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Title: Mixed mode fracture in adhesively bonded joints under quasi-static and fatigue loading
Author: Ali, Hafiz T.
ISNI:       0000 0004 2706 5541
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2011
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Quasi-static and fatigue tests have been carried out to investigate mixed mode crack growth behaviour in FM73 epoxy adhesive using Double Cantilever Beam (DCB) specimens. The DCB configuration used consisted of equal thickness mild steel adherends bonded with FM73 adhesive. The joints were tested under a range of both quasi-static and fatigue mixed-mode conditions from mode I to mode II using a relatively simple, variable-mode loading fixture. Values of GI and GII for the different test conditions have been obtained using both closed-form Linear Elastic Fracture Mechanics (LEFM) methods reported previously in the literature and finite element simulations, making use of the Virtual Crack Closure Technique (VCCT). The results for the total strain energy release rates (GT) and the components (GI and GII) for the two techniques agree very well over the range of loadings applied by the jig, provided that the adhesive deformation is considered. For the quasi-static tests, the strain energy release rate components at fracture have been presented in a conventional GI/GIC (mode I) - GII/GIIC (mode II) failure envelope. It is found that the fracture energy is enhanced for mode II (In-plane shearing) dominated loading compared to mode I (opening mode) dominated loading. The ratio of GIIC/GIC was found to be 1.8. Mixed mode fracture criterion of the following form is suggested which has been found to provide a reasonably good fit to the experimental data. 1 T IIC II H IC I                   G G G G Where GIC and GIIC represent pure mode I and pure mode II fracture energies respectively and H=0.5 and T=1 are material constants obtained through experimental fit of the dataFor fatigue characterisation, the testing was carried out at a load ratio (R) of 0.1, in displacement control such that the initial maximum fatigue load was 70% of the corresponding quasi-static fracture load. The fatigue load decreased as the fatigue crack grew and this load was recorded. Crack growth was monitored and measured using a video microscope. The strain energy release rate components were determined using corresponding values of fatigue crack length and fatigue load. The results suggest that crack initiation in the test specimens is controlled by the mode I strain energy release rate component. The fatigue crack growth rates were characterised using Paris law approach from which it appears that the total strain energy release rate range, ΔGTotal, is a more dominant factor in controlling crack growth than the mode I component of strain energy release rate range, ΔGI. For a quantitative description of the mixed-mode fatigue crack growth, generalised forms of the Paris relation are developed in which the Paris constants vary with the mode-mixity in a non-monotonic way
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available