Aircraft Fatigue Management

Fatigare (L)—to tire (translated term first used for metals in 1839, in France, and 1854, in England). Fatigue occurs under cyclic loading and can significantly degrade the operational capability and safety of metallic aircraft components and structures. Fatigue is also important in other engineering sectors. This book provides summaries of some metallic aircraft structural integrity issues; innovative but established examples of maintaining operational capability (airworthiness) and assessments of safe in-service fatigue lives. These topics are based on a through-life fatigue management philosophy that ensures safe and continued operation, including during life extensions that are almost inevitably required. This philosophy is underpinned by observations of the behaviour of fatigue cracks in actual structures subjected to realistic service loading conditions. More specifically, the book chapters consider aircraft design requirements, individual aircraft fatigue loads monitoring, airframe fatigue testing, sources of fatigue-nucleating discontinuities and prediction of fatigue crack growth from these discontinuities. All these aspects contribute to discussing methods of assuring the structural integrity and operational capability of realistically cracked structures. Detailed subjects include the exponential behaviour of lead or dominant cracks (i.e. those leading to first failure), and the practical significance of differences between fatigue fracture topographies produced under constant amplitude and variable amplitude loading. Also, the many observations of lead crack growth have enabled comparing the efficiencies of alternative (or competing) aircraft design standards, and developing procedures for countering in-service cracking. The book described has been developed through the pragmatic need to safely manage in-service aircraft structures for extended periods and with limited resources. The principles and analyses described in this book have contributed significantly to the efficient and effective structural integrity management of aircraft operated by the Royal Australian Air Force.

The study of metal fatigue dates back to 1844. Despite many breakthroughs in understanding, there is still much debate over the significance of some actual and potential influences, and the ability to accurately predict fatigue lives. This chapter attempts to demystify fatigue in aircraft structures.

Aircraft design standards provide the requirements and/or guidance to assist a manufacturer in providing a safe and durable airframe for a specific period of operation or an inspection-free period. The standards also advise on the design and in-service tasks or general aspects/elements that are required to achieve and maintain structural airworthiness. However, in some instances operational objectives mean that the original certification basis may be inadequate and further remedial action is required to maintain airworthiness as discussed.

Many years of QF on metallic airframe components from service and FSFTs have consistently shown that the dominant fatigue cracks (those first leading to failure) grow in an approximately exponential manner. These observations have lead to the Lead Crack Fatigue Lifing Framework (LCFLF). The LCFLF supporting the simplified lifing method is considered to provide conservative fatigue life estimates, albeit that considerable engineering expertise is required for a judicious choice of the FCG curve parameters.

Following on from the lead crack fatigue lifing framework, a number of fatigue crack growth tools have been developed:

Compared to the carefully prepared surfaces typical of traditional laboratory fatigue specimens, production aircraft structures have many surface discontinuities. These discontinuities are generally very small, of the order of 0.01 mm deep. When they are sufficiently crack-like and the local cyclic stresses generated by the service loads are sufficiently high, the nucleation and growth of fatigue cracks from the larger discontinuities will commence very early—almost immediately—in an aircraft’s service life or in an FSFT. The importance of the surface finish is stressed. A method of characterizing these discontinuities as equivalent cracks is presented.

The lead fatigue crack growth (FCG) concept has been developed into a methodology that can be used to estimate virtual test endpoints. These test endpoints can be obtained from FSFTs and large component fatigue tests; in-service fleet cracking, including disassembled (teardown) parts taken from the fleet and sometimes from highly representative coupon tests.

The following first presents some lessons learnt from a career in structural integrity fleet management of military aircraft. Some typical fatigue enhancing techniques are then summarised with the lead crack framework applied to assess the resulting life of some. Finally, the lead crack framework is used to justify delaying the repair of pitting corrosion to the next scheduled maintenance period.

This book summarizes some innovative contributions to the field of aircraft structural integrity, with emphasis on the fatigue of metallic airframes. It also emphasizes the Lead Crack Fatigue Lifing Framework (LCFLF), which has been used to produce effective and efficient fatigue lifing models which can be used for analyzing in-service detected cracks or for fatigue life extension purposes.