SBIR-STTR Award

Fatigue-related effects are a significant cost driver for maintenance of military aircraft systems. Typically, inspection intervals are defined based on expected damage accumulation or crack initiation and propagation rates. When damage is found a decision must be made for whether the damaged component should be removed, repaired, or left alone until the next inspection. Repairing may include material removals and/or the installation of a patch which requires additional engineering decisions as to the extent of the removal or the type and size of patch to maintain structural integrity. The objective of this project is to provide a repeatable and reliable methodology for damage tolerance analysis of wing structures prior to and after the installation of repair patches. This objective will be met through a collaborative approach between ESRD and AP/ES; ESRD brings expertise in modeling of damage and best practices in application of the finite element method, and AP/ES brings expertise in damage tolerance analysis as applied to military aircraft structures. Ultimately this methodology will better equip the Bonded Repair Center of Excellence to more effectively evaluate damage, repair patches, and fatigue life.

Airframe maintenance cycles are one of the key cost drivers in any military aircraft program. In order to minimize costs associated with maintenance operations as well as time an aircraft spends on the ground, engineering teams must seek out methods of optimizing the maintenance schedule, performing repairs as infrequently as is safe while having confidence that prior repairs will not fail before the next maintenance cycle. This requires engineers to have access to accurate and reliable analysis methodologies and simulation tools to evaluate and predict repair performance under a number of service conditions. In the Phase I effort, a preliminary methodology was established to evaluate a repair’s effectiveness at restoring static strength and arresting fatigue crack growth in a section of wing skin. The methodology’s effectiveness was demonstrated on a reduced-size model and showed great promise for prediction of life expectancy in repaired C-130 wing skins. Phase II of the project will focus on expanding the scope of the work done in Phase I. In order to accurately address the specific needs of the Warner-Robins engineering team, full scale FEA models are needed with accurate representations of wing geometry and loading patterns. Additionally, the damage tolerance analysis will be expanded to encompass multiple crack configurations and provide more accurate estimates of fatigue life. Finally, to support validation of the methodology’s predictions an experimental testing program will be undertaken using AP/ES existing laboratory equipment to test pre-made repair specimens and compare their fatigue life to the models’ predictions.