## 1 Introduction  
Transport is an integral and indispensable part of human lives, joining everything into one harmonic whole [1]. However, along with the increase in the intensity of transport, we feel its negative effects in the form of exhaust emissions, noise, vibrations and, above all, threats to the safety of road users in the form of road accidents. A tire is an element of a car that directly affects the road surface, transmitting driving, braking and lateral forces. The structural requirements for a tire are related to the necessity to transfer the above-mentioned forces, and therefore it is required to have adequate stiffness. This feature results from the complex construction of the tire, in particular the textile carcass, steel belt, butyl cover and fillers. Steel wire or products containing it are used to strengthen tires in the form of strands or ropes, the so-called steel cord [2; 3; 4], which is embedded in the elastomer to stiffen the structure while giving it adequate strength to tensile forces. Single steel wires have a diameter from 0.15 to 0.38 mm [5], and most often, they are produced as brass-plated or galvanized surfaces. Wires intended for the production of steel cords are made of quality unalloyed steel, containing from 0.70 to 0.95% of carbon. They have a pearlite structure and are suitable for drawing or cold rolling [6; 7; 8]. The chemical composition and mechanical properties of the wires should comply with EN 10323 [9]. The fatigue strength of the steel cord affects the operational properties of tires and, consequently, the safety of vehicle traffic. Due to the fact that the steel cord is covered with a layer of rubber, identification of possible adverse phenomena and cord degradation during tire operation is difficult [5]. Steel cords used for the production of passenger car tires have strands of two to five fibers (e.g., 2 + 2 \(\times\) 0.25; 2 + 1 \(\times\) 0.30--the first number means the number of middle wires and the second number of braided wires with adequate their diameter in mm), while cords used for tires intended for trucks and special construction machines are of multi-wire constructions (e.g., 2 \(\times\) 0.30; 3 \(\times\) 0.25 + 6 \(\times\) 0.35) [5; 10; 11]. Depending on the manufacturer and load of the tire, the number of wires and their diameter varies. Regardless of this, the cord reinforcement should meet the appropriate strength requirements. The technical condition of the tire has a significant impact on road safety, which is confirmed by numerous literature reports [12; 13; 14; 15; 16; 17; 18; 19; 20]. Unfortunately, it is often stated that vehicle operators disregard the technical condition of pressure, which has a significant impact on driving comfort, fuel consumption and road safety [20]. Vehicle tire pressure monitoring systems [21; 22] and vehicle inspection systems in transport companies are also helpful. The relationship between the drop of pressure in passenger cars and lifetime was studied, among others, by [PERSON] and [PERSON] [23] and [PERSON] [24] as well. Their research shows that the use of motor vehicles with reduced pressure by about 7 kPa shortens the service life by almost 2%. When the tire pressure is too low, the temperature of the tire increases while the vehicle is running, which may cause changes in its structure, permanent damage or even an explosion [22]. Moreover, low tire pressure increases fuel consumption and rolling resistance due to the greater contact area of the tire with the ground [25; 26; 27]. In these cases, the forces associated with the vehicle wheel load and increased frictional forces resulting from improper tire contact with the ground act synergistically on tire wear [28; 29]. In turn, a pressure that is too high causes excessive wear of the tread, reducing the time of safe operation [21; 22; 27; 30]. A high vehicle load combined with incorrect tire pressure also contributes to the vehicle's controllability.