**Abstract**

The present work deals with the experimental study on rehabilitation of damaged steel pipes. The process of rehabilitation was done by using adhesive bonded CFRP patch on damaged sites of the pipes with the help of vacuum bagging set-up. The reference optimal parameters to rehabilitate the damaged pipes were considered from the tensile test (tensile shear load) conducted on plates. The rehabilitated pipes were tested under hydrostatic pressure. 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 may lead to improper penetration of adhesive through open pores of the adherent surface. The viscosity of the adhesive was reduced in two ways: addition of a low viscous two-component nonstructural adhesive (Araldite LY-1564 and Aradur-22962) and heating the adhesive. Vacuum pressure, bond length and pre-bond surface preparation of the adherents were considered as parameters to evaluate the bond strength. The roughness of different samples was studied using a 3D microscope. The surface morphology of adherent was studied using scanning electron microscopy (SEM). Based on the experimental studies, it is observed that the optimal conditions of the tensile data of the plates hold good for the rehabilitated pipe under hydrostatic loading.

**Keywords:** vacuum bagging process, adhesive viscosity, surface pre-bond preparation, carbon fibre, SS-304 plates and pipes

### **1. Introduction**

Fibre-reinforced polymer (FRP) composites are made of a polymer matrix reinforced with fibres. They are different types of fibres generally carbon, glass, aramid fibres, etc., while the polymers are generally an epoxy, polyesters, vinyl esters, etc. Polymer composites are used in automotive, marine, construction and aerospace industries. Carbon fibre-reinforced polymer (CFRP) composites have high strength to weight ratio and are resistant to most chemical attacks.

There is a huge demand for repairing or strengthening of existing structures in many applications either to sustain increased loads or to repair deteriorated structures. Many structural members like metallic pipelines, bridges and such other structures may deteriorate as they age, where the conventional method of repairing involves replacement or welding another plate over the damaged plate. The plates

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 quality products with the help of vacuum pressure.

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 process.

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 interlocking between the adhesive and adherent.

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

**53**

**Table 1.**

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

To rehabilitate the damaged SS pipe using a composite patch (CFRP) prepared with vacuum bagging process, the important aspect to study is the bond interface between SS and CFRP [15]. Single-strap adhesive joint is the least strength joint configuration than any others [16]. The present work aims to enhance the adhesive bond strength (tensile shear load capacity) of single-strap SS-CFRP joint using vacuum bagging process. The parameters for investigating the effect on load capacity of the joint are adhesive combination and temperature, vacuum pressure, bond length and surface of the adherent with different surface textures. These optimal parameters are applied to rehabilitate the damaged SS pipes and investigate its

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,

**[Pa s]**

30–50 Not

**Tensile strength [MPa]**

given

Aradur-22962 0.005–0.02 0.89–0.90

20–35 0.94–0.95

1.2–1.4 75–80 2.8–3.3 1.10–1.20

NA 3500 230 1.76

**Elastic modulus [GPa]**

Not given 1.14–1.15

**Density at 25°C [g/ cc]**

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

4 Stainless steel SS 304 NA 550 200 7.85

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

and the sandblasted surface with open surface cavities.

**S. no Materials Viscosity** 

Araldite-AW 106

Hardener-HV 953 U

Araldite LY 1564

300 Gsm UD-carbon fibre

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

hydrostatic pressure resistance.

**2. Materials and methods**

**2.1 Materials**

etc. were also used.

1 Structural

2 Nonstructural

adhesive (10:8 mix ratio)

> adhesive (10:2.5 mix ratio)

3 Fibre 12KUD-

**2.2 SS plate preparation**

*Materials and properties.*

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

To rehabilitate the damaged SS pipe using a composite patch (CFRP) prepared with vacuum bagging process, the important aspect to study is the bond interface between SS and CFRP [15]. Single-strap adhesive joint is the least strength joint configuration than any others [16]. The present work aims to enhance the adhesive bond strength (tensile shear load capacity) of single-strap SS-CFRP joint using vacuum bagging process. The parameters for investigating the effect on load capacity of the joint are adhesive combination and temperature, vacuum pressure, bond length and surface of the adherent with different surface textures. These optimal parameters are applied to rehabilitate the damaged SS pipes and investigate its hydrostatic pressure resistance.
