**2. Materials and methods**

#### **2.1 Materials**

*Adhesives and Adhesive Joints in Industry Applications*

ity products with the help of vacuum pressure.

interlocking between the adhesive and adherent.

which in turn reduces the resin content in the final product.

used to rehabilitate the structure are usually heavy, the process is difficult and chances of corrosion and fatigue are high as they are subjected to high temperatures while welding. Adhesive bonded CFRP patch can be a good replacement for conventional methods as it has a high strength to weight ratio with design flexibility [1, 2]. Adhesive bonding can be done for similar, dissimilar and thin components. There are many applications in marine structures for adhesive bonding such as bonding of the hull to the deck, the channels running through the deck, the sea chests, engine compartment and the exhaust system. Repairing with CFRP patch can give a long life to the components as they have high resistance to chemical attacks and corrosion. The cost of the structural repairing with adhesive bond using wet lay-up method can be reduced [3, 4]. In this process, fibre mats were wetted by an adhesive/resin until the desired thickness is achieved. Rollers are used to promote good impregnation of the fibre and reduce the voids entrapped in the adhesive mixture. The use of wet/hand lay-up results from the ease of application, minimal tooling cost and low cost of raw material. But the emissions from the adhesive/resin, low fibre volume fraction and high void content may limit its use [5–8]. The hand/wet lay-up process along with vacuum bag can prevent the harmful emissions and produce better qual-

Vacuum bagging process is used in many repairing applications in construction, marine and aerospace industries. Generally, scarf repair uses wet lay-up vacuum bagging process to bond CFRP patch on the surface of the damaged components. However, generally, the bond between the adherents is weaker than adherents in most cases due to the inability to apply sufficiently high-pressure during curing process. Thus, piles are added to the patch to attain the desired stiffness and loadcarrying capacity [9]. The increased thickness of the patch enhances the bending stiffness and leads to premature failure at the edges under bending loads [10]. The properties of the repair patch can be enhanced by reducing the void content and increasing the fibre volume fraction which may be possible by vacuum bagging

Adhesive bonding can be done for similar, dissimilar and thin components. But the problem with adhesive bonding of metal plates with CFRP is its low strength. The major adhesion mechanisms that occur between CFRP and metals are (a) chemical bonding such as van der Waals forces, (b) mechanical interlocking between adhesive and substrate and (c) diffusion bonding. Joint strength for interdiffusion phenomena depends on diverse aspects, namely, contact temperature, time, nature and molecular weight of the polymers, etc. In order to vary the mechanical interlocking of surfaces, pre-bond surface preparation is needed, which includes surface abrasion, sand blasting, etching, etc. [11]. Mechanical interlocking is provided by allowing adhesive to wet the cavities and asperities of adherent surface. However, the surface asperity dimension should be controlled to avoid the formation of air bubbles. Air bubbles can generate regions of stress concentrations which are not desirable. In most cases, rough surface is good for better mechanical

The evaporation of resin is another source of void formation in composites. The evaporation was high at the beginning of curing at room temperature. After 30 min, the evaporation settles to a constant rate. The high evaporation speed at the beginning was probably caused by air dissolving in the resin. The air bubbles can be removed with the application of vacuum to the mould [12, 13]. The application of vacuum pressure may pull the air bubbles formed during the process out of the mould. If the vacuum pressure is too high, the carbon fibre preform arrests the air bubbles formed between the laminates and creates voids [14]. The increased vacuum pressure may increase the compaction pressure on the carbon fibre preform

**52**

process.

Major materials used for the current study along with their properties were given in **Table 1**. The properties of the adhesives were taken from the Huntsman Advanced Materials, Switzerland data sheet. Fibre properties were considered form the Toray Composite Materials America, Inc. data sheet. Along with major materials, consumable like ferric chloride, distilled water, sticker tape, vacuum bagging, etc. were also used.


#### **Table 1.**

*Materials and properties.*

#### **2.2 SS plate preparation**

SS plates were considered in the following dimensions: 125 × 25 × 3 mm. The surface of the samples was prepared with different surface preparation methods. The surface preparation includes surface without preparation cleaned with acetone (plane surface), surface prepared with etching, surface prepared with sand blasting and the sandblasted surface with open surface cavities.

The open surface cavities were made on the SS plates using chemical etching process. To perform chemical etching on the samples, the part of material to be etched should be opened to the chemical interaction, and the rest of the material was masked with sticker tape. Ferric chloride and distilled water were mixed in the ration of 1.5:10. The masked samples were dipped and rinsed in the etchant (ferric chloride and distilled water mixer) for 8 min to get 80 ± 0.1-micron deep surface cavities. The etching process and the etched 3D microscopic image were shown in **Figure 1a**. The pipe surface was also prepared with plane surface cleaned with acetone, sandblasted and chemical etched and a combination of sandblasting and surface cavities.

The plates prepared with different methods show different surface morphologies. These morphologies can be seen using SEM images as shown in **Figure 2a**–**c**. From SEM images it was evident that the roughness of chemically etched surface is lesser than the roughness of the sandblasted surface. There are some locations on the sandblasted surface where the sand particulates diffused onto the surface of the specimens.

