**3. Results and discussion**

#### **3.1 Adhesive viscosity**

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.

#### *3.1.1 Addition of low viscous nonstructural adhesive*

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

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 adhesion strength.

**57**

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

The adhesive joint with pure nonstructural adhesive has a load capacity of 4.3 kN. It reaches to a maximum of 9.1 kN with 20% NSA addition and 8.8 kN with pure structural adhesive. The viscosity with 20% NSA addition would be enough to penetrate through the open pores. Hence the mechanical interlocking and load

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

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

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 pre-

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

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].

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

the tensile test can be seen in **Figure 5b**.

(80°C) during tensile test.

pare the sample.

of 950 mPa-s.

**3.2 Effect of bond length**

capacity of the joint increases.

*3.1.2 Heating effect*

The adhesive joint with pure nonstructural adhesive has a load capacity of 4.3 kN. It reaches to a maximum of 9.1 kN with 20% NSA addition and 8.8 kN with pure structural adhesive. The viscosity with 20% NSA addition would be enough to penetrate through the open pores. Hence the mechanical interlocking and load capacity of the joint increases.
