*3.1.2 Heating effect*

*Adhesives and Adhesive Joints in Industry Applications*

*3.1.1 Addition of low viscous nonstructural adhesive*

**3. Results and discussion**

and it was shown in **Figure 4a**.

adhesion strength.

**3.1 Adhesive viscosity**

Microscope). The surface morphology of the prepared samples was analysed using SEM micrographs taken with the help of "TESCAN VEGA3 LMU SEM". All tests

The two-component structural adhesive (Araldite AW105 and Hardener HV 953U) used in this study has high viscosity. It is not desirable for CFRP composite making and penetration of adhesive through open pores of the surface of the adherents. The adhesive viscosity was reduced in two ways: (1) addition of low viscous nonstructural adhesive and (2) heating the adhesive to get required viscosity.

The adhesive mixture was taken with different proportions by adding nonstructural adhesive in structural adhesive. The samples were prepared as per ASTM D638 standards as shown in **Figure 5c**, and tensile test was performed. The change in the viscosity of the mixture with NSA addition was calculated using Gambill method,

Single-strap SS-CFRP adhesive bonded samples were prepared by adding a low viscous nonstructural adhesive (NSA) from 0 to 100% in structural adhesive (SA). The variable parameters like surface preparation, bond length and vacuum pressure were taken at random as sand blasted, 100 and 350 mm of Hg. The samples were cured in atmospheric conditions and de-moulded after 24 h of curing. The samples were machined to remove the excess material. The samples were tested in the UTM for tensile shear load capacity of the adhesive bonded joints prepared. The results were plotted between % NSA and tensile shear load capacity as shown in **Figure 4b**. The addition of NSA decreases the load capacity of the joint and the viscosity of the adhesive mixture [17]. Generally, NSA does not take any loads they meant for holding the fibres or components together, while the SA is capable of bearing loads. Hence the addition of NSA reduces the

*Effect of NSA addition on (a) viscosity of the adhesive mixture; (b) load capacity of the SS-CCRP joint.*

were performed at room temperature except viscosity measurement.

**56**

**Figure 4.**

Curing of the adhesive makes the resin to form a cross-linked network of polymers. The structure and its mechanical properties can be changed by the way the chain network forms, which depends on the curing process. During solidification of adhesive, there is a shrinkage which creates some internal stresses in the adhesive and leads to failure of the joint well below its designed load [17]. An attempt has been made to find the effect of adhesive pre-cure temperature on tensile strength. The viscosity of adhesive was reduced by increasing the temperature of the adhesive. The adhesive was heated to 45, 55, 65, 72 and 80°C in a furnace. The viscosity at each temperature was measured using "Brookfield Viscometer". The results were plotted between temperature and viscosity as shown in **Figure 5a**. The increased curing temperature may also change the curing rate, which in turn affects the bond strength. A series of experiments were performed on adhesive samples cured at different temperatures (45, 55, 65, 72 and 80°C) to evaluate the curing temperature effect on tensile strength of the adhesive. The sample dimensions were considered as per ASTM D638 standards. The results obtained from the tensile test can be seen in **Figure 5b**.

From **Figure 5b** and **d**, it was observed that the tensile strength of the adhesive and tensile shear load capacity of the SS-CFRP joint were increased with the increase in precuring temperature [18]. The adhesive cured at lower temperatures (45°C) has shown a brittle nature than the adhesive cured at higher temperature (80°C) during tensile test.

From **Figure 5a** and **b**, it is evident that the adhesive has shown a viscosity of 560 mPa-s at 80°C and a tensile strength of 21.1 MPa which is optimal when considering both viscosity and tensile strength as a function of temperature. In case of adhesive, joint preheat temperature limits the use of adhesive preheat temperature to 55°C. Further heating reduces the gel time, and curing it might not be good for vacuum bagging process. The supplier's data states that the increased temperature reduces the minimum curing time ("At 20°C the minimum curing time is 15 hrs, whereas at 100°C it is 10 min"). Hence the adhesive curing temperature was considered as 55°C where the minimum curing time is about 2 hrs (gel time is directly proportional to curing time) which is enough to prepare the sample.

The addition of NSA in the adhesive mixture reduces the adhesive bond strength. Heating the adhesive mixture reduces the gel time. Hence the adhesive was considered as 20% NSA + 80% SA mixture heated to 55°C which has a viscosity of 950 mPa-s.

#### **3.2 Effect of bond length**

The stress generated at the edge is maximum in the joint. As the bond length increases, the stress generated at the edge reduces. But it is up to a certain length beyond which the addition of bond length has no significance. Then the increased load that may act at the edge undergoes an elastic-plastic transition [19].

