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Introduction to Nature Composite Fibre. According to the face value, the term composite will signify anything in the real world. If closely analyzed, materials are nothing more than a mixture of various subunits. When a material is built by using fiber, it is known as matrix material in present material engineering. For example, Fiber Reinforced Plastic (FRP) points that it is a thermosetting polyester matrix which consists of glass fibers and it present market composite has a major share. The below figure shows that, by Crossplying unidirectionally a laminated fiber is built as layers in a stacking (90) sequence. Material Technology has produced various composites which are available in current market. These composites are in much demand because of it cost and performance, they are used in highly demanding applications like spacecrafts. From many years nature has been combining various dissimilar elements to form heterogeneous material. In olden days, people used to copy the combinations methods from the nature. For example, in brick making they used straw to strengthen the mud; with the absence of straw bricks will not have any strength. This method is taken from the Book of Exodus. However, these materials are not utilized as fiber, and generally they are combined with matrix material which helps in transferring masses to the fiber, and it also provides protection to the fiber from environmental attacks and attrition. (AK, 1980)
The main drawback by using the matrix material is that it reduces materials properties to some extent. However, some high specific properties like weight-adjusted materials are available. As matrix materials glass and metals are available, but they are very costly and their use is limited to Research and Development laboratories. Most of the low to medium performance applications use polymers because of its unsaturated styrene-hardened polyesters. Thermoplastic matrix composites materials are very attractive, but the major limitation is its processing difficulty. (AK, 1980)
The term composites points to a wide range of individual combinations that is present in the class of materials. There are other familiar ones which provide a clear plan or idea on the possibility to be creative. This is usable to the Design Engineer and Material Scientist and their customers. Firstly, in a group of materials Ceramic, Polymeric and Metallic; there are other familiar materials which come under composites. Many members in a community and a group of engineering materials, steel commonest consists of combination of hard ceramic compounds in a softer metallic matrix. These particles are available in many different shapes like plates, needle-shape, polygonal or spherical. (Curtis PT and Morton J, 1982)
Polymers are always two phased, it consists the matrix of one polymer which may have softer or harder particles within it. The most common example is wood. And the classic example for ceramic-ceramic composite is concrete. It consists of sand particles and collection of matrix in graded size of hydrate Portland cement. These materials are familiar from many years, and Material Scientist has found out various methods to control their microstructure which helps to control their properties. Changing the structure of a material may affect the form, the distribution and the quantity it is also known as reinforcing phase. (A, 1996)
Natural extension of materials is achieved by integrating components in class boundaries. To make the material harder, cheap and fire proof. Ceramic powders are converted to plastic to make a filled polymer. Cement is created by adding hard, thermally or abrasive stable ceramic particle to metals. It also helps in producing machine tool tips which are capable of cutting hard metals at high temperature or at high speed. The major principle behind the expansion is to integrate ceramics, polymers and filamentary to make fiber composites or reinforced plastics, like GRP and CFRO, MMCs (metal mixing components) like silicon carbide fiber reinforced aluminum, and use of ceramic matrix composites (CMCs) like carbon fiber reinforced glass. (B.W.., 1965)
Figure Relationships between classes of engineering materials
Conventional Materials and Limitations
Grouping materials characteristic and judging their strengths and weaknesses of plastic, ceramics and metals in a table is very difficult. As these three terms covers the entire families of materials where the properties range is as big as the difference among these three classes. In a general comparison some of the advantages and disadvantages can be identified in these three classes of materials. (Dickson RF, 1968)
Identifying at an unsophisticated levels, then:
Density of plastics: Plastic has a very low density, though the plastic can with stand short term chemical reaction but it lacks at thermal stability and has modest resistance towards environmental degradation mainly photo-chemical effects which is caused by sunlight. It can be easily jointed and manufactured, but it has poor mechanical properties. (DRH, 1980)
Density of ceramics: Some ceramics are of low density; however some ceramics are very dense. But ceramics are very good at thermal stability and they are resistant to most of the environmental degradation such as corrosion, abrasion and wear etc. Due to their chemical bonding ceramics are very strong and rigid; they can only be formed in shapes which are very difficult procedure. (B.W.., 1965)
Density of Metals: Most of the metals are of medium to high density. Some metals like beryllium, aluminum and magnesium have low density as plastic. And metals are having good thermal stability. (Atzori B, 1994)
Metals and Alloys: steels, aluminium alloys, copper & brasses, titanium, etc.
