384 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju Figure 9.17 shows enhanced micrographs of G30-500/PMR-15 uni- directional composites after 407,1,200,and 2,092 h of aging at 288C that clearly show that oxidation substantially increases in both the axial and transverse directions with aging time.Next,the overall level of oxidation is quantified by measuring the percentage of the specimen cross-sectional surface area that is oxidized.Using the constructed enhanced micrograph image,a histogram of the image is used to determine the ratio of white to black pixels and the ratio is used to quantify the amount of surface area of the composite cross section that is oxidized.Quantification of the oxidation area using this process is a two-dimensional computation and does not account for stochastic variations of the oxidized cross section through the thickness of the specimen. Figure 9.18 shows the average of the extent of the axial oxidation along the fiber direction as a function of aging time at 288C.The figure also includes the average of the extent of the transverse oxidation from the enhanced micrographs.Figure 9.18 shows the dominance of the axial oxida- tion degradation as compared to the transverse degradation in composites which is attributed to the higher effective diffusivity on the S3 surfaces. Figure 9.19 compares the oxidation growth in the transverse direction (from S surface)of the composite to the oxidation growth in neat resin PMR-15.It appears that the presence of fibers may initially retard growth of oxidation in the transverse directions.Figures 9.18 and 9.19 show the significant differences between the oxidative behavior of the constituents and the composite owing to the fiber-matrix microstructure that may cause increases in diffusion and reaction rates. 407hr 1200hr 2092hr Fig.9.17.Unidirectional G30-500/PMR-15 at 407,1,200 and 2,092 h of oxidation at 288C
Figure 9.17 shows enhanced micrographs of G30-500/PMR-15 unidirectional composites after 407, 1,200, and 2,092 h of aging at 288°C that clearly show that oxidation substantially increases in both the axial and transverse directions with aging time. Next, the overall level of oxidation is quantified by measuring the percentage of the specimen cross-sectional surface area that is oxidized. Using the constructed enhanced micrograph image, a histogram of the image is used to determine the ratio of white to black pixels and the ratio is used to quantify the amount of surface area of the composite cross section that is oxidized. Quantification of the oxidation area using this process is a two-dimensional computation and does not account for stochastic variations of the oxidized cross section through the thickness of the specimen. Figure 9.18 shows the average of the extent of the axial oxidation along the fiber direction as a function of aging time at 288°C. The figure also includes the average of the extent of the transverse oxidation from the enhanced micrographs. Figure 9.18 shows the dominance of the axial oxidation degradation as compared to the transverse degradation in composites which is attributed to the higher effective diffusivity on the S3 surfaces. Figure 9.19 compares the oxidation growth in the transverse direction (from S1 surface) of the composite to the oxidation growth in neat resin PMR-15. It appears that the presence of fibers may initially retard growth of oxidation in the transverse directions. Figures 9.18 and 9.19 show the significant differences between the oxidative behavior of the constituents and the composite owing to the fiber–matrix microstructure that may cause increases in diffusion and reaction rates. Fig. 9.17. Unidirectional G30-500/PMR-15 at 407, 1,200 and 2,092 h of oxidation at 288°C 407 hr 2092 hr 1200 hr 384 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju
Chapter 9:Predicting Thermooxidative Degradation 385 250 () 1600 1400 200 1200 Neat resin 1000 Axial growth 150 x沙指 800 from S surface 目 600 Transverse growth 100 400 from S.surface uoneziplxO y 50 Transverse growth 200 一一 from S,surface 0 5001000150020002500 5001000150020002500 Aging Time,hr Aging Time,hr Fig.9.18.Comparison of oxidation growth Fig.9.19.Comparison of oxidation in S and S3 directions of composite growth in S direction of composite and in neat resin PMR-15 A Philips XL30 environmental scanning electron microscope (ESEM) is used to document and characterize the surfaces of the aged specimens Figure 9.20 shows the photomicrographs of polished cross sections of axial surfaces S3 for both nonoxidized and oxidized specimens of the PMR-15 unidirectional composites.It is commonly known that,when composite specimens are polished for imaging,the stiffer fibers wear at a slower rate than the parent matrix,leading to topographic differences between the fibers and the surrounding matrix.For the oxidized specimen (Fig.9.20b), oxidative degradation of the fibers exacerbates the uneven polishing or rounding of the fibers as compared to the nonoxidized specimen shown in Fig.9.20a.The PAN-based G30-500 fiber has a typical skin-core structure in which crystalline sheets are oriented radially in the skin but form a random granular-like structure in the core [71].This construction of the fiber's cross section is not expected to polish evenly Fiber ends (a) (b) Fig.9.20.SEM micrograph of the specimen cross section in(a)nonoxidized and (b)oxidized region of the composite
