Long-Term Durability of Polymeric Matrix Composites

This chapter focuses on the chemistry of polymer resins and the changes that occur when resins are exposed to environmental factors that can cause degradation. The exposures that are considered are elevated temperatures (with or without oxygen); contact with water and other fluids; radiation; and mechanical loads. Some general observations about the types of effects to be expected for each exposure condition are outlined first. Then, the chemistries of the various classes of resins that are in current use as matrices for high-performance composites are described. Separate sections treat epoxies, bismaleimides, PMR-type thermosets, phenyethynyl-terminated imides, and high-temperature thermoplastics. For each resin type, the formulation and curing are briefly discussed. This discussion is followed by a review of the available literature on mechanisms of long-term degradation as determined by spectroscopy, chromatography, and other analytical techniques. Consequences to constituent properties are also described: increases or decreases in glass transition temperatures; shrinkage and cracking; and changes in mechanical stiffnesses, strengths, and toughnesses.

Material use is often governed by the properties it brings to an application, but in some cases those desirable properties are rapidly degraded by their intended use environment. Heat, abrasion caused by part-on-part wear, and particulate impact can damage material properties, especially polymer–fiber-reinforced composites. To ensure that the benefits of polymer composites can be utilized in these extreme environments, protection is needed. The simplest form of protection is the use of additives to the polymer matrix, such as antioxidants, thermal stabilizers, and flame retardants. Of newer interest is the use of nanocomposite technology, which provides enhanced thermal and mechanical durability, which sometimes brings multifunctional performance to the composite. Barrier coatings represent an engineering solution to protect the composite part, but newer research focuses on incorporation of the barrier coating during composite fabrication so that the protection is engineered to be a covalently bound part of the polymer rather than a post-fabrication add-on coating produced via painting or adhesive bonding. This chapter provides a survey of the broad range of protection solutions available for composites, with an emphasis on approaches that yield thermal and/or abrasion protection in polymer composites.

During the cure of thermosetting polymer–matrix composites, the presence of reinforcing fibers significantly alters the resin composition in the vicinity of the fiber surface via several microscale processes, forming an interphase region with different chemical and physical properties from the bulk resin. The interphase composition is an important parameter that determines the composite micromechanical properties and the durability of the products in service. Historically, the description of the interphase has been rather empirical and because of the complexities of the molecular level mechanisms near the fiber surface, few studies have been carried out on the prediction of the interphase evolution as function of the process parameters. This chapter provides an overview of the existing thermodynamic and kinetics-based models for describing the interphase in composites and related experimental studies. The models are used to develop the processing–interphase and interphase–property linkages, which form the basis for guiding the process design for tailored composite properties and performance.

Damage treated here is a multitude of surfaces formed within a composite that permanently change its response to external impulses. Examples of such damage are matrix cracking at different scales, fiber breakage, fiber/matrix debonding, and interply cracking (delamination). The response affected could be mechanical (stiffness properties), thermal (expansion and conductivity), time-dependent (viscoelastic), and in general any that is sensitive to the presence of internal surfaces. The representation of damage is by internal variables and the response functions are formulated in a thermodynamics framework. Although specific cases considered here are composite laminates with multiple sets of intralaminar cracks, the formulation has sufficient generality to treat other composite configurations and other energy-dissipating mechanisms such as thermal oxidation and radiation-induced morphological changes in polymers.

The damage tolerance of composite structures is intimately linked to the morphology and extent of flaws or damage. Impacts are a commonly occurring source of threat to composites that can produce a “seed” damage state which inherently controls the subsequent durability and damage tolerance of the affected structure. Transverse impact to composites is of particular concern due to the possibility of exciting damage modes that are difficult, or even not possible, to visually detect from the exterior (impact-side) surface. Some examples of such damage are delamination, backside-only fiber failure, debonding of internal substructure (e.g., stringers and stiffeners, doublers, joints), and crushing and separation of sandwich panel core. Impact damage is highly dependent upon the nature of the threat and conditions associated with the impact event. This chapter will provide an overview of impact damage threats that are common to composite aircraft structures and describe the relationship of these threat sources to the damage that is seeded. An historical overview of the impact damage tolerance methodologies developed by military aircraft programs, and subsequently widely adopted across the composite structures community, is also provided.