Plastics: metal-matrix composites, ceramic-matrix composites, (including ordinary reinforced concrete and steel-fibre reinforced concrete) resins (epoxies, etc.), thermoplastics, rubbers, foams, textile fibre.
Ceramics& Glasses: glass, fired ceramics, concrete, fibre-reinforced plastics (including GRP, CFRP, glass/PTFE coated fabrics), FRP-reinforced concrete metal-filled plastics (particulate and fibre fill). Relationship between classes of engineering materials, showing evolution of composites taken up based on nature of composites resistant by alloying. They have useful mechanical properties and high toughness, and they are moderately easy to shape and join. It is largely a consequence of their ductility and resistance to cracking that metals, as a class, became and remain preferred engineering materials. (Ashton, 1969)
Structural or load bearing engineers use these materials, and are aware of its important feature its solidity. But they are unaware that it's the chemical bond which holds them together and makes it strong. The main reason is that all materials have some faults/flaws of different types, and these faults cannot be eliminated entirely in realistic manufacturing operations. For instance, the strength is not decided by its ionic or covalent bond for materials like ceramics, bulk glass. Although some sharp cracks or petite pores many exist on surface or in the interior. (AK, 1980)
Ceramics which are extremely refined and dense may have a thousandth less strength with is mention in theoretical prediction. In the same way, metals have some drawbacks in heaping of atoms in their crystalline arrays. Imperfections and dislocations is the most damaging drawback. ((Editors, 1996)
The general potency level of the material can be increased if there is a careful check on the faults in the manufacturing process and the inconsistency reduced. Glass fibres are made by quick melting of the glass that forms fine filaments that can arrange ten microns (10Î¼m) in diameter. As the fibre that is formed is of very fine quality, it does not contain any defect which is generally found in large quantity of glasses. The quality of the newly made glass fibres has a 5GPa when compared with bulk glass which has 100MPa. Exposure to moisture in the environment may decay glass fibre. The fibre should be carefully protected from hard objects as it may wear away when comes in contact with that may again result in the fault of surface and decrease the quality. The polyacrylonitrile fibres that are used to manufacture acrylic textiles restrict the quality of the polymeric filaments through feeble bonding of chemicals, amid the molecular chain and by the manufacturing defects. The process of stretching, oxidation and carbonization are controlled by the study of such filaments. Thought the conversion of polymers into fibres is chemically carbon having a crystal formation of graphite, the proportion C-C that stretches out to the direction of fibre axis are very enormous. These filaments have high load-bearing capability of 7Î¼m in diameter and the remaining defect is minute. (A, 1996)
Functions of the matrix
The matrix combines the fibres by holding them in an aligned direction. Matrix enables the composites to resist the compression, flexible force and load tension. The matrix transfers the composite into fibres for applied load, inturn toughened with fibres of different kinds of load.
The matrix separates the fibres to as a single unit from each other. Several hard fibres tends break with changeable force. The fibres are powerful when compared to monolithic form by utilizing such fine fibres. (A, 1996)
The ability of composites reinforced with short fibres to support loads of any kind is dependent on the presence of the matrix as the load-transfer medium, and the efficiency of this load transfer is directly related to the quality of the fibre/matrix bond. (RF, 1980)
The fibre has less inconsistent compared with a monolithic rod that an equal capacity of load bearing. The matrix helps to understand the merit of the combined fibre, by separating them to avoid the cracks that pass without any hindrance through the chain of fibres that come in contact and finally with an outcome of fragile composites. ((Editors, 1996)
The matrix safeguards the strong filaments from harmful mechanism and ecological hit. Resins are utilized as matrices for the glass fibres that allows to disperse the water, but is not satisfied by various GRP materials that is harmful for the ecology. The alkaline nature of the matrix in the cement damages the glass fibre. Hence, the glasses containing of zirconium which has the ability to resist the alkali is developed (Proctor & Yale, 1980). The matrix safeguards the fibres form oxidation for few composites like MMCs and CMCs they are controlled at eminent heat. (AK, 1980)
The matrix that is malleable provides the prevention of cracks that has started from a broken fibre. On the other hand, breakable matrix takes the support of fibre to work as break stopper.
The matrix helps to increase the hardening of the composite with its grip feature on the fibres (the interfacial bond strength).
