Fatigue crack strengthening using pre-stressed composite patch
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI-COLUMBIA AT AUTHOR'S REQUEST.] The fatigue life of steel bridges can be extended by externally bonding carbon fiber reinforced polymer (CFRP) patches to the damaged elements arrest the crack propagation. Bonding a composite patch to a cracked steel section is an efficient technique to reinforce cracked members or to delay fatigue crack growth in the structural elements. In this thesis, the influence of the patch pretension level, patch stiffness ratio, and patch fiber orientation on the stress intensity factor levels, crack tip opening displacement and the plastic zone are investigated. A three-dimensional finite element model of the double sided CFRP patch-strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of loading modes I and II. It is found that the introduction of a compressive stress by pre-tensioning of the CFRP patch prior to bonding produced a significant reduction of up to 70% in SIFs for different crack lengths, which led to increasing the remaining fatigue life of the steel member. Also, it produces a significant reduction in the crack tip opening displacement and plastic zone. The increase in the CFRP patch-to-steel axial stiffness ratio, reduced the SIFs by up to 30%. The optimum benefit of using the CFRP patch to reduce mode I SIF was accomplished when the patch axial stiffness was about 50% or below the steel plate stiffness. Moreover, the increase in the patch stiffness ratio reduces the amount of plastic zone and opening displacement at the crack tip. Furthermore, orienting the fibers perpendicular to the crack direction is not necessarily the most effective in arresting the crack propagation. The effectiveness of the fiber orientation in the composite patch depends mainly on both the loading direction and the crack inclination angle. Concerns over means to improve fatigue behavior of cracked steel beams exist. This work presents the details of strengthening schemes that was carried out using prestressed ultra-high modulus carbon fiber reinforced polymer (UHM-CFRP) plates. Furthermore, an experimental investigation included the prestressing procedure. Steel W-sections were strengthened using different configurations with UHM- CFRP plates. The repaired specimens and control specimens were tested in cyclic four-point bending until fatigue failure. The study also considered the effects of CFRP end clamping force. A set of fatigue tests were conducted and results indicated that prestressed UHM-CFRP technique could increase fatigue crack propagation life 16 times longer than that of unrepaired one. The results also indicated that UHM- CFRP is effective at increasing fatigue life.
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