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A COMPOSITE MATERIAL is a macroscopic combination of two or more distinct materials, having a recognizable interface between them. However, as a common practical definition, composite materials may be restricted to emphasize those materials that contain a continuous matrix constituent that binds together and provides form to an array of a stronger, stiffer reinforcement constituent. The resulting composite material has a balance of structural properties that is superior to either constituent material alone.
Composites are commonly classified at two distinct levels. The first level of classification is usually made with respect to the matrix constituent. The major composite classes include organic-matrix composites (OMCs), metal-matrix composites (MMCs), and ceramic-matrix composites (CMCs) .The second level of classification refers to the reinforcement form-particulate reinforcements, whisker reinforcements, continuous fiber laminated composites, and woven composites (braided and knitted fiber architectures are included in this
In order to provide a useful increase in properties, there generally must be a substantial volume fraction (~10% or more) of the reinforcement. A particulate reinforcement is considered to be a "particle" if all of its dimensions are roughly equal. Thus, particulate-reinforced composites include those reinforced by spheres, rods, flakes, and many other shapes of roughly equal axes. Whisker reinforcements, with an aspect ratio typically between approximately 20 to 100, are often considered together with particulates in MMCs. Together, these are classified as discontinuous" reinforcements, because the reinforcing phase is discontinuous for the lower volume fractions typically used in MMCs. 
2. FRP Composites-
FRP composites are defined as the materials that consist of fibers embedded in a resin matrix. The aim of combining fibers and resins that are different in nature is to take advantage of the distinctive material features of either component to result in an engineered material with desired overall composite action for specific applications. Continuous fiber-reinforced composites contain reinforcements having lengths much greater than their cross-sectional dimensions. Such a composite is considered to be a discontinuous fiber or short fiber composite if its properties vary with fiber length.
. On the other hand, when the length of the fiber is such that any further increase in length does not, for example, further increase the elastic modulus or strength of the composite, the composite is considered to be continuous fiber reinforced. Most continuous fiber (or continuous filament) composites, in fact, contain fibers that are comparable in length to the overall dimensions of the composite part. As shown in Fig. 1, each layer or "ply" of a continuous fiber composite typically has a specific fiber orientation direction. These layers can be stacked such that each layer has a specified fiber orientation, thereby giving the entire laminated stack ("laminate") highly tailorable overall properties.
Engineering properties of FRP composites for structural applications, in most cases ,are dominated by fiber reinforcements. More fibers usually give rise to higher strength and stiffness. Excessively high fiber/matrix ratios may ,however, lead to strength reduction or premature failure due to internal fracture. Fiber lengths and orientation also affect the properties considerably.
Resin matrix is an adhesive that supports the fibers from buckling under compressive stress ,binds the fibers together through cohesion and adhesion ,protects the fibers from physical and chemical attacks and micro-cracking during service, and provides shearing strengths between FRP laminas. Shearing strength is essential to resist delamination , lap joint failure and impact forces.
Structural FRP composites are generally high strength , reasonably stiff environmentally resistant and significantly lighter than conventional construction materials such as concrete and steel.
Choice of particular types of fibers and resins depends on the specific applications. Load-bearing capacity, level of exposure , wear resistance , temperature and frequency ranges , fire and water resistance and costs are some of the important issues that need to be thoroughly considered. Various advantages of composite materials are its high specific stiffness and high specific strength . These properties are generally used in structural application such as aerospace and sporting goods .
2.1 FIBER REINFORCEMENT :
A great majority of materials are stronger ad stiffer in fibrous form than as bulk materials.
A high fibre aspect ratio (length: diameter ratio) permits very effective transfer of load via matrix materials to the fibres, thus taking advantage of there excellent properties. Therefore, fibres are very effective and attractive reinforcement materials.
Types of fibers used in fibre reinforced polymer composites
2.1.1. GLASS FIBERS:
The most common reinforcement for the polymer matrix composites is a glass fibre. Most of the fibres are based on silica (SiO2), with addition of oxides of Ca, B, Na, Fe, and Al. The glass fibres are divided into three classes -- E-glass, S-glass and C-glass. The E-glass is designated for electrical use and the S-glass for high strength. The C-glass is for high corrosion resistance, and it is uncommon for civil engineering application. Of the three
fibres, the E-glass is the most common reinforcement material used in civil structures. It is produced from lime-aluminaborosilicate which can be easily obtained from abundance of raw materials like sand. The glass fibre strength and modulus can degrade with increasing temperature. Although the glass material creeps under a sustained load, it can be designed to perform satisfactorily. The fibre itself is regarded as an isotropic material and has a lower thermal expansion coefficient than that of steel.
