Bonded CFRP/Steel Systems, Remedies of Bond Degradation and Behaviour of CFRP Repaired Steel: An Overview
Abstract
:1. Introduction
2. Bond Behaviour between CFRP and Steel
2.1. Bond Test Fabrication and Method
2.2. Failure Modes of CFRP/Steel Systems
- (a) Steel and adhesive interface failure.
- (b) Cohesive failure (adhesive layer failure).
- (c) CFRP and adhesive interface failure.
- (d) CFRP delamination (separation of some carbon fibres from the resin matrix).
- (e) CFRP ruptures.
- (f) Steel yielding.
2.3. Factors That Affect the Performance of Bonded CFRP/Steel Systems
2.3.1. Adhesive Selection and Application
- Tensile strength and modulus, to ensure the strength is sufficient and the stiffness is compatible with the FRP material.
- Shear strength and ductility, in order to provide the necessary load transfer capabilities, deformability, and toughness.
- Fatigue resistance, which is critical in tension members subjected to cyclic loads such as bridge trusses or transmission line poles in windy areas.
- Environmental durability, especially when repairing structures that operate in aggressive environments.
- Curing time and temperature, in order to attain the design strength rapidly and preferably without the need of artificial heating.
- Workability, as adhesives must be viscous enough to remain in place during bonding.
- Pot life, where higher values benefit constructability by facilitating installation over large areas.
2.3.2. Galvanic Corrosion
2.3.3. Sustained Loading
2.3.4. Fatigue Loading
2.3.5. Elevated Temperature
2.3.6. Moisture and Saturation
3. Current Remedies for Bond Degradation
3.1. Material Insulation
3.2. Surface Priming
3.3. Adhesive Modification
4. Behaviour of CFRP Repaired Steel
4.1. Experimental Repair of Fatigued Steel Plates
4.2. Strengthening of Steel Members under Static Bending
4.3. Strengthening of Steel Members under Bending Fatigue
4.4. Experimental Repair with Environmental Exposure
5. Prospects
- Several steel/CFRP configurations successfully survive fatigue loading and environmental exposures, when applied consecutively. However, it may be more accurate and relevant to investigate the simultaneous application of environmental exposure with fatigue loading. Concurrent conditioning removes the potential for systems to experience levels of recovery once isolated from the harsh environment. Simultaneous exposure or wet/dry cycling, combined with fatigue loading would better replicate the potential extreme industrial scenarios. However, this process may create an unrealistically short exposure time, therefore, specimens may need to undergo pre-exposure to reach saturation before simultaneous loading.
- As high modulus materials exhibit superior fatigue performance, investigations into ultra-high modulus and pre-stressed CFRP laminates under environmental exposure may be valuable. Currently, more research is required to determine if the laminated materials are better at resisting environmental degradation while maintaining good fatigue resistance.
- Adhesive performance remains critical to the strength and durability of wet layup composite systems. The common epoxy adhesives used for wet layup fabrication degrade during elevated temperature seawater exposure. Hence, it may be beneficial to investigate the performance of techniques that minimise the quantity of applied adhesives, such as laminates or unbonded CFRP systems, under environmental exposure. The challenge with unbonded systems, is to find an anchorage system that maintains strength during submergence without environmental degradation.
- Real time structural health monitoring, primarily of adhered CFRP systems. With adhesive and CFRP layers preventing inspection of repaired surfaces, it would be beneficial to investigate techniques to accurately view and monitor surfaces underneath composite patches.
- Silane pre-treatment occasionally creates varying signs of improved longevity and durability of CFRP/steel systems. Silane’s improvement depends heavily on the failure mode, which makes it important to consider silane pre-treatment to various CFRP configurations, to better determine its effectiveness at restricting degradation during moisture exposure. Configurations involving laminates or normal modulus materials may result in more significant improvements from silane pre-treatment as they experience more interfacial and cohesion failures. To further validate the use of silane, it may be required to quantify the proportion of bond strength provided by both the mechanical and chemical components of pre-treated adhered CFRP/steel systems. The portion provided by chemical bonding, for certain CFRP configurations, may pre-determine the potential effectiveness of silane to provide improved adhesion.
