Repair of Fatigue Cracks in Steel Members Using Adhesively Bonded Carbon Fiber Reinforced Polymers (CFRP)

  • Author / Creator
    Mobeen, Syed
  • Bonding fiber reinforced polymer (FRP) composites is emerging as an alternative to the fatigue crack repair techniques because of its several advantages over the conventional fatigue crack repair methods and materials. Its major advantages include its higher stiffness and strength, choice of directional stiffness or strength, high strength-to-weight ratio and ease of application. Most of the previous research work on the use of FRP in repair of fatigue cracks exists mostly in the repair of aluminum, its alloys, and concrete/masonry members but the use of FRP in repair of steel members and its related research work is limited.

    In current research work an attempt has been made to study in-depth the use of FRP in repair of fatigue cracks in steel members. More emphasis was put on the impact of bonded patch delamination on the stress intensity factor (SIF), which is a governing parameter for prediction of fatigue crack growth. It was so because most of the available research works on the repair of fatigue crack in steel structures using bonded FRP or CFRP patches showed the governing failure mode of such repairs as the progressive delamination of the FRP patch from the steel plate. Therefore, initial part of current research work focused on studying the impact of different patch parameters on initiation of the FRP patch delamination. It followed by the study of the impact of patch delamination on SIF at crack tip of the repaired plate. Properties of the interface adhesive layer were found to be affecting the interface stress distribution and its failure under high shear stress around the crack was found to be enhancing the SIF at the crack tip of repaired plate. Adhesive shear stiffness (GA/tA) was found to be the governing parameter in controlling the interface shear stress distribution near the crack, instead of adhesive thickness (tA) or modulus (EA) alone, within identical ETR repairs. ETR is the relative axial stiffness of bonded patch to the steel plate. It was also found, from the results of a numerical study of impact of patch delamination, that a stiffer CFRP patch could become less effective than a less stiff patch, if it delaminates more because of weaker adhesive.

    An experimental phase was also carried out to study the fatigue behavior of CFRP repaired edge-cracked steel plates under constant amplitude fatigue cycles. Its results showed that all the specimens were failed in the progressive patch delamination mode, but, the delamination itself found to be initiated from either the patch-end or from the near-crack locations. Specimens failed by the governing patch-end delamination achieved short and unexpected fatigue life while the specimens failed by the near-crack delamination achieved higher fatigue life. A methodology was developed to predict fatigue life of the edge-cracked steel plates, repaired with double sided bonded CFRP patches, incorporating the impact of patch delamination. It mainly included the use of linear elastic fracture mechanics (LEFM) to predict the fatigue life of cracked steel plates, but modified for the presence of bonded CFRP patch by including the numerically developed geometric factors (fa/b) in the governing LEFM equations. The geometric factors were developed using finite element analysis of the cracked steel plate with bonded CFRP patches, incorporating the impact of patch delamination, through modelling of the adhesive shear failure. The developed methodology predicted well fatigue life of the tested specimens, failed by the near-crack patch delamination. Fatigue life of specimens failed in the governing patch-end delamination failure mode could not be predicted with the developed methodology, because of less sensitivity of the SIF or geometric factors on patch-end delamination.
    After validating the developed methodology from the experimental results, an extensive parametric study was carried out to develop the geometric factors (fa/b), which could be used to predict the fatigue life of cracked steel plates, repaired with double sided bonded CFRP patches, having different patch properties. These geometric factors were developed for the two more common cases of edge crack and central crack steel plates, and these were also incorporating the impact of patch delamination around the crack. The developed geometric factors were based upon the patch ETR, adhesive shear stiffness (GA/tA) and the shear strength of adhesive (Ƭ)

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.