FATIGUE LIFE TIME PREDICTION OF POAF EPSILON TB-30 AIRCRAFT - PART I: IMPLEMENTATION OF DIFERENT CYCLE COUNTING METHODS TO PREDICT THE ACCUMULATED DAMAGE B. A. S. Serrano 1 , V. I. M. N. Infante 2 , B. S. D. Marado 3 1 Direcção de Engenharia e Programas da Força Aérea Portuguesa, Estado Maior da Força Aérea Av. Leite de Vasconcelos 2614-506 Amadora E-mail: baserrano@emfa.pt 2 Área Científica de Projecto Mecânico e Materiais Estruturais, Instituto Superior Técnico Av. Rovisco Pais, 1 1049-001 Lisboa E-mail: virginia.infante@ist.utl.pt 3 Direcção de Engenharia e Programas da Força Aérea Portuguesa, Estado Maior da Força Aérea Av. Leite de Vasconcelos 2614-506 Amadora E-mail: bsmarado@emfa.pt ABSTRACT The prediction of fatigue lifetime can be calculated by analyzing the accumulated damage of the aircraft structure through Miner's rule and vertical acceleration spectra. Two different cycle counting methods were used to analyze the vertical acceleration signal which was recorded during 72 flight hours. The first method was the rainflow counting method and the second one the level cross counting method, which is the method similar to the one used by the Portuguese Air Force (PoAF). The results of these two counting methods were compared with the spectrum used by Epsilon manufacturer. Once the spectra were obtained, the damage was also calculated using two methods: the method that considers the influence of the mean stress and the method in which the damage is calculated according to the trapezoid rule. At the end all the spectra were used to calculate the damage through these two methods. The main conclusion was that the operation of the PoAF Epsilon aircraft is more severe than the reference used by the manufacturer, and consequently the lifetime predicted for the aircraft should be 12 % lower than the life defined by the manufacturer. KEY WORDS: fatigue, spectrum, damage, vertical acceleration, cycle counting methods 1. INTRODUCTION In order to predict the fatigue lifetime of the aircraft, the manufacturer of Epsilon carried out a real scale fatigue test at the Centre d'Essais Aeronautique de Toulouse (CEAT). During these tests the manufacturer realized that the aircraft lifetime is determined by the fracture of the second bulkhead beam, which occurred after 89458 simulated flight hours (FH) [1]. In this test the manufacturer used a spectrum that was considered characteristic of the typical aircraft operation. In order to define the secured fatigue lifetime of operation for the PoAF, one of the squadron's aircraft was previously instrumented to measure and record vertical acceleration and local stress in critical areas. The objective of this study was analyse the collected data through two different counting methods and two different ways of assessing the damage. Finally, the objective is to compare the PoAF damage with the manufacturer's damage in order to predict the lifetime of PoAF aircraft, because the spectrum of vertical acceleration of the manufacturer is different from the PoAF one. 2. FATIGUE CONCEPTS According to ASTM [2] the fatigue phenomenon is related to dynamics solicitations, which structurally and permanently changes the material in a specific location where cracks will appear. The process involves 4 phases: the nucleation of the crack, the microscopic growing of the crack, the propagation phase, and finally the rupture (fracture) of the material.