《纺织复合材料》课程参考文献（Multiscale Modeling and Simulation of Composite Materials and Structures）Chapter 9 Predicting Thermooxidative Degradation and Performance Matrix Composites

Chapter 9:Predicting Thermooxidative Degradation and Performance of High-Temperature Polymer Matrix Composites G.A.Schoeppner',G.P.Tandon2and K.V.Pochiraju3 US Air Force Research Laboratory,Wright-Patterson Air Force Base, OH.USA 2University of Dayton Research Institute,Dayton,OH,USA Stevens Institute of Technology,Hoboken,NJ,USA 9.1 Introduction Polymer matrix composites (PMCs)used in aerospace high-temperature applications,such as turbine engines and engine-exhaust-washed structures, are known to have limited life due to environmental degradation.Predict- ing the extended service life of composite structures subjected to mechanical, high temperature,moisture,and corrosive conditions is challenging due to the complex physical,chemical,and thermomechanical mechanisms involved.Additionally,the constituent phases of the material,in particular the matrix phase,continuously evolve with aging time.It is the aging- dependent evolution of the constituent properties that makes prediction of the long-term performance of PMCs in high-temperature environments so challenging.A comprehensive prediction methodology must deal with several complications presented by the highly coupled material aging, damage evolution,and thermooxidation processes. While carbon fibers may be more resistant to oxidation and have longer relaxation times than the polymer matrix,the mechanical performance of the fiber-matrix interface at high temperatures may be critical to the evolution of damage and composite failure.The properties of the fiber-matrix interface

Chapter 9: Predicting Thermooxidative Degradation and Performance 1 2 3 1 US Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA 2 University of Dayton Research Institute, Dayton, OH, USA 3 Stevens Institute of Technology, Hoboken, NJ, USA 9.1 Introduction Polymer matrix composites (PMCs) used in aerospace high-temperature applications, such as turbine engines and engine-exhaust-washed structures, are known to have limited life due to environmental degradation. Predict￾ing the extended service life of composite structures subjected to mechanical, high temperature, moisture, and corrosive conditions is challenging due to the complex physical, chemical, and thermomechanical mechanisms involved. Additionally, the constituent phases of the material, in particular the matrix phase, continuously evolve with aging time. It is the aging￾dependent evolution of the constituent properties that makes prediction of the long-term performance of PMCs in high-temperature environments so challenging. A comprehensive prediction methodology must deal with several complications presented by the highly coupled material aging, damage evolution, and thermooxidation processes. While carbon fibers may be more resistant to oxidation and have longer relaxation times than the polymer matrix, the mechanical performance of the fiber–matrix interface at high temperatures may be critical to the evolution of damage and composite failure. The properties of the fiber–matrix interface of High-Temperature Polymer Matrix Composites G.A. Schoeppner , G.P. Tandon and K.V. Pochiraju

360 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju or interphase region are a function of the interaction of the fiber and matrix during the manufacturing process,the chemical compatibility of the fiber,and the sizing(if applicable)on the fiber. To deal with the aforementioned complex interactions,modeling is expected to incorporate mechanisms active at multiple scales (constituent, microstructural,lamina,and laminate scales)and multiple domains of res- ponse (chemical,thermal,and mechanical).The motivation is to develop robust,life-performance,prediction modeling capabilities for designers who currently use cost prohibitive experimentally based design allowables and use weight loss to evaluate the thermal-oxidative stability of material systems. In the face of such complexity and lack of knowledge regarding material behavior,industry currently relies on the use of environmental "knockdown factors"for the design of composite aerospace structures which operate in aggressive environments.One of the current methodologies used for determining environmental knockdown factors for designing composite aerospace structures is to determine reduced material allowables for each of the lamina properties (laminate ultimate strengths,notched strengths,fatigue,flexure properties,bearing strengths,etc.)for each of the material forms used in the structure.The material forms may include unitape, woven cloth,fiber preforms with mold filling,etc. The large variety of tests performed on a statistically sufficient number of replicates for the various environmental conditions adds tremendous cost to the materials qualification and design allowables process.In addi- tion,the worst case environmental extreme conditions are typically used for determining reduced materials allowables to help circumvent risk asso- ciated with the variability and inaccurate prediction of service environ- ments.In addition to the environmental knockdown factors,damage tolerance knockdown factors are also applied to the designs.The damage tolerance factors may be determined through damaging full-scale components and testing them for a predetermined number of life cycles in a hot/wet envi- ronment.Using this approach,the end-of-life properties are measured and used in the allowables determination process for designing components. The aerospace engine community (where high-temperature polymer matrix composites(HTPMCs)are most prevalent)often designs to end-of- life properties,and it is accepted that at the end of life for certain lightly I Although fiber-matrix interface and fiber-matrix interphase are often used interchangeably,we formally define the fiber-matrix interface as the two-dimensional surface defined by the common fiber and matrix surfaces.We define the fiber- matrix interphase as the three-dimensional matrix region directly around the fiber that may have properties distinct from the bulk matrix properties