- Experiments involving silane pre-treatment also suggested hydrolysis occurred from moisture ingress during environmental submergence. However, this phenomenon requires investigative confirmation by examining the chemical composition of pre-treated steel surfaces, after submergence, to determine if hydrolysis has definitively taken place. This will require the development of a technique to successfully remove residual adhesive from the steel surface, or utilise CFRP systems that undergo steel/adhesive interfacial failure. The surface must then be examined as soon as possible to limit the chemical changes transpiring from atmospheric exposure.
- With environmental exposure occasionally causing levels of debonding it may be applicable to investigate the formation of the debonding region of CFRP/steel systems caused by environmental conditioning and fatigue loading.
- Analytical models require further development to incorporate the significant number of variables related to industrial applications. For example, as failure modes can potentially change as a result of environmental degradation, their fatigue performance significantly alters. Hence, the enhanced model would benefit from a further modification that integrates expected failure modes of CFRP configurations to accurately mimic the reduced performance.
6. Summary
- The galvanic interaction of CFRP/steel systems and their potential to create areas of isolated pitting, which can become high stress regions and the site of premature structural failure.
- The combined effects of environmental exposure and fatigue loading on CFRP/steel joints. Primarily investigating the effect of fatigue stress range, number of applied cycles as well as the exposure temperature and duration. The influence of these variables will allow the design, performance and durability of such bonds to be better understood under industrial conditions.
- The use of adhesive modifiers and chemical primers to restrict the amount of bond degradation witnessed after environmental exposure and fatigue. With durability of such systems being a big threat to their implementation bond strength optimisation is a key to their success.
- The fatigue performance of CFRP repaired steel exposed to environmental conditioning, utilising techniques that proved successful in previously conducted CFRP/steel joint investigations.
- The theoretical prediction of the fatigue life of CFRP repaired steel incorporating the influences of environmental exposure on existing fracture mechanics theories.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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Steel Type | Carbon Fibre and Carbon Fibre Reinforced Polymer (CFRP) Specification | Test Method | Highlights | Ref. |
---|---|---|---|---|
Low-alloy steel (16-gauge A242 cold-rolled steel) | Unimpregnated Fibre (in tow-sheet form), C1:20 (weight: 200 g/cm2, Tensile strength: 3480 MPa, Tensile modulus: 228 GPa). C1:30 (weight: 300 g/cm2, Tensile strength: 3480 MPa, Tensile modulus: 228 GPa). C5:30 (weight: 300 g/cm2, Tensile strength: 2940 MPa, Tensile modulus: 370 GPa). | Wedge test: The modified wedge-crack specimens (of nominal size 25.4 by 203 mm), which were used to simultaneously evaluate the adhesive steel and adhesive composite bonds. |
| [1] |
A36 steel bar (1/2″ × 1.5″ × 36″) | CFRP plate (0.21″ × 1.44″ × 18″) | Tensile test: A series of increasing tensile loads was applied using an Instron Model 1332 testing machine and the accompanying strain data was recorded. A constant strain rate of 3000 lbs./min was used. |
| [26] |
Fatigue test: A series of small-scale double reinforcement specimens was tested under cyclic loads at a stress range corresponding to the fatigue threshold for common fatigue-sensitive conditions. Double reinforcement specimen is fatigued at a stress range of 12 ksi for 2.55 million cycles. |
| |||
