Characterization of fatigue crack growth in unitized construction

The objectives of this research were to analyze and predict the influence of damage containment features that enhance damage tolerance in unitized structures, and to characterize the internal three dimensional crack growth behavior in integrally stiffened panels. To accomplish these research goals, both surface and internal crack growth were measured and analyzed, and a new analysis concept method was introduced to characterize the internal crack growth behavior. Unitized structure is an important issue in the aerospace industry, since it has a significant economic advantage. There are, however, potential damage tolerance problems. Damage Containment Features (DCF), that retard or arrest fatigue crack growth, are one proposed solution for enhanced damage tolerance in unitized structures. For DCF design applications, it is essential to understand the relation between surface and internal fatigue crack growth behavior for obtaining maximum structural advantage. Four DCF configurations were fatigue tested to investigate the characteristics of the surface and internal fatigue crack growth behavior in unitized structures. The results of surface and internal crack growth analyses clearly showed the characteristic reduction in fatigue crack growth rate as the crack grows into DCF. Subsequent stress intensity factors analysis also supported this DCF effect. A parametric study with various DCF geometries suggested a design factor that might be considered in designing unitized structures. The study suggested that internal fatigue crack shape change patterns could be determined by employing an Ellipse model approach. Moreover, these reconstructed elliptic crack model could provide the numerical representations of crack fronts for FEM analyses. The results of this study are used to characterize and explain surface and internal fatigue crack growth behavior in unitized structures. All of the experimental data analyses were presented in tabular forms and figures