Merged Mode TFM with Mode Conversion Artifact Suppression

Abstract

The objective of the paper is to compare the merits of the multiplication and max value schemes for producing a total focusing method (TFM) representation that combines multiple acoustic modes. We have chosen to compound phase coherence imaging (PCI) images because it eliminates the difficulties associated with handling vastly different acoustic sensitivities across different propagation modes. We also introduce an apodization-based algorithm that can filter out unwanted mode-conversion artifacts that are caused by similarities in time of flights. Experimental data were gathered on an electric resistance welding (ERW) hook crack and a natural bottom-surface-connected vertical crack. Experimental results indicate the multiplication scheme suppresses defect echoes that can only be imaged by a subset of acoustic modes. This can potentially lead to false negatives or underestimation of the extent of the defect. In contrast, the max value scheme retains all indications that appear in a subset of acoustic modes. For the vertical crack configuration imaged by shear wave acoustic modes, the max value scheme retains all mode-conversion artifacts and therefore complicates the defect analysis. The apodization-based filtering method significantly suppressed mode-conversion artifacts without affecting the imaging of the vertical crack. The combination of PCI with max value merging and apodization-based mode-conversion filtering is a promising methodology for merged mode TFM in shear wave weld inspections.