Increasing Remaining Fatigue Life of Steel Plates Using Bonded Ultrahigh-Modulus CFRP Plates: The Effect of Cyclic History and Bond Length

This study evaluates the effectiveness of externally bonded small-patch ultrahigh-modulus (UHM) carbon fiber–reinforced polymer (CFRP) plates of 460 GPa modulus in extending fatigue life and slowing crack propagation in steel plates with simulated fatigue cracks. The study investigates a range of very short bond lengths for applications where bonding space may be limited or accessibility restricted, together with the effect of single- versus double-sided application, on fatigue performance. It also compares short UHM CFRP plates with similar length normal modulus (NM) (165 GPa) CFRP plates with regard to extending fatigue life and reducing stress concentration at the crack tip. Distributed fiber-optic sensors (DFOS) and digital-image correlation (DIC) are used to capture the full strain field and track crack growth. Results showed that fatigue life was increased by up to 2.0 and 2.34 times for single- and double-sided UHM CFRP repairs. As bond length increased from 25 to 100 mm, fatigue life increased from 1.36 to 2.0. UHM CFRP more effectively reduced stress concentration at the crack tip, by 60% compared with 37% for NM CFRP, indicating that it has the potential for superior fatigue life gains, relative to NM CFRP if the necessary bond length to prevent or delay debonding is provided. As debonding failure governed in this study, NM CFRP plates achieved comparable fatigue life extension. DIC tracking of crack growth matched the traditional “beach marking” technique with less than 6% difference.

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The fatigue life of steel bridges can be extended by externally bonding carbon fiber reinforced polymer (CFRP) patches to the damaged elements arrest the crack propagation. Bonding a composite patch to a cracked steel section is an efficient technique to reinforce cracked members or to delay fatigue crack growth in the structural elements. In this thesis, the influence of the patch pretension level, patch stiffness ratio, and patch fiber orientation on the stress intensity factor levels, crack tip opening displacement and the plastic zone are investigated. A three-dimensional finite element model of the double sided CFRP patch-strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of loading modes I and II. It is found that the introduction of a compressive stress by pre-tensioning of the CFRP patch prior to bonding produced a significant reduction of up to 70% in SIFs for different crack lengths, which led to increasing the remaining fatigue life of the steel member. Also, it produces a significant reduction in the crack tip opening displacement and plastic zone. The increase in the CFRP patch-to-steel axial stiffness ratio, reduced the SIFs by up to 30%. The optimum benefit of using the CFRP patch to reduce mode I SIF was accomplished when the patch axial stiffness was about 50% or below the steel plate stiffness. Moreover, the increase in the patch stiffness ratio reduces the amount of plastic zone and opening displacement at the crack tip. Furthermore, orienting the fibers perpendicular to the crack direction is not necessarily the most effective in arresting the crack propagation. The effectiveness of the fiber orientation in the composite patch depends mainly on both the loading direction and the crack inclination angle. Concerns over means to improve fatigue behavior of cracked steel beams exist. This work presents the details of strengthening schemes that was carried out using prestressed ultra-high modulus carbon fiber reinforced polymer (UHM-CFRP) plates. Furthermore, an experimental investigation included the prestressing procedure. Steel W-sections were strengthened using different configurations with UHM- CFRP plates. The repaired specimens and control specimens were tested in cyclic four-point bending until fatigue failure. The study also considered the effects of CFRP end clamping force. A set of fatigue tests were conducted and results indicated that prestressed UHM-CFRP technique could increase fatigue crack propagation life 16 times longer than that of unrepaired one. The results also indicated that UHM- CFRP is effective at increasing fatigue life.