â€¢ E-glass (electrical)
Family of glassed with a calcium aluminum borosilicate composition and a maximum alkali
composition of 2%. These are used when strength and high electrical resistivity are required.
â€¢ S-glass (tensile strength)
Fibres have a magnesium alumino-silicate composition, which demonstrates high strength and
used in application where very high tensile strength required.
â€¢ C-glass (chemical)
It has a soda lime borosilicate composition that is used for its chemical stability in corrosive environment. It is often used on composites that contain or contact acidic materials .
Less than 2
Table1.3. Composition of E-Glass
Young's Modulus (GPa)
Tensile Strength (GPa)
Tensile Elongation (%)
184.108.40.206. Structure of glass fibre
Glass fibres have high tensile strength, impact strengths and high chemical resistance.
But these have relatively low modulus, self-abrasiveness, low fatigue resistance and poor
adhesion to matrix composites.
The three dimensional network of structure of glass results in isotropic properties of glass
fibres, in contrast to those of carbon and Kevlar aramid fibres which are anisotropic. The elastic modulus of glass fibres measured along the fibre axis is the same as that measured in the transverse direction, a characteristic unique to glass fibres.
220.127.116.11. Surface Treatment of Reinforcing Materials
Surface treatment is done to improve the adhesion of fillers and fibres to matrix resin by
modifying the surface of the solid. Often, chemical structure and sometimes topology of the
surface change upon the treatment.
Chemistry of Surface Treatment and Interfacial Structure
Glass Fibres and Inorganic Fillers
Inorganic materials like glass fibres and many fillers have poor compatibility at the
fibre/matrix or filler/matrix interface. In order to improve adhesion at the interface, compounds with dual property, i.e. molecules having chemical functionalities similar to the fibre and the matrix resin, are used. Due to the coupling action of the fibre and the resin by the compatibilizing compound, this type of compound is often referred to as a coupling agent, such as a silane coupling agent. In addition to the adhesion promotion, coupling agents aid in protecting fibre surfaces and prevent inhibition of polymerization by the solid surfaces. A small amount of a coupling agent can often dramatically improve the mechanical and physical properties of composites. .
Fig.1. Chemical process during surface treatment silaceous material by a silane coupling agent. Regardless of the treatment methods, the silane loses its alkoxy groups and chemically reacts with the hydroxyl groups of the mineral surfaces.
2.1.2. Carbon fibres
Carbon fibre is the most expensive of the more common reinforcements, but in space
applications the combination of excellent performance characteristics coupled with light weight make it indispensable reinforcement with cost being of secondary importance. Carbon fibres consist of small crystallite of turbostratic graphite. In a graphite single crystal the carbon atoms in a basal plane are arranged in hexagonal arrays and held together by strong covalent bonds. Between the basal planes only weak Van-der-waal forces exist. Therefore the single crystals are highly anisotropic with the plane moduli of the order of 100 GPa whereas the molecules perpendicular to the basal plane are only about 75 GPa. It is thus evident that to produce high modulus and high strength fibres, the basal planes of the graphite must be parallel to the fibre axis. They have lower thermal expansion coefficients than both the glass and aramid fibres. The carbon fibre is an anisotropic material, and its transverse modulus is an order of magnitude less than its longitudinal modulus. The material has a very high fatigue and creep resistance. Since its tensile strength decreases with increasing modulus, its strain at rupture will also are much lower. Because of the material brittleness at higher modulus, it becomes critical in joint and connection details, which can have high stress concentrations. As a result of this phenomenon, carbon composite laminates are more effective with adhesive bonding
that eliminates mechanical fasteners .
Fig: Carbon fibre
Table1.5. Typical properties of Carbon Fibre
2.0 - 2.1
520 - 620
1.03 - 1.31
2.2 Types of Matrix phase used in fibre reinforced polymer composites
Fibres, since they cannot transmit loads from one to another, have limited use in
engineering applications. When they are embedded in a matrix material, to form a composite, the matrix serves to bind the fibres together, transfer loads to the fibres, and damage due to handling.
