Natural-frequency reduction model for matrix-dominated fatigue damage of composite laminates

A new non-destructive fatigue prediction model for composite laminates is developed. The natural frequencies of fatigue-damaged laminates under extensional loading are related to the fatigue life of the laminates by establishing the equivalent flexural stiffness reduction as a function of the elastic properties of sublaminates. In particular, the stiffness degradation in 90° laminae is recast by exploring the 90°-ply elastic modulus reduction, and by using the concept of a linear fracture mechanism for the cracks in 90° plies, where the crack-growth displacement is defined to give the correct average crack density in the cross-ply laminate. This crack density is then used in the stiffness reduction model. The flexural stiffness is derived by relating the 90°-ply elastic modulus reduction, and using the laminate plate theory to the degraded elastic modulus and the intact elastic modulus of other laminae. The natural-frequency reduction model, in which the dominant fatigue mode can be identified from the sensitivity scale factors of sublaminate elastic properties, provides natural frequency vs. fatigue curves for the composite laminates. Vibration tests were also conducted on [902/02]s carbon/epoxy laminates to verify the natural-frequency reduction model. Correlations between the predictions of the model and experimental results are good.