Aircraft life management using crack initiation and crack growth models – P-3C Aircraft experience

Abstract In this paper, we present our experience in applying crack initiation and crack growth models to aircraft life predictions and fleet management. The first section of the paper discusses general fleet life assessment methods followed by a description of traditional US Navy method of fleet fatigue life assessment and individual aircraft fatigue tracking. The next section focuses on the history of life estimation methods of P-3C aircraft, with brief descriptions of recently conducted full-scale fatigue test. We then discuss limitations of safe-life methods for aging aircraft such as P-3C and then describe a deterministic total life approach for continued operations beyond crack initiation up to a safe limit using full-scale test as a basis. In the context of crack growth sensitivity to various factors, the need to conduct risk assessment supported by service examples is then presented. Details of risk assessment effort using service data as a basis are briefly described next. Equivalent mathematical formulations for probability of failure analysis are presented with lessons learnt in applying this to field problems. The paper discusses lessons learnt mainly from the standpoints of equivalent initial flaw size distributions, final crack length distributions, and probability of failure calculation methods. The last section of the paper focuses on current crack initiation and crack growth models and their use in aircraft life assessment. Recent coupon test results are used to make general observations about the use of crack initiation and crack growth models. The limitations of applying crack initiation models for different classes of spectrum such as tension dominated and tension–compression type spectrum are then discussed. Similarly, limitations of current crack growth models for application to wide classes of spectra and stress severities are presented. We show the need for calibration of crack growth models when applying to long spectrum histories, using results from recent crack growth coupon tests. The paper concludes with a discussion on the need of better predictive models for both crack initiation and growth for application to a wide variety of aircraft problems.