When compared with metals, polymers have stable thermal capacity. Polymides (ether ketone) that are known as PEEK have a discredit as it is exposed to the high temperature that is above 300Â°C. The common characteristic for all the polymers is the support that fights chemical humiliation in connection with the twist of decrease in strength and raise of time. To support the fibre is delayed in resin system compared to thermo plastics. (AK, 1980)
Defects in Manufactured Polymeric Composites
The composites of all realistic resistant plastics probably have defects of numerous types arising from the manufacturing processes. Only when the composites have been created under highly meticulously controlled circumstances, composites become prominent for the changeability of exhibits of mechanical properties. The method consisting of changeability of substance manufactured by hand lay-up method is more distinct rather the one formed from automated processes. The rigorousness and precise nature of the defects encountered in any manufactured product will too be attributed to the process of manufacture. Also, any composite on cooling can develop stresses of residual adequately high enough to break a fragile matrix. This happens when on heating; the composite contains constituents of extensively other coefficients of thermal expansion. The manufactured composites may contain defects such as: (DRH, 1980)
Inaccurate fiber volume portion on the whole
Irregular distribution of fiber resulting in matrix-rich regions
Inaccurate resin cure condition, particularly resulting from limited exothermic temperatures in difficult and wide sections during autoclaving
Broken or uneven fibers
Occurrence of overlaps, gaps and other defects in the plies arrangement
Interlaminar debonded regions
Occurrence of voids or pores in matrix-rich regions
Incidence of mechanical damage around holes done through machine
Transverse ply cracks or resin cracks resultant of stresses of thermal variance
Disbonds in composites of thermoplastics resulting from breakdown of divided flows to re-weld throughout moulding
Breakdown of local bonds in adhesively bonded components of composites. (Ashton, 1969)
Some of these defects will be discussed in detail perspective to specific manufacturing processes.
Two major types of defects occur likely during the process of shaped artifacts molding from materials like sheet-molding compounds and dough-molding compounds (SMC and DMC). The first defect results due to breakdown of process controlling flow and treatment of resin. This may be possibly being an outcome due to utilization of some molding compound that is through with his shelf-life or due to inaccurate control of temperature/pressure cycle. Re-amalgamation in complicated moldings, failure of divided flow streams, partial mould filling are the consequences of this defect. The second defect may occur due to either irregular filling of the mould cavity preceding the pressing or lack of proper attention given to the flow patterns occurring during pressing. In either of the above situations, there may again occur irregularity in fiber content in completed artifact, and/or limited favored orientation of short fibers resulting in anisotropy related to mechanical properties. The constituents may fall short to meet up the design specification if this anisotropy is not permitted for in the design. Like considerations and assumptions relate obviously to injection molding of thermoplastics with short-fiber- reinforced characteristics. (JA, 1977)
Distinctive defects found in RTM moldings are:
Movement or wash out of perform particularly near injection corners or points, reduction in wetting out of the fibers by the resin Porosity which is a result from bubbles injected with the resin. Malfunctioning of the resin to break through perform evenly dispersed porosity early gelation. (Beer F. P., 1992)
From the time when the process depends on the flow of resin to remove the air enclosed inside the mould, porosity has become a meticulous difficulty. Also sometimes there is a chance that the filler particles are filtered out of the resin when resins of the filled matrix are being injected into a strongly packed fiber perform. By decreasing air pressure in the cavity of the mould, the quality of the RTM can be enhanced significantly. (AK, 1980)
Figure Variation of mechanical properties of GRP
In view to enhance the wetting out of the perform significantly and also decrease the level of remaining porosity in the molding to a portion of a chapter two resulting the composite materials to 26 percent that too for comparatively slight additional expenditure, it is necessary to preserve the pressure at 0.7 bar in the supposed so called vacuum-assisted RTM process (VARTM). The variation of mechanical properties of a general glass/polyster composite with the levels of porosity features pertaining to various processing circumstances. The average levels of porosity achieved in VARTM moldings can be then compared with those obtained from autoclaved resources of high quality. (B.W.., 1965)