Composites provide advantages over conventional structural upgrade systems by offering up to 50% first-cost savings and lower life-cycle costs, often with additional benefits such as easier installation and improved safety. Fiber-reinforced polymer (FRP) composites are finding increasing applications as primary structural components in aerospace and automotive applications, bridges, building repair, and the oil and gas pipeline industry. These composites are typically exposed to a variety of aggressive environments, such as extreme temperature cycles, ultraviolet (UV) radiation, moisture, alkaline/salt environments, etc. However, no capability currently exists for reliably projecting the future state and conditions of composites used in various environments. The accurate determination of diffusivity and moisture uptake in a polymer composite is a key step in the accurate prediction of moisture-induced degradation. With this in mind, the chapter is subdivided into three sections: (1) the combined influence of damage and stress on moisture diffusion within the (bulk) polymer matrix in a polymer composite, (2) the combined influence of strain gradient, relative humidity, and temperature on moisture diffusion at the fiber–matrix and/or interlaminar interface, and (3) a simple mechanism-based model to predict strength degradation in a composite due to moisture ingress. The discussions presented in this chapter are primarily directed toward thermoset resins, such as epoxy.

Physical aging is observed in all glassy materials because of the fact that they are out of equilibrium. The ways in which aging manifests itself are the results of the thermal history of the materials, the environment, and even the constraint of, e.g., fibers or particles. In the present chapter, the fundamentals of aging of glasses are summarized by considering first structural recovery, which is the kinetics of the thermodynamic-type variables such as volume or enthalpy, and its impact on the mechanical response, which is the physical aging. Linear viscoelastic and nonlinear viscoelastic properties as well as yield behaviors will be considered. Furthermore, we will consider environmental effects on physical aging behaviors. The work will end with a perspective on aging in composites and where further research is needed.

It is now well recognized that during thermal aging at moderate temperatures, for example, typically below the glass transition temperature, organic matrix composites perish mainly by matrix embrittlement resulting from its thermo-oxidation. The present chapter aims to briefly introduce this domain. The chapter consists of a brief history of polymer oxidation and description of mechanisms and kinetics. The radical character of oxidation processes; the main elementary steps: propagation, termination, initiation processes, and initial steps; structure–property relationships; the nature of oxidation products; and experimental methods for the study of oxidation mechanisms are also discussed. The standard kinetic scheme, case of oxygen excess and general shape of oxidation kinetic curves, the induction period, departure from Arrhenius law, and case of oxygen lack are described. Consequences of oxidation on matrix thermomechanical properties including chain scission and cross-linking physical approaches are presented.

An understanding of the effects of thermo-oxidation in high-temperature PMCs for structural components subjected to arbitrary service environments is critical to life-performance predictions. Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. The thermo-oxidative behavior of the composite is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphases/interfaces, architecture, and ply layup introduce anisotropy in the diffusion and oxidation behavior. In this work, light microscopy and scanning electron microscopy techniques are used to characterize the oxidative process in laminated and textile carbon-fiber-reinforced polyimide composites. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage through the use of fluorescence imaging using dye impregnation. It is shown that alternative pathways for transport of oxygen into the interior of the composite are fiber–matrix debonds and matrix cracks that propagate with the oxidation front. It has been further determined through closer examination of the oxidation front and the crack front for discrete regions of the various composite specimens, that the oxidation front consistently precedes the crack front. This mechanism for accelerated oxidation is an excellent example of the intrinsic coupling of chemical oxidative aging and damage, which needs to be properly represented in predictive models.

Long-term durability and use-life of polymeric matrix composites operating at elevated temperatures are limited by their thermo-oxidative stability. Although weight loss testing is traditionally performed to characterize the oxidative degradation of composite systems, the results of such tests are neither translatable to other composites architectures with the same constituents nor scalable to longer exposure times or higher temperatures. A comprehensive modeling framework for understanding the morphological changes in the composites and degradation of the mechanical performance is described in this chapter. A thermo-chemo-mechanics model that defines and utilizes an oxidation state parameter for each constituent is formulated. The effect of oxygen diffusion in the fiber and fiber–matrix interphase on the oxidation of the composite is simula-ted. The role of damage in accelerating the oxidation growth along the fiber direction leading to high orthotropy in lamina oxidation is also addressed. The stiffness changes due to oxidation as well as the strains induced due to shrinkage are explicitly modeled leading to a detailed simulation of oxidation growth around discrete cracks. Oxidation growth in laminated composites is predicted using microscale and homogenization techniques. The model is applied to study the long-term thermal oxidation of polyimide composites.