In steel members strengthened by carbon-fiber reinforced polymer (CFRP) plates, the thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. As so far, the authors proposed a technique to reduce the thermal stress in steel members strengthened by CFRP plates, which involves bonding aluminum alloy plates with CFRP plates. In the proposed method, the thermal stress in steel member can be reduced so that there are negligible levels of stress in steel member when the cross sectional areas of CFRP and aluminum plates are designed to correspond to the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. In this study, to confirm the maintaining the thermal stress reduction in steel member by proposed method, thermal stress measurement in steel plate strengthened by CFRP and aluminum plates was carried out about 21 months. In this research, the thermal stress introduced in the steel plate strengthened by CFRP plates was also measured. Furthermore, to assume the thermal shear and normal (peel) stresses in adhesive layers, FE analysis with plane stress element was employed. As the result, it was shown the thermal stresses in steel plate with CFRP plate were able to calculate by using composite theory and measured temperature. Furthermore, in steel plate strengthened by CFRP and aluminum plates, the thermal stress introduced in steel plate was negligible-small through the all-season. It was found the thermal stresses in steel plate with CFRP plates as well as CFRP and aluminum plates were also estimated by using composite theory and measured temperature. In the steel plate strengthened by CFRP and aluminum plates, the thermal shear and normal stresses in adhesive layer glued to steel plate become smaller than that in the conventional CFRP bonded specimen. However, the shear stress in adhesive layers between CFRP and aluminum plates in proposed method was higher than the thermal stress in adhesive layers between CFRP plates in conventional method.

The traditional Euler’s buckling theory of slender columns indicates that column capacity depends on flexural rigidity (EI), rather than material strength. As such, the availability of ultrahigh modulus carbon fiber-reinforced polymer (CFRP) plates, which could be much stiffer than steel, can offer a unique alternative for strengthening slender steel columns, in lieu of welding or bolting steel plates. In this study, twelve 2.6 m long S75×8 steel columns of 197 slenderness ratio that represents the upper limit permitted by code were tested under concentric axial loading using pin-ended conditions. The columns were allowed to buckle around their weak axes. CFRP plates were adhesively bonded to the flanges of the steel I-shape sections in nine of the columns. The main parameters studied were the level of initial out-of-straightness [length(L)/8,387 to L/1,020], CFRP modulus (168–430 GPa), CFRP reinforcement ratio (13–34%) and the length of CFRP plate (33–95% of L). The gain in axial strength due to ...

The purpose of this study is to investigate the mechanism of improving fatigue performance and the estimation model of fatigue life for corroded steel plate strengthened with CFRP plates. A new two-stage fatigue crack propagation prediction model for the corroded steel plate strengthened with CFRP plates was proposed; moreover, the identification of critical rust pits and the equivalent method of initial cracks, and the calculation method of stress intensity factor (SIF) values at the crack tip were established. The accuracy of the proposed model was verified by comparing the predicted and tested fatigue life of the corroded steel plate strengthened with CFRP plates. Finally, the proposed two-stage crack propagation model was applied to carry out a parameter analysis to investigate the effect of weight loss rate, equivalent initial crack size, adhesive thickness, CFRP stiffness and CFRP prestress level on the fatigue crack propagation of the corroded steel plate strengthened with CFRP plates. Results showed that the maximum depth and the average width of the rust pits were suggested to be taken as the equivalent dimensions of the initial semi-elliptical surface crack for the fatigue crack propagation prediction of corroded steel plate strengthened with CFRP plates. Increasing the weight loss rate of the corroded steel plate, the initial crack size or the adhesive thickness would accelerate the crack growth and reduce the fatigue life, whereas increasing the stiffness or prestress level of the CFRP plate would significantly reduce the crack growth rate and increase the fatigue life. The smaller the initial crack size, the more sensitive the crack propagation life was to the variation of equivalent initial crack size. The influence of adhesive thickness on the fatigue life was limited and convergent, and the application of prestressing could significantly improve the utilization rate of CFRP materials and the fatigue strengthening effect of the corroded steel plate.

