Prediction of crack growth in aircraft engine bleed air ducts using finite element and crack growth analysis

The bleed air system in an aircraft engine is one of the most important systems for flight operation and is the heart of the aircraft engine pneumatic system. It consists of ducts and components that supply high pressure bleed air from the engine compressor to the other systems. The functioning of bleed air system plays a vital role in flight safety, ground operations, and coordination with other existing systems [1] including fuel system, lubrication system, ignition system, and anti-ice system. A prior Wichita State University Research and Creative Award (URCA) research project entitled "Aircraft Pneumatic Leak Detection Methods" was the motivation for this thesis work. In this research, different inspection techniques were studied to detect leaks in bleed air ducts. Existing conditions such as continuous high pressure and temperature loading within a duct can make it prone to leak because of high stress concentration areas, pre-existing flaws, or manufacturing defects. A general-purpose finite element package, ABAQUS was used for stress analysis of cracked structures such as tubular T-joints. The high stress concentration region at the intersection of the duct joints was assumed to have certain initial flaws. This initial crack flaw was analyzed using line-spring elements to compute the stress intensity factor (SIF). The values of SIF obtained from the present analysis were compared with appropriate results in the literature. Stresses from the finite element analysis were used as input in AFGROW, a crack growth software package consisting of extensive material properties and all necessary fracture mechanics parameters required for crack-growth analysis. Damage tolerance was applied to estimate the life of the duct. The leak-before-break concept was used to determine whether crack growth in the duct would lead to any kind of leak or catastrophic failure. Crack and leak detection techniques to evaluate the crack size are discussed.