The matrix has a strong influence on several mechanical properties of the composite such as transverse modulus and strength, shear properties, and properties in compression.. Commonly used matrix materials are described below:
2.2.1. Epoxy resin
Epoxy resins are relatively low molecular weight pre-polymers capable of being
processed under a variety of conditions. Two important advantages of these resins are over
unsaturated polyester resins are: first, they can be partially cured and stored in that state, and second they exhibit low shrinkage during cure. Approximately 45% of the total amount of epoxy resins produced is used in protective coatings while the remaining is used in structural applications such as laminates and composites, tooling, moulding, casting, construction, adhesives, etc .
Epoxy resins are characterized by the presence of a three-membered ring containing two
carbons and an oxygen (epoxy group or epoxide or oxirane ring). Epoxy is the first liquid
reaction product of bisphenol-A with excess of epichlorohidrin and this resin is known as
diglycidylether of bisphenol A (DGEBA). DGEBA is used extensively in industry due to its high fluidity, processing ease, and good physical properties of the cured of resin.
Ethylene diamines are most widely used aliphatic amines for cured epoxy resins. These are highly reactive, low molecular weight curing agents that result in tightly cross-linked network. One primary amino group reacts with two epoxy groups. The primary and secondary amines are reactive curing agents. The primary amino group is more reactive towards epoxy than secondary amino groups are consumed (95%), whereas only 28% of secondary amino groups are consumed.
The primary amino-epoxy reaction results in linear polymerization while secondary
amino-epoxy reaction leads to branching and cross-linking. The cured epoxy resins find a variety of applications as adhesives, laminates, sealants, coatings, etc. The optimum curing temperature and the thermal stability of epoxy resin depend on the type of curing agent. Epoxies are used as binders in materials for construction. Filling of cracks in concrete structures is achieved by epoxies. In construction industry, for bonding and coating purposes, low temperature curing of epoxies is achieved by using thiols that exhibit higher curing rates.
The composites industry has begun to recognize that the commercial applications of
composites promise to offer much larger business opportunities than the aerospace sector due to the sheer size of transportation industry. Thus the shift of composite applications from aircraft to other commercial uses has become prominent in recent years .
Increasingly enabled by the introduction of newer polymer resin matrix materials and
high performance reinforcement fibres of glass, carbon and aramid, the penetration of these advanced materials has witnessed a steady expansion in uses and volume. The increased volume has resulted in an expected reduction in costs. High performance FRP can now be found in such diverse applications as composite armouring designed to resist explosive impacts, fuel cylinders for natural gas vehicles, windmill blades, industrial drive shafts, support beams of highway bridges and even paper making rollers. For certain applications, the use of composites rather than metals has in fact resulted in savings of both cost and weight.
Unlike conventional materials (e.g., steel), the properties of the composite material can be
designed considering the structural aspects. The design of a structural component using
composites involves both material and structural design. Composite properties (e.g. stiffness, thermal expansion etc.) can be varied continuously over a broad range of values under the control of the designer. Careful selection of reinforcement type enables finished product characteristics to be tailored to almost any specific engineering requirement.
3.1. Composites: The Future Trends
Armed with a wide gamut of advantages, composites have a key role to play in the
growing market in India. Composites have made an entry into diverse end-use segments and the developmental efforts for finding newer composites for existing and novel applications is an area of top priority.
3.1.1. Transportation Sector
Despite the potential benefits of lighter weight and durability resulting from corrosion
resistance, advanced composites is not recognized as a material of choice in the near term for automotive applications. The principal barrier is the high cost of the raw and fabricated materials when compared to existing options.
