Evaluation of SMART Layer Sensor Inspection for Cracking Around Chemical Milled Area Under Simulated Environment for Commercial Airplane

The occurrence of multiple cracking instances at the edges of chemical milled pockets between the stringers and frames of the interior surface of the skin of Boeing 737 component has led to the implementation of service bulletins and Airworthiness Directives mandating Eddy Current inspections at the chem-mill line. These inspections are known to be time-consuming, costly, and often require extensive dismantling of the aircraft's interior. In response to these challenges, Delta Airlines has expressed interest in exploring Structural Health Monitoring (SHM) as a cost-effective alternative to avoid expensive teardowns. To evaluate the effectiveness of SHM for mill line crack inspections, a test segment was introduced with a deliberately induced crack at the mill line. Acellent Technologies, a leading supplier of SHM solutions, has collaborated in a cost-sharing initiative to study the feasibility of achieving FAA certification by utilizing SMART layer technology to establish a sensor network for real-time damage assessment through Damage Index (DI) analysis. The SMART Layer sensor system developed by Acellent Technologies is a versatile solution comprising a network of PZT sensors integrated into a thin carrier film, integrated with built-in wiring and connectors. This system allows for effortless installation on a wide range of structures, offering choices between individual sensors, standard layers, and customized configurations to meet various needs. Known for their robustness, utility in structural health monitoring, and adaptability to different shapes and surroundings, these layers stand out for their durability and practicality. This approach enables precise crack detection and localization, utilizing DI values from sensor data to monitor crack progression and identify structural irregularities. The research highlights the potential of SHM systems in enhancing maintenance efficiency and improving aircraft safety targeting chem-mill areas of Boeing 737 structural elements. The results emphasize the significant advantages of SMART layer based SHM systems in transforming aircraft maintenance practices by enabling proactive and accurate structural health assessments. Chem-mill fuselage panels with pre-initiated saw cut cracks were fabricated and tested under biaxial testing machines at FAA testing center. The objective is to monitor the crack growth after each cyclic fatigue load using onboard sensors. There were concerns about potential issues related to the chem-milled area and the proximity of a structural stringer to the chem-mill area. The Federal Aviation Administration (FAA) facilitated and conducted the test using Acellent’s PZT sensors layout. Using the PZT sensor data the Damage Index (DI) values were calculated to measure the changes in ultrasonic signal as the crack grow due to incremental load cycles. Our research utilized the capabilities of Acellent’s SHM Patch software to accurately diagnose the cracks using DI growth with respect to increasing load cycle. The incorporation of SHM Patch software into our analysis process marks a significant advancement in the field, allowing for a more intuitive and detailed visualization of structural flaws. By converting DI values into a visual representation of crack locations, we streamline the assessment process, which is essential for timely maintenance and repair strategies. The precision achieved in localizing cracks through this approach underscores the significance of software algorithms in enhancing the interpretability of SHM data. The results are very promising and provides clear evidence that acoustic-ultrasound SHM techniques could provide an alternative means of compliance for monitoring cracks in Chem-mil structures. The study showcases the potential for SHM systems to progress beyond mere data collection instruments, evolving into comprehensive diagnostic tools that can guide maintenance decisions.