Some of the defects occurring during lying up of prepare resources is shown as a graphic representation. The features required for fixed dimensions of prepared in periodic intervals of time are that joins:
(1) It needs to be created. Strongly butted joins are evidently more acceptable than overlapped
(2) Broadly divided
(3) The results in resin rich regions which end prepare are amalgamation of inclusions such as bits of waste
(4) It should be averted by implementing precise procedures enduring quality control. However general problem associated with prepare is the incidence of small bundles of fiber rubble called as 'whorls' which get occasionally attached to the exterior of the sheet and consequently gets integrated in the shield in the form of small regions of disoriented configuration of the fiber. These are supposed to damage the composite strength. Such type of defects should not take place rather once in ten meters of prepare roll as usually stated by an aerospace producer. Samples of [(0/90)2] S glass/epoxy laminate which is hoy-pressed and is a result of business prepare contains calculated concentrations of whorls to the amount of 3 defects per prepare ply fitted in a 150mm x 150mm x 1mm plate. This resulted in reduction in tensile strength by only 6% and the failure damage by about 2% unchanging the stiffness (Ellis, 1985). Incorrectly bonding of piles in small regions is due to the failure to attain a consistent pressure level above the entire surface of a sheet at the time of autoclaving (5). Faults in laminates subjected to impact forces or compression are sourced by these disbonds. Hydrostatic pressure may build up within the resin region and push out the gel coat or surface layer to form blisters (6). This can be a result due to voids or incorrectly treated resin regions near the laminate surface which may likely soak moisture during wet circumstances. Inadequately stored prepares or those that are draped over the previous sharply rounded may develop wrinkles (7), which can result in weak service quality or weaknesses from rich in resin regions in the laminate. The last aspect depicted n figure 2.2 is shown intentionally 'defect'. Dimensions of a configuration is frequently needed to be altered and while each ply efficiently represents a 'quantum' of laminate thickness, such type of changes can only be applied when the amount of plies in a cross section is increased or decreased gradually. (Ashton, 1969)
Implementation of 'ply drop-offs' leads to this design process (8) yet encounters a difficulty for the stress engineer as the ply leads to the consequence in local stress concentrations, results of which must be considered for (Curry et al, 1985; Cui et al, 1994). Defects can also be sourced from the prepares on the condition that the strict methods of quality control are not implemented. Examples of prepare defects leading to weak quality of the finished laminate are accumulations of rubble ( eg 'whorls'), broken fiber tows, and expired resin.
Since most composites consist of materials of widely different thermal expansion coefficients. If these heat different thermal expansion coefficients are cooled during manufacture, develop residual stresses sufficiently high to crack a brittle matrix. Early high-performance CFRP laminates were frequently found to have suffered multiple internal cracking unless proper care was taken to control the production cycle.
By advent of 'modified' or 'toughened' resins, these types of problems can be eliminated to a large extent for polymer. Perhaps, such problems can be more concern for CMCs. Though structural and economical advantages can be obtained of using hybrid laminates, but there are few potential disadvantages. As the temperature rises above 150Â°C, axial thermal expansion coefficient carbon fibers becomes close to zero, where as paramid fibre like Kevlar-49 is negative, about -3 x 10 -6 K -1 , and that of E-glass is positive, 0.7 x 10 -6 K -1. Thus, thermal stress is developed in hybrid laminates, which may cause marked undulations in the plies to compression - the Kevlar plies in a carbon/Kevlar hybrid. For example, wavy plies are unable to contribute their full share of load-bearing in tension, and the compression strength of a hybrid may also be reduced because the waviness facilitates a common compression kinking mechanism. (Ashton, 1969)
Manufacturing of complex shapes in single shot operation is one of the key advantages of hybrid polymer. This can be done by the elimination of need for the joining operations which differentiates most conventional manufacturing with metallic materials. Though it not possible to avoid joining and matching completely, But there is a possibility of bonding failures in improper adhesive joints, and while machining damage will at cut edges, and hence poor joints can be easily detected.
