## 4 Tire Testing Results and Discussion
This clearly indicates that before the accident, there were corrosion centers on the wires of the steel cord, which weakened the tire structure (Figure 14). On the cross-section of the wires, no traces were noticed that would indicate interference of a sharp metal element with the tire. The presence of corrosion of the steel cord at the fracture site (which has been proven before the accident) allows the conclusion that in this place (marking 4, Figure 5), the tire sidewall was weakened. This fact cannot be interpreted as the cause of an impact fracture but only as a necessary condition for such a fracture to occur at all. It cannot be ruled out that with such a weakening of the tire structure, its breakage may have occurred while driving. Figure 14: View of selected steel wires: (**a,b**)—sample 3; (**c,d**)—sample 5. Figure 13: Places of sampling and method of marking samples. The surfaces of the wire fractures (Figure 14b,d) are rough and intercrystallite. There was also observed small necking at the sectional area. However, in the steel of pearlitic structure, there is an expected visible cup or cone shape of the material deformation. Due to the lack of occurrence of these breakthrough features, it can be concluded that the damage to the wires took place at a relatively high speed or that the damage was caused by the presence of intercrystallite corrosion. Non-destructive tests to control the occurrence of this type of corrosion are practically not carried out even for elements to which there is unlimited access. In the case of a cord placed as reinforcement in the tire material, it is not possible to conduct direct corrosion tests at all. Even small cracks in the tire carcass can cause the penetration of moisture by capillary action and the development of corrosion of the steel cord, which can lead to its significant weakening and, at the same time, an increase in the temperature of the tire material in the area of corrosion. Subsequent tests concerned the formation of irregular sidewall abrasions on the so-called belt on both sides of the tire (Figure 5). A thesis was formulated which assumed the appearance of abrasions as a result of operation with too low air pressure in the tires. In order to confirm the probability of this thesis, the key test was to examine how the sides of the tire pressed to the surface by the wheel rim deform. During the analysis of the successive stages of loading the tire, it was noticed that the side of the tire was deforming in the place where the \"belt\" was located (Figure 5). The static deflection of the tire profile, loaded with a mass of 603 kg, was equal to 250-140 = 110 mm. The results of deflection vs. load of the tire tests are shown in Figure 15. For a sample size of 20, in five levels of tire deformation, a regression function was found as well as a 95% confidence interval for the regression equations. These parameters were determined in accordance with the rules applicable in scientific works [56; 57]. The method of deformation of the profile (and \"belt\") with a mass of 603 kg is shown in Figure 16a. With such deflection, the side of the non-rotating tire did not touch the ground surface. The static deflection of the tire profile, loaded with a mass of 676 kg, was equal to 250-120 = 130 mm. The method of deformation of the profile (and the \"belt\" of the tire), when loaded with a mass of 676 kg, is shown in Figure 16b. Figure 15: Results of testing load mass for particular deformation of tire up to contact of the “belt” with ground. The way the profile deformed during the static load test indicates that the \"belt\" on the sidewall of the tire was not making contact with the road surface. In this context, it should be considered unlikely that the scratches on the side of the tire (\"belt\") were related to the long-term operation of the tire with a lower tire pressure value. In such a case, the abrasions would be below the \"belt\" and would be uniform.