Multi-mode excitation and data reduction for fatigue crack characterization in conducting plates

Abstract

Advances in the technique of fatigue crack characterization by resonant modal analysis have been achieved through a new excitation mechanism and data reduction of multiple resonance modes. A non-contacting electromagnetic device is used to apply a time varying Lorentz force to thin conducting sheets. The frequency and direction of the Lorentz force are such that resonance modes are generated in the test sample. By comparing the change in frequency between distinct resonant modes of a sample, detecting and sizing of fatigue cracks are achieved and frequency shifts caused by boundary condition changes can be discriminated against. Finite element modeling has been performed to verify experimental results.

Fatigue Crack Characterization in Conducting Sheets by Non Contact Stimulation of Resonant Modes

“…The fourth peak corresponds to that of the n=l, m=l eigen-mode, were the main wave crest is now perpendicular to the direction of fatigue crack growth. The work here focused on monitoring the frequency of the second (2,1/2) mode as a function of fatigue crack length, although a multi-mode approach to the characterization of fatigue damage has also been studied [4].…”

“…The cause of the increased error has been traced to boundary condition variations. This boundary condition dependence, however, can be reduced by a multi-mode approach to data acquisition and reduction [4]. A study is currently under way to determine the optimum test configuration for the technique, and to incorporate these findings into a portable hand held instrument operating on the principles described above.…”

“…The new design removes the permanent magnets from the sample surface, making use of a normal Lorentz force in order to induce resonant vibration [4]. Access to only one side of the structure is required, and no couplant need be applied to the surface.…”

“…Access to only one side of the structure is required, and no couplant need be applied to the surface. A detailed explanation of the operational principle is given elsewhere [4], so that only the configuration of the device in the experimental setup shall be presented here. Figure I shows a schematic representation of the experimental configuration.…”

“…A larger scatter in the experimental data is therefore encountered due to small changes in the boundary conditions between runs. It has recently been found that this boundary condition induced error can be minimized through a multimode approach to data acquisition and reduction, as is explained in [4].…”