#### **Figure 5.**

*(a) Viscosity of adhesive as a function of temperatures; (b) tensile strength of the adhesive as a function of temperatures; (c) adhesive tensile test sample; (d) SS-CFRP joint with different preheat temperatures.*

**Figure 6.** *(a) Samples with different bond lengths; (b) effect of bond length on load capacity.*

To evaluate the effect of bond length on joint load capacity, the adhesive bonded samples were prepared with different bond lengths as shown in **Figure 6a**. The parameters considered for this study are 20% NSA + 80% SA mixture heated to 55°C, sandblasted surface and 350 mm of Hg vacuum pressure. The samples were tested for tensile load capacity, and the obtained results were plotted as shown in **Figure 6b**.

**59**

**Figure 7.**

*Load capacity for various vacuum pressures.*

*Overhauling of Steel Pipes Using Vacuum Bagging Processed CFRP Patch*

ered as the adequate bond length for this experimental condition.

From **Figure 6b**, it has been observed that the bond load capacity of the adhesive joint increases from 5.2 to 10.5 kN, as bond length increases from 30 to 100 mm; hence the increased 70 mm bond length carries 5.3 kN load capacity. During this stage the addition of bond length will carry load linearly and reduces the stress concentration at the edge. From 100 mm to 130 mm, it has been observed that the 40 mm bond length contributes to increase 0.4 kN load capacity. During this stage, the load induces the stresses at the edges, which are greater than the elastic limit of the adhesive, so elastic plastic transition takes place, and further addition of bond length is not going to have any significance. 100 mm bond length has been consid-

Compaction pressure was applied on the wet fabric with the help of the vacuum bag which is under vacuum pressure. An attempt has been made to find the effect of vacuum pressure on the adhesive bond strength of the single-strap SS-CFRP joint. Like in hand/wet lay-up, the fibres were impregnated. Initially the mould is under atmospheric pressure. Different vacuum pressures were introduced in the mould cavity. As the vacuum pressure increases, the compaction pressure acting on wet fibres increases. Hence more amount of adhesive comes out of the patch and is absorbed by the breather fabric. The air voids present in between the layers would also come out along with the excess adhesive. During this study, the other parameters considered are 20% NSA + 80% SA mixture heated to 55°C, 100 mm bond length and sandblasted surface. The prepared samples were tested on UTM for its

From **Figure 7**, it was evident that the increased vacuum pressure increases the load capacity of the prepared joint. With 100 mm of Hg vacuum pressure, the obtained joint thickness and load capacity were 4.84 mm and 9.4 kN. With 700 mm of Hg vacuum pressure, the obtained joint thickness and the load capacity were 4.32 mm and 12.6 kN. With more compaction, more amount of adhesive can be

*DOI: http://dx.doi.org/10.5772/intechopen.87074*

**3.3 Vacuum pressure effect on bond strength**

tensile shear strength.

*Overhauling of Steel Pipes Using Vacuum Bagging Processed CFRP Patch DOI: http://dx.doi.org/10.5772/intechopen.87074*

*Adhesives and Adhesive Joints in Industry Applications*

**58**

in **Figure 6b**.

**Figure 6.**

**Figure 5.**

To evaluate the effect of bond length on joint load capacity, the adhesive bonded

samples were prepared with different bond lengths as shown in **Figure 6a**. The parameters considered for this study are 20% NSA + 80% SA mixture heated to 55°C, sandblasted surface and 350 mm of Hg vacuum pressure. The samples were tested for tensile load capacity, and the obtained results were plotted as shown

*(a) Viscosity of adhesive as a function of temperatures; (b) tensile strength of the adhesive as a function of temperatures; (c) adhesive tensile test sample; (d) SS-CFRP joint with different preheat temperatures.*

*(a) Samples with different bond lengths; (b) effect of bond length on load capacity.*

From **Figure 6b**, it has been observed that the bond load capacity of the adhesive joint increases from 5.2 to 10.5 kN, as bond length increases from 30 to 100 mm; hence the increased 70 mm bond length carries 5.3 kN load capacity. During this stage the addition of bond length will carry load linearly and reduces the stress concentration at the edge. From 100 mm to 130 mm, it has been observed that the 40 mm bond length contributes to increase 0.4 kN load capacity. During this stage, the load induces the stresses at the edges, which are greater than the elastic limit of the adhesive, so elastic plastic transition takes place, and further addition of bond length is not going to have any significance. 100 mm bond length has been considered as the adequate bond length for this experimental condition.

