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Analysis of Lamina Hygrothermal Behavior Hygrothermal – combined effects of temperature and moisture

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Effects of changes in hygrothermal environment on mechanical behavior of polymer composites: 1. Hygrothermal deformations change the stress and strain distributions. 2. Matrix – dominated properties under transverse, off-axis and shear loading are altered, particularly in the glass transition region.

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Variation of stiffness with temperature for a typical polymer showing the glass transition temperature T g and the effects of absorbed moisture on T g wet Dry Glassy Region Increasing Moisture Content Rubbery Region Temperature Stiffness T gw T go Typical operating temperature below T g for structural polymers Glass Transition Region

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Operating temperature ranges for different types of polymers Stiffness Temperature TgTg Glassy structural polymers Rubbery polymers (elastomers)

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Arrow indicates hot gasses leaking from O-ring in joint on solid rocket booster Practical example of the importance of the glass transition temperature, T g, of polymers 1986 Space Shuttle Challenger explosion

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Space shuttle system Joints in solid rocket booster tank

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Joint in Challenger solid rocket booster showing the problem O-rings

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From The failure of an O-ring seal was determined to be the cause of the Space Shuttle Challenger disaster on January 28, A contributing factor was cold weather prior to the launch. This was famously demonstrated on television by Caltech physics professor Richard Feynman, when he placed a small O-ring into ice-cold water, and subsequently showed its loss of pliability before an investigative committee. O-rings are now examined under high-power video microscopes for defects. The material of the failed O-ring was FKM (a fluoroelastomer) which was specified by the shuttle motor contractor, Morton-Thiokol. FKM is not a good material for cold temperature applications. When an O-ring is cooled below its T g (glass transition temperature), it loses its elasticity and becomes brittle. More importantly, when an O-ring is cooled near, but not beyond, its T g, the cold O-ring, once compressed, will take longer than normal to return to its original shape. O-rings (and all other seals) work by creating positive pressure against a surface thereby preventing leaks. On the night before the launch, exceedingly low air temperatures were recorded. On account of this, NASA technicians performed an inspection. The ambient temperature was within launch parameters, and the launch sequence was allowed to proceed. However, the temperature of the rubber O-rings remained significantly lower than that of the surrounding air. During his investigation of the launch footage, Dr. Feynman observed a small out-gassing event from the Solid Rocket Booster (SRB) at the joint between two segments in the moments immediately preceding the explosion. This was blamed on a failed O-ring seal. The escaping high temperature gas impinged upon the external tank, and the entire vehicle was destroyed as a result.

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Challenger O-ring stiffness vs. temperature O-ring Stiffness Temperature TgTg Desired O-ring temperature is above T g so that material is pliable enough to fill gaps and form seal proper seal against metal rocket motor case as it expanded under pressure Actual O-ring temperature at launch was below T g and material was too stiff to fill gaps and form proper seal against metal motor case as it expanded under pressure

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Dr. Feynman's famous C-clamp experiment during meeting of the Presidential Commission investigating the Challenger disaster

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Conclusions When designing with polymers, knowledge of the glass transition temperature, T g, and the operating temperature range are essential Structural polymers should operate below T g, and rubbery polymers (elastomers) should operate above T g The operating temperature should not be in or near the glass transition region, as stiffness and strength are very sensitive to temperature in this region

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Free Volume concept used to interpret glass transition Total sample volume where V o = volume occupied by molecules V f = free volume due to space between molecules

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Temperature TgTg Volume VfVf V VoVo Glassy region Rubbery region Free Volume concept used to interpret glass transition

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MaterialSupplier Saturation Moisture Content, M m (Weight %) T go (Dry) [°F (°C)] T gw (Wet) [°F (°C)] Maximum Service Temperature (Dry) [°F (°C)] Hexply ® F655 bismaleimideHexcel4.1550(288)400(204) Hexply ® epoxy Hexcel3.1315(157)240(116)200(93) Hexply ® 8552 epoxyHexcel392(200)309(154)250(121) Hexply ® 954-3A cyanateHexcel400(204) CyCom ® 2237 polyimideCytec4.4640(338)509(265)550(208) CyCom ® 934 epoxy Cytec381(194)320(160)350(177) Avimid ® R polyimideCytec2.8581(305)487(253)550(288) Derakane ® vinylester Ashland1.5250(120)220(105) Ultem ® 2300 polyetherimide Sabic0.9419(215)340(171) Victrex ® 150G polyetherether-ketone Cetex ® polyphenylene sulfide Victrex plc Tencate (143) 194(90) 356(180) 212(100) Hygrothermal Properties for Various Polymer Matrix Materials

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Stress–strain curves for 3501–5 epoxy resin at different temperatures and moisture contents. (From Browning, C.E., Husman, G.E., and Whitney, J.M Composite Materials: Testing and Design: Fourth Conference, ASTM STP 617, pp. 481–496. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. Reprinted with permission.)

