Proceedings of the ASME Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2008 October 28-30, 2008, Ellicott City, Maryland, USA SMASIS2008-646 ON-LINE LIFE PREDICTION OF A STRUCTURAL HOTSPOT Subhasish Mohanty Aditi Chattopadhyay Pedro Peralta Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ, 85287, USA ABSTRACT Current aerospace practice follows an engineering model based on damage-tolerant reliability whereby structural components are regularly inspected and replaced. Under this practice, engineering designs are generally based on a physics- based fracture mechanics approach, in which the life of structural component is estimated using an assumed initial damaged condition. However, in a real time environment, keeping track of the damage condition of a complex structural component manually is quite difficult and requires automatic damage state estimation. The real-time damage state information can be regularly fed to a prognosis model to update the residual useful life estimation in event of a new prevailing situation. The present paper discusses the use of an adaptive hybrid prognosis model, which estimates the residual useful life of a structural hotspot using information on the damage condition obtained in real time. The hybrid prognosis model has two modules: an off-line prognosis module that forecasts the future damage state, and an on-line state estimation module, which regularly predicts the current damage state and feeds into the off-line module in real time. Both the off-line and on-line modules are probabilistic models and use the concept of Bayesian inference based on input-output mapping through a Gaussian process. INTRODUCTION Aircraft maintenance must balance labor, logistic, and equipment budget constraints with the competing requirements of fleet readiness, reliability, and safety. Recently, stringent Diagnostic, Prognostic, and Health Management (PHM) [1, 2] capability requirements are being placed on new applications, like the Joint Strike Fighter (JSF), in order to enable and reap the benefits of new and revolutionary Logistic Support concepts. Although the PHM is the name given to the capability being developed by the JSF, to enable the vision of autonomous logistics and therefore meet the overall affordability and supportability goals, similar PHM systems can be developed and implemented for residual life prognostics and health management of any civil, mechanical or aerospace structure. The usual practice for defining the structural life ceiling limits of any current aircraft structural component is based on either of the following three distinct approaches: safe-life, fail- safe, and damage-tolerant approach. Out of the above three approaches the damage-tolerant approach is quite popular in the aircraft industry, either for designing a new aircraft or maintaining an aging one. The damage-tolerant approach assumes the presence of initial defects, regardless of how small they may be, which will be eventually grow in service to be large cracks. Generally, inspection intervals are derived by using appropriate safety factors on the life spent to grow a crack from the detectable crack size to the critical crack length, which would provide a certain number of opportunities to find a crack. These safety factors are empirical or based on experience and will ensure that cracks will be detected at least once before reaching critical size. The United States Air Force also provides guidelines on crack size assumptions to calculate the crack growth lives needed to determine inspection intervals [3, 4]. Once the initial defect condition is known, any suitable crack growth model can be used with the available loading history data to estimate the future crack length and corresponding remaining life to reach that crack length. To work with different available crack growth models, there exist model dependent parameters that have to be fine tuned [5] to make consistent life predictions for the material selected, load histories and geometries to be analyzed. It is noted that the accuracy of the residual useful life estimation at a typical fatigue cycle, depends on proper determination of the damage condition at that fatigue cycle. Manual inspection of the damage condition is generally uneconomical and also undermines the mission capability due to long overhauling time requirement. The current research on structural health monitoring [6, 7] can lead to a paradigm shift in condition based maintenance (CBM) and residual useful life (RUL) estimation procedures. The structural health monitoring based on a damage estimation approach will help to 1 Copyright © 2008 by ASME