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Shahanara, Kaniz & Fawzia, Sabrina(2012)Stress distribution of CFRP strengthened steel hollow sections under ten-sion.In Islam, K, Hassan, K M, Hossain, Q S, Ali Md S, S, Adhikary, S K, RashidMd H, H, et al. (Eds.) Proceedings of the 1st International Conference onCivil Engineering for Sustainable Development.Khulna University of Engineering and Technology (KUET), Bangladesh,pp. 1-9.

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Proceedings of the 1st International Conference on Civil Engineering for Sustainable Development (ICCESD-2012), 2~3 March 2012, KUET, Khulna, Bangladesh

STRESS DISTRIBUTION OF CFRP STRENGTHENED STEEL HOLLOW SECTIONS UNDER TENSION

Kaniz Shahanara1*and Dr. Sabrina Fawzia2

1 School of Urban Development, Faculty of Built Environment & Engineering ,Queensland University of Technology, Gardens Point campus, 2 George Street, Brisbane 4000, Queensland, Australia, e-

mail:[email protected]. 2 Lecturer in Civil Engineering , School of Urban Development, Faculty of Built Environment & Engineering, Queensland University of Technology, Gardens Point campus, 2 George Street, Brisbane 4000, Queensland,

Australia, e-mail:[email protected].

ABSTRACT

Carbon fibre reinforced polymer (CFRP) sheets have many outstanding properties such as high strength, high elastic modulus, light weight and good durability which are made them a suitable alternative for steel in strengthening work. This paper describe the ultimate load carrying capacity of steel hollow sections at effective bond length in terms of its cross sectional area and the stress distribution within bond region for different layers CFRP. It was found that depending on their size and orientation of unidirectional CFRP layers, the ultimate tensile load was different. Along with these tests, non linear finite element analysis was also performed to validate the ultimate load carrying capacity depending on their cross sections. The predicted ultimate loads from FE analysis are found very close to the laboratory test results. The validated model has been used to determine the stress distribution at bond joint for different orientation of CFRP. This research shows the effect of stress distribution and suitable wrapping layer to be used for the strengthening of steel hollow sections in tension. Keywords: CFRP, steel hollow section, wrapping layer, stress distribution

1. INTRODUCTION

A composite material is one that attains its physical and mechanical characteristics through the integration of other materials. Generally, a composite material combines the most desirable characteristics of its constituents to create a superior material. A well-known example of a composite material is CFRP (carbon fibre reinforced polymer). This advanced composite material provides greater strength at lighter weights than traditional construction materials thus offering distinct advantages in many engineering applications. The use of carbon fibre reinforced polymer in civil infrastructure for repair and strengthening of reinforced concrete structures and also for new construction has become common practice [Teng et al, 2001]. With the introduction of CFRP materials, the possibility for providing a solution to the ongoing problem of infrastructure deterioration may be extended to steel structures as well. Conventional welding used to repair cracks in steel structures by adding new material to the crack area will typically lead to poor fatigue performance. Welding can also cause metallurgical changes to the parent material, resulting in premature failure. These conservative methods are less effective and could increase maintenance costs [Seica et al, 2007]. Thus there is a need for more efficient and reliable retrofit and restoration methods. CFRP strengthening is attractive for steel bridges and structures since it avoids field welding, particularly overhead welding. Bond stresses may be much more critical for steel structures than for concrete structures since more strengthening material is needed for steel structures to achieve a similar increase in strength due to the inherent high strength of steel and also since the debonding failure does not occur in the substrate as in concrete structures. Despite these challenges, since many structures built in the post-World War II era are already past their design life, the inventory of deteriorated steel structures and bridges in need of rehabilitation are extremely significant [Hollaway, 1994; Schwartz, 1997; Barbero, 1999; Peters, 1998]. A significant amount of research has been undertaken to determine bond characteristics of CFRP strengthened double strap joints under tension [Fawzia et al, 2004a, 2004b, 2005a, 2005b, 2006a, 2006b, 2007, 2008 and 2010]. But those researches were limited to steel plate double strap joint.

