Detection of mode frequencies for spherical cavity embedded within a mild steel plate

Abstract

The modern era of structures has observed a vast increase in non-destructive evaluation techniques in order to detect unwanted damages or porosities within the structure. Detection of defect frequencies will play a vital role in local defect resonance–based defect imaging in the future. In this paper, a technique for porosity and cavity detection in metals is discussed. A numerical investigation is carried out first on a square mild steel plate with a spherical cavity and a square mild steel block with multiple cavities. Different resonance frequencies and corresponding mode shapes in the case of the single cavity are calculated using steady state dynamic analysis. Six fundamental mode shapes are identified as Rocking (R), Torsional (T), Bouncing (B), 4-noded (4 N), 6-noded (6 N) and 8-noded (8 N) modes. Moreover, an explicit dynamic analysis is performed by exciting the model with a chirp signal for both cases of single and multiple cavities. The numerical study is supported by experimental results, performed on a mild steel plate embedded with a single spherical cavity and previously published literature. It is observed that the analytical, numerical as well as the experimental results obtained for all the modes are in close agreement with each other. The bouncing mode is found to be the promising one for pore imaging due to its uniform displacement across all the directions.