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Fracture Analysis of debonding mechanism for FRP Plates

P.M.M. Achintha

Inevitable flaws in the concrete-FRP interface govern plate debonding, and are not amenable to finite element analysis because the models require far more detail than will ever be available for the interface.  This thesis describes a global-energy-balance based fracture-mechanics model for the debonding mechanism of externally bonded FRP plates attached to concrete beams. 

The model investigates the possible propagation of an existing interface crack by considering the energy balance of the beam during a small potential crack extension.  The crack will extend if the energy release rate is greater than the interface fracture energy.  Despite the fact that the crack-tip stress field is not amenable to precise analysis, its influence on the energy balance of the beam is insignificant because of the small volume of the “uncertain zone”, whereas the crack tip stress field would solely govern an analysis based on linear elastic fracture mechanics.

The plate end and the locations where the widening of flexural and flexural/shear cracks cause interface cracks are most susceptible to initiation of debonding.  The model analyses debonding that initiates from either location.  With the small extension of the interface crack the compatibility between the beam and the FRP alters, consequently causing changes in the stress states, and hence the energy states, of zones in the vicinity of the crack. 

The change in energy state of a beam section upon interface crack extension is determined from a revised version of Branson's model.  The strain state when the FRP is fully or partly debonded needs to be considered.  The mechanics of stress transfer from the concrete to the FRP differs from that with conventional steel reinforcing bars for which the accuracy of the original Branson’s model was validated.  So, the moment–curvature model considers the force in the FRP as an external compressive force on the concrete beam section; the separation of the effects of the axial force and the moment is done by defining an equivalent centroid. 

Debonding will propagate in whichever of the concrete, adhesive, or at an interface that provides the least resistance; thus, the interface fracture energy is that of the weakest phase.  Experimental observations confirm that the concrete substrate just above the interface is most likely to fail, in particular, when the FRP manufacturer-recommended adhesives are used with appropriate curing procedures.  Fracture energy of concrete is determined from Hillerborg’s cohesive-crack-model-based experimental and approximate theoretical models.  Premature debonding propagation within the adhesive layer can also be analysed but the knowledge of that fracture energy is required.  

Energy release rate is calculated for assumed interface crack lengths and locations, from which the critical state is determined when it equals the interface fracture energy. Comparisons with test data reported in the literature demonstrate that the model is accurate against all modes of plate debonding.  The analysis gives the critical plate curtailment location and the critical crack length which trigger debonding at the plate end and the high moment zone respectively.  The model allows the inclusion of all properties of the concrete beam, adhesive, FRP and the loading arrangement and hence can be used as an optimisation tool in design.  The model also provides a frame work for the design of more complex real life applications, and highlights subjects that require further research.