Durability of composites not only depends on environmental factors such as moisture, temperature, physical and chemical exposure, but also to some extent on structural factors. The factors that influence structural properties are: 1) type of fibers (glass, aramid, carbon etc.), 2) type of resins (epoxy, vinyl ester, polyester etc.), 3) type of composites (epoxy/glass, vinyl ester/glass etc), 4) fiber orientations, 5) fiber volume fractions, 6) thickness of the composites, 7) interfacial bond, 8) manufacturing techniques 9) others. Performance of a composite as function of some of the structural factors is addressed, herein.
Fibers
The performance of composites under aging conditions depends on the type of fiber reinforcement. Since the fibers are the main load resisting constituents of composites, the early degradation of the fiber should be avoided. The fiber should be selected in such a way that they are alkali resistance, because alkali solution breaks the glass bonds in the fiber leading to fiber breakage. Generally, boron free glass fibers are corrosion resistant and perform well when compared to the traditional E-glass fibers. As mentioned earlier, the penetration of alkali occurs more in glass fibers than in aramid or
chemical solutions, carbon composites do absorb moisture, when formulated with polymer matrices. Although the degradation of carbon fibers may not occur by itself, the oxidation reaction at the carbon cathode degrades the matrix material in that location causing degradation of the composites. Similarly, ECR-glass has an enhanced chemical resistance, especially in acid environment [EUROCODE, 1996].
A chopped strand mat is used for a smooth finish of a composite. The chopped strand mats are actually resin rich surfaces and increase the moisture absorption and diffusion coefficient. Diffusion coefficient for laminates with chopped strand mats at the surface was found to be considerably higher than with continuous fabrics. The fiber volume content is one of the factors responsible for the durability of composites. Interlaminar shear strength and impact strength of the laminates were measured and found to have the properties of the composites with higher fiber content degraded faster than those with lower fiber content [Singh, 1991]. The fiber orientation plays a vital role in the moisture absorption. In one study, Karbhari et al. [1998] found higher moisture absorption but lower strength degradation in triaxial fabrics compared to the uniaxial fabrics. The extra moisture absorption effects in specimens with triaxial fabrics were hypothesized due to absorption along the fiber-resin interfaces with increased directionality resulting in increased crossover or contact points.
Resins
The durability of composites varies with the type of resin used in the composites. Failure in the composites primarily initiates at the resin level. Since resins have more void contents, they are easily attacked by the aging solutions. Most of the aging solutions penetrate the resin and reach the core, thus degrading the fibers or the interface of fiber/matrix. Hence, one should be careful in selecting the type of resins. In one study, bisphenol polyster was found to perform much better as compared to the isophthalic polyester, because the former was more corrosion resistant. Also several studies have proven that vinyl ester has good stability against harsh environments compared to the polyester. This is mainly attributed to the chemical structure of vinyl ester resins. Polyesters have double bonds at about 250g/mol level while vinyl esters
have reactive double bonds at about every 500-1000 g/mol. In addition, performance characteristics of composites with vinyl ester change with cure time.
Manufacturing Techniques
Manufacturing techniques play a partial role in the durability of composites. Possibility of high void content exists in a composite during manufacturing. The presence of voids in the components increases the moisture absorption and diffusion coefficient, which eventually leads to degradation in strength and stiffness on a long- term basis. Controlling the line speeds can reduce the void fractions during pultrusion. Pultruded samples made at different line speeds (4, 8, and 12 ipm) were aged, then different conditions and mechanical properties were evaluated [Garland, 2000]. Based on the microscopy results, no difference was observed in the void fraction due to the effect of line speeds; hence, no difference was observed in the rate of moisture absorption. Pull speeds at higher line speeds may provide some differences in void fractions, fiber wet-out and degree of cure, which may then show the rate of degradation in the strength and stiffness when exposed to environmental conditions. Hence, the void contents in the composites should be kept as low as possible during manufacturing process, i.e. less than 0.5% if possible and certainly no more than 1%.
4.3.3.4 Knockdown Factors
The mechanical properties of a composite material, such as strength in tension, bending, shear etc. can be obtained by conducting experiments at coupon, component or system level on non-aged specimens or by using the analytical formulae given in section 1.4.1. In order to calculate the nominal strength and stiffness values for design, the base values obtained should be multiplied with modification factors for actual field and environmental conditions. These modification factors are called knockdown factors. Equations 4.32 and 4.33 are equations used to calculate nominal strength and stiffness. (AASHTO LFRD Bridge Design Specifications)
E=EoCm …..(4.33)
where
F= Nominal resistance in bending (b), or torsion (t), or compression (c), or shear (s)
F0 =Base resistance of b,t,c, or v
E = Nominal modulus for b,t,c,or v Eo = Base modulus for b,t,c,or v
Cf = Size effect factor for dimensions of width, depth, span etc.
Cm = Moisture content factor and/or humidity factor with pH variation
Cc = Environmental factor, which varies with the FRP material exposure to
different temperature levels
Ca = Physical aging factor that varies with number of years of service
Cst = Sustained load factor
These knockdown factors or reduction factors are established through different tests and field evaluations. These factors can be used when no test data are available. For example, if the tensile strength of a bigger diameter bar is required and the data are not available then the size effect factor can be used to establish these values. Vijay, (1998) has given a table to account for various knock down factors by conducting tests on 3 GFRP bars each, weathered under natural atmospheric exposure for 3, 7 and 10 years and tested in tension for this research.
Table 4-5: Knockdown Factors (Vijay,1998)
Factor Notation
Eqn.1.1 & 1.2 Parameter
Knockdown Factor (Reduction Coeff.)
Size effect Factor Cf Diameter
1.00-#4 0.85-#5 0.70-#6 0.65-#7 0.60-#8 Salt (Ph ≈ 7) 0.9-0.75 Moisture Content Factor Cm Alkaline (Ph ≈ 13) 0.8-0.65 Salt/Water 0.85-0.70 Sustained Load Factor (20%-40% on GFRP bar) Cst Alkaline 0.70-0.40 Temperature Factor to be used with (Cm and Cst) Cc Mean Annual Temperature (T0F) (In combination with alkalinity and
stress) (52.5 92.5 ) 100 ) 5 . 52 ( 1 ) 5 . 52 ( 1 0 0 F T for T F T ≤ ≤ − − ≤ Physical Aging Factor Ca 0.90 Notes:
size effect factor can be used only for interpolating bigger bar diameter strengths when smaller diameter bar (in this table #4 is chosen as reference) is tested.
values in this investigation were correlated for a mean annual temperature of 52.50F and hence knock-down factor of 1 is chosen at that temperature.
If mean annual temperature is more than 52.50F then a minimum reduction of 0.1 is applied for every 100F increase in the mean annual temperature. This is an empirical approach purely based on strength reductions in GFRP bars under
results of Litherland et al., 1981. Mean annual temperatures above 900F are not expected in any part of the globe.
A limit of 0.4 is provided as the reduction factor for combined effects (Vimala, 2002). This factor is given considering that all the environmental factors that cause aging do not act at the same time and place.
4.3.3.5 Durability Models
Various analytical models used to predict the effect of environmental factors on the durability of the composite materials is collected and cited in the paragraphs, which follow.