or interphase region1 are a function of the interaction of the fiber and matrix during the manufacturing process, the chemical compatibility of the fiber, and the sizing (if applicable) on the fiber. To deal with the aforementioned complex interactions, modeling is expected to incorporate mechanisms active at multiple scales (constituent, microstructural, lamina, and laminate scales) and multiple domains of res￾ponse (chemical, thermal, and mechanical). The motivation is to develop robust, life-performance, prediction modeling capabilities for designers who currently use cost prohibitive experimentally based design allowables and use weight loss to evaluate the thermal-oxidative stability of material systems. In the face of such complexity and lack of knowledge regarding material behavior, industry currently relies on the use of environmental “knockdown factors” for the design of composite aerospace structures which operate in aggressive environments. One of the current methodologies used for determining environmental knockdown factors for designing composite aerospace structures is to determine reduced material allowables for each of the lamina properties (laminate ultimate strengths, notched strengths, fatigue, flexure properties, bearing strengths, etc.) for each of the material forms used in the structure. The material forms may include unitape, woven cloth, fiber preforms with mold filling, etc. The large variety of tests performed on a statistically sufficient number of replicates for the various environmental conditions adds tremendous cost to the materials qualification and design allowables process. In addi￾tion, the worst case environmental extreme conditions are typically used for determining reduced materials allowables to help circumvent risk asso￾ciated with the variability and inaccurate prediction of service environ￾ments. In addition to the environmental knockdown factors, damage tolerance knockdown factors are also applied to the designs. The damage tolerance factors may be determined through damaging full-scale components and testing them for a predetermined number of life cycles in a hot/wet envi￾ronment. Using this approach, the end-of-life properties are measured and used in the allowables determination process for designing components. The aerospace engine community (where high-temperature polymer matrix composites (HTPMCs) are most prevalent) often designs to end-of￾life properties, and it is accepted that at the end of life for certain lightly 1 Although fiber–matrix interface and fiber–matrix interphase are often used interchangeably, we formally define the fiber–matrix interface as the two-dimensional surface defined by the common fiber and matrix surfaces. We define the fiber– matrix interphase as the three-dimensional matrix region directly around the fiber that may have properties distinct from the bulk matrix properties. 360 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju

Chapter 9:Predicting Thermooxidative Degradation 361 loaded structures,the composites may have cracks.On the other hand,for highly loaded or critical structures,the presence of cracks,even at the end of life,is not accepted.This traditional approach for determining material design allowables,taking into account extremes in-service environments, requires significant testing (cost and schedule).For cost savings and risk mitigation,designers often resort to previously qualified materials,thereby negating the potential benefits of new material advancements.By develop- ing a life prediction/performance modeling capability,the cost and time associated with developing material design allowables can provide oppor- tunities for the insertion of new advanced materials. This chapter focuses on predicting the performance of PMCs in high- temperature environments where thermooxidative conditions are extreme and the material is used near its tolerance limits.For the current design methodology used by aerospace structure designers,composites are con- sidered to be "chemically static,"ignoring the evolution of the chemical/ environmental degradation and nonelastic mechanical response.How- ever,the empirical design allowables are based on the expected end-of life properties by testing specimens that have been aged under representative service environments for the design life. This chapter outlines the current state of the art,the shortcomings of existing capabilities,and future challenges for addressing modeling needs for PMCs in high-temperature oxidative environments.It is believed that the approach presented provides the methodology to accurately model both short-and long-term environmental effects on composite laminates for facilitating design allowables generation (including history-dependent failure prediction)and for predicting life expectancy(degradation state)for fielded composite structures. A valuable resource for understanding the high-temperature behavior of PMCs is the tremendous volume of long-term high-temperature aging data generated in the NASA High-Speed Research (HSR)program.The HSR effort was a national effort to develop the next-generation supersonic passenger jet designed for a 60,000-h life with temperatures approaching 177C.To meet the vehicle requirements,PMCs with high glass transition temperatures T and,thus,high-temperature use capabilities were thermo- mechanically loaded for very long-aging times. The long-term testing conducted in this program demonstrates the challenge of using PMCs in high-temperature environments.An important contribution to understanding the high-temperature performance of the polymer composites evaluated in the program is the use of viscoelastic formulations to model long-term behavior [21,42,49,68,115,116].The assumption inherent in the use of these viscoelastic formulations is that the material is chemically static or the original chemical structure is thermally

