Experimental and numerical studies of a cracked thin-walled box-beams

This paper discusses the problem of detection and location of cracks in a cantilever beam. To solve this problem, a damage index based on the modal displacement method is applied in both experimental and numerical studies. Thin-walled composite box-beams with rectangular cross-section are tested. Two types of beam configuration are examined: circumferentially asymmetric stiffness (CAS) and circumferentially uniform stiffness (CUS). The CAS configuration is characterized by the coupling between bending in a flexible direction and torsion, while the CUS configuration is characterized by coupled bending in two perpendicular directions (flexible and stiff). Numerical models of the tested beams are made by the finite element method. A series of simulations are performed for different damage locations and ply orientations. The Lanczos method is used to solve the eigenvalue problem. Natural frequencies and free vibration modes of the beams are determined. To validate the numerical models, a test stand is build. The test specimens are made of unidirectional carbon-epoxy prepreg by the autoclave technique. The damage of the reinforcement fibres is induced by making a crack in the beam using a circular saw. Selected numerical results are validated with two different experimental methods. First, the experimental test is conducted using a non-contact laser vibrometer provided with a scanning head and a data acquisition-visualization unit. Next, a modal analysis is performed by the contact method using the LMS analyzer with a modal hammer and an acceleration sensor. All numerical and experimental results of the beams with cracks are compared with the findings obtained for the specimens without any damage.