Vibration-Based NDE Technique for Identifying Non-Uniformities in Manufactured Parts With Degeneracies

A novel method for characterizing non-uniformities in homogeneous precision symmetric parts using pseudo-repeated modal frequencies is discussed in this paper. It is demonstrated that the spacing between pseudo-repeated roots (modal frequencies) in dominantly symmetric manufactured parts is a direct indicator of non-uniformities (i.e. asymmetries) in the stiffness or mass distributions. Smaller differences between split-peaks indicate less non-uniformity, and hence, higher quality-greater precision parts. By establishing a tolerance for precision manufactured parts based on a collection of vibration modal parameter data from acceptable parts for a given process, this work shows that poorly manufactured parts with density discontinuities (e.g. inclusions, voids) can be identified based on the degree to which their pseudo-repeated roots are separated. A simple low-order lumped parameter model is used to develop this technique analytically and acceleration response data from impact vibration tests on deep drawn hemispherical shells is used to verify the validity of the approach experimentally.