An overview of the durability design requirements and substantiation approaches, with an emphasis on aerospace applications, is provided in this chapter. The service history of both commercial and military composite aircraft structures has provided numerous examples of both good and bad designs from which current design specification and guidance are derived. Issues to be considered in the design of composites used in primary and secondary composite structures including corrosion prevention measures associated with joining composite and metallic components are described. Design details, material selection, and demonstration that designs meet performance criteria are dependent on a structure’s thermal and mechanical loading environments, service and economic life requirements, manufacturing constraints, and inspectability requirements. Based on an assessment of the durability of composite materials at the coupon level through modeling and testing, a description of the building block approach used to validate durability predictions of elements, subcomponents, components, and full-scale structural testing is given. The unique requirements for developing load spectra for accelerated full-scale durability testing for both composite and combined composite/metallic structures are discussed.

Structural joints are where the durability, or lack of durability, is usually most evident in composite structural response to combined environmental effects and mechanical loads. The overall durability or degradation of a structural joint is a function of the response of all of the constituents of the joint: individual joined members, fasteners and/or adhesive, shims, sealants, coatings, etc. The objective of this chapter shall be to provide a brief overview of observed durability behavior of composite structural joints, experimental techniques for exploring the durability of joints, analytical predictive methods, and certain empirical case studies, mainly from the aerospace industry. The chapter is logically divided into separate bolted and bonded sections.

The use of fiber-reinforced composites in aircraft structural components has significantly increased in the past few decades due to their improved specific strength and stiffness and superior resistance to both corrosion and fatigue with respect to their metal counterparts. Furthermore, current economic conditions require the use of most military and commercial aircraft beyond their original design service objectives; therefore, it is necessary to understand composite aging and in-service-induced damage to ensure the airworthiness and structural integrity of these airframes. Most aging aircraft studies conducted thus far have focused on metallic structures; however, as more composite components are being certified and used on aircraft structural components, it is necessary to address this aging concern for composite components as well. The primary objective of this chapter is to summarize the findings of the teardown conducted on a B-737 CFRP composite stabilizer after 18 years of service. The B-737-200 stabilizer was developed by Boeing as part of the NASA ACEE program initiated in July 1977. Five B-737 horizontal stabilizer shipsets were manufactured and certified in August 1982. As of March 2011, three shipsets have been retired from service, one has been sold to a foreign carrier, and one is owned by a commercial aircraft part supplier and has been reported for sale. This chapter provides highlights of the ACEE program, a summary of the B-737 horizontal stabilizer teardown activities, and results of the aging study. Results found indicate that the composite structure maintained its structural integrity over its service life and did not show significant degradation or detrimental signs of aging.

Advancements in polymer matrix composites have made them attractive to developers of power and propulsion equipment for spaceflight and aeronautic applications. However, many of these applications have very unique operational environments that are not easily found in the available design databases. Rapid insertion of these materials through prototype development and concept demonstration programs are hampered by the absence of relevant design data. In such cases, development programs conducted by the NASA Glenn Research Center have found it beneficial to employ pathfinder experimental methods designed to focus on the specific application and operational environment at hand. This chapter describes specialized experimental investigations of composite durability for applications that include flywheel energy storage, combustion chamber, and fan case structures. The experiments were designed to investigate complex thermomechanical and hygrothermal environments posed by these technologies. Beyond cycles-to-failure and residual strength, dimensional stability and stiffness degradation are key, if not primary, concerns in preserving functional capability for the cited applications.

Aircraft engine components are subjected to high temperatures and complex stress states, thus requiring a much more rigorous assessment of durability. This chapter provides a brief introduction to durability assessment of polymer–matrix composites in gas turbine engine applications. The engine environment exposes the composite material to combined missions of hygrothermal exposure, mechanical loading, and fatigue. The complexity of the actual engine conditions is not easily duplicated in the laboratory and simplified simulated exposure missions are often used for durability characterization. Full-term durability testing to determine end-of-life properties is also not practical and accelerated testing and modeling is often considered as an alternative. Thus, a combined experimental and analytical approach is typically used to characterize durability of engine composites. A specific example of a woven composite material is described in this chapter to demonstrate how simple mechanics-based modeling can be used to study the evolution of the different damage mechanisms under fatigue and to predict fatigue life. Finally, the building block approach used to characterize the durability of engine components is discussed along with a specific example of its application.