Glass-reinforced thermoplastic polymer is a promising material for weight reduction because of the relatively low cost of the fibre, its fast cycle time and its ability to facilitate parts integration. Carbon fibre reinforced polymer is another candidate but will require breakthroughs in cost and manufacturing techniques to be cost effective for high volume production. The likely future business opportunities in automotive sector are mentioned below:
â€¢ Pultruded Driveshafts
â€¢ RTM Panel
â€¢ Fiber Glass/Epoxy Springs for Heavy Trucks and Trailers
â€¢ Rocker Arm Covers, Suspension Arms, Wheels and Engine Shrouds
â€¢ Filament-Wound Fuel Tanks
â€¢ Electrical Vehicle Body Components and Assembly Units
â€¢ Valve Guides
Composite bicycle frames have been a largely American phenomenon, as a spin-off
technology from the aircraft and boating industries. Carbon composite bike frame is a complex structure with performance characteristics that include lightness, rigidity, durability, shock absorption etc. As composites fabrication offers variation over the length of the tube providing different fiber angles, different plies, different ply thickness, different combinations of materials. So the properties of the end product made from composites can be tailored to specifications. Hybrid fibre (carbon and aramid), carbon/kevlar epoxy materials areideal composite materials for bicycle components. The composites are finding application in bicycle components such as
â€¢ Handle bars and Connecting bar ends
â€¢ Seat posts
With composites exhibiting excellent resistance to the marine environment, their applications have made good inroads in the marine sector worldwide. Complex configurations and the advantages of seamless hulls were the main driving factors in the development of FRP boats.
Racing power-boats employ advanced and hybrid composites for a higher performance craft and driver safety. Major structural elements viz. deckhouses, hatch covers, kings posts and bow modules appears to be very well suited for FRP construction. The consumption of composites by this industry is mainly glass fibre reinforced polyesters. Advanced composites materials on vessels have a potential to reduce fabrication and maintenance cost, enhance styling, reduce outfit weight and increase reliability. Potential ship applications for composite materials are:
â€¢ Shafting Overwraps
â€¢ Life rails, Handrails
â€¢ Masts, Stacks and Foundations
â€¢ Propellers vanes, Fans and Blowers
â€¢ Gear cases
â€¢ Valves and Strainers
â€¢ Condenser shells
3.1.2. Chemical Industry
Supplemented by the advantages of composites of lightweight, mouldability, fire
resistance properties, resistance to chemicals has made the material popular in the chemical industry. Composites are extensively used in industrial gratings, structural supports, storage tanks, scrubbers, ducting, piping, exhaust stacks, pumps and blowers, columns, reactors etc. for acidic and alkaline environments.
Some of the potential applications are:
â€¢ Composite vessels for liquid natural gas for alternative fuel vehicle
â€¢ Racked bottles for fire service, mountain climbing
â€¢ Double-wall FRP vessels with an early warning system for leakage detection
â€¢ Underground storage tanks
â€¢ Casings for electrostatic precipitator
â€¢ Drive shafts
â€¢ Fan blades (for both axial and centrifugal fans)
â€¢ Ducts and Stacks
â€¢ Aerial man-lift device
3.1.3. Electrical and Electronics
Composites equipped with good electric insulation, antimagnetic and spark-free, good
adhesion to glue and paint, self-extinguishing qualities are used for the construction of
distribution pillars, link boxes, profiles for the separation of current-carrying phases to prevent short circuits etc. The other potential applications of composites in this sector are:
â€¢ Third rail covers for underground railway
â€¢ Structurals for overhead transmission lines for railway
â€¢ Power line insulators
â€¢ Lightning poles
â€¢ Power pole cross arms
â€¢ Fibre optic tensile members
â€¢ Switchgear frames
â€¢ Aerial lift-truck booms
Construction holds priority for the adaptation of composites in place of conventional
materials being used like doors and windows, paneling, furniture, non-structural gratings, long span roof structures, tanks, bridge components and complete bridge systems and other interiors. Components made of composite materials find extensive applications in shuttering supports, special architectural structures imparting aesthetic appearance, large signages etc. with the advantages like corrosion resistance, longer life, low maintenance, ease in workability, fire retardancy.
Other critical applications of composites in the civil engineering area are:
â€¢ Tunnel supports
â€¢ Supports for storage containers
â€¢ Airport facilities such as runways and aprons
â€¢ Roads and bridge structures
â€¢ Marine and offshore structures
â€¢ Concrete slabs
â€¢ Power plant facilities
â€¢ Architectural features and structures such as exterior walls, handrails, etc.