In steel members strengthened with carbon-fiber reinforced polymer (CFRP) plate, thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. Therefore, the thermal stress caused by temperature change has to be considered when designing the repair or strengthening of a steel member with CFRP plates. With this in mind, the authors proposed a technique to reduce thermal stress in steel members strengthened by CFRP plate on both side, which involves bonding aluminum alloy plates with CFRP plates. In this proposed method, the thermal stresses in steel member can be reduced so that there are negligible levels of stress when the cross sectional areas of CFRP and aluminum plates are designed to correspond the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. So far, a thermal bending moment was not considered in the proposed method, because the steel members strengthened by CFRP-aluminum laminated plate on top and bottom sides were assumed. However, if the proposed method is applied on one side of steel member, the thermal stress in the steel member might not be reduced completely by generated thermal bending moment. Therefore, to confirm the effectiveness of the proposed method for one-sided bonding, heat tests were conducted on a steel plate with a laminated plate bonded on one side. Additionally, to verify test results and calculate the shear and peel stresses in the adhesive layers, a numerical model that used Eigenvalue analysis was proposed and applied. The tests revealed that using the proposed method to create a two-layered laminated plate consisting of CFRP and aluminum plates could not reduce the thermal stress completely in a thin steel plate. However, it was found that the thermal stress in steel plate can be completely reduced, even in thin steel plate, when the proposed method is used to create a three-layered laminated plate consisting of one CFRP plate between two aluminum plates, which when composited has a thermal bending moment equal to zero.

The use of Carbon Fiber Reinforced Polymer (CFRP) in nuclear industry is very limited and has not been used for nuclear safety related application until recently. In 2019, a new ASME Boiler and Pressure Vessel (BPV) Code Case N-871 was approved for internal repair of buried Class 2 and 3 nuclear safety related piping using CFRP for Service Levels A, B, C and D for a service period of 50 years. However, US Nuclear Regulatory Commission (NRC) has not yet accepted the Code Case. This Code Case provides a very detail guidance on materials, fabrication, installation, and design of CFRP repair for Class 2 and 3 nuclear safety related piping. There are two aspects in designing CFRP repair for nuclear safety related piping application in terms of service duration — short-term use condition and end-use (end of service life) condition. The final design must satisfy both conditions in terms of available safety margin for the entire period of service. In contrast to end-use condition, some of CFRP materials’ degradation phenomena such as time effect factor due to sustained loading, material adjustment factors due to environmental exposure do not affect design calculation for short-term use. However, while end-use condition design criteria may control the final CFRP repair design, it is equally important to understand the available safety margin for short-term use. In this paper, the available safety margin in ASME BPV Code Case N-871 was evaluated for internal CFRP repair of nuclear safety related piping for short-term use by conducting a full-scale hydrostatic test. The full-scale hydrostatic test was conducted on 40” outside diameter steel pipe. The pipe contains a postulated flaw (cut-out) with an internal CFRP repair designed according to ASME BPV Code Case N-871. The internal CFRP repair consisted of three unidirectional CFRP layers applied over the flaw 360° around the circumference at the inside surface of the pipe. Two glass fiber reinforced polymer (GFRP) layers were also applied as dielectric barrier and watertightness layer. The design pressure and temperature for the hydrostatic test was 84 psi and 72F. The pipe with internal CFRP repair was pressurized until failure (leak) occurred. The details of CFRP repair installation and hydrostatic test are presented in this paper. Finally, the available safety margin for short-term use was evaluated. Detailed analysis of strain gage data, bulging measurements and the video revealed that potential failure mechanism in the full-scale hydrostatic test of pipe with an internal CFRP repair and a postulated flaw (cut out).

Fatigue behavior of notched steel beams reinforced with bonded CFRP plates: Determination of prestressing level for crack arrest

The bond–slip relationship between carbon fiber reinforced polymer (CFRP) and aluminum alloy (AA) is an important factor in predicting the performance of CFRP-strengthened AA structures. Despite its importance, research on this relationship remains limited. In this study, thirty specimens were tested under double shear loading to explore the effects of adhesive type, the stiffness ratio between CFRP and AA plates, and the thickness of the CFRP plates on the CFRP-AA bonded interface. The analysis focused on key parameters such as failure modes, load–slip behavior, and strain distribution. The results revealed that CFRP delamination was the predominant failure mode for the CFRP-AA bonded interface. Moreover, the ultimate load of the specimens initially increased with greater CFRP plate thickness. However, variations in the stiffness ratio between the CFRP and AA plates had little impact on interfacial shear stress. Specimens with identical AA thicknesses exhibited larger initial and ultimate slip when the non-linear Araldite_2015 adhesive was used, compared to the linear two-component epoxy resin. Building on these findings, an interfacial shear bond–slip model that incorporates the adhesive type used in this study was developed. The model demonstrated strong correlation with the experimental results, offering a reliable framework for predicting the behavior of CFRP-AA bonded interfaces under shear loading.

