Load-enhanced lamb wave techniques for characterization of scatterers in structures with complex geometries

Ultrasonic guided waves, called lamb waves in plates, are a promising tool for in-situ monitoring of metallic structures. It is well known that varying environmental conditions such as temperature and loads significantly change guided wave signals and can thus obscure the detection of damage. However, applied tensile loads open cracks and can enhance their detectability. Previous research by the authors demonstrated a load-differential technique for detection and localization of fatigue cracks in simple structures that leveraged the effect of loading on crack opening. Here, load- enhanced concepts are combined with minimum variance imaging to characterize scatterers in structures with complex geometries. First, minimum variance images of cracks and benign scatterers were generated by incorporating assumed scattering patterns. Then, orientation curves were obtained by plotting the peak amplitude of preselected regions in the images as a function of the assumed orientation angle of the scatterers. The discrimination of cracks from several types of benign scatterers was accomplished by identifying characteristic patterns in the orientation curves. The efficacy of the proposed technique was demonstrated using data from fatigue tests performed on aluminum plate specimens with bonded doublers, multiple fastener holes and installed fasteners. It is shown that the load-enhanced techniques can discriminate cracks from drilled holes and fastener tightness variations, and that the approximate orientation of cracks can be estimated.