Figure some typical defects in composite
Methods of Non-Destructive Evaluation for Polymer Composites
In presence of some defect in reinforced plastics at initial phase, there is more damage to the service provided. The users and the manufacturers need to be more sensitive in taking up better techniques to identify defects. For effective handling of fibers, most of the technologies for metallic engineering materials and structures are developed with appropriate modifications. Few back up techniques are useful for good practice than to rely on a single method. While writing, some of the common methods in current use have high level of activity in R & D work on fibre-optic sensors. Specifically in using reinforced plastics. Brief details of some of these NDT tools are as follows. (Atzori B, 1994)
In translucent GRP, inspection by transmitted light indicates the presence of pores, poor wetting-out, delimitation, and gross inclusions. There is a possibility of correlating light transmission with fibre content, Loss of transparency (stress-whitening) is related with the development of fibre/resin de bonding and resin cracking. To observe the surface damage feasible visual inspection techniques are associated in non-translucent composites. Usually dye-penetrant methods are used in detection of surface cracking may be enhanced. Few methods require photographic grids to be printed on the surface of the composite and irradiated with coherent light. The resulting interference fringes give clear indications of local stress concentrations and deformation, including sub-surface damage. For non-translucent materials, Laser holographic methods and electronic speckle-pattern interferometry (ESPI) are also used. (A, 1996)
Radiography is not easily applied in the field, but information about composite quality can be obtained by x-ray inspection. Contact radiography with sources of 50kV or less yields high-contrast photographs from low-density materials like GRP because of their low inherent filtration. The linear absorption coefficient of glass is about twenty times that of most resins, and film density measurements can be correlated with VF if the material is unpigmented. The fibre distribution, quality of weave, and the presence of large laminating defects can be easily investigated. Contact radiographs showing fibre distributions have also been analyzed by optical diffraction methods to give quantitative information about the fibre distribution. The sensitivity of contact radiography can be improved by impregnating the composite with a radio-opaque material, usually an alcoholic solution of zinc iodide. By this technique, resolution of cracks of the order of a few mm long and 0.1 mm deep is feasible in GRP and CFRP, and delimitations are easily resolved. (AK, 1980)
When a uniform heat flux is supplied to a plate, any anomalous variations in the resulting temperature distribution in the plate are indications of structural flaws in the material. Similarly, in an otherwise uniform material working under variable loads, local damage will give rise to changes in the hysteretic thermal losses in the body of the material. Thermal imaging, with limiting detection of temperature differences of about 0.2Â°C, is easily carried out by means of infra-red television photography. Damage has been detected in glass/epoxy laminates at low stresses and frequencies, but in materials of higher thermal conductivity detection is more difficult. (B.W.., 1965)
Model for Composite Fiber
Simple Micromechanical Model
The simplest method of estimating the stiffness of a composite in which all of the fibre is aligned in the direction of the applied load (a unidirectional composite) is to assume that the structure is a simple beam, as in Figure 3.1, in which the two components are perfectly bonded together so that they deform together. We shall ignore the possibility that the polymer matrix can exhibit time-dependent deformation. The elastic (Young) module of matrix and reinforcement are and, respectively. We let the cross-sectional area of the fibre 'component' be and that of the matrix component be. If the length of the beam is L, then we can represent the quantities of the two components in terms of their volume fractions, Vf and Vm , which is more usual, and we know that their sum . The fibre volume fraction is the critical material parameter for most purposes. The subscript 'c' refers to the composite. The load on the composite, Pc, is shared between the two phases, so that =, and the strain in the two phases is the same as that in the composite, Îµc = Îµf = Îµm (i.e. this is an 'is-strain' condition). Since stress = load/area, we can write: (JA, 1977)
And from the is-strain condition, dividing through by the relevant strains
Figure Confirmation of the rule-of-mixtures relationship
This equation is denoted as the Voigt estimate, but is acknowledged as the rule of mixtures in a more familiar manner. It makes the unquestioning supposition that the Poisson ratios of the two components are equal (Î½f = Î½m), consequently not taking elastic constraints induced by differential lateral contractions in to consideration. More complex or intricate models have been formulated which facilitate such consequences, the most recognizable is that of Hill (1964) which demonstrates that the accurate rigidity of a unidirectional composite beam would be higher than the forecast of an equation 3.1 by an amount which is relative to the square of the difference in Poisson ratios, (Î½f - Î½m) 2, but for most realistic reasons this difference is so little as to be insignificant. (AK, 1980)
Figure Effect of randomizing the fibre orientations on the
Figure : Out-of-plane distortion in an unsymmetrical
The designer should be conscious of a few noticeable concerns. To the extent temperature is a taken in to consideration, several consequences might be cited. The first is that at increased temperatures movement might take place in one or both of the components. Measurable creep will not take place in glass, carbon, or boron fibre over the temperature limits within which most resins, including the ones which are more thermally-stable continue to be intact. Consequently, in incessantly strengthened plastics, distortion depending on time will be restricted by the degree to which the uninterrupted fibre holds up the load to the maximum extent. In short-fibre composite moldings, however, the upper usable temperature limit may be much lower than that at which resin degradation begins to occur. (Atzori B, 1994)
