TY - JOUR

T1 - Determination of minimum CFRP pre-stress levels for fatigue crack prevention in retrofitted metallic beams

AU - Ghafoori, E.

AU - Motavalli, M.

AU - Nussbaumer, A.

AU - Herwig, A.

AU - Prinz, G. S.

AU - Fontana, M.

N1 - Funding Information: This study was funded by the Swiss Commission of Technology and Innovation (CTI) (Grant No. 12993.1 PFIW-IW). Financial and technological support from S&P Clever Reinforcement AG Company and the Swiss Federal Railways (SBB) are also acknowledged. Furthermore, the authors would like to thank Prof. Xiao-Ling Zhao from Monash University, Australia, for his helpful comments on this study while he was visiting Empa.

PY - 2014/12/1

Y1 - 2014/12/1

N2 - The majority of fatigue strengthening studies focus on reducing propagation rates of existing cracks, ignoring the crack initiation stage. Many existing metallic bridge members however do not contain existing cracks, but rather are nearing their design fatigue life. Limited research exists on the prevention of crack initiation using carbon fiber reinforced polymer (CFRP) materials. In this paper, constant life diagrams (CLDs) are used to determine the minimum level of CFRP pre-stress required to indefinitely extend the fatigue life of existing metallic beams. It is shown that by applying a compressive force to an existing fatigue-susceptible detail using pre-stressed CFRP plates, the mean stress level can be reduced such that the detail is shifted from the 'finite life' regime to the 'infinite life' regime. The proposed fatigue strengthening approach is advantageous particularly when the stress history from the prior traffic loadings is not known. To validate the proposed method, a pre-stressed un-bonded CFRP reinforcement system is introduced and tested on four metallic beams. The proposed un-bonded CFRP system is advantageous over traditional bonded CFRP systems as it can be applied to rough or obstructed surfaces (surfaces containing rivet heads or corrosion pitting for example). Additionally, the new un-bonded CFRP system offers a fast on-site installation (no glue and surface preparation are required) and an adaptive pre-stress level. Experimental results show that strengthening using pre-stressed CFRP plates are capable of shifting the working stresses from a finite fatigue-life zone to an infinite fatigue-life zone preventing crack initiation. Although according to many structural standards, the stress range is the main parameter that affects the fatigue life of a metallic detail, the results of this study clearly show that the mean stress level also plays a significant rule in the detail fatigue life. Based on the proposed CLD approach in this paper, the combined effects of the stress range and mean stress level can be taken into account for prediction of fatigue life of metallic members.

AB - The majority of fatigue strengthening studies focus on reducing propagation rates of existing cracks, ignoring the crack initiation stage. Many existing metallic bridge members however do not contain existing cracks, but rather are nearing their design fatigue life. Limited research exists on the prevention of crack initiation using carbon fiber reinforced polymer (CFRP) materials. In this paper, constant life diagrams (CLDs) are used to determine the minimum level of CFRP pre-stress required to indefinitely extend the fatigue life of existing metallic beams. It is shown that by applying a compressive force to an existing fatigue-susceptible detail using pre-stressed CFRP plates, the mean stress level can be reduced such that the detail is shifted from the 'finite life' regime to the 'infinite life' regime. The proposed fatigue strengthening approach is advantageous particularly when the stress history from the prior traffic loadings is not known. To validate the proposed method, a pre-stressed un-bonded CFRP reinforcement system is introduced and tested on four metallic beams. The proposed un-bonded CFRP system is advantageous over traditional bonded CFRP systems as it can be applied to rough or obstructed surfaces (surfaces containing rivet heads or corrosion pitting for example). Additionally, the new un-bonded CFRP system offers a fast on-site installation (no glue and surface preparation are required) and an adaptive pre-stress level. Experimental results show that strengthening using pre-stressed CFRP plates are capable of shifting the working stresses from a finite fatigue-life zone to an infinite fatigue-life zone preventing crack initiation. Although according to many structural standards, the stress range is the main parameter that affects the fatigue life of a metallic detail, the results of this study clearly show that the mean stress level also plays a significant rule in the detail fatigue life. Based on the proposed CLD approach in this paper, the combined effects of the stress range and mean stress level can be taken into account for prediction of fatigue life of metallic members.

KW - Constant life diagram (CLD)

KW - Fatigue crack

KW - Fatigue damage prevention

KW - Mean stress influence

KW - Metallic beams

KW - Pre-stressed carbon fiber reinforced polymer (CFRP)

KW - Steel

KW - Strengthening

UR - http://www.scopus.com/inward/record.url?scp=84914163996&partnerID=8YFLogxK

U2 - 10.1016/j.engstruct.2014.11.017

DO - 10.1016/j.engstruct.2014.11.017

M3 - Article

AN - SCOPUS:84914163996

VL - 84

SP - 29

EP - 41

JO - Engineering structures

JF - Engineering structures

SN - 0141-0296

ER -