Fatigue crack propagation in functionally graded materials .

摘要

Interest in the development and application of functionally graded materials (FGM) has increased in recent years, leading to their limited use in a number of commercial and military products. More recently, interest in the use of FGMs in aircraft fuselage structures as integrated thermal protection systems has also begun to develop. However, our limited ability to predict the nucleation and fatigue propagation of cracks in these materials limits the application of FGMs to non-fracture critical structures, reducing the potential benefits of their incorporation. A first step toward addressing this technology gap is to evaluate whether linear elastic fracture mechanics methods, appropriately modified to account for material non-homogeneity, can be used to predict fatigue crack propagation in an arbitrarily graded metal/ceramic FGM.;This dissertation documents new developments for characterization and modeling of fatigue crack propagation in FGMs. Unique precracking and characterization methods, and a Paris equation dependent upon stress intensity and material phase volume fraction and gradients are developed for FGMs. Hybrid numerical experiments are used to develop and verify a multivariable regression (MVR) method for identifying the effective crack tip coordinates and accurately recovering stress intensity from 2-D crack tip displacement field measurements in FGMs. Numerical and experimental results for Ti-TiB FGM specimens validate the MVR method for recovery of stress intensity in FGMs when allowing for the presence of manufacturing-induced residual stresses. Predicted fatigue crack propagation rates also compare favorably with experimental results, but are limited in accuracy by the effects of residual stresses. Residual stresses modify crack tip stress intensities and their effects are only accountable here by using the MVR recovered stress intensities. The results suggest that a Paris equation including material gradients as independent variables is viable. However, an accurate means of accounting for residual stresses in arbitrary FGM forms must be developed if linear elastic fracture mechanics methods are to be used effectively, otherwise excessive experimental characterization of fatigue crack propagation properties would be required.