in S1 and S3 directions of composite growth in S1 direction of composite and in neat resin PMR-15 A Philips XL30 environmental scanning electron microscope (ESEM) is used to document and characterize the surfaces of the aged specimens. surfaces S3 for both nonoxidized and oxidized specimens of the PMR-15 unidirectional composites. It is commonly known that, when composite specimens are polished for imaging, the stiffer fibers wear at a slower rate than the parent matrix, leading to topographic differences between the fibers and the surrounding matrix. For the oxidized specimen (Fig. 9.20b), oxidative degradation of the fibers exacerbates the uneven polishing or rounding of the fibers as compared to the nonoxidized specimen shown in Fig. 9.20a. The PAN-based G30-500 fiber has a typical skin–core structure in which crystalline sheets are oriented radially in the skin but form a random granular-like structure in the core [71]. This construction of the fiber’s cross section is not expected to polish evenly. Fig. 9.20. SEM micrograph of the specimen cross section in (a) nonoxidized and (b) oxidized region of the composite 0 50 100 150 200 250 0 500 1000 1500 2000 2500 Oxidization Thickness (µm) Aging Time, hr Neat resin Transverse growth from S1 surface 0 200 400 600 800 1000 1200 1400 1600 0 500 1000 1500 2000 2500 Oxidization Thickness (µm) Aging Time, hr Axial growth from S3 surface Transverse growth from S1 surface Figure 9.20 shows the photomicrographs of polished cross sections of axial Fig. 9.18. Comparison of oxidation growth Fig. 9.19. Comparison of oxidation Chapter 9: Predicting Thermooxidative Degradation 385
386 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju Development of damage at the fiber-matrix interface m田 Fig.9.21.Debonding at fiber-matrix interface at the oxidized fiber end Higher magnification of an oxidized axial surface,as seen in Fig.9.21, shows details of the rounding of the oxidized fibers and the opened inter- facial fiber-matrix cracks [80]. In Fig.9.22,a high magnification view of a transverse cross section S2 for a specimen aged in air at 288C for 2,092 h shows significant fiber- matrix debonding indicative of that shown in Fig.9.21.Additionally during sectioning and polishing,the PMR-15 matrix resin fragmented in the highly oxidized regions.Figure 9.23 shows cracking on the axial S3 surface that included both fiber-matrix debonding and matrix cracking that progressively open during the aging process.The fiber-matrix debonds that are evident on both the axial and transverse sections are a result of mass loss,shrinkage, and increased brittleness of the resin due to oxidation.The cracks provide added surfaces and pathways for oxidants to penetrate deeper into the com- posite during aging.Therefore,once cracking initiates on the axial surface at the fiber-matrix interface,an acceleration of the degradation occurs. oxidized Development of damage at the fiber-matrix interface 起出Fan品 Fig.9.22.Debonding/delamination at fiber-matrix interface in unidirectional composite
Chapter 9:Predicting Thermooxidative Degradation 387 (a) (b) Fig.9.23.Photomicrographs of axial surface S3 of a unidirectional specimen aged for 1,864 h at 288C showing(a)matrix cracking perpendicular to fiber ends and (b)close-up view of a matrix crack providing pathway for enhanced diffusion 9.3 Accelerated Aging/Oxidation Accelerated aging methods are needed to evaluate materials which are to be used under long-term exposure to elevated temperature in oxidative environments.The need for accelerated test methods for HTPMCs is mani- fested in the requirement to characterize these materials for their expected service life which can be several thousands of hours.The cost of aging these materials for this extended period is often prohibitive.A good accelerated test method neither introduces extraneous damage/degradation mechanisms nor omits any actual mechanisms at its use temperature.Moreover,the method must be validated by comparing mechanical properties,damage modes,and physical aging parameters,such as weight loss,with those from specimens tested under real-time conditions.Since the use temperature of HTPMCs is often near the glass transition temperature of the material, the ability to accelerate aging by increasing the temperature is limited.The use of pressure to accelerate the oxidative aging process is used in the air- craft engine community based in part on the fact that engine parts experience elevated pressures during use.Other conditioning factors reported in the literature include aging in different gaseous environments,aging under load, exposure to ultraviolet radiation,and exposure to moisture.An extensive assessment of accelerated test methods conducted for the HSR program is found in the HSR Materials Durability Guide [2,41]. The only way to know if an aging mechanism can be properly accelerated is to identify and understand the mechanism and how it will be affected by acceleration [41].Although we do not fully understand the oxi- dation mechanism in HTPMCs,we can determine some of the functional relationships that govern aging.For example,elevated pressure aging should not change the oxidative chemical mechanism but should accelerate it
(a) (b) Fig. 9.23. Photomicrographs of axial surface S3 of a unidirectional specimen aged for 1,864 h at 288°C showing (a) matrix cracking perpendicular to fiber ends and (b) close-up view of a matrix crack providing pathway for enhanced diffusion 9.3 Accelerated Aging/Oxidation Accelerated aging methods are needed to evaluate materials which are to be used under long-term exposure to elevated temperature in oxidative environments. The need for accelerated test methods for HTPMCs is manifested in the requirement to characterize these materials for their expected service life which can be several thousands of hours. The cost of aging these materials for this extended period is often prohibitive. A good accelerated test method neither introduces extraneous damage/degradation mechanisms nor omits any actual mechanisms at its use temperature. Moreover, the method must be validated by comparing mechanical properties, damage modes, and physical aging parameters, such as weight loss, with those from specimens tested under real-time conditions. Since the use temperature of HTPMCs is often near the glass transition temperature of the material, the ability to accelerate aging by increasing the temperature is limited. The use of pressure to accelerate the oxidative aging process is used in the aircraft engine community based in part on the fact that engine parts experience elevated pressures during use. Other conditioning factors reported in the literature include aging in different gaseous environments, aging under load, exposure to ultraviolet radiation, and exposure to moisture. An extensive assessment of accelerated test methods conducted for the HSR program is found in the HSR Materials Durability Guide [2, 41]. The only way to know if an aging mechanism can be properly accelerated is to identify and understand the mechanism and how it will be affected by acceleration [41]. Although we do not fully understand the oxidation mechanism in HTPMCs, we can determine some of the functional relationships that govern aging. For example, elevated pressure aging should not change the oxidative chemical mechanism but should accelerate it. Chapter 9: Predicting Thermooxidative Degradation 387
