20-06-2014, 11:58 AM
FIBRE REINFIRCED COMPOSITES
FIBRE REINFIRCED COMPOSITES.docx (Size: 1.8 MB / Downloads: 54)
Introduction
Reinforced concrete structures often have to face modification and improvement of their performance during their service life. The main contributing factors are change in their use, new design standards, deterioration due to corrosion in the steel caused by exposure to an aggressive environment and accident events such as earthquakes. In such circumstances there are two possible solutions: replacement or retrofitting. Full structure replacement might have determinate disadvantages such as high costs for material and labour, a stronger environmental impact and inconvenience due to interruption of the function of the structure, e.g. traffic problems. When possible, it is often better to repair or upgrade the structure by retrofitting.
In the last decade, the development of strong epoxy glue has led to a technique which has great potential in the field of upgrading structures. Basically the technique involves gluing steel laminates or fibre reinforced Hybrid Laminates (FRHL) to the surface of the concrete. The laminates then act compositely with the concrete and help to carry the loads.
OBJECTIVES OF THE PROJECT
The objectives of this research project are:
1. To implement the technology and design of FRHL (fibre reinforced hybrid laminate) in the retrofitment of beams
2. To assess the short and long-term performance of FRHL (fibre reinforced hybrid laminate) under different service loading and environmental conditions.
3. To make direct comparison with steel bars under identical loading and environmental conditions.
4. To enhance the confidence of engineers, governmental authorities and end-users to use these new technologies, which introduce a potential solution to the corrosion problems of reinforced concrete structures
Definition of composite material
The word composite in the term composite material signifies that two or more materials are combined on a macroscopic scale to form a useful third material. The key is the macroscopic examination of a material wherein the components can be identified by the naked eye. Different materials can be combined on a microscopic scale, such as in alloying of materials, but the resulting material is, for all practical purposes, macroscopically homogeneous, i.e., the components cannot be distinguished by the naked eye and essentially act together. The advantage of composite materials is that, if well designed, they usually exhibit the best qualities of their components or constituents and often some qualities that their constituent possesses. Some of the properties that can be improved by forming a composite material are
FIBRES
A fibre is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.
They are very important in the biology of both plants and animals, for holding tissues together.
Human uses for fibers are diverse. They can be spun into filaments, string, or rope, used as a component of composite materials, or matted into sheets to make products such as paper or felt. Fibers are often used in the manufacture of other materials. The strongest engineering materials are generally made as fibers, for example carbon fiber and Ultra-high-molecular-weight polyethylene.
Synthetic fibers can often be produced very cheaply and in large amounts compared to natural fibers, but for clothing natural fibers can give some benefits, such as comfort, over their synthetic counterparts.
CELLULOSE FIBERS
• Cellulose fibers are a subset of man-made fibers, regenerated from natural cellulose. The cellulose comes from various sources. Modal is made from
. MINERAL FIBERS
Mineral fibers can be particular strong because they are formed with a low number of surface defects.
• Fiberglass, made from specific glass, and optical fiber, made from purified natural quartz, are also man-made fibers that come from natural raw materials, silica fiber, made from sodium silicate (water glass) and basalt fiber made from melted basalt.
• Metallic fibers can be drawn from ductile metals such as copper, gold or silver and extruded or deposited from more brittle ones, such as nickel, aluminum or iron
. POLYMER FIBERS
• Polymer fibers are a subset of man-made fibers, which are based on synthetic chemicals (often from petrochemical sources) rather than arising from natural materials by a purely physical process
CARBON FIBRES
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment gives the fiber high strength-to-volume ratio (makes it strong for its size). Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric.
The properties of carbon fibers, such as high flexibility, high tensile strength, low weight, high resistance, high temperature tolerance and low thermal expansion, make them very popular in aerospace, civil engineering, military, and motorsports, along with other competition sports. However, they are relatively expensive when compared to similar fibers, such as glass fibers or plastic fibers.
[b]CASE STUDY OF FIBRE REINFORCED HYBRID LAMINATES[/b]
Introduction
A lamina or ply is formed by a combination of a large number of fibres in a thin layer of matrix. Fibres in the lamina may be continuous or discontinuous, arranged in a specific direction or in a random orientation .A unidirectional lamina is one where the fiber’s in a lamina run parallel to one another in a particular direction. It is natural that discrete fibre composite will have lower strength and modulus than continuous fibre composite. However, with the random orientation of the fibre, it is possible to obtain nearly equal mechanical and physical properties in all direction in the plane of the lamina.
It is a flat (or sometimes curved) arrangement of unidirectional (or woven) fibers suspended in a matrix material. A lamina is generally assumed to be orthotropic, and its thickness depends on the material from which it is made.
For example, a graphite/epoxy (graphite fibers suspended in an epoxy matrix) lamina may be on the order of 0.127 mm thick. For the purpose of analysis, a
Flexural strengthening of beams
Recently, FRHL has started to be used to increase the flexural strength of members. To increase flexural capacity, the FRHL should be glued to the member in the way that fibers are parallel to the direction of the principal stress.