1st International Conference on Civil Engineering for Sustainable Development (ICCESD-2012)

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Bambach et al. (2008) studied the axial compressive behaviour of CFRP strengthened cold-formed square hollow sections using experiments. They showed that the application of CFRP delayed local buckling, and that the elastic buckling strength of slender sections is increased by up to 4 times. However they only considered two layouts of CFRP. More tests are required to derive an optimal combination of fibre orientation, number of layers and sequence in applying CFRP layers. Shaat and Fam (2006) found that transverse CFRP layers are effective in confining the outward local buckling of short columns and that the load capacity increased by 18% for short columns and 13%–23% for long columns. A thorough understanding of bond characteristics is essential for CFRP strengthen of different types of steel structures to establish a suitable design guideline. A series of laboratory testing were conducted to find out the ultimate load carrying capacity of CFRP strengthened circular hollow sections (CHS) for different orientation of CFRP layers under tensile load. This paper showed the stresses of different layers of CFRP strengthened hollow steel structure under tensile load by using non-linear finite element models. The main findings of the paper show a new way to strengthen steel hollow sections with cost effective CFRP wrapping.

2. MATERIALS PROPERTY

A series of laboratory tests were conducted to investigate the bonding characteristics between CFRP and circular hollow sections of steel. There were three materials used in the bonding process, they were CFRP, adhesive and mild steel circular hollow sections. Normal modulus of CFRP CF130 is a high tensile, unidirectional carbon fibre sheet of nominal strength. According to the manufacturer’s specifications, these carbon fibres have a Young’s Modulus of 240 GPa and a tensile strength of 3800 MPa [BASF, 2006]. The adhesive MBRACE Saturant is a two part epoxy resin. According to the manufacture’s specifications this adhesive has a Young’s Modulus greater than 3000 MPa [BASF,2006]. The resin should be mixed 3:1 ratio of Part A to Part B, and will be applied using a brush in a Wet Lay Up method [BASF, 2006]. Mild steel circular hollow sections 200 mm long, 2.9 mm thick and diameters 48.3mm and 60.3 mm were used in this test program. The materials, used in the bonding process, should undergo quality assessment by material property testing to find the degree of strengthening. It is common practice to obtain the measured material properties such as tensile strength, modulus of elasticity and ultimate strain rather than relying on the values supplied by the manufacturer. In this case, the material properties of MBRACE Fibre and MBRACE Saturant which was measured by Fawzia[2008] using the coupon test was used. The properties of the three materials used in FE analysis has shown in Table 1.

Table 1: Material properties

Properties CFRP Adhesive Steel

Tensile modulus(Gpa) Tensile strength(MPa)

Yield stress(Mpa) Tensile strain

Poisson’s ration

230 2675

1.2% 0.28

2.028 24.8

1.46% 0.32

195 484 359

1.7% 0.25

3. EXPERIMENTAL PROGRAM

Total six specimens having two different diameters of circular hollow sections (CHS) were prepared with normal modulus CFRP. A schematic view of the specimen is shown in Figure 1.


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Figure 1: A schematic view of specimen (not to scale).

The surfaces were cleaned with acetone to remove grease, oil and rust. Two CHS were aligned in position before applying adhesives and CFRP. Three layers of CFRP sheets were wrapping around the steel CHS in three different wrapping schemes. First wrapping scheme was all layers longitudinal in direction denoted as 3L, second wrapping scheme was two layers longitudinal and one layer transverse in direction denoted as (2LT) and the rest one was one layer longitudinal and two layers transverse in direction denoted as (L2T) as showed in Figure 2.

Figure 2: Layout of CFRP sheet for L2T specimen

The specimens were cured for two weeks at room temperature. Each specimen was loaded in tension in a 500 kN capacity Instron testing machine with a loading rate of 2 mm/min. The details of the tests procedure can be found in Fawzia et al. [2004(b)]. The observed failure mode for the normal modulus CFRP was bond failure. Table 2 gives test results of ultimate load carrying capacity for different wrapping scheme at effective bond length.