When less amount of consideration is given to chemical debasement of matrix resins which may start in the polymers which are less constant, at comparatively small temperatures, a second consequence of temperature is in the material response connected with variations in temperature, instead of the consequences of exposure at steady temperature. It is observed in Chapter 2 that considerable levels of remaining stress can be formed as an outcome of disparity in thermal expansion. Under such state of affairs these can be so immense as to cause harm to fibre, cause internal fissures and transverse ply fissures. In aerospace structures, specific apprehension is the harmful consequences of quick excursions to high temperature, an observable fact known as 'thermal spiking', which can lead to noteworthy decrease of the remaining potency of the spiked laminate. Collings and Stone (1985), for instance, remarked that the thermal spiking of dry, cross-plied XAS/914 CFRP lead to a 7% decrease in the ILSS, even though no noticeable indications of abjection took place. Alternatively, they observed that same kind of spiking of laminates consisting dampness created large-scale harm in the form of microscopic interlaminar fissures which decreased the ILSS to some 75% of the dry room-temperature value, and approximately the balance dampness content of the laminates was increased twofold. The collective result of temperature and dampness is often noticed to be more harmful than the result of either independently. The conflicting reply of cross-plied epoxy-based composites reinforced with glass, paramid, and carbon fibre to joint temperature/humidity cycling are demonstrated by some results of Dickson et al (1984) depicted . (Ashton, 1969)
Figure Reductions in the strengths of 0/90 laminates
In metal-matrix composites, an aqueous environment may result in stern deterioration if the amalgamation of matrix/reinforcement gives a suitable galvanic couple. In the early seventies, Carbon-fibre- reinforced aluminum alloys were recognized as being predominantly having a tendency to assail (see, for instance, the appraisal by Pfeifer (1977). An extensive range of resistance to corrosion was recognized in a variety of systems analyzed at that time, much of the work related to in this appraisal concerning to the consequences of marine, salt-spray and relative-humidity exposures. It was described that the most important form of corrosion was pitting, beginning at the interface at position where the fibre were exposed, at cut or machined surfaces for instance, the pitting is said to be frequently followed by rigorous scurf resulting in total damage of test panels. The composite corrosion resistance was influenced to a greater extent by the mixture of ingredients and composition of the matrix alloy, and was maximum in composites in which a high-quality fibre/matrix bond was attained. Same kind of corrosion problems may be anticipated when usage of carbon-fibre-reinforced plastics in combination with metallic components in the incidence of the approximately unavoidable aqueous environment. (Ashton, 1969)
Hydrothermal sensitivity of Reinforced plastic
When glass is considered, this is a type of environmental stress cracking, or stress-corrosion, which is a result from the percolating out of the network modifier, NaO2, from the glass structure. The resulting alkaline environment subsequently assails the generally- static SiO2 network and lessens the effectiveness of the glass fibre. Fundamentally, stress-corrosion is a result of the two or more associated outcomes of the corrosive environment and the applied stress. The outcome takes place more quickly in acid or alkaline environments when compared to water alone, but even in pure water the rate of loss of potency is relatively high and the degree of the deteriorating is considerable. The loss of potency of ordinary E-glass fibre in the alkaline environment of a moist concrete matrix critically restricted the efficient utilization of glass-fibre reinforced cement products (GRC) in the building industry. This difficulty resulted in improvement at the UK Building Research Establishment, and succeeding mistreatment by Pilkington, of a zirconia-consisting glass called as 'Cem-FIL', which is more resistant to alkali in comparison to other glass compositions (Majumdar, 1970). Effects on the stress-corrosion of this Cem- FIL glass fibre in aqueous alkaline environments (Proctor and Yale, 1980) recommended that the material presented a far-from-perfect solution, but it has been made known that under the more practical circumstances of service exposure of thin-walled Cem-FIL-reinforced cement pipe ,the realistic life of such materials is superior than the results obtained in laboratory.
One of the problems with composites is that there is such a diverse range of materials, and it is necessary to have a thorough understanding of each class: we cannot simply read across from the characterization of one group of composites to another without carrying out extensive checks of the validity of the old models for the newer materials. Theories of the elastic behavior of composite materials are very well developed, and predictions of elastic response are often very satisfactory. Problems arise, however, when attempts are made to predict processes of failure. The failure of composites is almost always a complex process. Damage accumulates in a widespread fashion in composites, and many individual processes occur at the microstructural level. The strength of a composite is determined by the conditions which determine the climax of this damage accumulation process, and modeling of these conditions is far from perfect at the present time. As a consequence, there is still a certain lack of confidence on the part of designers, a reluctance to use some of these new materials for which we have not, as yet, the operating experience that instill confidence, and this leads to a measure of inefficient (and often uneconomic) design for which the only cure is better understanding.