# The Influence of Fatigue Loading on the Durability of the Conveyor Belt
## 3 Testing and Modeling of Conveyor Belt Fatigue Behavior  
The uniaxial tension/tension test is the most commonly used test. Servo-hydraulic test devices or devices with servo-electric motors with adequate drivers are used. Alignment of the specimens in the jaws is extremely important. Bending loading should not occur, so a ball-and-socket joint is used between the jaw and the force sensor in order to transmit only forces and not torques. It is necessary to measure the deformation of the specimen, and for this, high-precision displacement sensors--extensometers--are used, or data from the driver of the servo-electric motor can also be used as displacement sensors. The highest possible test frequency is chosen in order to shorten the duration of the test, but it is necessary to take into account that the increase in the temperature of the specimen due to high frequencies leads to the degradation of the material characteristics. During the test, it is possible to monitor a large number of parameters--loading amplitude, deformation, test frequency, changes in axial stiffness, variations in Poisson's ratio, etc. The change in the axial stiffness can be obtained as the quotient of the measured loading and the measured strain from the displacement sensor. Depending on the type of fiber, the stiffness degradation can range from just a few percent to several tens of percent [32]. By analyzing valid standards concerning testing of specimens of conveyor belts [33], it was determined that there are no direct guidelines regarding fatigue testing of conveyor belt specimens. Therefore, it is possible to apply higher level standards related to tension/tension fatigue testing of composite materials. Those are ASTM D 3479/D 3479M-96 Standard Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials [34] and ISO 13003 Fibre-reinforced plastics--determination of fatigue properties under cyclic loading conditions [35] standards. These standards provide general principles of fatigue testing of textile composites and therefore can be applied to the testing of conveyor belt specimens. There are two technical reasons why modeling fatigue damage in textile composites is difficult and expensive. The first reason is that there are multiple levels at which damage mechanisms are present--micro level (fibers and matrix), medium level (single layer), and macro level (textile composite). The second reason is the impossibility of producing identical specimens due to the nature of the production of textile composites [36]. There are two divisions of existing fatigue models for textile composites. Figure 2: Gradual development of textile composite damage due to fatigue: (**a**) initial phase–no failure due to fatigue; (**b**) first damage phase – delamination of welt threads and the matrix, transverse cracking; (**c**) second damage phase – delamination spreads between welt threads and the matrix, meta delamination between welt and warp threads; (**d**) third damage phase – crack propagation from welt to warp threads and fatigue fracture occurs [31]. According to [37], it is possible to divide fatigue damage models according to fatigue criteria:  
* Fatigue criterion based on macroscopic strength;
* Fatigue criterion based on residual strength;
* Fatigue criterion based on residual stiffness;
* Fatigue criterion based on actual damage mechanisms. According to [38], fatigue models for textile composites can be classified into three major categories:  
* Fatigue life models that do not consider the actual mechanisms of material deterioration, but are based on S (S--stress)-N (N--number of cycles) or Goodman diagrams;
* Phenomenological models of residual stiffness/strength;
* Progressive damage models that use one or more variables that are associated with measurable damage representations. Regarding the first category, there are many models based on established S-N diagrams for metals. This approach requires a large number of experiments for each individual material, for different ways of layering, different loading conditions, etc.