PSI - Issue 59

M. Karuskevich et al. / Procedia Structural Integrity 59 (2024) 642–649 M. Karuskevich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Two options are possible: attachment of the aluminum single crystal foil sensitive element and attachment of the polycrystalline aluminum foil sensitive element. Both materials are sensitive to cyclical loading and demonstrate surface deformation relief (extrusion/intrusion structure). Dimensions of the sensitive element are 5 mm diameter; 0.15 mm thickness. The cyanoacrylate glue provides attachment, widely used in the aviation industry for strain gauge attachment at fatigue tests or aircraft flight tests. The geometry can be varied according to the need for fatigue indicator sensitivity. 5. Field of application Fatigue monitoring is an actual problem for a large number of engineering structures. During fatigue indicator research and development, particular features of inspected objects must be considered. These are: inspected metals, bearing element design, loads, and expected lifespan. Discussed above indicators are aimed at aircraft fatigue monitoring, as metal fatigue is still an actual problem for aviation due to the requirements of weight reduction, high level of safety, and long service life. According to the Advisory Circular No: 25.571-1D (2011) issued by Federal Aviation Administration, special attention must be drawn to the so-called Principa l Structural Elements. As defined in this A.C. “ a principal structural element is an element of structure that contributes significantly to the carrying of flight, ground, or pressurization loads and whose integrity is essential in maintaining the overall structural integrity of the airplane. Principal structural elements include all structure susceptible to fatigue cracking, which could contribute to a catastrophic failure.” One of the principal structural elements vulnerable to biaxial cyclical loading is a pressure bulkhead, where the fatigue indicator could be installed to avoid unexpected failure. A review of the aircraft bulkhead fatigue failures may be found in the paper by Li (2021). The accident investigation includes six failures of Tri-Star Lockheed L1011 pressure bulkheads. The fatigue cracks explained at least three accidents. These happened in 1989 and twice in 1995. Fatigue crack in the pressure bulkhead initiated the accident of Boeing 747 in 1985. In 1971 fatigue cracks in the pressure bulkhead damaged by corrosion caused the failure of the Airbus A-340. A pressure bulkhead is only one of many examples of pressure vessels prone to biaxial fatigue. It is known that ultrasonic testing and radiography are widely used for diagnostics. At the current state of the art, the biaxial fatigue indicator described above can be considered a supplementary nondestructive method to ensure pressure vessels’ structural integrity. 6. Conclusions Aircraft accident case studies show that despite the development of analytical methods for multiaxial fatigue assessment, primary structural elements are still prone to fatigue failure. Surface relief fatigue indicators for uniaxial loading developed early have been tested at a wide range of loading conditions and have proved their efficiency. Based on the success of the uniaxial fatigue indicators, the conceptual design of the biaxial fatigue indicator has been proposed. For both types of fatigue indicators, namely uniaxial and biaxial, the fatigue damage is estimated by the intensity of the surface deformation relief, i.e., extrusion/intrusion structure, having crystallographic character. The proposed concept of the biaxial fatigue indicator can be adapted for different engineering structures, aircraft, ships, piping or pressure vessels. References Advisory Circular No: 25.571-1D, Damage Tolerance and Fatigue Evaluation of Structure. U.S. Department of Transportation, FAA, 2011. https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentid/865446 Blacha , Ł., 202 . Non - Linear Probabilistic Modification of Miner’s Rule for Damage Accumulation. Materials 14(23), 7335. https://doi.org/10.3390/ma14237335 Boller, C., 2001. Ways and options for aircraft structural health management. Smart Mater. Struct. 10, 432. DOI 10.1088/0964-1726/10/3/302 Corten, H.T. & Dolan, T.J., 1956. Cumulative fatigue damage. In: Proceedings of the International Conference on Fatigue of Metals. Institute of Mechanical Engineering, ASME. 235-246. Chen, D. G., 1996. The method of determining the exponent d in the Corten –Dolan’s fatigue damage formulary. J. Mech. Strength 18, 21 – 24.