388 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju The acceleration of aging would be a result of increased transport of the oxygen to the interior of the specimen.Since the oxidation process is not reaction rate limited but diffusion rate limited,the elevated pressure would effectively increase diffusion rate,thereby accelerating the oxidation process Specimens subjected to accelerated aging conditions will not necessarily have the same degradation state as specimens aged in standard conditions This is evidenced by the fact that the elevated pressure aging specimens have a significantly greater oxidation layer thickness [103]than the ambient pressure-aged specimens.The accelerated aging test must be accompanied by analysis methods to relate the nonaccelerated aging data to the acce- lerated aging data.These analysis methods and tools will provide capa- bilities to predict the performance of the material in more general service environments. 9.3.1 Elevated Temperature Aging Thermally activated rate processes are typically accelerated by increasing temperature.Acceleration by temperature occurs by reducing the activation energy of chemical bond rupture in the polymer macromolecule.Unfor- tunately,elevated temperature may promote degradation processes that do not occur at application temperatures.Temperature can also affect the rate of degradation by increasing the thermal stress in polymer composites caused by differences in the thermal expansion coefficient of the constituents. A series of tests was conducted on two carbon/epoxy systems by Tsotsis [108,109]and Tsotsis and Lee [110]to assess key factors in the characteri- zation of the thermooxidative stability of composite materials.Mechanical properties were determined from specimens aged in air at 177C up to several thousand hours.After 1,000 h at different temperatures,differential scanning calorimetry (DSC)as well as glass transition temperature T measurements suggest that higher aging temperatures lead to more highly crosslinked microstructures which may not be representative of actual aging at use temperatures.Aging below,but near the Ts,gives degradation rates that are nonlinear with respect to temperatures,thereby making estimates of useful lifetimes difficult at best. Recent work by Ripberger et al.[78]and Tandon et al.[103]also examines the use of elevated temperature to accelerate the rate of thermo- oxidative degradation in PMR-15 resin.While the oxidized layer thickness is similar for all of the aging temperatures up to approximately 1,000 h,as shown in Fig.9.24,the effect of elevated temperature on the weight loss
oxygen to the interior of the specimen. Since the oxidation process is not reaction rate limited but diffusion rate limited, the elevated pressure would effectively increase diffusion rate, thereby accelerating the oxidation process. Specimens subjected to accelerated aging conditions will not necessarily have the same degradation state as specimens aged in standard conditions. This is evidenced by the fact that the elevated pressure aging specimens have a significantly greater oxidation layer thickness [103] than the ambient pressure-aged specimens. The accelerated aging test must be accompanied lerated aging data. These analysis methods and tools will provide capabilities to predict the performance of the material in more general service environments. 9.3.1 Elevated Temperature Aging Thermally activated rate processes are typically accelerated by increasing temperature. Acceleration by temperature occurs by reducing the activation energy of chemical bond rupture in the polymer macromolecule. Unfortunately, elevated temperature may promote degradation processes that do not occur at application temperatures. Temperature can also affect the rate of degradation by increasing the thermal stress in polymer composites caused by differences in the thermal expansion coefficient of the constituents. A series of tests was conducted on two carbon/epoxy systems by Tsotsis [108, 109] and Tsotsis and Lee [110] to assess key factors in the characterization of the thermooxidative stability of composite materials. Mechanical properties were determined from specimens aged in air at 177°C up to several thousand hours. After 1,000 h at different temperatures, differential scanning calorimetry (DSC) as well as glass transition temperature Tg measurements suggest that higher aging temperatures lead to more highly crosslinked microstructures which may not be representative of actual aging at use temperatures. Aging below, but near the Tg, gives degradation rates that are nonlinear with respect to temperatures, thereby making estimates of useful lifetimes difficult at best. Recent work by Ripberger et al. [78] and Tandon et al. [103] also examines the use of elevated temperature to accelerate the rate of thermooxidative degradation in PMR-15 resin. While the oxidized layer thickness is similar for all of the aging temperatures up to approximately 1,000 h, as shown in Fig. 9.24, the effect of elevated temperature on the weight loss The acceleration of aging would be a result of increased transport of the by analysis methods to relate the nonaccelerated aging data to the acce- 388 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju