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Significance of static and fatigue strength testing
The fatigue of a material is a phenomenon of breaking, i.e. fracture of the body due to loads variable over time.
The notion of the fatigue of material (metals) was most likely introduced by J.V. Poncelet. In the middle of the last century femenological fatigue tests were introduced by A. W?hler. The constant increase in theoretical and experimental fatigue-themed research accompanies continuous technological progress in material engineering, the construction of machines and vehicles, and most of all in aircraft and spacecraft. The interdisciplinary nature of the textile industry causes that materials and polymers are used in designing machines and equipment for a growing range of applications. Special textile products are more commonly used in the military industry,automotive industry and aviation. These materials are expected to have special mechanical properties, for which classical theories and research methods of static strength are insufficient. The requirements for modern textile materials are often compared to those for metal or steel materials.
In the case of the variable stress of textile materials, even if its maximum value does not exceed the value of the yield strength, this may cause local microfractures in laden material after some time. In the case of composite materials for designing machines or steel products,fractures may develop over a period of time (propagation of fracture fissure) and end with a sudden brittle fracture (with out previous permanent deformation of the whole element).
As for textile materials, assessment of the fatigue of material is extremely complex due to the variety of constructions and structures.
In real conditions of using the product,loads acting on structural elements are often of a variable as well as steady (sinusoidal) or unsteady (random) nature over time. Such loads are called variable loads, and stress corresponding to them is defined as variable (Figure 1).Variable loads cause a number of complex physical phenomena in a material, depending on the values of the load and number of cycles. A large variety of these phenomena and their accumulation lead to the weakening and ultimately damage of an element. Damage caused in such a way is called the fatigue of a material, and variable loads which led to such damage are called fatigue loads.
The first works in the scope of the fatigue strength of materials began to appear about 170 years ago, focussing on the relationship between load and fatigue life. Since then, a lot of scientific publications discussing the phenomenon of fatigue have appeared. However, no effective method of fatigue life estimation which would take into consideration all aspects investigated by scientists has been developed. This phenomenon is examined for a number of aspects such as material,shape of the subject, type of load and even the state of stress [32]. In some cases,mechanical stresses are compounded by significant thermal stresses, which cannot be avoided, e.g. at the time of starting-up and shutting-down of equipment or in the cooperation of two elements of different thermal expansion. In a number of elements, we deal with a multiaxial state of stresses, while the components of these stresses do not occur phase-consistently.In practice, we deal with very different stress passes of random nature resulting from varying exploitation conditions.Fatigue is the result of lots of changes taking place in differently loaded parts of equipment in which, depending on the time and the value of the load, an element is damaged as the final result.
Material loads are usually of complex nature because multiaxial states of load generate spatial stress and deformation states. The complex nature of fatigue processes forced the need to search for methods and tools useful in the process of predicting the fatigue life. Lots of hypotheses might be found in literature enabling the reduction of a complex, spatial state of stresses to a uniaxial one.The criteria of multiaxial fatigue related to those hypotheses might be divided in terms of the physical nature of the parameter influencing the damage of an element into stress-related, displacementrelated and energy-related (stress- and displacement-related).
Methods based on the stress-related record are used mostly in the scope of the high-cycle fatigue strength of materials, whilst those based on the displacement-related record are used primarily in the low-cycle scope. Energy-related methods are used both in the case of a small and large number of cycles. It is impossible to distinguish one universal criterion in the form of a multiaxial criterion of fatigue that could be used for all types of materials due to their diversity and continuous development.
One of the main purposes of research in the field of the fatigue life prediction of textile products is the development of methods to estimate strength already at the design stage. Thanks to analyses carried out on the basis of experimental research as well as with the use of calculations and simulations based on a computer model, it is possible to predict fatigue in an early phase of the life of a product. Even more often, numerical calculation methods are also used in predicting the fatigue strength of textile products.
The fatigue phenomenon of materials appears both in the engineering industry and light industry. With the development of materials science and technology of manufacturing, new materials with specific physical and chemical properties are being developed, as a result of which it is possible to develop models allowing to estimate the fatigue life of materials with greater precision.
The work analyses literature on the subject matter in the scope of fatigue strength assessment methods of textile products as well as provides the scientific systematisation of problems related to fatigue life.