Strengthening of metallic members under mixed mode fatigue loading using prestressed CFRP plates

The strengthening of existing steel structures against high-cycle fatigue (HCF) loading has become one of the most important topics of interest in structural rehabilitation. Apart from the conventional strengthening techniques that incorporate bulky and heavy steel reinforcements, the application of carbon fiber reinforced polymer (CFRP) composites has attracted much interest for the strengthening of fatigue-prone (or fatigue-damaged) steel members. Over the last two decades, considerable research studies have been conducted to further understand the static and/or fatigue behavior of CFRP-strengthened steel members. However, existing knowledge on the issue is more focused on the strengthening of small-scale steel members using nonprestressed CFRP patches subjected to the simple mode I (tensile mode) loading condition. Nevertheless, it is quite evident that a combination of modes I and II (shear mode), hereafter referred to as the mixed mode I/II loading condition, often acts on structural members; such a complex loading is responsible for the failure of fatigue-critical details. Although some limited research studies have been conducted on the mixed mode I/II fatigue behavior of bare mild steel, which is relevant for civil structural applications, the existing literature does not provide any solution for the strengthening of fatigue-cracked members subjected to mixed mode I/II loading condition. Therefore, the main objective of the current PhD work is to propose a strengthening design approach to arrest an existing mixed mode I/II fatigue crack in a structural steel member by using prestressed CFRP reinforcements. To achieve this objective, first extensive analytical and experimental studies are conducted to develop a viable prestressing system by comparing the performance of the prestressed bonded reinforcements (PBRs) with that of a novel prestressed unbonded retrofit (PUR) system. Second, a strengthening design approach is proposed for the mixed mode I/II fatigue crack arrest in existing structural steel members using the developed PUR system. Through the analytical formulation of mode I and II stress intensity factor ranges, a design model is proposed to determine the strengthening solution, including the required prestressing level and/or the cross-sectional area of the reinforcement, which would ensure the complete arrest of an existing mixed mode I/II fatigue crack in a steel member. The proposed model is validated by performing sets of stepwise HCF tests on reference unstrengthened and prestressed CFRP-strengthened precracked steel plates of grade S355J2+N under various mode mixities. From a practical point of view, it deemed necessary to develop a reliable system for the prestressed strengthening of real-scale metallic members. Therefore, an alternative configuration of the PUR system, named the flat prestressed unbonded retrofit (FPUR) system is developed for the strengthening of real-scale metallic I-girders using multiple prestressed CFRP plates. An analytical model is proposed to accurately determine the stress levels in a metallic I-girder strengthened with the developed FPUR system, and sets of real-scale pull-off tests, as well as static and fatigue four-point bending tests are conducted to confirm the excellent performance of the system. The practical application of the FPUR system is demonstrated for the strengthening of a 122-year-old roadway metallic bridge in Australia.

Fatigue strengthening of cracked steel plates using prestressed unbonded CFRP reinforcements

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

Fatigue strengthening of existing steel structures is of great importance and has attracted lots of interest recently. Apart from the conventional strengthening technique, i.e. utilizing bulky and heavy steel plates and its disadvantages, carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential to be used as external reinforcements in strengthening of steel structures. As a result, during the last two decades, considerable research studies have been done to further investigate static and fatigue behavior of CFRP-strengthened steel members under different loading configurations. Nevertheless, very limited investigations can be found in the literature on the performance of prestressed CFRP plates for fatigue strengthening of cracked steel members; while, it is obvious that using prestressed reinforcements is more effective than non-prestressed ones. Consequently, the main intention of the current study is to show the capability of fatigue crack arrest in steel plates using a novel prestressed unbonded reinforcement (PUR) system, including high performance mechanical clamps. This paper presents a set of fatigue tests on precracked steel plates with and without the PUR strengthening that has been performed. Based on the obtained experimental results it can be concluded that utilizing the proposed PUR system with a certain prestressing level, an existing fatigue crack in a steel member can be completely arrested.

2017

Flat prestressed unbonded reinforcement (FPUR) system for strengthening of steel I-Beams

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

This paper proposes a novel retrofitting system for the strengthening of steel I-beams using prestressed unbonded carbon fiber reinforced polymer (CFRP) plates. The system relies on a pair of mechanical clamps. Each clamp holds two flat CFRP plates (each 50 × 1.4 mm) and anchors their prestressing forces to the steel I-beam via friction. The proposed system does not require any adhesive between the CFRP plates and the steel substrate; therefore, surface preparation and adhesive curing are not needed, which reduces the time and cost of retrofitting. The design considerations of the mechanical clamps, the most important elements of the proposed unbonded system, are explained. A finite element (FE) model was established to optimize the design of the required mechanical parts, and a set of pull-off tests was performed to evaluate the capacity of the optimized system. The experimental results show that the proposed system is capable of transferring almost the entire tensile capacity of the normal modulus CFRP plates to the steel substrate via friction, while no sliding of the joint was observed. Moreover, the proposed flat prestressed unbonded reinforcement (FPUR) system was applied on a 6.4-m-long steel I-beam and the high performance of the system was confirmed based on the results of a set of four-point static and fatigue tests. Relying on the laboratory experimental tests, the proposed FPUR system was applied on the cross-girders of an almost 120-year-old steel bridge in Australia to monitor its long-term performance as a strengthening solution to increase the capacity and/or fatigue life of existing metallic I-beams.

Taylor & Francis2018

Development of prestressed unbonded and bonded CFRP strengthening solutions for tensile metallic members

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

In this study, a novel unbonded mechanical clamping system was developed for the strengthening of tensile metallic members using prestressed carbon fiber reinforced polymer (CFRP) plates. The system clamps a pair of prestressed CFRP reinforcement to a metallic substrate and provides an almost uniform contact pressure over the CFRP plate along the anchorage length. A finite element simulation was used to optimize the design of the mechanical components of the system. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized design. Experimental results revealed that the proposed mechanical clamping system is capable of transferring the entire tensile capacity of the CFRP plates to the steel substrate, even after experiencing 10 million fatigue cycles. The comparative performance of the developed clamps was further investigated by a set of static tests on steel plate specimens strengthened with the prestressed bonded reinforcement (PBR) and the newly developed prestressed unbonded reinforcement (PUR) systems. Furthermore, simple analytical models are proposed to formulate the stress state in prestressed unbonded and bonded CFRP-strengthened tensile metallic members. The accuracy of the proposed analytical formulations was verified by the experimental results obtained during the current study. Experimental results revealed that the efficacy of having relatively high prestressing forces in the normal modulus (NM) CFRP reinforcements is much higher than the stiffness improvement obtained by using ultra-high modulus (UHM) CFRPs. However, the available capacity of the PBR system before debonding failure is far lower than that of the developed PUR solution.

2019