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Stress–strain curves for AS/3501–5 graphite/epoxy composite under transverse loading at different temperatures and moisture contents. (From Browning, C.E., Husman, G.E., and Whitney, J.M Composite Materials: Testing and Design: Fourth Conference, ASTM STP 617, pp. 481–496. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. Reprinted with permission.)

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Percent weight gain due to moisture pickup vs. soaking time for several E-glass/polyester sheet-molding compounds. Materials described in table 5.2. (From Gibson, R.F., Yau, A., Mende, E.W., and Osborn, W.E Journal of Reinforced Plastics and Composites, 1(3), 225–241. Reprinted by permission of Technomic Publishing Co.)

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Variation of flexural modulus of several E-glass/polyester sheet-molding compounds with soaking time in distilled water at 21 to 24°C. (From Gibson, R.F., Yau, A., Mende, E.W., and Osborn, W.E Journal of Reinforced Plastics and Composites, 1(3), 225–241. Reprinted by permission of Technomic Publishing Co.)

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Description of Composite Materials for Figures 5.4 and 5.5 from Gibson, et al [1982] Material Weight of Percentage of constituents Chopped E-glass Fibers Continuous E-glass Fibers Polyester Resin, Fillers, etc. PPG SMC-R PPG SMC-R PPG XMC ( x- pattern) 25 OCF SMC-R OCF C20/R (Aligned)50 1 Manufactured by PPG Industries, Fiber Glass Division, Pittsburgh, PA Manufactured by Owens-Corning Fiberglas Corporation, Toledo, OH

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Schematic representation of temperature and moisture distributions through the thickness of a plate which is exposed to an environment of temperature T a and moisture concentration C a on both sides. Plate h z z Thickness TaTa CaCa T C Ambient Temperature T a and Ambient Moisture Concentration C a

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Distribution of Temperature One dimensional case Fourier Heat Conduction Equation: where = density of material C = specific heat of material K z = thermal conductivity of material along z direction T = temperature t = time (5.1)

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Ficks Second Law of Diffusion: where c = moisture concentration D z = mass diffusivity along z direction Equations solved subject to initial and boundary conditions Distribution of Moisture One dimensional case (5.2)

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For typical composites, the values of C, K z, D z and are such that T approaches equilibrium ~ 10 6 times faster than C, so temperature can usually be assumed equal to ambient temperature T a. Moisture concentration requires further analysis, However,if diffusivity is constant, Ficks second law is Distribution of Temperature and Moisture – one dimensional case (5.3)

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Solution where c m = moisture concentration at surface Equation (5.4) gives local concentration c(z,t), but we normally measure total amount of moisture averaged over sample. Average concentration is (5.4) (5.5)

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Weight percent moisture M, is what we actually measure, and since c is linearly related to M, we can write, (5.6) Where M = average weight percent of moisture at time t M i = initial average weight percent moisture M m = average weight percent moisture at fully saturated equilibrium condition Series converges rapidly.

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Predicted moisture profiles through the thickness of a graphite/epoxy plate after drying out for various periods of time. From Browning, C.E., Husman, G.E., and Whitney, J.M Composite Materials: Testing and Design: Fourth Conference, ASTM STP 617, pp. 481–496. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. With permission.)

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Comparison of predicted (eq. [5.6]) and measured moisture absorption and desorption of T300/1034 graphite/epoxy composites. Open symbols represent measured absorption and dark symbols represent measured desorption. (From Shen, C.H. and Springer, G.S Journal of Composite Materials, 10, 2–20. Reprinted by permission of Technomic Publishing Co.)

