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Motor Brake
Published in Wei Tong, Mechanical Design and Manufacturing of Electric Motors, 2022
In material science, cyclic loading at moderate or high stresses can lead to the formation of fatigue crack and possibly ultimate failure of material. The fatigue cracking process can be made up of two stages: cracks initiation and crack propagation. In electric machines, brakes are exposed to repetitive thermomechanical load cycles due to simultaneous variations of temperature and mechanical strain during the operation of the machines. The stresses formed in the course of braking and subsequent cooling attain the plasticity limit of the brake materials in tension and compression. It is shown that, for locally heated and cooled discs, the cycles of thermomechanical loading and unloading are responsible for the growth of fatigue cracks [7.63]. Practically, thermomechanical fatigue is the main failure type of brake disc, apart from wear.
Introduction
Published in Vladimir V. Bolotin, Mechanics of Fatigue, 2020
In a narrow sense, the term fatigue of materials and structural components means damage and fracture due to cyclic, repeatedly applied stresses. In a wide sense, it includes a large number of phenomena of delayed damage and fracture under loads and environmental conditions. The systematic study of fatigue was initiated by Wöhler, who in the period 1858–1860 performed the first systematic experimentation on damage of materials under cyclic loading. In particular, Wöhler introduced the concept of the fatigue curve, i.e., the diagram where a characteristic magnitude of cyclic stress is plotted against the cycle number until fatigue failure. Up to now, the Wöhler curve has been used widely in the applied structural analysis. At the same time, fatigue and related phenomena are considered as a subject of mechanics of solids, material science, as well as that of basic engineering.
Introduction
Published in M. Rashad Islam, Civil Engineering Materials, 2020
Fatigue refers to the damage in a material due to applied cyclic/repeated loading. Cyclic loading is very common in civil engineering structures, e.g. vehicle loading on roads and bridges. Repeated cyclic loading to a stress (or strain) level less than the ultimate (or even the yield stress) can lead to failure. For example, bending a hair clip backward and forward a single time may not break it. However, the repetition of bending the clip a hundred times may break it. This is called the fatigue failure. The fatigue failure can be both sudden and brittle, although it occurs after many years of satisfactory service. It is therefore potentially very dangerous for civil engineering structures. Fatigue life is defined as the number of cycles of loading leading to fatigue failure. It is not the time to failure, although this can of course be calculated if the frequency of loading is known. Another term commonly used in fatigue testing is called the fatigue endurance limit, which is the stress or the strain level at which no fatigue damage occurs. More specifically, it is the stress or the strain level at which, if a material undergoes cyclic loading, it never fails or is able to withstand a predetermined number of loadings. Figure 1.14 shows the fatigue testing setup for asphalt materials. The outer two clamps hold the asphalt beam specimen in position, and the middle two clamps force the beam down and/or up to produce repeated bending stress. The number of load repetitions to cause the failure is recorded.
Enhancing shear behaviour of the exterior beam-column joint using sufficient reinforcement details or ultra-high-performance fiber-reinforced concrete
Published in European Journal of Environmental and Civil Engineering, 2023
Sayed Ahmed, Heba A. Mohamed, Talaat Ryad, Ayman Abdo
BCJ samples were placed on the test device to enable testing. The displacement measuring device was placed at the free end of the top, and the loading device was placed below the free end of the beam (Figure 5). This setup provides suitable boundary conditions for BCJ as in practice. The specimen’s column is loaded with a constant axial load of 100 kN to match the dead and live loads of the prototype structure. A servo-controlled hydraulic jack is applied to simulate a gravitational load from the upper floors. Then a 350 kN actuator is used to apply cyclic loading at the beam’s free end in the displacement control mode. The cyclic load pattern used in this study is one-way at the end of the beam (Ahmed et al., 2019). A cyclic load consists of repeated loading and unloading cycles (the load returns to zero), as shown in Figure 6c. The actuator applied each target displacement in a semi-constant and monotonous position (5 kN, 10 kN, 15 kN, ……) until sample collapse occurred. The LVDT was attached to the beam’s free end to measure the deflection, as shown in Figure 5a-65.
Assessment of remaining fatigue life based on temperature-evolution measurements
Published in Nondestructive Testing and Evaluation, 2022
Atsushi Akai, Yuka Kojima, Yasumoto Sato
Engineering structures and components are often subjected to cyclic loading, which ultimately leads to fatigue failures. Under time, cost and labour constraints, engineering structures and components are inevitably worked out until their effective performance limits are reached. To guarantee the long-term use of engineering structures and components, their remaining fatigue lives must be assessed. Since a part of the energy imparted to a material under cyclic loading above the fatigue limit (fatigue loading) dissipates in the form of heat, fatigue loading increases the material temperature. In the recent studies pertaining to thermography-based techniques, the fatigue behaviour has been investigated in terms of the energy dissipation derived from temperature signals measured using infrared thermography. The corresponding energy dissipation component was obtained using several approaches, such as lock-in thermography [1–6], calorimetric analysis [7,8], specific heat loss [6,9,10] and thermoelastic phase analysis (TPA) [3,11].
A comparative study on hybrid fatigue stress-life model of RC beams strengthened with NSM CFRP
Published in Mechanics Based Design of Structures and Machines, 2022
Fatigue is an accumulative process of minor damages to a material followed by a sudden failure due to cyclic loading. The structural components of bridges are frequently subjected to repetitive application of traffic loads. A 40-years old Class-A highway bridge might experience no less than 58 × 108 cycles of traffic loading over its design service life (Heffernan 1997); even at stresses below the yield strength of the used material, this might result in cyclic stresses that lead to physical microscopic damage. As load cycles continue, microscopic damage builds up until it becomes a crack (Dowling 2007). Once these minor cracks combine into a major crack, the cross-sectional area of the structural member will be essentially decreased. Over time, the cumulative damage can lead to a sudden failure when the applied load causes stress higher than what the cross-sectional area can withstand (Sobieck, Atadero, and Mahmoud 2015). Fatigue failures are dangerous because they occur without any warning.