3.1.5. Offshore Oil and Gas Industry
Steel and concrete are the materials of choice for offshore oil and gas production platforms,
with steel dominant in the topside applications. Composites have found their way into limited applications, particularly where corrosion and the need to reduce high maintenance costs have been an issue. As the industry moves to greater water depths, the significance of weight saving has become increasingly important in conjunction with the application of buoyant tension for the leg structures. Composites may find excellent usage in fabrication of the following:
â€¢ Profiles for oil pollution barriers
â€¢ Gratings, ladders and railings on oil-drilling platforms and ships
â€¢ Walkway systems
â€¢ Sucker rods
3.1.6. Consumer and Sports Goods
The optimum design of sports equipment requires the application of a number of disciplines, not only for enhanced performance but also to make the equipment as user-friendly as possible from the standpoint of injury avoidance. In designing sports equipment, the various characteristics of materials must be considered. Among these characteristics are strength, ductility, density, fatigue resistance, toughness, modulus (damping), and cost. Following are the general consumer and sports goods where there is lot of potential for composites in the near future:
â€¢ Canoes and Kayaks
â€¢ Vaulting Pole
â€¢ Golf and Polo rods
â€¢ Archery equipment
â€¢ Hand gliders
â€¢ Wind surfer boards
4. MECHANICAL AND OTHER PROPERTIES
Mechanical properties of FRP composites are the weighted average of those of fibres and resins. Rule of mixtures are normally applied to estimate strength and stiffness of a FRP composite material that is composed of a particular type of fiber and resin. Owing to the relative brittleness of a FRP composites (in the fibre direction), strength values from experimental testing normally exhibit higher variance than those of other construction materials like steel. The use of the rule of mixtures or the micro-mechanics approach does not predict strength very well. Statistical approaches such as Weibull analysis are thus required for appropriate quantification .On the contrary, stiffness values from the testing are relatively uniform and can be predicted with high accuracy using micro-mechanics formulation.
Compared to steel and concrete,FRP composites are about 1.5to 5 times lighter. For example,carbon/epoxy composites has a density of 1.6g/cm3 compared to 7.9g/cm3 of steel and 2.4g/cm3 of concrete. This lightweight characteristic not only leads to very high specific strength and specific stiffness,hence high load taking efficiency with decreased structural weight,but has also strong implication on reduced costs of transportation, handling, and construction.
Tensile strength of FRP composites ranges from about that of mild steel more than that of pre-stressing steels depending on the fibre types,arrangements,orientation,and production.While compressive strength of steel is identical to its tensile strength,the compressive strength of FRP composites is normally less than the tensile strength,which is due to fibre buckling failure. Yet,the strengths of commercial FRP composites are tremendous compared to any conventional construction materials.In situations where high tensile and compressive strengths are considered assets such as tensile and flexural members,FRP composites offer strong incentives for use.Shear strength of all unidirectional laminates is relatively low since this parameter is mainly controlled by the strength of the resin in such FRP systems.however,for a laminate that is made up of multiple plies,fibres can be designed to the particular orientation that matches with the maximum shear stress.In such a case,shear strength is provided by the fibres ,which act in tension,instead of the resin.
Stiffness of FRP composites ranges widely and has strong correlation with fibre content, continuity and orientation. Glass FRP generally has lower stiffness than carbon FRP while the latter can have a magnitude from half of the stiffness of ordinary structural steel to higher than that of high strength steel. One unique feature about FRP composites is that the material exhibit linear or nearly linear stress-strain phenomenon throughout their load carrying range, meaning that the stiffness does not change over their load history. This behavior is significantly different from that of steel and concrete in which reduction of stiffness occurs even when the applied stress has not quite yet reached the ultimate material capacity.
4.4 Coefficient of thermal expansion
Unlike all the construction materials that civil engineers are acquainted with, many FRP composites(unidirectional laminates) shrink upon temperature increase. In particular, carbon and aramid FRP have negative coefficients of thermal expansion while that of glass is positive .although resins expand upon heating, carbon and aramid fibres resist thermal expansion. The magnitude of thermal shrinkage of carbon and thermal expansion of epoxy resins is about to cancel out each other and thus give rise to an overall near-zero coefficient of thermal expansion of carbon/epoxy FRP>this property is of great significance in terms of residual stress development in the bond interface.
4.5 Electrical conductivity
Electrical conductivity may be of great interest when provision of post-curing is desired for the bonding adhesives so as to boost higher the glass transition temperature to prevent viscoelastic responses during the service life .In fact, some companies who provide FRP retrofit materials have started to develop electrical heating systems that can be clamped onto the FRP plates to heat up the bond interface for further curing. As such, electrical conductivity of FRP composites is a parameter that is worth looking upto.