FRHL plates have been proved to increase the stiffness of the member and the load capacity, and reduce the cracking. The deflection of a retrofitted beam is considerably smaller than that of an un-retrofitted. This is due to the fact that stiffness is added to the member by the FRHL plate or sheet. Moreover, the number of FRHL sheet layers has considerable effect on the ultimate load and stiffness of a beam. The load carrying capacity was shown to increase with an increased number of layers of carbon fibre sheets for up to six sheets
Failure modes
There are three main categories of failure in concrete structures retrofitted with FRH that have been observed experimentally. The first and second type consist of failure modes where the composite action between concrete and FRH is maintained. Typically, in the first failure mode, the steel reinforcement yields, followed by rupture of . In the second type there is failure in the concrete. This type occurs either due to crushing of concrete before or after yielding of tensile steel without any damage to the FRH laminate, or due to an inclined shear crack at the end of the plate. In the third type, the failure modes involving loss of composite action are included. The most recognized failure modes within this group are debonding modes. In such a case, the external reinforcement plates no longer contribute to the beam strength, leading to a brittle failure if no stress redistribution from the laminate to the interior steel reinforcement occurs.
DESIGN OF FIBRE REINFORCED COMPOSITE LAMINATES
INTRODUCTION
The application of FRHL composites is increasing rapidly due to the vast strides in technology made in many disciplines. They are looked upon as potential replacements of the metal structure in naval, aerospace, civil and mechanical engineering industries. The micro mechanics, macro mechanics and lamination theory have been discussed in the earlier chapters. This chapter deals with the design process for composite structures with a few examples demonstrate the methods applied to calculate the design of composite structures.
The design of FRHL composite structures plays a vital role in ensuring the integrity of the structure without compromising set targets. By the term optimum design, the cost and the size become essential factors for the judicious application of the design methodology. To achieve this optimum, the need and the importance for a proper design goes without saying. Keeping this in view, the present chapter focuses on the general design philosophy, configuration selection, material selection and structural analysis that are pertinent to the composite laminate design process
Functional inputs
The input variables are interpreted as functional variables for the composite laminate design process. To this functional variable, performance and economic considerations are introduce to start the design process which is iterative in nature. The structural designer is required to identify the parameters correctly an alter them in each cycle to improve the performance. The criteria for the structural design are required to be clearly defined for the entire iterative design process. The design criteria and the other governing parameters of the design process form the basis for the design methodology.
Composite laminate design
composite laminate design can be defined as the process of finding an optimum configuration with structural dimensions and materials to support the given load and to economically perform the assigned task.
Any design process including the composite laminate design starts with an objective. Objectives are nothing but set targets which are required to be achieved at the end of the design process. The objective may be limited to a single target or multiple targets with certain conflicting requirements. The designer should be able to comprehend the design requirements, the goal and the failure criteria in terms of design objectives. The next step in the composite laminate design process is to identify the critical constraints, which affect the performance of the structures directly or indirectly. Most constraints in the structural design process appear to be in the form of size, load, weight, maintenance requirements and the cost depending on the area of application.
Design criteria
The composite laminate design is based on certain criteria. The design criteria are important to integrate various factors like loading, responses and durability with functional and operational requirements. While integrating these factors to derive the design criteria, the cost, the weight and the material availability are also taken into consideration along with safety and reliability as counter-checks to obtain a satisfactory design. Generally, the operational design criteria govern the basis structural configuration. Presently composite laminate designer employ the same design criteria as those used for metals. In general design criteria can be classified as operational criteria, environmental criteria and other miscellaneous criteria. The composite laminate design is either based on the allowable deflection or on the ultimate strength. To this, sustainability under the ultimate design load and fatigue behavior is imposed to further ensure satisfactory performance
Material selection in composite design
The material selection in composite structures is very complicated and is a difficult process. The difficulty arises mainly due to the possibilities of combining different types of fiber and resin to produce different types of composite laminae. Each type may possess different properties and be of varying cost. Composite material selection is an optimal decision making process. A material with some specific properties may be too costly from the economic point of view but may have very low specific weight. Here, the decision has to be made weather the cost or the weight is the overriding factor. Therefore, a material selection process requires a rational approach. The aim of the selection process is to choose the right material for the right application. Some of the factors, which need to be considered, are listed in below Fig
DESIGNING OF REINFORCED BEAMS
INTRODUCTION
In the existing scenario there are a number of laminates like FIBRE REINFORCED HYBRID LAMINATES are being used for retrofitting of structures. FRHL has inherent properties like lighter weight, ease of construction, low self weight; thinner section etc is gaining popularity. Few researchers had used FRH laminates for enhancing either the flexural or shear strength of the beam but in actual the beam may need strengthening in both shear and flexure.
Thus in the present study shear deficient beams are cast and subsequently stressed to 60%, 75%, and 90% of the safe load and are retrofitted with FRH laminates bonded to beam with EPOXY RESIN TO the beam. For the proposed work 4 real size beams (152.4 x 387 x 1219.2 mm) beams were cast. Out of these two are controlled beams tested to find out safe load carrying capacity of beams and subsequently two each of the rest of the six beams are stressed to 60%, 75%, and 90% of the safe load and then retrofitted with 10 mm thick ….FRH laminate
TEST PROGRAMME
The test programme is so devised so as to find out the properties of materials to be used for casting of beams and then the behavior of retrofitted beams. The test programme consists of:
1. Determination of basic properties of constituent materials namely cement, sand, coarse aggregates and steel bars as per relevant Indian standard specifications.
2. Casting of 20 beams of size (152.4 x 387 x 1219.2 mm) using M 20 grade concrete, the mix of which is designed with evaluated properties.
3. Computation of the ultimate failure load of the beams and subsequently the safe load from deflection criteria.
4. The beams then retrofitted with FRH laminates of thicknesses 10 mm using EPOXY as bonding agent to beam
The experimental program can be easily illustrated in a form of flow chart
CONCLUSION
Based on the experimental test results, following conclusion can be made: