Behavior of Deficient Steel Members Strengthened Using CFRP Under Combined Compressive Load and Torsional Moment

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.

Several reinforced concrete structures that get deteriorated by rebar corrosion are retrofitted using Carbon Fiber Reinforced Polymer (CFRP). When rebar comes in direct contact with CFRP, rebar may corrode, as iron is more active than carbon. Progression of corrosion of rebar in strengthened RC structures has been carried out when rebar comes in direct contact with CFRP. The experimentation is carried out in two phases. In phase I, corrosion of bare steel bar is monitored by making its contact with CFRP. In phase II, concrete specimens with surface bonded CFRP were casted and subjected to the realistic exposure conditions keeping direct contact between rebar and CFRP. Progression of corrosion has been monitored by various parameters: Half-cell potential, Tafel extrapolation and Linear Polarisation Resistance. On termination of exposure, to find residual bond stress between rebar and concrete, pull-out test was performed. Rebar in contact with CFRP has shown substantially higher corrosion. The level of corrosion will be more with more area of contact.

Investigation on the behaviour of CFRP strengthened CHS members under monotonic loading through finite element modelling

Design rules for web crippling of CFRP strengthened aluminium rectangular hollow sections

Excellent mechanical and physical properties make carbon fiber reinforced polymer (CFRP) the best options for repair, retrofit, and rehabilitation of civil engineering structures. A great success on application of this material in reinforced concrete (RC) structures has attracted much attention from many researchers to develop it in combination with steel. The number of studies on the use of CFRP composites for strengthening steel structures has still been limited and needs to be more explored. To date, the research in this field has mainly focused on CFRP strengthening with adhesively-bonded technique. This paper reports an experimental study to investigate the performance of slender axial compression steel members partially strengthened with unbonded CFRP composites. The requirements for stiffener to prevent buckling occurred in stiffening region are derived from structural equilibrium conditions. Vacuum-assisted Resin Transfer Molding (VaRTM) method is adopted to form CFRP laminates in the strengthened specimens. Totally eight small scale specimens are tested, and it is clear from the test that improvement in load-carrying capacity can be achieved by using CFRP.

Carbon fibre reinforced polymer (CFRP) has been used in strengthening different types of steel members in bending and compression. However there is still a lack of understanding on the behaviour of CFRP reinforced steel beams subject to torsion. This paper presents a theoretical approach to investigate CFRP strengthening of thin-walled steel square hollow section (SHS) subjected to torsion. Various CFRP strengthening configurations including vertical, spiral and reverse-spiral wrapping are considered. Laminate composite mechanics is introduced to incorporate the contribution of CFRP strengthening to the torsional stiffness and strength of the SHS. By introducing reasonable failure criteria, the torsional behaviour of CFRP strengthened SHS can be predicted corresponding to different failure modes, i.e. CFRP shear, CFRP fracture, CFRP buckling, steel yielding. The relationship between torsion and twist angle have also been identified. The theoretical solutions are validated with lab testing results and reasonable agreement has been achieved.

In engineering, many pressure pipes are made of steels with good plasticity, which are subject to internal pressure, axial force, shear force, bending moment, torsion moment or their combined loads. The plastic limit load is an important indicator of the load capacity of pressure pipe. According to Hill yield function, the theoretical solutions of limit load of orthotropic cylindrical pipe under various combined loads under internal pressure, axial force, shear force, torsion moment, and bending moment have been derived on the basis of elastic perfectly plastic constitutive model. The effects of radial stress on different combined limit loads of cylindrical pipe are explored and these results show that the radial stress should be considered about the limit load calculation especially for thick-walled cylindrical pipe. The interactions of various load combination are analyzed in detail and drawn with the interaction curves. For isotropic cylindrical pipe, the limit load increases with the yield strength. For the orthotropic cylindrical pipe, the limit loads of cylindrical pipe under axial force, bending moment, shear force, and torsion moment without internal pressure are only related to the axial yield strength. The limit bending moment is mainly dependent on the axial yield strength when internal pressure is lower, while the impact of the circumferential yield strength of orthotropic cylindrical pipe is obvious when internal pressure is some higher. When the axial yield strength of orthotropic cylindrical pipe is the same, the circumferential yield strength can enhance the limit axial load, limit torsion moment, and limit shear load. Under the different load conditions including internal pressure, bending moment, axial force, shear force, and torsion moment or their combined loads, the relation of limit bending moment with yield strength ratio is diverse, which is decide by the load combination, the circumferential yield strength, and the axial yield strength.

During earthquake excitations, reinforced concrete bridge columns can be subjected to a combination of axial load, shear force, flexural moments, and torsional moments. The torsional moment can be much more significant in columns of bridges that are skewed, curved, have unequal spans, or unequal column heights. Combined loading including torsion can result in complex flexural and shear failure of these bridge columns. This paper presents an experimental study on the seismic behavior of square reinforced concrete columns under combined cyclic flexural and torsional moments. The columns in this study were designed with an aspect ratio of six and tested under various loading conditions: cyclic flexural moment and shear force, cyclic pure torsion, and combined cyclic shear force, bending, and torsional moments. Test results reveal that (1) the flexural and torsional capacity is decreased due to the effect of combined loading, (2) the failure modes and deformation characteristics are changed, and (3) the damage zone tends to move upwards from the typical flexural plastic hinge zone due to the effect of additional torsional moment. The effects of combined loading on the hysteretic loading-displacement response, and damage characteristics are discussed.

CFRP (Carbon Fiber Reinforced Polymer) are widely used in steel structural reinforcement. For steel structures strengthened with CFRP, except the cases the structures have defects before strengthening, the adhesive interface is the weakest part and CFRP debonding is the most common failure mode. In order to investigate the failure mechanism of CFRP strengthened steel structures, this paper presents an experimental study on shear performance of adhesive interface between steel plate and CFRP by twin shear model. Six steel plates strengthened with CFRP are divided into two groups, one has no damage, another has a gap at the mid. The specimens are tested under tensile loadings. The study results show that, the plates with a gap failed for CFRPs debonding, the cracking loading and breaking loading are 14.85kN, and 17.88kN respectively; the strain-loading curves had long linear stages, two strains decrease and other strains of another side increased rapidly at the cracking loading, then they both rose until the plates failed.

O.082 Dynamic ultrasonic examination in evaluation of the mandible regenerate development in children

Cracked cross-beam connections made of thin-walled rectangular hollow sections (RHS) are repaired with carbon fiber-reinforced polymer (CFRP) sheets. Constant amplitude fatigue experiments are conducted on the repaired specimens. A pilot test is conducted on a T-connection of square hollow sections (SHS) to explore the effective method of applying CFRP. Due to the peeling effect at the corner region of the SHS-to-SHS connection, early debonding happens in the pilot test that only results in a slight extension in fatigue life. In the following experiments of repairing RHS-to-RHS cross-beam connections, circumferential or transverse restraining CFRP patches are applied in the corner region that prevents early debonding successfully and leads to significant increase in fatigue life. Finally, an improvement is made with the addition of steel strengthening plates that increase both the flexural stiffness and fatigue life of the cracked specimens significantly. The proposed retrofitting method may be useful for repairing other similar fatigue-cracked welded tubular connections.

Advanced composite patching has been used to rehabilitate cracked structures. In this work, the effects of bonded carbon fiber-reinforced polymer (CFRP) patch on reducing the crack tip driving forces, namely J-integral, crack tip plastic zone, and crack tip opening displacement, of a crack emanating from a fastener hole, under cyclic loading, was studied. The number of CFRP layers, the orientation of the fiber with respect to the direction of the path of the crack, and the pinned joint configurations are the main parameters studied in this research. A three-dimensional finite element model was employed in this work, using ABAQUS. The double-sided CFRP patch-repaired pinned joint is simulated in order to study crack retardation behavior under two different combinations of loading, i.e. Modes I and II. The results of the study show that the stiffness of the cracked pinned joint is increased by using a CFRP composite patch. The reduction in the J-integral ranged from 25% to 45%, depending on the length of the crack and the mode of the crack. Moreover, the monotonic and cyclic crack tip plastic zones decreased by 5% to 45%, depending on the length of the crack.

Underwater strengthening and repairing of tubular offshore structural members using Carbon Fibre Reinforced Polymers with different consolidation methods

Analytical study of CFRP strengthening steel plates with a semi-elliptical surface crack

Carbon fiber reinforced polymer (CFRP) composites have exhibited a great potential for strengthening of steel structures. In the current study, an innovative prestressed unbonded reinforcement (PUR) system is introduced for fatigue strengthening of existing steel members. The system relies on a pair of mechanical clamps; each holds multiple CFRP plates and anchors their prestressing forces to the steel substrate via friction. A finite element model was established to optimize the design of the required mechanical components of the system. A set of static and fatigue tests was conducted on the developed mechanical clamps as the key elements of the proposed PUR system. The performance of the PUR system was then evaluated using a set of fatigue tests on two precracked steel plate specimens, one without any strengthening system and the other one strengthened with the proposed PUR system. In the latter specimen, the CFRP plates were prestressed up to about 800 MPa (approximately 30% of the CFRP tensile strength), which resulted in a complete fatigue crack arrest in the precracked steel plate. Furthermore, neither slippage of the mechanical clamps nor any prestress loss in the CFRP plates was observed after 7.5 million fatigue cycles. Based on the promising experimental results, obtained from the sets of fatigue tests performed in the current study, it can be concluded that the proposed PUR system can be considered as an efficient alternative to the conventional bonded reinforcement solutions for fatigue strengthening of damaged steel members.