Table 1: Results of specimen testing

Specimens Label Ultimate Load (kN) Failure Mode

C48.33L C48.32LT C48.3L2T C60.33L

C60.32LT C60.3L2T

60.151 47.990 30.870 70.770 56.967 53.973

Bond failure Bond failure Bond failure Bond failure Bond failure Bond failure

4. FINITE ELEMENT ANALYSIS

In this study a finite element model for circular hollow steel sections wrapped with CFRP composites was developed. The model was simulated using Strand7 finite element software. The simulation was done by running nonlinear FE analysis solver to account for the nonlinear properties of the materials. To create the FE model a cylindrical coordinate system was selected. All materials were modelled as brick elements. First cross sectional areas of all materials were created as plate element by adding nodes and then extruded with different thickness along the longitudinal direction.


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Generally the applied strain had been considered for calculating the damage rate. For composite materials, the stress of fibre and matrix were different for their volume and elastic modulus but they were functioned for the same strain. So strain was chosen to explain failure mode. In FE model all different layers of CFRP is created as an individual layer to find the stresses of each layers in different directions. The FE model has validated by experimental results and then carried out the comparative study.

4.1 The boundary conditions and loadings

In the present model, the Z-axis of the coordinate system coincided with the axis of the cylinder. The R and T axis represented the radial and hoop directions of the cylinder respectively. Top end of the steel section was fixed rotation but free in translation, i.e., the three degrees of freedom on that surface were constrained and others were free. Figure 3 was representing the boundary conditions of FE model at longitudinal section. A uniform displacement was applied at the bottom end nodes in the axial direction. The axial displacement load was increased gradually until the CFRP fails. In FE model the thickness of adhesive layers were considered constant throughout the bond length and it was taken as all layers of adhesive were same i.e. 0.345 mm [Smith, 1973]. Each layer of CFRP has thickness 0.176 mm as given by the manufacturer was used in FE model.

Figure 3: Details of the boundary conditions of the model

4.2 Material Property

The properties including tensile strength, strain and modulus can be found from Table 1. In the FE model, one layers of CFRP and one layers of adhesive were considered as full CFRP layer having an equivalent modulus of one layer was taken equal to 75.78 GPa [Fawzia et al.,2006(b)].

4.3 Failure mode and Ultimate Loads


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Figure 4: Bond Failure in FE Analysis

Strand7 uses information about the nominal size of the structure to automatically give a reasonable displacement scale, one that clearly shows the deformation. Figure 4 showed bond failure of the CFRP sheets in FE analysis. In case of experiment the failure mode was similar as with the FE. Figure 5 showed a typical experimental bond failure. This was similar to those observed previously from similar tests on normal modulus CFRP and steel tubes [Jiao and Zhao, 2004].

Figure 6 showed the comparison of the load caring capacity of CFRP strengthened CHS having diameter of 48.4mm and 60.3 mm at different CFRP orientation of experiment and FE analysis. It observed that 3L specimens attained maximum load carrying capacity and L2T specimens carried minimum load. They are differences are significant.

Steel Adhesive & CFRP



Figure 5: Bond failure of the specimen (experiment)


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Figure 6: Load carrying capacity of CHS48.3

In Table 3 the comparison of load carrying capacity between experimental and FE results is shown with reasonable accuracy. The mean ratio is 0.9823 with coefficient of variables is 0.0238. In experiment the thickness of adhesive layer was not absolutely constant throughout its length but in FE model it is same for the entire length which made the difference between FE results and experimental results. Therefore there is a reasonably good agreement between the experimental and the proposed FE model results.