#### **3.3 Vacuum pressure effect on bond strength**

Compaction pressure was applied on the wet fabric with the help of the vacuum bag which is under vacuum pressure. An attempt has been made to find the effect of vacuum pressure on the adhesive bond strength of the single-strap SS-CFRP joint. Like in hand/wet lay-up, the fibres were impregnated. Initially the mould is under atmospheric pressure. Different vacuum pressures were introduced in the mould cavity. As the vacuum pressure increases, the compaction pressure acting on wet fibres increases. Hence more amount of adhesive comes out of the patch and is absorbed by the breather fabric. The air voids present in between the layers would also come out along with the excess adhesive. During this study, the other parameters considered are 20% NSA + 80% SA mixture heated to 55°C, 100 mm bond length and sandblasted surface. The prepared samples were tested on UTM for its tensile shear strength.

From **Figure 7**, it was evident that the increased vacuum pressure increases the load capacity of the prepared joint. With 100 mm of Hg vacuum pressure, the obtained joint thickness and load capacity were 4.84 mm and 9.4 kN. With 700 mm of Hg vacuum pressure, the obtained joint thickness and the load capacity were 4.32 mm and 12.6 kN. With more compaction, more amount of adhesive can be

**Figure 7.** *Load capacity for various vacuum pressures.*

squeezed out of the patch which increases the fibre volume fraction and reduces adhesive layer between SS plate and carbon fibre. Hence the adhesive bond strength increases [20].

## **3.4 Effect of surface texture**

Surface preparation is very essential in adhesive bonding; a proper surface may provide a high bond strength. In order to find the proper surface to the adhesive bonding, the adherent surfaces were prepared with plane surface cleaned with acetone, chemical etched surface, sandblasted surface and surface texture created in the form of circular cavities at different densities with a depth of 80 ±5 μm as shown in **Figure 1b**. The surface roughness was measured using a 3D microscope, and the values can be seen in **Table 2**. The surface morphology of the samples was studied using SEM images as shown in **Figure 2**.

The samples for tensile test was prepared with the optimal conditions like 100 mm bond length, 700 mm of Hg and 20% NSA + 80% SA mixture heated to 55°C. Tested results can be seen in **Table 2**. The specimens were failed by delamination between steel and carbon fibre interface (no residues of fibres). The failed surfaces can be seen in **Figure 8**.

From the results, it was evident that the circular surface cavities spread over the 33% of the bonded area were shown a maximum bond strength of 14.15 kN, which is 26% higher than plane surface, 38% higher than etched surface and 12% higher than sandblasted surface. Surface texturing increases the surface roughness of the adherent which in turn increases the mechanical interlocking, and hence the adhesive bond strength increases [11].


#### **Table 2.**

*Load capacity of different pre-bond surfaces.*

**61**

**Table 3.**

*Hydrostatic pressure of pipes.*

**Figure 9.**

*Overhauling of Steel Pipes Using Vacuum Bagging Processed CFRP Patch*

The damaged SS pipes were considered for rehabilitation. The rehabilitation capacity was evaluated using hydrostatic pressure test. Here, a man-made through-all defect with 10 mm diameter was machined over an 200 mm

diameter pipe. This through-all hole defect was covered with a two-component solid state adhesive (M-Seal). The pipe was tested for hydrostatic pressure with M-Seal adhesive after 24 hrs. A pressure of 500 ± 30 kPa was observed with

The pipe surface around the defect was prepared as plane surface cleaned with acetone, etched surface, sandblasted surface and sandblasted surface with circular cavities spread over 33% bonded area. The rehabilitation was done on defect filled with M-Seal adhesive; then the composite patch proposed during the present study as 100 × 100 mm bond area (the fibres in the adhesive bonding of SS plates are aligned in the loading direction with a length of 100 mm) with [0/90]3 carbon fibre layers, 700 mm of Hg and 20% NSA + 80% SA mixture

These hydrostatic tests were conducted on the rehabilitated pipes as shown in **Figure 9**, and the results were given in **Table 3**. From the results, it is evident that the failure pressure of a pipe can be changed with surface texture. A maximum of 3852 ± 50 kPa was achieved with a pipe surface prepared with the combination of sandblasting and circular cavities. It is 62.8% higher than the

*Pipe hydrostatic pressure testing (left side) and surface prepared before patching (right side).*

**S. no Pipe bonded area condition Failure pressure (kPa)** Etched 1525 ± 50 Plane 2365 ± 50 Sandblasted 3150 ± 50 Surface cavities 3852 ± 50

*DOI: http://dx.doi.org/10.5772/intechopen.87074*

**4. Rehabilitation of pipe**

heated to 55°C was applied.

M-Seal adhesive.

plane surface.

**Figure 8.** *Surface of the adherent after failure in tension test.*

*Overhauling of Steel Pipes Using Vacuum Bagging Processed CFRP Patch DOI: http://dx.doi.org/10.5772/intechopen.87074*
