Issue 38

M.V. Karuskevich et alii, Frattura ed Integrità Strutturale, 38 (2016) 198-204; DOI: 10.3221/IGF-ESIS.38.27 199 I NTRODUCTION eing first emerged in aviation, Structural Health Monitoring (SHM) systems recently have been spread into the different areas of mechanical engineering applications. Among the highly responsible components of SHM systems, the fatigue sensor should be considered as a key one. The history of research and developments of metal- based fatigue sensors started together with awareness of the fatigue problem importance. Unfortunately, it should be stated that the practical application of metal fatigue sensors remains to be pretty limited even in the spheres were sensors are of high demand. The low level of industrial applications, to our mind is related to the poor scientific basis of many proposed approaches and designs in this area. To an even greater degree this problem is of crucial importance for multiaxial fatigue monitoring. The multiaxial loading is the mostly spread mode for many engineering structures. The complexity of phenomena to develop there has led to numerous theories, models, experimental studies, etc. [1-5]. The papers [6-10] present some recent reviews and approaches for multiaxial fatigue and fracture problems. Being general ones for multiaxial fatigue analysis, these criteria might be classified into three groups, being based on fatigue damage parameter used, i.e. stress, strain and strain energy density criteria [11]. Each one was described mainly by the critical plane orientation. Critical plane concept usually applies different loading parameters within the critical plane whose orientation is determined by i) only shear loading parameters; ii) only normal loading parameters or sometimes iii) mixed loading parameters [12]. The variety of loading conditions gives rise to the development of new concepts, providing rather accurate prediction of the fatigue life. In some cases the unique properties of particular materials might favour a possibility for the prediction their service life. These approaches demand for the obtaining new experimental data. Experimental studies of multiaxial fatigue are based on testing specimens of rather complicated shape. The most commonly used specimens have the pipe shapes (similar to ones used in bulge test) as well as cross-shaped (cruciform) ones. In so doing, metal specimens were tested. The available experimental data related to the multiaxial loading of aircraft structure are somehow unavailable. For example, the report on biaxial tests of aircraft component might be found in [10]. The data presented in the paper are the part of investigations aimed at development of the creation for accumulated fatigue damage assessment for structures subjected to multiaxial fatigue. The monitoring of the process was performed with the use of computer aided optical technique based on observation of surface deformation relief of aircraft components made of cladded aluminium alloys [13-15]. Since the strain-induced relief results from localized plastic deformation this approach might be referred to as the strain criteria. Recently it has been suggested that the deformation relief might be as well taken into consideration within a criterion of the biaxial fatigue damage. L OADS TO ACT ONTO AIRCRAFT IN FLIGHT AND ON THE GROUND ircraft structure is considered as a typical one being subjected to biaxial loading since it experiences the spectrum of loads in air and on the ground. Among them are: wind gusts, loads from airdrome unevenness, buffeting motor vibration, acoustic vibration, loads at maneuvers, pressurization, etc. The complexity of analytical and experimental estimation of aircraft components fatigue life is determined by the complexity of their loading patterns, i.e. by irregular (random) character of the loads sequence, multiaxial stress state, etc. The fuselage skin is subjected to the simultaneous action of loadings coming from pressurization, bending and torsion. In the case of airwings the stresses caused by the combined action of bending and torsion are considered as dominant ones. These loads give rise to normal and tangent stresses to act in the bearing components (Fig. 1). In doing so, normal stresses are caused by the bending of the wing, while tangent ones result from the shear force and torque moment. The assessment of the accumulated fatigue damage can be performed analytically as well as by instrumental inspection. Both approaches need further improvement especially under multiaxial fatigue. D EFORMATION RELIEF FORMATION UNDER UNIAXIAL FATIGUE he aircraft skin is usually made of cladded aluminium (alclad) alloys (2024T3, 7075T6, etc.). The detailed description of the deformation relief nature, its evolution and methods for the analysis can be found in [13-15]. Let us point out some key aspects of the nondestructive methods aimed at the analysis of the strain-induced relief. The 2D–optical images of the latter were registered on aluminum alloy specimens under fatigue tests. In order to protect B A