loaded structures, the composites may have cracks. On the other hand, for highly loaded or critical structures, the presence of cracks, even at the end of life, is not accepted. This traditional approach for determining material design allowables, taking into account extremes in-service environments, requires significant testing (cost and schedule). For cost savings and risk mitigation, designers often resort to previously qualified materials, thereby negating the potential benefits of new material advancements. By develop￾ing a life prediction/performance modeling capability, the cost and time associated with developing material design allowables can provide oppor￾tunities for the insertion of new advanced materials. This chapter focuses on predicting the performance of PMCs in high￾temperature environments where thermooxidative conditions are extreme and the material is used near its tolerance limits. For the current design This chapter outlines the current state of the art, the shortcomings of existing capabilities, and future challenges for addressing modeling needs for PMCs in high-temperature oxidative environments. It is believed that the approach presented provides the methodology to accurately model both short- and long-term environmental effects on composite laminates for facilitating design allowables generation (including history-dependent failure prediction) and for predicting life expectancy (degradation state) for fielded composite structures. A valuable resource for understanding the high-temperature behavior of PMCs is the tremendous volume of long-term high-temperature aging data generated in the NASA High-Speed Research (HSR) program. The HSR effort was a national effort to develop the next-generation supersonic passenger jet designed for a 60,000-h life with temperatures approaching 177°C. To meet the vehicle requirements, PMCs with high glass transition temperatures Tg and, thus, high-temperature use capabilities were thermo￾mechanically loaded for very long-aging times. The long-term testing conducted in this program demonstrates the challenge of using PMCs in high-temperature environments. An important contribution to understanding the high-temperature performance of the polymer composites evaluated in the program is the use of viscoelastic formulations to model long-term behavior [21, 42, 49, 68, 115, 116]. The assumption inherent in the use of these viscoelastic formulations is that the material is chemically static or the original chemical structure is thermally methodology used by aerospace structure designers, composites are con￾sidered to be “chemically static,” ignoring the evolution of the chemical/ environmental degradation and nonelastic mechanical response. How￾ever, the empirical design allowables are based on the expected end-of life properties by testing specimens that have been aged under representative service environments for the design life. Chapter 9: Predicting Thermooxidative Degradation 361

