## 1 Introduction  
Helicopter blades are essential elements that can withstand high loads and, at the same time, need to be as light as possible. To meet these requirements, helicopter blades are manufactured using composite materials: carbon fibre, fibreglass, and Kevlar fibre [1]. In order for the helicopter blade to be used safely and with maximum efficiency, several structural criteria must be taken into account [2, 3]: high resistance to forces in the axial direction, which is the main stress, as well as the secondary stress coming from the bending forces acting through the lift force generated by the aerodynamic profile of the blade and its angle of attack; low mass, which is necessary for two reasons: the first is the additional fuel consumption that the helicopter has to sustain to generate the force required to rotate the blades and the second is that through the action of the centrifugal force the mass effect of each fraction is amplified by at least one or two orders of magnitude, which further stresses the blade structure in the axial direction; and the fatigue strength of the material is also a very important factor as it is part of a dynamic assembly, the blade is constantly subjected to a load-unload cycle, which leads over time to fatigue cracks [4] and failure. The core of the helicopter blades [3] can be made of foam or a honeycomb structure (metallic, composite, and Nomex). Composite materials are replacing the conventional ones (metals and wood) in the structure of helicopter blades due to their ease of processing and productivity. The main advantages of carbon fibre and fibreglass composites are the following [5, 6]: high strength-to-weight ratio, excellent fatigue resistance compared to metal alloys, high strength, high corrosion resistance, and anisotropy (different physico-mechanical properties in different directions). These properties are the main reasons for the use of composites in the manufacture of aircraft parts, which are improved performance resulting from the ability to optimise the shape, structure, and mechanical properties of the parts, and weight reduction, which improves the efficiency of the parts and the aircraft and allows a higher load or operating range for the aircraft [7]. Due to the fierce competition between companies, there is a desire to introduce superior products to the market in terms of operational performance, which can be achieved using accelerated reliability testing for blades [8] but also by determining reliability indicators for the spar structure of helicopter rotor blades [9]. An important aspect of helicopter maintenance and flight safety activities is the determination of blade life using fatigue stands and tests [3,10]. Fatigue is the process of localised, progressive, and permanent structural changes in one or more areas of the blade under stress that produce variations in stress and strain and may culminate in cracking or complete failure after a sufficient number of oscillations [11,12]. The vital components (blades) of the helicopter structure are subject to particularly rigorous testing, as they cannot be doubled and their failure can lead to serious accidents [13,14]. Another method used since the early stages of aeronautical products is finite element analysis (FEA). Various studies [15-17] determine the distribution of equivalent stresses and total displacements for helicopter blades made of composite materials by FEA. Another research direction dedicated to the study of helicopter blades is aerodynamic optimisation using neural networks based on CFD solutions [18]. In order to design blades with the longest life and the best stress resistance, engineers very closely analyse the behaviour of blades in service; therefore, non-destructive methods [19,20] and failure mode analysis [21,22] are current activities in the life cycle of aircraft parts.