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Consider a piezoelectric transducer that is attached to the back of a plate-like structure, such as is the case for guided wave structural health monitoring. This transducer may be excited to produce guided waves that propagate outward from the source through the structure. These waves interact with impedance discontinuities in the structure at defects, bond lines, plate boundaries, and other geometrical features. These propagating waves may be measured on the surface of the specimen using either a scanning laser vibrometer or a scanning air coupled transducer. When full wavefield data are obtained by recording complete waveforms on a spatial grid, these measurements contain a wealth of information that can be used for identification, localization and characterization of damage. Results are presented here from a study that uses full acoustic wavefield images of propagating guided waves to detect and size different defect types in aluminum plate specimens. Several detection and sizing algorithms have previously been considered, including frequency domain analysis, incident wave removal, and acoustic wave crest analysis. The acoustic wave crest analysis method, which was developed by the authors to study corrosion type defects in aluminum plates, is applied here to analyze wavefield interactions with fatigue cracks in an aluminum plate specimen. The methodology demonstrated here on aluminum plate specimens is appropriate for application to critical metallic structural components as part of an integrated health management system.