PSI - Issue 24

Filippo Cianetti et al. / Procedia Structural Integrity 24 (2019) 526–540 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1: 3D model of the frame

increased safety and lower costs Moore (2013). In this scenario, the safety of airplanes, declined in aeronautical form as airworthiness, though imposed by mandatory rules and maintaining the character of a fundamental ethical question, becomes a legitimately pursued commercial purpose De Florio (2016). Airworthiness, referring to an aircraft or part of it, is defined by the Technical Regulations of the ENAC (Fa 54 of 8.5.2008) as: ”[...] the possession of the requisites necessary to be able to fly safely within the limits allowed. ” De Florio (2016). There is therefore a process, commonly called ”certification”, which has the ultimate aim of demonstrating to the in charge authority the compliance of an aeronautical product with the requirements of the reference standard Purton and Kourousis (2014), De Florio (2011). It is a complex path, driven by the thrust of conflicting pressures: on one hand the need to produce solid, robust evidence, capable of convincing the authority; on the other the constant constraint of times and costs that will inevitably fall within the development of the products De Florio (2016). Among the requirements to be demonstrated at this stage, fatigue resistance is particularly addressed by the certifiers for having been the cause of disastrous accidents in the past De Florio (2016), Braga et al. (2014). For critical components, designed with the ”safe life” criterion, the standards require demanding, and very time consuming, experimental tests De Florio (2016). In this case the certifier requires that at least one year of safe life be first demonstrated and that the tests continue, demonstrating the persistence of the requirement, taking as reference the flight hours of the oldest aircraft De Florio (2016). The scenario that is generated when the experimental simulations reveal, after the entering in service, the inadequacy of the structure to cover the expected life of the aircraft, is catastrophic in terms of costs and logistical impact Schijve (1994). Hence the need to numerically simulate the experimental tests as accurately as possible, already in the prototype phase, creating a strong mitigation action to the aforementioned risk Archibald (1977); Braccesi et al. (2015); Cianetti et al. (2019); Georgiadis et al. (2008); Guida et al. (2013); Ru˚zˇek and Beˇhal (2009); Xu et al. (2012). The proposed work fits into this context, taking as its starting point the certification of a minor structural component, conceived in a ”safe life” perspective, to adopt a numerical estimation procedure of durability that adheres as much as possible to reality. The object of the study is a support frame (Fig. 1), designed to accommodate di ff erent devices, on a military helicopter. The component has now reached the prototyping stage, therefore already sized and built it is subjected to the functional and flight tests required for certification purposes. In this phase, the authors, intend to virtually replicate the qualification test of the endurance section reported in the MIL-STD-810-H standard to the ”vibrations” method. The applied theoretical methodology is illustrated in the flow chart of Fig. 2, and distinguishes four processes: the choice of the material, the CAD / FEM modeling of the part, the laboratory tests and the functional tests in an operational environment. The estimation of fatigue life, along the four processes described above, meets two implementation phases: • a first phase where, starting from the numerical modeling of the object, experimentally validated, through mea sured and calculated load spectra, returns as output the stress in the component; • a second phase, i.e. the assessment of the so-called fatigue life, in which from stress, damage model and S-N curve, a map of the fatigue damage or the fatigue life is obtained.