Investigation of Fatigue Behavior of Steel and GFRP Double-Strap Joints under Varied Cyclic Loading at Given Temperatures Jie Liu 1 ; Tong Guo, M.ASCE 2 ; Matthew H. Hebdon, M.ASCE 3 ; and Junfeng Jia 4 Abstract: This paper examines the fatigue behavior of steel/glass fiber reinforced polymer (GFRP) double-strap joints under a variation of cyclic shear loading at different given temperatures. Initial experiments were performed at a constant amplitude force, which demonstrated a reduction in fatigue strength at increasing temperatures. The results were used for cumulative relative damage calculation purposes. Sub- sequent tests were then performed under two-stress level fatigue (TSLF) tests with a given temperature. The TSLF tests demonstrated the following: (1) relative to results at 20°C, the damage was retarded at lower temperatures (-10°C and 0°C), and the damage accelerated at higher temperatures (40°C); and (2) linear damage accumulation models, such as the Palmgren-Miner model, are not appropriate and tend to overpredict fatigue life. By using the nonlinear strength wearout and linear cycle mix models for bonded joints, an improved prediction method is proposed, and the fatigue results of the TSLF tests were discussed in which it was found that the proposed method can accurately predict the fatigue lifetime. DOI: 10.1061/(ASCE)MT.1943-5533.0003098. © 2020 American Society of Civil Engineers. Author keywords: Steel/GFRP joint; Fatigue behavior; Two-stress level fatigue; Strength wearout; Cycle mix. Introduction The strengthening of existing steel structures using fiber-reinforced polymer (FRP) composites has become a commonly used tech- nique owing to the unique advantages of the material, such as its light weight, corrosion resistance, increased strength, and fatigue endurance (Hosseini et al. 2018; Razaqpur et al. 2019; Yu et al. 2013). It has been reported that applications of FRP composites can enhance the ultimate and serviceability states of steel structures (Ghafoori and Motavalli 2015; Liu et al. 2018; Yu and Wu 2018). Externally bonded reinforcement is the most commonly used strengthening technique through which FRP materials are applied on the surface of a steel or concrete member using proper epoxy adhesive (Batuwitage et al. 2017; Irshidat and Al-Saleh 2016). The technique has attracted increased interest as a fast and relatively easy solution for the static and fatigue strengthening of existing structural elements, and noticeable effects have been recorded by several researchers (Colombi et al. 2015; Hu et al. 2016; Yu et al. 2019; Zheng et al. 2018). Generally, the fatigue failure of the steel/FRP bonded structures are categorized into six modes of failure: (1) steel and adhesive interface debonding, (2) adhesive layer failure (cohesive failure), (3) FRP and adhesive interface debonding, (4) FRP delamination, (5) FRP tensile rupture, and (6) steel yielding. Debonding is a key failure mode in terms of FRP-strengthened steel structures because load transfer between CFRP and the substrate mainly relies on the bond behavior (Zhao and Zhang 2007), and it has been a major concern for bonded structures (Hosseini et al. 2018; Pang et al. 2019). However, certain disadvantages are also associated with adhe- sive bonding, such as sensitivity to temperature (Schneider et al. 2018). Several researchers have reported that high temperatures have detrimental effects on the bonding system (Ashcroft and Shaw 2002; Banea and da Silva 2010). Besides, up to this date, the most reported experimental and predictive studies are for con- stant amplitude fatigue (CAF) in which a sinusoidal load or dis- placement amplitude is usually used (Sarfaraz et al. 2013; Zhang et al. 2010), and the authentic behaviors of adhesively bonded structures in service status have not been fully learned, indicating the need for the more progressive study on the bonding behav- iors under variable amplitude fatigue (VAF) loading and different temperatures. As to the stress-life fatigue analysis, the S-N curve and the Palmgren-Miner (P-M) rule (Palmgren 1924; Miner 1945) have been extensively applied (Bond 1999; Sonsino et al. 2004). How- ever, several studies have shown that the inaccurate prediction of VAF performance for bonded structures could be caused when us- ing the P-M rule due to the ignorance of the nonlinear damage ac- cumulation or load history effects (Erpolat et al. 2004; Shenoy et al. 2009). Alternatively, a new approach, which is to characterize fatigue damage as a function of the reduction (wearout) in the strength of the bonded structures during the fatigue test, is proposed (Shenoy et al. 2009). In the strength wearout method, the strength of a bonded structure is initially equal to the static strength but de- creases as damage accumulates through the application of fatigue cycles (Schaff and Davidson 1997a, b). Accordingly, failure occurs 1 Ph.D. Candidate, Key Laboratory of Concrete and Prestressed Con- crete Structures, Ministry of Education, School of Civil Engineering, Southeast Univ., Nanjing 210096, PR China. Email: [email protected] 2 Professor, Key Laboratory of Concrete and Prestressed Concrete Struc- tures, Ministry of Education, Southeast Univ., Nanjing 210096, PR China (corresponding author). ORCID: https://orcid.org/0000-0001-9228-4941. Email: [email protected] 3 Assistant Professor, Charles E. Via Jr. Dept. of Civil and Environmen- tal Engineering, Virginia Tech, Blacksburg, VA 24061. ORCID: https:// orcid.org/0000-0002-9115-0279. Email: [email protected] 4 Associate Professor, Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing Univ. of Technology, Beijing 100124, PR China. Email: [email protected] Note. This manuscript was submitted on June 4, 2019; approved on September 4, 2019; published online on January 23, 2020. Discussion per- iod open until June 23, 2020; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE 04020035-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2020, 32(4): 04020035 Downloaded from ascelibrary.org by Beijing University of Technology on 02/26/20. Copyright ASCE. For personal use only; all rights reserved.
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