Reinforcement fatigue data

Table 7.11 Reinforced thermoplastic short glass and carbon fiber (by weight) compounds fatigue data...

The U.W. measurements are believed to be the first 77 and 4.2 K compressive fatigue data reported on unidirectional fiberglass reinforced polyester. 

Fatigue data are normally presented as a plot of the stress (S) versus the number of cycles (N) that cause failure at that stress the data plotted defined as an S-N curve. Test results are illustrated graphically in the form of curves. Examples of fatigue curves for unreinforced and reinforced plastics are shown in Figure 3-65. The values for stress amplitude and the number of load cycles to failure are plotted on a diagram with logarithmically divided abscissa and English or metrically divided ordinates. 

The data bands for short length fibre thermoplastics and long fibre reinforced thermosets lie adjacent to one another with a similar shape if the fatigue data for each composite is divided by the ultimate strength (Table 10.3). 

Figure 9.19. Tension-tension S-N fatigue data. Filled circles refer to SWCNT-epoxy composites, while the grey band covers most S-N data for unidirectional carbon fiber reinforced composites [48]...

The contours for flex fatigue life are complex. The test is run such that the specimens are about equally strained however, there is some question as to whether the tests should be run at equal strain or at equal strain energy (Gent et al., 1964 McCall, 1969). For some cases, where strain is restricted by fabric reinforcement, fatigue test data should be compared at equal strain amplitude. For other applications, where the strain is not limited, the tests should be run at equal strain energy. The contours as presented here can be interpreted in terms of either constant strain or constant strain energy All points on the chart can be compared at an approximately equal strain per cycle however, if we interpolate... 

The available data for unidirectionally reinforced laminates are typically interpreted such that mode I values seem to constitute a lower bound compared to the other modes, specifically mode II, at least for quasi-static tests. For cyclic fatigue delamination tests, it has been shown by Martin and Murri [59] that cyclic delamination fatigue for CFRP... 

Figure 8.13 Double logarithmic plot of delamination rate da/dN versus Gt-a Irom mode I cyclic fatigue under load control with an R-ratio of 0.1 of Z-pin reinforced carbon fibre epoxy (IM7/977-2). Shown are selected data points for loads of 50, 100 and 200 N.

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