, for beams with a breathing crack, a two degrees of freedom model is necessary to characterize its dynamic behavior. Finally, a relation between frequency changes and damage geometry and location is contrived

Direct comparison of distinct naive pluripotent states in human embryonic stem cells

Stem cells & developmental biolog