Fatigue life prediction of thick-section S2-Glass fiber reinforced Vinylester composites has been studied analytically using fatigue modulus concept. Flexural fatigue tests were conducted under three point bend configuration. A stress ratio of R = 0.1 and a frequency of 3 Hz has been used for the fatigue tests. Fatigue data have been generated at five load levels; 85%, 80%, 70%, 60% and 55% of the ultimate flexural strength. Using these fatigue data, S-N diagram has been generated. Fatigue modulus has been determined by the slope of the line drawn on a plot of applied stress vs. resultant strain at specific loading cycle. Since fatigue modulus degrades with cycle number, it was assumed that the degradation rate follows a power function of fatigue cycle. Using this concept, a practical and applicable equation for predicting fatigue life is established.

The Fatigue Life Prediction method used in this investigation requires two distinct parameters, namely applied stress level and two material constants. These material constants have been determined from the fatigue test data. A comparison has been made between the analytical prediction and the experimentally obtained S-N curve. The correlation between the two has been observed to be excellent. Flexural failure modes have also been identified as extensive delamination, fiber fracture and fiber kinking. Microscopic observation shows that all failures are predominantly on the tensile side with slight fiber kinking and matrix crushing on the compression side.

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