Any component employed in day to day applications is expected to have considerable durability in order to improve the overall service life of the component. The durability of FRP systems can be studied in terms of thermal cycling, humidity, exposure to ultraviolet light, weathering, creep and fatigue which have been described as follows:-
4.6.1 Thermal cycling
Thermal cycles such as that from thermal extremes over seasonal changes can contribute to cyclic thermal fatigue. It has been observed that unless void content in the resin is high, freeze thaw effect on strength within normal civil engineering applications temperature range of (30 degree cent to -20 degree cent) is insignificant although thermal aging at temperatures higher than300 degree cent can degrade the mechanical properties due to oxidation of fibres and resins. So resin systems that can exist at extreme temperatures for aerospace applications have been developed.
4.6.2 Moisture attack
It has been found that FRP is resistant to many chemicals and metal corrosive agents. Yet moisture uptake could occur through the resin. Moisture absorption is a function of void content, resin type, fibre type, temperature, applied stress and presence of microcracks. Improperly manufactured FRP composites may result in high void content and development of microcracks upon curing. This will promote water ingress which will cause swelling and property change of the resin.
Glass fibres are prone to moisture attack upon prolonged contact due to hydroscopic nature of the alkaline metallic oxides in the silica of glass. Aramid fibres can absorb large amount of water and can swell considerably. Carbon fibres do not exhibit any degradation due to water. However, carbon FRP when in contact with metals (except platinum, gold, titanium) will make metals corrode due to galvanic action.
4.6.3 UV Radiation
Ultraviolet radiation is known to harden and discolor polymeric resin matrix. However, degradation takes place only at the skin of FRP laminates as the material has a screening effect although topcoat delamination may occur. For thicker composites degradation effects on mechanical properties is minimum. Use of UV resistant coating has been a popular tactic to substantially slow down or even eliminates the associated degradation. UV screeners such as carbon black and titanium dioxide are the oldest form of UV protection materials for polymers. They have the ability to absorb light and increase photo stability. Light stabilizing oxidants such as hindered amine light stabilizers (HALS) are used to terminate the free radicals resulted from the bond breaking due to unreleased energy absorbed, hence retarding thermal oxidation. The combination use of UVA and HALS is observed to provide the greatest UV protection effect.
Weathering is the deteriorating action that alters the surface color, texture, and material properties of the structure. For FRP composites, resin matrix is responsible of protecting the structure from degrading upon prolonged exposure in outdoor environments. Degradation of the resin will not affect the mechanical properties of the FRP composites such as stiffness and strength in general. Moreover, weathering degradations mostly take place at the material surface.
Creep is the time dependent deformation under a constant load. Upon load removal, unrecoverable deformation occurs after some nominal recovery. Creep in FRP is a resin dominant behavior and is dependent on temperature, moisture and the magnitude of applied stress. Thermosets such as epoxy resin are more creep resistant than thermoplastics due to their inherent cross linking structures. More creep is usually observed at higher temperature.
4.6.6 Fatigue It has been observed that FRP possess good fatigue resistance in tension cycling. Amongst all FRP composites, carbon FRP exhibits the highest fatigue resistance. With a high fibre content, between 50% and 70%, carbon FRP almost does not fatigue due to its high strength and stiffness. Fatigue behavior of carbon FRP is superior to that of steel. However, fatigue degradation occurs more substantially in glass FRP. Glass fibres are comparatively less stiff and the stress transfer mechanism from the matrix to the fibres is less effective, making the matrix more prone to large stresses and strains, promoting crack development under fewer no. of load cycles.
5. ENVIRONMENTAL DEGRADATION OF FRP:
Almost all studies in this area have been based on accelerated testing of meso-scale specimens(in the cm range),unlike short term debonding investigations that often involve large scale beams(in the m range). Specimen types include frp bonded plane concrete beams , frp bonded reinforced concrete beams, lap shear specimens, and peel specimens. Most investigations involve characterization of strength parameters such as ultimate strength, stiffness and deformation but it has also been found that fracture characterization by means of fracture toughness parameters also play a major role in degradation studies. With little physical understanding developed so far modeling of environmental degradation of frp bonded system is virtually non existent. Environmental parameters that have been tested usually include the following:
Extreme temperature cycles, continuous frozen states ,continuous fresh water conditioning, continuous salt water conditioning, wet-dry cycles using salt water freeze/thaw cycles and UV exposure under sustained load, from which wet /dry cycling is more often tested.