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Table 2: Results of specimen testing

Specimens Label Ultimate Load (kN)Experimental(Exp)

Ultimate Load (kN)FE Analysis(FE)

FE/Exp Failure Mode

C48.33L C48.32LT C48.3L2T C60.33L

C60.32LT C60.3L2T

60.151 47.990 40.870 70.770 56.967 53.973

59.450 48.546 40.332 69.844 55.855 50.725

0.9883 1.0115 0.9868 0.9869 0.9804 0.9398

Bond failure Bond failure Bond failure Bond failure Bond failure Bond failure

Average 0.9823 COV 0.0238

5. DISTRIBUTION OF LONGITUDINAL STRESS FOR EACH LAYER OF CFRP

Figure 7: Longitudinal stress distribution for different orientation of CFRP layers for CHS48.3

Steel hollow sections especially circular sections are widely used in marine structures and liquid retaining structures for their high hoop stress along with longitudinal stress. With the increment of diameter of CHS load also increase but depends on their slenderness ratio. The effect of longitudinal stress distribution of different layers of CFRP sheet is investigated at failure load at the joint of CFRP strengthened CHS. Figure 7 show that C48.33L specimen has almost same longitudinal stress at all layers of CFRP. For C48.32LT specimen 2nd layer of CFRP attains largest longitudinal stress for their interlayer shear stress between 2nd and 3rd layer because of their different orientation. For C48.3L2T specimens 1st layer carries the largest longitudinal stress for their interlayer shear stress between 1st and 2nd layer because of their different orientation. It can be concluded that longitudinal stress sharply decreased due to change of CFRP layer orientation. The same phenomenon observes in Figure 8 which represents for CHS60.3.


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Figure 8: Longitudinal stress distribution for different orientation of CFRP layers for CHS60.3

6. DISTRIBUTION OF HOOP STRESSES FOR EACH LAYER OF CFRP

Circular hollow sections were subjected to tensile force along the longitudinal direction in this FE analysis. So their hoop stress distributions were same along the cross section but vary in longitudinal direction. The effect of hoop stress distribution of different layers of CFRP sheet was investigated at failure load at the joint of CFRP strengthened CHS. Figure 9 showed that C48.33L specimen has almost same hoop stress at all layers of CFRP and the value is unimportant. For C48.32LT specimen 1st and 2nd layer of CFRP attains very tiny hoop stress as they are oriented in longitudinal direction and unidirectional CFRP sheet has no stress in transverse direction. But 3rd layer oriented in transverse direction so this layer attains more hoop stress. For C48.3L2T specimens 3rd layer carries the largest hoop stress for their same orientation in 2nd and 3rd layer.

Figure 9: Hoop stress distribution for different orientation of CFRP layers for CHS48.3

The same phenomenon observes in Figure 10 which represents for CHS60.3. But hoop stress of C48.32LT specimens more than that of C60.32LT and hoop stress are same for C48.3L2T and C60.3L2T. It can be concluded that the L2T wrapping scheme is better for slender CHS.


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Figure 10: Hoop stress distribution for different orientation of CFRP layers for CHS60.3

7. CONCLUTION

In this paper the experimental test results from CFRP wrapped specimens under tension were presented. CFRP sheet was used in different orientation at effective bond length. The investigated variables were load carrying capacity and stress distribution at effective bond length of circular steel hollow sections having diameters of 48.3 mm and 60.3 mm. The findings from the comparative studies are summarised as follows:

Load carrying capacity is maximum when there layers of CFRP oriented in longitudinal direction. Maximum longitudinal stress developed in 2nd layer of CFRP for 2LT specimens. Due to change of

orientation of CFRP, the longitudinal stress distribution sharply decreased. Maximum hoop stress developed in 3rd layer of CFRP for L2T specimens. Due to change of orientation

of CFRP, the hoop stress distribution sharply increased. L2T specimens are good for circular hollow sections having large slender ratio. Due to they are large

hoop stress, the effect of slenderness could be minimized 2LT specimens are better for circular hollow sections due to load carrying capacity and both stresses are

high compare with other combinations.

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