Fatigue load level

The evolution of damage through these phases depends on the cyclic fatigue-loading level three loading levels can be identihed with respect to the damage produced in the composite [10] ... 

In the case of continuous damage growth, which will be observed at higher fatigue load levels, a continuous increase in specimen temperature is also measured. Figure 5.15(a) shows the temperature change for four different tests. All tests were performed until specimen rupture occurred. A continuous temperature increase can be observed throughout the tests. This corresponds to the parallel observation of transverse crack development... 

In order to accurately model the fatigue behavior of rubber, fatigue analysis methods must account for various effects observed for rubber during constant amplitude testing. Effects associated with load level, 7 -ratio (ratio of minimum to maximum loading level), and crack closure are presented in this section. 

The relationship between load level and fatigue crack nucleation lives is clearly evident from the e-N and S-N plots for the material. A sample e-N plot for natural rubber is presented in Figure 25.4. An increase in the load level of the applied cycles results in a shorter fatigue life. Strain levels below the fatigue life threshold produce inhnite fatigue lives. The relationship between the load and the fatigue life follows a linear relation when plotted on a log-log scale. 

Tests for the J R curve and the determination of can be conducted by employing multiple specimens or a single specimen. With the first method, identical multiple specimens arc tested to different load levels and then unloaded. The extent of stable crack growth is marked by staining or fatigue cracking after the test to facilitate the crack extension measurements. Multiple measurements enable the construction of a load vs. crack-extension curve. The single-specimen test technique relics on the determination of... 

Loading Level 1 Matrix cracks begin to develop while the hbers remain intact. Under cyclic loading, these cracks propagate until they reach a hber—matrix interface or some other inhomogeneity. If the cyclic load is sufficiently low, these cracks will not propagate any further, and no more fracture surfaces will be created until the fatigue limit of the hbers is reached and overall composite failure occurs. 

Under fatigue loading in stage I, the formation of transverse cracks can be observed very early, depending on the cyclic strain level. The formation of transverse cracks dominates the stiffness reduction ascertained in early fatigue life in stage I and is proportional to the transverse crack density [22]. 

Figure 5.14 Fatigue test of a cross-ply laminate at a low load level. Stiffness reduction, development of transverse cracks and temperature change are plotted versus the number of load cycles. Specimen did not fail. Number of transverse cracks taken from X-ray radiographs. Fibre T300, resin LY 556, MY 720 (50 50), lay up [Oj, 90j, O2, 9O2],. R= -1, f = 10 Hz, = 340Nmm ...

Figure 5.18 Total temperature and fatigue lifetime variation with load level, 10Hz, unidirectional laminate (Dyneema-epoxy O/OA))...

As with the work of Swain [7] and Subramanian [9], the S-N curve for the 8 series shows substantial differences between the performance of the A and O fiber sized composites. Reviewing the S-N curve for the 8 series compression-compression, R = 10 fatigue, a best fit line through each of the data sets assists in delineation of the difference in performance of the A and O interphases, as shown in Fig. 11.1. The apphed cyclic stress levels are presented as a percentage of their ultimate notched compression strength, as reported in Table 11.1. The 810-A composite exhibited a slightly lower rate of fife reduction with increasing load level. However, all lifetimes... 

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