Steel beam (S355J0 (ST 52-3)) | CFRP plate (150/2000, width: 50 mm and thickness: 1.2 mm) | Pull-Off Test: Three CFRP plates are gripped inside a friction clamp. Each CFRP plate is pulled using a single-FRP clamp, which is connected to an actuator. The actuators are connected to a hydraulic jack that provides equal pressure for each actuator. An inclined test setup was used due to the deviation of the CFRP plates about 12° in the proposed trapezoidal PUR system. |
| [29] |
Flexural Test: Three steel beams (one unstrengthened reference and two strengthened with 15% and 31% prestress levels, respectively), were statically tested until failure. A symmetric four-point bending setup is used. The loading span is 1700 mm, whereas the support span is 5000 mm. The test is carried out using a hydraulic testing machine (Pulsator P960) with 250 kN actuator capacity and a force control system. |
| |||
Steel beam (type IPE 120) yield strain: 1.9 mm/m. Young’s modulus: 199.3 GPa. Yield strength: 383 MPa. Tensile strength: 462 MPa. | CFRP laminates - Normal modulus (150/2000 50/1.4, width: 50 mm, thickness: 1.4 mm, cross-sectional area 70 mm2, Young’s modulus: 165 GPa). - High modulus (200/2000 50/1.4, width: 50 mm, thickness: 1.4 mm, cross-sectional area 70 mm2, Young’s modulus: 205 GPa). - Ultra-High modulus (Carbolam THM 450 50 × 1.2, width: 50 mm, thickness: 1.4 mm, cross-sectional area 60 mm2, Young’s modulus: 440 GPa). | Simply supported four-point bending set-up: Two bearings at the right and left sides of the beam to restrain the vertical and lateral displacements. Only one is free to move longitudinally. Rotations about the longitudinal axis is prevented using fork constraints at both ends of the beam. The test specimens were then loaded vertically using two hydraulic actuators, each having a 100 kN static load capacity. The support span is 1200 mm, while the actuators produce a constant bending moment over a length of 400 mm in the middle of the beam. |
| [31] |
Steel plates (210 mm long, 50 mm wide and 5 mm thick). Mechanical properties (mean elastic modulus: 195 GPa, yield stress: 359 MPa and tensile strength: 484 MPa). | Carbon fibre sheets, MBrace CF 130 (elastic modulus: 240 GPa, ultimate tensile strength: 3800 MPa and ultimate tensile elongation: 1.55%). MBrace CF 530 (elastic modulus: 640 GPa, ultimate tensile strength: 2650 MPa and ultimate tensile elongation: 0.4%). | Fatigue test: number of fatigue cycles (N) ranging from 0.5 million to 6 million at different levels of constant amplitude stress ranges. |
| [38] |
Two steel plates (12 mm thick) are welded to a two rectangular hollow sections (70 mm × 50 mm) of 3 mm thickness. | CFRP plate | Two pull-off tests are carried out on each steel block, one on each of the two thick steel plates. |
| [39] |
Steel plates (hot rolled structural steel HA300). The nominal yield stress is 300 MPa. The steel plates are all 20 mm thick and 50 mm width. | CFRP laminates (MBRACEs LAMINATE 460/1500). It is an ultra-high modulus laminate with a nominal elastic modulus of 460 GPa and a nominal tensile strength of 1500 MPa. The laminate thickness is 1.45 mm. | Tension test: Baldwin Universal Testing machine is used (loading rate is 2 mm/min). |
| [40] |
Stress Range (MPa) | Un-Retrofitted | Retrofitted Beams | Ratio of Fatigue Lives |
---|---|---|---|
Fatigue Life (No. Cycles) | Fatigue Life (No. Cycles) | ||
207 | 119,140 | 379,824 | 3.2 |
241 | 71,278 | 241,965 | 3.4 |
276 | 35,710 | 105,345 | 3.0 |
310 | 30,216 | 75,910 | 2.5 |
345 | 19,068 | 54,300 | 2.9 |
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Borrie, D.; Al-Saadi, S.; Zhao, X.-L.; Raman, R.K.S.; Bai, Y. Bonded CFRP/Steel Systems, Remedies of Bond Degradation and Behaviour of CFRP Repaired Steel: An Overview. Polymers 2021, 13, 1533. https://doi.org/10.3390/polym13091533
Borrie D, Al-Saadi S, Zhao X-L, Raman RKS, Bai Y. Bonded CFRP/Steel Systems, Remedies of Bond Degradation and Behaviour of CFRP Repaired Steel: An Overview. Polymers. 2021; 13(9):1533. https://doi.org/10.3390/polym13091533
Chicago/Turabian StyleBorrie, Daniel, Saad Al-Saadi, Xiao-Ling Zhao, R. K. Singh Raman, and Yu Bai. 2021. "Bonded CFRP/Steel Systems, Remedies of Bond Degradation and Behaviour of CFRP Repaired Steel: An Overview" Polymers 13, no. 9: 1533. https://doi.org/10.3390/polym13091533