Fatigue strengthening of cracked steel plates with CFRP laminates in the case of old steel material

Finite element analysis of fatigue crack growth in CFRP-repaired four-point bend specimens

Fatigue behavior of corroded steel plates strengthened with CFRP plates

Applying pre-stressed Carbon Fiber Reinforced Polymers (CFRP) plate to steelconcrete composite beams is considered an active strengthening technique that can create permanent internal stresses in the beam opposite to the internal straining actions due to the service loads. This study presents a numerical analysis of steelconcrete composite beams stiffened with pre-stressed CFRP plates. The pre-stressing effect on CFRP plate is enforced in the finite element (FE) model as an initial stress on the solid element that used to simulate the strengthening plate and the epoxy is considered as a thin layer between CFRP plate and the steel flange. The deterministic and stochastic analysis is performed to obtain the long-term random responses of the strengthened beams as well as plain beams due to the effect of shrinkage. In addition, the effect of CFRP plate rigidity and strengthening schemes are investigated. By using CFRP plates, strengthening can be achieved up to 25% for the system explored. During the post-elastic range, stiffness is improved when adding pre-stressed CFRP plates and deflection of strengthened beams is reduced by 20% in comparison to the reference beam at the ultimate capacity. Also, a slight drop in ductility occurs with the addition of CFRP plates.

The use of fiber-reinforced polymer (FRP) composites in retrofitting and strengthening reinforced concrete (RC) slabs has gained substantial attention due to their durability, high strength-to-weight ratio, and ease of application. The objective of this study was to theoretically investigate the flexural behavior of RC slabs strengthened with carbon fiber-reinforced polymer (CFRP) plates applied in different percentages and patterns using finite element methods (FEMs) in comparison with the experiment outcomes available in the literature using the ABAQUS software (version 2020). This study focused on understanding the influence of the CFRP configuration on the structural behavior, including the load-carrying capacity, flexural performance, crack patterns, and failure modes, under static loading on seventeen RC slabs of 1800 × 1800 mm and 150 mm thickness. A comprehensive program was adopted, where RC slabs were strengthened using CFRP plates with different coverage percentages (0.044, 0.088, 0.133, 0.178, and 0.223) and arrangements (unidirectional, cross-hatched, and grid patterns) to evaluate the slabs’ performance under realistic service conditions. After comparison, the results validate that the percentage and pattern of CFRP plates influence the performance of RC slabs. Higher CFRP plate percentages yielded greater strength enhancement, while optimized patterns guaranteed a uniform stress distribution and delayed crack initiation. This study hypothesizes that the flexural strength, stiffness, and failure behavior of RC slabs are significantly affected by the percentage and arrangement of CFRP strengthening, with certain configurations providing superior structural performance. The use of CFRP cross-hatched plates improved the load–deflection behavior, increasing the ultimate loads by 35% (452 kN) while reducing ultimate deflection, with the cross-hatched CFRP specimen showing the highest deflection among all the CFRP specimens. This study provides engineers and practitioners with valuable information on choosing appropriate strengthening plans for RC slabs using CFRP plates, leading to more cost-effective and ecologically friendly structural rehabilitation methods.

The use of CFRP (Carbon Fiber Reinforced Polymer) plate to strengthen RC (Reinforced Concrete) structure has become popular, nowadays. Meanwhile, bonding behavior between CFRP plate and concrete is an important issue in application. Many research works had been done to improve the bonding performance of this method. In prior study, it was reported that the bond strength could be increased by increasing the stiffness of CFRP or using a soft adhesive layer with a small shear stiffness. With this assumption, the bond strength will be enhanced by putting a soft layer (polyurea) between CFRP plate and usual adhesive (epoxy). To obtain the complete view of bonding behavior of CFRP plate-concrete, a double face tensile test setup was conducted in this research. Both high modulus type and high tension type of CFRP plate with soft layer and without soft layer were tested. CFRP plate used in this study had a thick of 1mm, 2mm and 4mm, respectively, and had a width of 50mm for all specimens. The results showed that soft layer system has a big influence for high tension type.