362 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju recoverable [98].Although these formulations are an important aspect of the overall methodology to model the performance of HTPMCs,a com- prehensive review of such is beyond the scope of the current discussion. The discussion here is limited to modeling the thermooxidative aging of HTPMCs and the role of the various constituents.Although hygrothermal degradation,which is a significant concern for HTPMCs,is not speci- fically addressed here,the methodology described herein can be applied to modeling hygrothermal degradation behavior.As with oxidation,hygro- thermal degradation involves transport and chemical reactions of the polymer with the diffused media.The primary challenge for modeling the coupling of oxidative and hygrothermal degradation of HTPMCs is an understanding of the numerous degradation mechanisms. Several major aging mechanisms that lead to weight loss and damage growth and,hence,degradation of the performance of the polymer resin, fibers,and their composite can be identified.They are: Physical aging.The thermodynamically reversible volumetric response due to slow evolution toward thermodynamic equilibrium is identified as physical aging.The decreased molecular mobility and free-volume reduction lead to strain and damage development in the material. Chemical aging.The nonreversible volumetric response due to chain scission reactions and/or additional crosslinking,hydrolysis,deploy- merization,and plasticization is classified as chemical aging.A dominant chemical aging process for HTPMCs is thermooxidative aging-which is the nonreversible surface diffusion response and chemical changes occurring during oxidation of a polymer (hence a modality of chemical aging).The oxidative aging may lead to either the reduction in molecular weight as a result of chemical bond breakage and loss in weight from outgassing of low molecular weight gaseous species,or chain scission and formation of dangling chains in polymer networks. Mechanical stress-induced aging.Mechanical and thermal fatigue loading cause micromechanical damage growth within the material. The damage evolution,in turn,exacerbates the physical aging and thermooxidative response of the material.This aging mechanism may be the least understood and least modeled by researchers. The capability to predict the performance of HTPMCs for both primary and secondary structural applications is elusive.The highly coupled physi- cal,chemical,and mechanical response of these materials to the extreme hygrothermal environments provides formidable challenges to the com- posite mechanics community.Polymers (in particular amorphous polymers) have been studied from the standpoint of physical aging [98],chemical aging [66],and strain-dependent aging [118].Although models to predict

recoverable [98]. Although these formulations are an important aspect of the overall methodology to model the performance of HTPMCs, a com￾prehensive review of such is beyond the scope of the current discussion. The discussion here is limited to modeling the thermooxidative aging of HTPMCs and the role of the various constituents. Although hygrothermal degradation, which is a significant concern for HTPMCs, is not speci￾fically addressed here, the methodology described herein can be applied to modeling hygrothermal degradation behavior. As with oxidation, hygro￾thermal degradation involves transport and chemical reactions of the polymer with the diffused media. The primary challenge for modeling the coupling of oxidative and hygrothermal degradation of HTPMCs is an understanding of the numerous degradation mechanisms. Several major aging mechanisms that lead to weight loss and damage growth and, hence, degradation of the performance of the polymer resin, fibers, and their composite can be identified. They are: – Physical aging. The thermodynamically reversible volumetric response due to slow evolution toward thermodynamic equilibrium is identified as physical aging. The decreased molecular mobility and free-volume reduction lead to strain and damage development in the material. – Chemical aging. The nonreversible volumetric response due to chain scission reactions and/or additional crosslinking, hydrolysis, deploy￾merization, and plasticization is classified as chemical aging. A dominant chemical aging process for HTPMCs is thermooxidative aging – which is the nonreversible surface diffusion response and chemical changes occurring during oxidation of a polymer (hence a modality of chemical aging). The oxidative aging may lead to either the reduction in molecular weight as a result of chemical bond breakage and loss in weight from outgassing of low molecular weight gaseous species, or chain scission and formation of dangling chains in polymer networks. – Mechanical stress-induced aging. Mechanical and thermal fatigue loading cause micromechanical damage growth within the material. The damage evolution, in turn, exacerbates the physical aging and thermooxidative response of the material. This aging mechanism may be the least understood and least modeled by researchers. The capability to predict the performance of HTPMCs for both primary and secondary structural applications is elusive. The highly coupled physi￾cal, chemical, and mechanical response of these materials to the extreme hygrothermal environments provides formidable challenges to the com￾posite mechanics community. Polymers (in particular amorphous polymers) have been studied from the standpoint of physical aging [98], chemical aging [66], and strain-dependent aging [118]. Although models to predict 362 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju

