A study of the effect of microcrack on the vibration mode shape

This paper presents the modal analysis of an aluminum 6061 sheet in the presence of microcracks. Microcracks were introduced at the center of the sheet by compression loading and monitoring the deformation using the Acoustic Emission Technique (AET). Modal analysis was conducted on the sheet under free boundary condition. The mode shapes were mapped using the vibration pattern imager (VPI) when the sheet was vibrated at the modal frequency. The overall mode shape did not provide any information regarding the elastic constraints introduced due to the presence of microcracks. Scans were made at distances of 5 mm intervals on either side of the plastically deformed region on the plate in the horizontal and the vertical directions. It was found that by making line scans around the area of deformation the effect of microcracks on modal constraint could be detected. Using this information a map showing the constrained region surrounding the deformed region was plotted. The elastic constraints introduced on the modes due to the presence of microcracks were directional depending on the mode position. Frequency response analysis was performed around the deformed region. The second modal frequency measured at the edge of the sheet was very close to that of the first response frequency at the region near the deformed region. Metallurgical analysis and plastic material flow studies using an X-ray imager were made on the deformed region. The direction of material flow correlates with the curve generated for the elastic constrained area.