Splitting initiation and propagation in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading

The initiation and propagation of splitting damage in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading were investigated experimentally. One-hundred-and-twenty [0°]₈ specimens, notched at the center with a 3·175, 6·350, or 9·525 mm hole or slit, were tested. Thirty specimens were loaded monotonically to failure to determine the static splitting stress. Ninety specimens were cycled at maximum loads corresponding to 80%, 70%, and 60% of their predicted static splitting stress. The stress ratio of the load cycle was maintained at a nominal value of $\text{R}{}_{\mathrm{\sigma }}\text{= 0·1}$, while the frequency was changed with the stress amplitude to maintain the same loading rate. The results show that the static splitting stress is sensitive to both notch type and size and that the split length varies linearly with the natural logarithm of the number of accumulated load cycles. The intercept of this linear relation depends on the notch type, notch size, and stress level while its slope depends only on the notch size and stress level. Empirical functional forms are derived using multiple linear regression analyses of the test data.