Field Operation of PZT Sensor Based SHM System on Commercial Aircraft in Detecting Cracking Around Chem-Mill Areas

The integrity of Boeing 737 aircraft structures, particularly at the edges of chemically milled pockets between stringers and frames on the interior skin surface, is compromised by the occurrence of multiple crack formations. These cracks, often initiated at the chem-mill line, have prompted regulatory agencies and manufacturers to enforce rigorous inspection protocols, including Eddy Current and Phased Array Ultrasonic Testing (PAUT). While effective, these NDT methods are labor-intensive, costly, and often necessitate extensive aircraft disassembly, posing safety risks to inspectors, creating logistical challenges, and incurring significant operational downtime. The sheer volume of inspection sites, coupled with the limited availability of qualified personnel and the difficulty of accessing critical areas, underscores the need for more efficient, reliable, and less invasive inspection techniques. To address these challenges, Delta Air Lines has shown interest in investigating Structural Health Monitoring (SHM) as a cost-effective solution that could eliminate the need for extensive access requirements. This study presents an evaluation of SHM technology's feasibility in detecting cracks along the chem-mill line on Boeing 737 structures in service condition.