Fracture and fatigue analysis of adhesively bonded composite joints
Tang, Jiann Haur
Date of Issue2014
School of Mechanical and Aerospace Engineering
Glass and carbon fiber reinforced polymer composite materials are predominantly finding wider use in civilian aircraft and wind turbine rotor designs. Adhesive bonding is commonly used to realize large composite structures due to their limited manufacturability to realize complex shapes. A successful bonded assembly entails a good understanding from design and mechanics standpoints. The design aspect often involves decision making in adhesive selection for chosen adherend material systems, geometrical constraints and intended failure modes in operative loading conditions. Whereas the mechanics aspect often requires a better design analysis tool for greater reliability and damage tolerance of these composite joints. A better design efficiency could be reached, if both aspects were incorporated together.The use of composite materials has also imposed additional challenges, especially assembly with composite sandwich structures or thick adhesive bondline. The first investigation concerns the interface stress analysis of a composite skin T-joint for describing its performance under tension and bending loading conditions. The web and flange are of sandwich construction with carbon fiber reinforced polymer matrix composite facesheets. As the sandwich core materials in the flange and web become stronger, interfacial failure at the junction becomes a dominant failure mechanism. Thin adhesive assumption allows a unique presentation of peel stress at the interface using strength of materials approach by considering the global equilibrium requirement. Critical failure initiation is identified at peel side of web’s near corner. Finite element based numerical model indicates that adhesive fillet sustains greater peel stress than closed-form prediction. An accurate contribution from the side skin and shape of fillet cannot be fully described merely based on global equilibrium. Analysis provided by this (peel stress) model is deemed conservative and useful in preliminary design of T-joints. Singularity behavior of the peel stress developed near the bi-material corner of a relatively thick composite single lap joint is investigated in the second part of this thesis. A failure criterion using stress intensity factors (SIFs) approach is adopted to understand the behavior of decreasing joint strength as bondline thickness increases. Classical stress analysis produces opposite interpretation that is of improvement in joint strength as bondline thickens. Analysis shows that one of the two eigenvalues is much dominant so that a single SIF term is conveniently preferred. Greater bending moment is experienced in joints with thicker bondline. Agreement of critical SIFs is obtained between two bondline thicknesses tested in quasi-static mode. Such approach allows for congregation of fatigue failure initiation data from different thicknesses collapse onto single fatigue band within experimental scatter. In the third study, fracture mechanics approach is used to demonstrate the development of a non-dimensional model describing the debonding and kinking problems of a single lap joint. The emphasis is on the integration of both design and mechanics aspects. The solution of transverse deflection makes the evaluation of non-dimensional energy release rate and mode mixity possible, which are then used to describe the debonding and kinking propensity. Selective adhesives are used to intentionally trigger or inhibit the desire kinking mode. Results show that crack tip closer to the loading end of thinner adherend is a preferable initiation side, and the debonding and kinking propensities are higher in joints with unequal adherend thicknesses. Significant mixed mode condition in asymmetrical configuration might lead to discrepancies between theory and experiment. The model is useful in selecting the optimum joint configuration for a given material system and loading conditions, vice-versa. Finally, the low-speed transverse impact analysis of a single lap composite joint with tension is investigated. A model incorporating the non-dimensional analysis is developed to relate the strain and potential energies stored to the kinetic energy in the event of low-velocity impact. Experiments are performed to validate consistency in stiffness responses and losses in impact energy level. Further analysis in numerical models using J-integral and virtual crack closure technique (VCCT) attempts to compare the energy release rate and mode mixity between various models. Good agreement of properties with experimental measurements and theoretical values is attained when non-dimensional transverse deflection in three-point bending specimens is less than 10 %. Decrease in tension load improves the agreements in energy release rate and mode mixity. The presence of tension load smooths the debonding propensity especially at higher impact energy level, in a clamped-clamped condition. Analytical model for predicting the critical impact energy absorption at debonding is thus established.
DRNTU::Engineering::Materials::Mechanical strength of materials