Date: Tuesday, December 2, 2003
Time: 1:30 p.m.
Place: N104 Walter Scott Engineering Center
Polymer matrix composites are used in repair of aerospace structures. Adhesively bonded composite patches are capable of minimizing balance and clearance problems on control surfaces and can be readily formed to complex aircraft contours. Reinforcement in a patch can be tailored to suit the loading configuration and to minimize undesirable stiffness increase. Adhesive joining is also attractive for future integrated manufacturing of large composite structures. One problem impeding wider use of adhesive joints is lack of understanding and reliable methods of their fatigue life prediction. The objective of this dissertation was systematic analysis of static and fatigue behavior of joints of several configurations and development of fundamentals of fatigue life prediction.
Behavior of adhesive lap joints with delaminated adherends of arbitrary lay-up was analyzed. Variations of the strain energy release rate with delamination size in the joints with unidirectional and cross-ply adherends were calculated and analyzed based on a modified analytical model and geometrically linear and non-linear finite element (FE) analysis. Effects of delamination location and size on the strain energy release rates under different loads were evaluated and compared with experimental analysis of crack growth in joints with embedded delaminations. The critical strain energy release rates for the delamination propagation were obtained for the first time from in-situ observations.
Static and fatigue analysis of double cantilever beam (DCB), end notch flexure (ENF), and crack lap shear (CLS) specimens was performed. A mixed mode fatigue fracture model was developed and verified on fatigue crack propagation in adhesive lap joints. A method of life prediction for joints of arbitrary configurations was proposed and demonstrated on the lap joints with embedded cracks. Simultaneous propagation of the bond crack and delamination crack was also analyzed.
A comprehensive acoustic emission (AE) analysis of mixed mode fatigue fracture of joints was performed. Pattern recognition analysis was used to classify the AE signals from different fracture tests. Dynamic finite element (FE) modeling was used to simulate the signals from different fracture micromechanisms. Scattered AE signals were also simulated numerically for he first time and analyzed using pattern recognition method. The results showed excellent correlation with the experimental data and results of mechanical calculations. A new method of nondestructive evaluation of the loading mode mixity in joints and other structures under fatigue was formulated.
A novel test method and specimen were developed for experimental evaluation of anisotropic fracture resistance of advanced composites. The method was demonstrated on a graphite-epoxy composite. The pioneering results showed the highest fracture anisotropy measured to date for a structural material.
The results of this dissertation improve the fundamental understanding of the complex fracture phenomena in adhesive composite joints. The experimental methods and predictive models developed can be used for further fundamental fracture studies as well as in design and development of integrated real-time health monitoring and life prediction systems for adhesive composite joints.