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Hygrothermal Degradation of Strength or Stiffness Chamis Equation (empirical) (5.7) Where F m = matrix mechanical property retention ratio P = degraded property of matrix P o = reference undegraded property of matrix T = temperature at which P is predicted T go = glass transition temperature for reference condition (dry) T gw = glass transition temperature for wet matrix material at moisture content corresponding to property P T o = test temperature at which Po was measured

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Empirical equation for T gw : where M r = percent moisture in matrix by weight Procedure: Use equations (5.8) and (5.7) to find degraded matrix property, than use degraded property in appropriate micromechanics equation to predict degraded composite property. (5.8) Example: longitudinal modulus (5.9) Where E mo = reference value of matrix modulus in dry condition Caution: these are empirical equations

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Variation of glass transition temperature with equilibrium moisture content for several epoxy resins. (From DeIasi, R. and Whiteside, J.B In Vinson, J.R. ed., Advanced Composite Materials Environmental Effects, ASTM STP 658, pp. 2–20. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. Reprinted with permission.)

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Comparison of predicted (eq. [5.7]) and measured strengths of several hygrothermally degraded composites. (From Chamis, C.C. and Sinclair, J.H In Daniel, I.M. ed., Composite Materials: Testing and Design (Sixth Conference), ASTM STP 787, pp. 498–512. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. With permission.)

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Effect of temperature on rate of moisture absorption in AS/3501–5 graphite/epoxy composite. (From DeIasi, R. and Whiteside, J.B In Vinson, J.R. ed., Advanced Composite Materials Environmental Effects, ASTM STP 658, pp. 2–20. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. Reprinted with permission.)

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Diffusion is a thermally activated process. Arrhenius Relationship for diffusivity: Where D o = material constant E a = activation energy R = universal gas constant T = absolute temperature log D Vs should be a straight line – see Fig (5.11) (5.10)

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Variation of transverse diffusivity with temperature for AS/ graphite/epoxy composite. (From Loos, A.C. and Springer, G.S In Springer, G.S. ed., Environmental Effects on Composite Materials, pp. 34–50. Technomic Publishing Co., Lancaster, PA. Reprinted by permission of Technomic Publishing Co.)

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Effects of applied stress on moisture diffusion in polymers and polymer composites Diffusivity D increases under tensile stress and decreases under compressive stress So, preferred diffusion path is through a tensile stress-field.

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Stress-Strain Relationships Including Hygrothermal Effects Thermal strains – isotropic material if i = 1, 2, 3 if i = 4, 5, 6 (5.11) Where i = 1, 2, 3, 4, 5, 6 (contracted notation) T = temperature change = T – T o T = final temperature T o = initial temperature where i T = 0 for all i = coefficient of thermal expansion (CTE)

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Assumption of linearity and constant is valid over sufficiently narrow T No shear distortion – uniform expansion or contraction Stress-Strain Relationships Including Hygrothermal Effects

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Thermal expansion vs. temperature for epoxy resin. (From Cairns, D.S. and Adams, D.F In Springer, G.S. ed., Environmental Effects on Composite Materials, vol. 2, pp. 300–316. Technomic Publishing Co., Lancaster, PA. Reprinted by permission of Technomic Publishing Co.)

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Hygroscopic Strains for Isotropic Material if i = 1, 2, 3 if i = 4, 5, 6 (5.12) Where i = 1, 2, 3, 4, 5, 6 (contracted notation) c = moisture concentration = mass of moisture in unit volume/ mass of dry material in unit volume = coefficient of hygroscopic expansion (CHE)

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Hygroscopic Strains for Isotropic Material Reference condition c = 0, i M = 0 Linearity and constant valid if range of c is not too wide Total hygrothermal strains – isotropic case (5.13)

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Hygroscopic expansion vs. moisture content for two epoxy resins. (From Delasi, R. and Whiteside, J.B In Vinson, J.R. ed., Advanced Composite Materials Environmental Effects, ASTM STP 658, pp. 2–20. American Society for Testing and Materials, Philadelphia, PA. Copyright, ASTM. Reprinted with permission.)

