Dynamic holographic modal analysis for NDT

Abstract

This paper describes crack and defect detection in structures through modification of the vibrational modal patterns and surface responses to stress. Features are made visible with dynamic holographic interferometery combined with parameter estimation. The procedure involves an unconventional, optimized, laser-illumination method. The methods are especially applicable to large structures and could prove pivotal to improved designs, monitoring and maintenance. Components and structures could be designed to better withstand operating stresses, and existing structures could be analysed to predict their response to stress. Since the modal characterization of a structure can act as a type of fingerprint, holographic interferometry can also be used to monitor structural degradation due to operating and aging. Modal characterization includes identification of resonant frequencies and also the corresponding mode shapes. Holographic interferometry provides for direct modal characterization of a structure as well as measuring its small loading dynamic response. The project demonstrated that a wide variety of defects can be located in structural components, vessels and pipes. An analytical exercise also demonstrates the ability to use global modal characteristics to determine the presence of local corrosion and erosion.