# The Influence of Fatigue Loading on the Durability of the Conveyor Belt
## 2 Fatigue of the Conveyor Belt as a Textile Composite  
Fatigue testing of textile composites is a relatively new concept when compared to fatigue examining of structural materials [21]. The building fibers of the conveyor belt are semi-crystalline materials which show highly elastic properties--they act partly as a viscous fluid, and partly as an elastic material. The elastic and plastic regions of visco-elastic materials and the corresponding yield point of the material and its limit are not precisely defined for such materials. Fibers are isotropic in nature, but reinforcement is anisotropic, which means that there are moderate constraints on molecular alignment. This feature directly affects the type of fracture due to the application of alternating loading to the fiber. Therefore, fiber fatigue fracture can be far more complex than fatigue fracture experienced by metal. It is not possible to consider any single fracture mode as solely responsible for the ultimate fracture of the fiber. The fiber can fail due to tension, bending, twisting, abrasion, environmental conditions, etc. There are over twenty different fracture mechanisms that have been identified that can lead to ultimate fiber fracture [22]. Fibers used for the carcass of conveyor belts have a slender and long structure. They behave rigidly during tensile tests, while they are very flexible during bending tests. The largest number of fiber tests are for tension [23]. The shape of the fracture that can be observed when the fiber fails due to fatigue can be used during diagnostics and provides insight into the mechanisms that led to the fracture. A comparison of fiber fracture due to tension and fatigue is shown in Figure 1.  
When a fiber fractures due to tension, an inclined fracture zone spreads from the place of the initial crack occurrence, which is caused by the plastic deformation of the fiber. During the crack propagation process, the cross-section of the fiber decreases, which leads to the ultimate fracture of the fiber. In the case of fiber fracture due to fatigue, the initial crack appears on the surface of the fiber as in the previous case, but the direction of crack propagation is different. While the crack in tension propagates towards the interior of the fiber, in the fatigue fracture mechanism the crack propagates along the fiber at a small angle to the axis of the fiber. The crack propagates to a critical point when the cross-section of the fiber decreases and damage occurs [24]. Figure 1: Fracture morphology of polyester and polyamide fibers: (**a**) due to tension; (**b**) due to fatigue [24]. In fiber-reinforced composites, which include conveyor belts, the fibers are load-carrying elements, while the matrix has the role of holding the fibers together and protecting them from harmful environmental influences. During testing of textile composites, two categories influence the obtained results: the parameters of the tested material and the parameters of the fatigue test method. As for the parameters of the tested material, one of the most important is thermal conductivity. When testing specimens at higher frequencies (5-10 Hz), the temperature of the tested object increases to over 100 \({}^{\circ}\)C [25]. When the thermal conductivity is low, heat accumulates in the sample, and the material deteriorates faster due to heating. This feature is extremely important when testing specimens of conveyor belts because the upper and lower protective rubber layers act as insulators that additionally lead to heat accumulation. During the manufacturing process of the textile composite, damage may occur that will later affect fracture due to fatigue. Significant in-plane shear may occur, leading to the formation of folds that later negatively affect the fatigue behavior of the specimen [26].