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Composite lamina – orthotropic in both mechanical and hygrothermal properties due to difference in fiber and matrix properties (see tables 3.1 and 3.2) Subscript needed on and Total hygrothermal strains for specially orthotropic lamina: If transversely isotropic, 2 = 3, 2 = 3 if i = 1, 2, 3 if i = 4, 5, 6 (5.14)

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Variation of measured longitudinal and transverse thermal strains for unidirectional Kevlar 49/epoxy and S-glass/epoxy with temperature. (From Adams, D.F., Carlsson, L.A., and Pipes, R.B., Experimental Characterization of Advanced Composite Materials. CRC Press, Boca Raton, FL. With permission.)

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Typical Thermal and Hygroscopic Expansion Properties. From Graves, S.R. and Adams, D.F Journal of Composite Materials, 15, 211–224. With permission.. Material Thermal Expansion coefficients (10 -6 m/m)/C Hygroscopic Expansion Coefficients (m/m) AS graphite/epoxy E-glass/epoxy AF adhesive steel

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Total strains (mechanical + hygrothermal) for specially orthotropic lamina (5.15) or in matrix notation, (5.16) so, stresses are (5.17)

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No shear coupling in mechanical or hygrothermal behavior for specially orthotropic case ( not so for generally orthotropic case as shown later.) If material is unrestrained during hygrothermal exposure, { } = 0 and the strains are (5.18) If material is completely restrained during hygrothermal exposure, { } = 0 and the stresses are found from (5.19) or since

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Total Strains (mechanical + hygrothermal) for generally orthotropic lamina (5.21) (shear coupling present) CTEs and CHEs must transform like tensor strains, not engineering strains. (5.22) And similarly for the CHEs

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Variation of lamina thermal expansion coefficients with lamina orientation for a lamina having 2 > 1 > 0.

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Micromechanics Models for Hygrothermal Properties Example: Longitudinal CTE, 1 Average composite longitudinal strain is (5.23) stress (5.24) Similarly, for fiber and for matrix

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Recall Eq. (3.23), rule of mixtures for longitudinal stress or (5.25) Recall Eq. (3.26) (3.25) And Eq. (3.27) (3.26)

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For isotropic constituents: (5.26) (5.27) Combining Eqs. 5.25, 3.26 and 3.27, and solving for 1 : (5.26)

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Variation of predicted longitudinal and transverse coefficients of thermal expansion with fiber-volume fraction for typical unidirectional graphite/epoxy composite. (From Rosen, B.W In Reinhart, T.J. ed., Engineered Materials Handbook, vol. 1, Composites, Sec. 4. ASM International, Materials Park, OH. Reprinted by permission of ASM International.)

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Micromechanics Models for Hygrothermal Properties Example: Transverse CTE, 2 Average composite transverse strain is (5.28) Substituting equations similar to equation (5.28) for composite, fiber and matrix, respectively, in the transverse geometric compatibility condition given by equation (3.37), the result is (5.29)

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Combining equation (5.29) with equation (3.39) and assuming that the stresses in the composite, fiber and matrix are all equal we get another rule of mixtures type equation (5.30) For the case of isotropic fiber and matrix, this reduces to (5.31) However, due to the assumption of equal stresses in composite, fiber and matrix, Equations (5.30) and (5.31) are only rough estimates

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Finite element analysis (FEA) micromechanical models for CTEs Typical FEA unit cell for prediction of composite longitudinal and transverse CTEs. (From Karadeniz, Z., and Kumlutas, D Composite Structures, 78, Reprinted with permission of Elsevier. FEA unit cells similar to this are subjected to a temperature change,, then the resulting FEA-calculated thermal displacements are used to determine the CTEs

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Examples: FEA-calculated longitudinal CTE FEA-calculated transverse CTE (5.33) (5.34)

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Comparison of analytical and FEA numerical predictions of longitudinal and transverse CTEs for SiO 2 /epoxy composites of various SiO 2 volume fractions. From Gibson and Muller [27].

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Longitudinal CHE, (5.35) Longitudinal transport Properties Thermal Conductivity: (5.39) Diffusivity: (5.40) Transverse Transport Properties K 2, D 2, etc. can be found by using Halpin – Tsai eqs.

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Hygrothermal Degradation of,, K and c Effect of temperature on these properties is opposite from effect on mechanical properties. Chamis empirical equation is, Where F h = Matrix hygrothermal property retention ratio R = Matrix hygrothermal property after hygrothermal degradation Ro = Reference matrix hygrothermal property before degradation Degraded matrix property is used in micromechanics equations to estimate degraded composite property. (5.41)

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