Chapter 9:Predicting Thermooxidative Degradation 363 the aging response of polymers are available and the fiber response can be assumed to be elementary,the ability to model the thermooxidative aging of the fiber-matrix system (presence of fiber-matrix interface/interphase region complicates the issue with a coupling or sizing agent)is lacking. While the time-dependent physical,chemical,and damage-induced de- gradation mechanisms have been studied for some resin systems,polymer composite thermal oxidation studies from a mechanistic perspective are nascent.Notable exceptions to this are the recent works of the groups of Colin and Verdu [27],Colin et al.[32],Skontorp et al.[97],Wang et al. [1211,and Pochiraju and Tandon [73,741,Tandon et al.[1041,and past work by Wise et al.[125],Celina et al.[22],and McManus et al.[621. Equally important are the experimental characterization efforts by such groups as Bowles et al.[14,19],Tsuji et al.[113],Abdeljaoued [1],Johnson and Gates [49],and Schoeppner et al.[91].However,most literature is confined to thermal oxidation of neat polymer systems and there are limited studies that correlate the thermochemical decomposition of high-temperature polymers to the resulting mechanical performance.Such works include Meador et al.[63-65]and Thorp [107]for PMR-15 high-temperature composites and Lincoln [56]for 5250-4 bismaleimide(BMI)composites. Current emphasis is on the implementation and extension of multiscale models to represent the polymer behavior/properties as a function of the degradation state to include chemical degradation kinetics,micromechanical models with time-dependent polymer constitutive relationships,and ply- level and laminate level models to predict structural behavior.The behavior of the composite will,thus,be dependent on its current chemical,physical. and mechanical state as well as its service history.This multidisciplinary modeling approach will provide generalizable analytical and design tools for realistic prediction of performance,durability,and use life of HTPMCs. The following requirements are identified for the formulation of the predictive thermooxidation models: Appropriate mechanistic and kinetic modeling of polymer environ- mental degradation.For the highly crosslinked polyimide composite of interest,the reactivity of the end cap is often a primary concern [63, 1071.Although the primary,and in some cases secondary,oxidation and hydrolytic degradation mechanisms can be identified,determination of mechanisms up to the final state of degradation is difficult if not impossible.Predicting thermal,physical,and mechanical performance based on the chemical state of the polymer is currently impractical for all but the very simplest of polymer systems.In the absence of this pre- dictive capability,empirical correlation of the chemical state (if known) to mechanical properties is used to help define the constitutive models

the aging response of polymers are available and the fiber response can be assumed to be elementary, the ability to model the thermooxidative aging of the fiber–matrix system (presence of fiber–matrix interface/interphase region complicates the issue with a coupling or sizing agent) is lacking. While the time-dependent physical, chemical, and damage-induced de￾gradation mechanisms have been studied for some resin systems, polymer composite thermal oxidation studies from a mechanistic perspective are nascent. Notable exceptions to this are the recent works of the groups of Colin and Verdu [27], Colin et al. [32], Skontorp et al. [97], Wang et al. [121], and Pochiraju and Tandon [73, 74], Tandon et al. [104], and past work by Wise et al. [125], Celina et al. [22], and McManus et al. [62]. Equally important are the experimental characterization efforts by such groups as Bowles et al. [14, 19], Tsuji et al. [113], Abdeljaoued [1], Johnson and Gates [49], and Schoeppner et al. [91]. However, most literature is confined to thermal oxidation of neat polymer systems and there are limited studies that correlate the thermochemical decomposition of high-temperature polymers to the resulting mechanical performance. Such works include Meador et al. [63–65] and Thorp [107] for PMR-15 high-temperature composites and Lincoln [56] for 5250-4 bismaleimide (BMI) composites. Current emphasis is on the implementation and extension of multiscale models to represent the polymer behavior/properties as a function of the degradation state to include chemical degradation kinetics, micromechanical models with time-dependent polymer constitutive relationships, and ply￾level and laminate level models to predict structural behavior. The behavior of the composite will, thus, be dependent on its current chemical, physical, and mechanical state as well as its service history. This multidisciplinary modeling approach will provide generalizable analytical and design tools for realistic prediction of performance, durability, and use life of HTPMCs. predictive thermooxidation models: – Appropriate mechanistic and kinetic modeling of polymer environ￾mental degradation. For the highly crosslinked polyimide composite of interest, the reactivity of the end cap is often a primary concern [63, 107]. Although the primary, and in some cases secondary, oxidation and hydrolytic degradation mechanisms can be identified, determination of mechanisms up to the final state of degradation is difficult if not impossible. Predicting thermal, physical, and mechanical performance based on the chemical state of the polymer is currently impractical for all but the very simplest of polymer systems. In the absence of this pre￾dictive capability, empirical correlation of the chemical state (if known) to mechanical properties is used to help define the constitutive models. The following requirements are identified for the formulation of the Chapter 9: Predicting Thermooxidative Degradation 363