#### **2.3 Single-strap SS-CFRP adhesive joint preparation**

A flat marble was considered as a mould. The boundaries of the moulds were marked, and the sealant tape was attached. The pre-bond surface prepared samples were cleaned with acetone to remove chemical residues, dust and contaminations from the surface. The gap between the plates should be less than 1 mm in order to avoid the edge effect. Alternate layers of adhesive and carbon fibre were applied onto the surface of the plates (3 layers of CFRP and 4 layers of adhesive). Different accessory layers were placed over the carbon fibre layers to assist the vacuum bagging process. The accessory layers include a peel ply which is used to provide easy removal of other layers from the surface of the composite and a breather fabric to absorb excess resin during compression (vacuum pressure). Vacuum bag was applied with the help of the sealant tape, which forms a one-sided flexible mould. The mould was connected to the vacuum reservoir through vacuum hose pipe as shown in **Figure 3a** and **b**. The vacuum bag should be checked for vacuum drop (leakage). To do so the hose pipe should be closed, while the vacuum bag should be connected the vacuum gauge. A good vacuum bag should not drop more than 500 Pa in 5 min, and it is not recommended to be used if the vacuum loss is greater than 5000 Pa in 5 min. The mould should be isolated from the vacuum reservoir through proper clamping devices and allow the samples to cure for 24 hrs in the atmospheric condition. When demoulding, care must be taken as the excess adhesive would spread and stick over the entire mould cavity. Machining was done to remove the excess adhesive. The schematic of sample dimensions can be seen in **Figure 3c**.

**55**

**2.4 Characterisation**

**Figure 3.**

*(mm).*

The adhesive bond strength of SS plates was evaluated with tensile shear load capacity. It was measured using M/s. Jin Ahn Testing, China, with 100 kN load cell under a crosshead rate of 0.5 mm/min. The results shown here are the average values of three samples. The roughness of different pre-bond prepared surfaces was measured using 3D optical microscope (Huvitz Automatic 3D Measuring

*(a) Schematic of vacuum infusion set-up; (b) homemade vacuum bagging set-up; (c) sample dimensions* 

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

*SEM images (a) chemical etched surface; (b) plane surface; (c) sandblasted surface.*

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

**Figure 2.**

**Figure 1.** *(a) Chemical etching process and (b) cavities with different densities.*

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

#### **Figure 2.**

*Adhesives and Adhesive Joints in Industry Applications*

**2.3 Single-strap SS-CFRP adhesive joint preparation**

masked with sticker tape. Ferric chloride and distilled water were mixed in the ration of 1.5:10. The masked samples were dipped and rinsed in the etchant (ferric chloride and distilled water mixer) for 8 min to get 80 ± 0.1-micron deep surface cavities. The etching process and the etched 3D microscopic image were shown in **Figure 1a**. The pipe surface was also prepared with plane surface cleaned with acetone, sandblasted

The plates prepared with different methods show different surface morphologies. These morphologies can be seen using SEM images as shown in **Figure 2a**–**c**. From SEM images it was evident that the roughness of chemically etched surface is lesser than the roughness of the sandblasted surface. There are some locations on the sandblasted surface where the sand particulates diffused onto the surface of the specimens.

A flat marble was considered as a mould. The boundaries of the moulds were marked, and the sealant tape was attached. The pre-bond surface prepared samples were cleaned with acetone to remove chemical residues, dust and contaminations from the surface. The gap between the plates should be less than 1 mm in order to avoid the edge effect. Alternate layers of adhesive and carbon fibre were applied onto the surface of the plates (3 layers of CFRP and 4 layers of adhesive). Different accessory layers were placed over the carbon fibre layers to assist the vacuum bagging process. The accessory layers include a peel ply which is used to provide easy removal of other layers from the surface of the composite and a breather fabric to absorb excess resin during compression (vacuum pressure). Vacuum bag was applied with the help of the sealant tape, which forms a one-sided flexible mould. The mould was connected to the vacuum reservoir through vacuum hose pipe as shown in **Figure 3a** and **b**. The vacuum bag should be checked for vacuum drop (leakage). To do so the hose pipe should be closed, while the vacuum bag should be connected the vacuum gauge. A good vacuum bag should not drop more than 500 Pa in 5 min, and it is not recommended to be used if the vacuum loss is greater than 5000 Pa in 5 min. The mould should be isolated from the vacuum reservoir through proper clamping devices and allow the samples to cure for 24 hrs in the atmospheric condition. When demoulding, care must be taken as the excess adhesive would spread and stick over the entire mould cavity. Machining was done to remove the excess adhesive. The schematic of sample dimensions can be seen in **Figure 3c**.

and chemical etched and a combination of sandblasting and surface cavities.

**54**

**Figure 1.**

*(a) Chemical etching process and (b) cavities with different densities.*

*SEM images (a) chemical etched surface; (b) plane surface; (c) sandblasted surface.*

#### **Figure 3.**

*(a) Schematic of vacuum infusion set-up; (b) homemade vacuum bagging set-up; (c) sample dimensions (mm).*

#### **2.4 Characterisation**

The adhesive bond strength of SS plates was evaluated with tensile shear load capacity. It was measured using M/s. Jin Ahn Testing, China, with 100 kN load cell under a crosshead rate of 0.5 mm/min. The results shown here are the average values of three samples. The roughness of different pre-bond prepared surfaces was measured using 3D optical microscope (Huvitz Automatic 3D Measuring

Microscope). The surface morphology of the prepared samples was analysed using SEM micrographs taken with the help of "TESCAN VEGA3 LMU SEM". All tests were performed at room temperature except viscosity measurement.