5.1 EFFECT OF MOISTURE ABSORPTION ON FRP'S PROPERTIES:
It is now well known that the exposure of polymeric composites in moist environments, under both normal and sub-zero conditions, leads to certain degradation of its mechanical properties which necessitates proper understanding of the correlation between the moist environment and the structural integrity.
It is well known that there is a degradation of material property during its service life, as it is often subjected to environments with high temperature and humidity or having a sharp rise and fall of temperature (thermal spikes).
The deterioration that occurs is FRP during the service life is in general, linked to the level of moisture that is absorbed. The absorption of moisture can be attributed largely to the affinity for moisture of specific functional groups of a highly polar nature in the cured resin. The absorption of moisture causes plasticization of the resin to occur with a concurrent swelling and lowering the glass transition temperature of the resin. This adversely affects the fiber matrix adhesion properties, resulting debonding at fiber/matrix interfaces, micro-cracking in the matrix, fiber fragmentations, continuous cracks and several other phenomena that actually degrades the mechanical property of the composites.
6.1. Short Beam Shear Test
It is one of the most popular flexural test methods. In this test the flat specimen is simply
supported at the two ends and is loaded by a central load. Generally the flexural strength is
obtained by measuring the applied load and corresponding strain. In this case we are obtaining the interlaminar shear stress values. The most widespread method of testing ILSS is the short span flexural test. If despite the short span to thickness ratio, the specimen fails in the flexural rather than a shear mode, the result of the test should not be reported as ILSS. Short span flexural testing is applicable to composites with unidirectional and bi-directional reinforcement, but does not give satisfactory results with planer random and three dimensionally random short fibre composites.
ILSS = 0.75p/bt
It shows the set up of three point bend test. Here the specimen is simply supported at the
two ends and is loaded by a central load. Below is given the formula to obtain the ILSS value. Where p is the load at yield, b is the width of the sample and t is the thickness of the sample.
ILSS data are often used to specify the quality of the composites. It is considered as a
direct function of interfacial adhesion.
6.2. SEM Test
Fractography" has become synonymous with an investigation in which both the
mechanics of cracking are identified and the influence of the environment and/or the internal structures of the component on the mechanics of fracture are determined.
Examination of the fracture surface of a polymer is usually carried out sequentially using
low power optical microscopy, high power optical microscopy, and either scanning electron microscopy SEM applied directly to the surface or transmission electron microscopy TEM of replicas taken from the surface. The use of replicas has declined in the recent years due to the improvement and increased availability of scanning electron microscopes. The scanning electron microscope SEM, due to its very large depth of field, is ideally suited to the study of fractures in polymers and can focus on even the most fibrous fracture surfaces. Magnifications between 20Ã- and 10,000Ã- are possible and it allows correlation of the detailed structures observed at high magnification with the coarser optically visible features of the fracture. To understand the interfacial bonding condition between the fibre and the matrix and its effect on mechanical properties, photomicrographs were taken using a scanning electron microscope. Dramatic changes in the structure and properties of the composite when exposed to cryogenic temperatures, particularly in cyclical fashion, can be seen by studying the SEM micrographs. Generally cryogenic cycling leads to microcracking, delamination and potholing that are localized surface degradation . Increased thermal stresses are the underlying cause of
microcracking in composites at cryogenic conditions. As the temperature of the laminate falls below its stress free temperature, residual stresses develop in the material. These stresses are the result of difference in coefficient of thermal expansion between the fibres and the matrix. As the temperature deviates from the stress free temperature, the amount of thermal stresses increases. And when these residual stresses become large enough they are relieved through physical processes such as microcracking, delamination and potholing.
7. MATERIALS AND EXPERIMENTAL
7.1. Flexural test (Short Beam Shear Test)
The short beam shear tests were carried out for first batch of samples immediately after
exposure to cryogenic temperature. The former samples after exposure to room temperature andthe untreated as-cured samples were tested in short beam shear test at room temperature. All the mechanical flexural tests were performed at 2, 50, 100, 200 and 500 mm/min crosshead speeds. Then breaking load and strain at maximum load was measured from stress vs strain graphs for all the samples.
An Instron1195 tensile testing machine was used to perform SBS tests in accordance with
ASTM D 2344-84 standard. Multiple samples were tested at each point of experiment and the average value was reported.
Fig.3.12. Instron 1195
Fig.3.13. Short Beam Shear test set up, loading of the sample and fracture.