Vibration based Structural Health Monitoring (SHM) technologies have been the focus of significant development efforts for applications to a broad spectrum of transportation and civil structural systems. The need for development of embedded "on-board" sensors and systems for health and condition assessment is driven by a desired shift from scheduled maintenance to Condition Based Maintenance (CBM) in order to lower operational system costs and to improve system reliability / safety. This work presents recent results from a study aimed at developing embedded vibration based airframe damage detection technologies to detect and classify critical fatigue induced damage. Previous work described the application of Structural Intensity (SI) based approaches to the SHM problem. SI was shown to be an attractive damage detection feature, being sensitive in both its magnitude and direction to damage initiation and progression. A new advanced damage assessment technique using a Nonlinear Structural Surface Intensity (NSSI) feature has been developed. Sensitivity benefits were demonstrated using the NSSI the approach for certain types of critical airframe damage, including cracks and loose joints. Key experimental results from the development of the NSSI technique are presented including discrete sensor measurements from damage progression development / validation experiments. Scanning laser vibrometer results, used to visualize structure energy flow mechanisms during damage progression, are also presented which lend insight to the key aspects of the damage detection technology developed.