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Tribo-material Properties
Published in Ahmed Abdelbary, Extreme Tribology, 2020
A fatigue test is usually done by subjecting a test specimen for a given material to a fluctuating (axial, torsion, or bending) stress until a fatigue crack or other damage leading to complete failure is developed. The results of such tests (the magnitude of an alternating stress versus the number of cycles to failure) from a number of different stress levels may be plotted on a logarithmic scale to obtain a stress–life (or S-N) curve which is considered as the classical strength theory in fatigue analysis. Based on the theory, some other researchers brought in plastic strain amplitude as the calculating parameter and deduced the famous Manson-Coffin formula to calculate the fatigue life. Later, Paris (Paris and Erdogan, 1963) applied the theory of fracture mechanics to investigate the fatigue crack propagation. Figure 2.5 shows a typical S-N curve containing three different areas: Plastic, elastic and infinite life regions.
Introduction
Published in V. Karthik, K.V. Kasiviswanathan, Baldev Raj, Miniaturized Testing of ENGINEERING MATERIALS, 2016
V. Karthik, K.V. Kasiviswanathan, Baldev Raj
Specimens for fatigue tests are designed according to mode of loading, which may be axial stressing, rotating beam, alternate torsion, or combined loading. The specimen sizes and volumes in conventional fatigue are very similar to the tensile specimen configuration, with similar conditions for the radius of the fillet (eight times the gauge diameter) and gauge length of at least three to four times the gauge diameter (Figure 1.14a). A minimum cross-sectional diameter of 5 mm is recommended for fatigue specimens. For specimens tested in tension–compression, hourglass types of specimens (Figure 1.14b) are employed; this has the advantage of minimizing buckling in push–pull loading. Specimens for uniaxial creep tests are quite similar in shape and dimensions of tensile test specimens. The gauge length (GL) and diameter of the GL are typically 30–50 and 6–10 mm, respectively.
Properties of Engineering Materials
Published in Leo Alting, Geoffrey Boothroyd, Manufacturing Engineering Processes, 2020
Leo Alting, Geoffrey Boothroyd
The usual form of a fatigue test is where a cylindrical specimen gripped at one end is simultaneously rotated about its axis and loaded as a cantilever beam. The specimen is thereby subjected to alternating bending stresses, that is, a sinusoidal variation of stress with (depending on the axial loads) different mean stresses. Different types of test equipment have been developed, but these will not be described here. In all cases testing is carried out in accordance with the national standards, which can also help the selection of testing equipment.
Effect of eccentric loading on fatigue cracking mode and characteristics of diaphragm-to-rib welded joints in steel bridge deck
Published in Structure and Infrastructure Engineering, 2023
Liang Fang, Zhongqiu Fu, Bohai Ji, Yixun Wang
The crack-growth rate can be obtained by recording the crack length and corresponding cycles experienced in the fatigue test. The crack-growth rates of specimens loaded with 100 MPa and 80 MPa are shown in Figure 4a and b, respectively. Except for SJ9, the number of cycles required for crack initiation on NES of all specimens is much larger than that on ES under eccentric loading. The reason for the premature cracking of SJ9 on NES may be the small load eccentricity combined with the large residual stress of weld on NES. As shown in the cracking morphology of the failed specimen in Figure 4c, the weld toe on ES will crack ahead of NES. The specimens all failed with cracks on ES extending to the specimen edge. It is also found in Figure 4c that the shapes of DU cracks in real bridges are basically the same with that on ES in the test. Therefore, it is believed that the crack-propagation law obtained under out-of-plane bending loads in the test is in good accordance with that in real bridges.
Effect of test temperature on the shear and fatigue strengths of epoxy adhesive joints
Published in The Journal of Adhesion, 2022
K. Houjou, K. Shimamoto, H. Akiyama, C. Sato
Figure 5 shows the results of the creep tests at RT and 87°C. A constant stress (τap) was applied to the specimen in the electric furnace, and the time to failure (tf) was measured. The stress for which the specimen did not fail until t = 106 s was defined as the creep strength (τw). The shear strengths at RT and 87°C are shown on the left side of Figure 5. The decreases in the fatigue limit with respect to the tensile strength at RT and 87°C were 43% and 59%, respectively. The τw compared with shear strength at 87°C was significantly lower than that at RT. In addition, the breakpoints of the curve appeared in the early stages and were 3 × 104 s and 5 × 103 s at RT and 87°C, respectively. Figure 6 shows the effects of the stress waveforms and temperature on τw. The dark grey bar graph (III, IV) shows the cyclic fatigue limit with a stress ratio of R = 0.[4] In the cyclic fatigue test, the fatigue limit indicates the maximum point of the stress waveform. Figure 6 shows some interesting fatigue properties of the epoxy adhesive joints. For testing at RT, fatigue limit (III) with respect to (I) was only 57%. Therefore, when the adhesive joint undergoes a cyclic stress (strain), its strength is considerably reduced.
Analytical modelling of thixotropy contribution during T/C fatigue tests of asphalt concrete with the VENoL model
Published in Road Materials and Pavement Design, 2021
L. Coulon, G. Koval, C. Chazallon, J.-N. Roux
During its life, a road is subjected to repeated traffic which causes mechanical damage within the layers constituting it. To better understand this fatigue phenomenon and improve the roads design, cyclic loading tests are commonly carried out in the laboratory on bituminous mixtures specimens. There are several types of fatigue tests, the most common are: the direct tensile–compression (T/C) test on a cylindrical specimen, the two-point bending test on a trapezoidal specimen, the four-point bending test on a beam. From these tests, a typical decreasing curve of the norm of the complex stiffness modulus as a function of the number of cycles is obtained. The modulus decreases rapidly and sharply at the start (Stage I), then slowly but continuously (Stage II) before dropping sharply following the sample rupture (Stage III).