Estimating S-N curves for local fiber dominated fatigue failure in ring specimens representing filament wound pressure vessels with damage

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Modeling the effect of fatigue is important for predicting remaining life of damaged filament wound composite pressure vessels. This study shows how S-N curves can be measured that describe local fiber dominated fatigue failure on the scale of 0.25 mm near stress concentrations caused by a hole representing damage.High frequency Digital Image Correlation DIC was applied to measure strain fields in rings tested by the split disk method. The rings were cut from a glass fiber reinforced polymer pressure vessel. A hole was cut into the rings to simulate damage in the laminate of the pressure vessel.The Miner sum cumulative damage is calculated based on strain histories measured by DIC and several assumed S-N curves. The S-N curve giving Miner sum damage closest to the experimentally observed local failures in several samples is taken as the S-N curve describing the material´s behavior best. The local S-N curve was considerably less conservative than nominal S-N curves obtained from standard coupon testing. Its origin was a factor 2.5 higher.