Applications of 3D Scanning Laser Doppler Vibrometry to an Article with Internal Features

3D scanning laser Doppler vibrometry (LDV) systems continue to gain popularity for use in experimental modal analysis as the systems become more widespread. LDV is, by its nature, limited to measurements with line-of-sight visibility. This work presents an application of 3D scanning LDV to a test article with un-instrumented internal features that were not accessible to the lasers. The internal features, while not directly measurable, were known to contribute strongly to the modal characteristics of the test article. Initially, a traditional roving hammer test was conducted and modal parameters were extracted. The limited degrees of freedom inherent to this test method proved to be inadequate to correctly identify key mode shapes. It was found that by increasing the measurement point density and including all three translational degrees of freedom at each point, the key modal characteristics of the full system were able to be inferred from purely external measurements. These characteristics were essential in updating the mechanical behavior and material properties of the corresponding finite element model. The response measurements required for system identification were only practically achievable using the 3D LDV system. Comparisons of key experimental results to those of the calibrated analytical model are demonstrated.