**4. Conclusions**

Acoustic emission (AE) was successfully applied to full-scale, composite, fixed-wing aircraft spars during structural strength qualification testing. Correlations between AE activity and applied load were made that allow for the in-depth structural integrity assessment beyond the data available from pass/fail, legacy aircraft component testing. In addition to examining spar progressive failure, a criterion based on cumulative AE energy was introduced to characterize the recorded AE profile as indicating either significant or progressive behavior. Moreover, the criterion agreed well with particular instances during loading, and has potential for use during real-time AE data assessment, as there appears to be a link between AE data trends and ultimate failure loads. Using the knowledge of sensor placement and recorded AE activity at each sensor, damage critical regions were identified and determined to be generally concentrated at the spar root. Finally, statistical calculation methodologies for evaluating test performance using AE data were proposed, and feasibility was demonstrated. In summary, the presented investigation demonstrates that AE monitoring may assist in quantifying the effect of manufacturing or design differences on structural component performance parameters, which could lead to refined assessment criteria that are over and above the available legacy test methodologies.

**Author Contributions:** The authors contributed to the following tasks. Conceptualization, A.K. and J.G.; Methodology, A.K., P.A.V. and S.E.; Validation, S.E., B.J.W. and A.K.; Formal Analysis, P.A.V., S.E., B.J.W. and A.K.; Investigation, P.A.V., S.E. and A.K.; Resources, A.K. and J.G.; Data Curation, S.E., B.J.W. and A.K.; Writing-Original Draft Preparation, B.J.W., S.E. and A.K.; Writing-Review & Editing, B.J.W., S.E. and A.K.; Visualization, B.J.W. and S.E.; Supervision, A.K. and J.G.; Project Administration, A.K.

**Funding:** This investigation was supported financially by General Atomics Aeronautical Systems.

**Acknowledgments:** The authors acknowledge and thank Trilion Quality Systems, King of Prussia, PA, USA, MISTRAS Group Inc., Princeton Junction, NJ, USA, and General Atomics Aeronautical Systems, Adelanto, CA, USA for collaborating on this investigation.

**Conflicts of Interest:** The authors declare no conflict of interest.
