The hygrothermal aging characteristics of an epoxy resin were characterized over 1 year, which included 908 temperature and humidity cycles. The epoxy resin quickly showed evidence of aging through color change and increased brittleness. The influence of aging on the material's glass transition temperature (T g ) was evaluated by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). The T g remained relatively constant throughout the year long cyclic aging profile. The chemical composition was monitored by Fourier Transform Infrared Spectroscopy (FTIR) where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged. This property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions. This paper details the influence of processes such as: advancement of cure, chemical degradation, and physical aging on the chemical and physical properties of the epoxy resin. IntroductionEpoxy resins constitute an important class of matrix material for the aerospace industry. These materials are generally easy to process and can be tailored to provide a wide range of chemical and physical properties (Ref. 1). Composites based on epoxy resins are often used in structural applications where environmental conditions can include cyclic exposure to high heat and humidity. Repeated exposure to temperature/humidity cycles will be referred to as "hygrothermal aging" in this paper. Hygrothermal aging results in changes to the physical and mechanical properties of an epoxy resin (Ref. 2). When the epoxy resin is used as the matrix material in a composite, these changes in resin properties can contribute to microcracking and to reduced strength for matrix dominated composite failure modes.NASA/TM-2011-216999 2 Hygrothermal aging can induce permanent changes in an epoxy resin through non-reversible mechanisms that alter the chemical structure of the material. This process is called chemical aging. A change in the glass transition temperature (T g ) during aging is one indication of chemical aging in the form of continued cure or thermal degradation in the bulk resin. Chemical aging can also occur as oxidation (which is typically limited to exposed surfaces) and as hydrolysis (which is dependent on moisture absorption). There is a vast body of literature detailing the influence of thermal-oxidative aging on the chemistry of epoxy resins and epoxy based composites. It has been reported that thermal-oxidative environments can be detrimental to the integrity of the resin, and many papers report a reduction in T g with aging; attributed to oxidative chain scission within the structure (Refs. 3 and 4). The chemical reactions associated with oxidation have been extensively monitored by Fourier Transform Infrared Spectroscopy (FTIR), and a significant database of peak assignments has been collected (...

SummarySmart adaptive materials are an important class of materials which can be used in space deployable structures, morphing wings, and structural air vehicle components where remote actuation can improve fuel efficiency. Adaptive materials can undergo deformation when exposed to external stimuli such as electric fields, thermal gradients, radiation (IR, UV, etc.), chemical and electrochemical actuation, and magnetic field. Large strain, controlled and repetitive actuation are important characteristics of smart adaptive materials. Polymer nanocomposites can be tailored as shape memory polymers and actuators.Magnetic actuation of polymer nanocomposites using a range of iron, iron cobalt, and iron manganese nanoparticles is presented. The iron-based nanoparticles were synthesized using the soft template (1) and Sun's (2) methods. The nanoparticles shape and size were examined using TEM. The crystalline structure and domain size were evaluated using WAXS. Surface modifications of the nanoparticles were performed to improve dispersion, and were characterized with IR and TGA. TPU nanocomposites exhibited actuation for~2wt% nanoparticle loading in an applied magnetic field. Large deformation and fast recovery were www.nasa.gov 20 observed. These nanocomposites represent a promising potential for new generation of smart materials.

A notched coupon geometry was evaluated as a method for tensile testing of 2D triaxial braid composites. Edge initiated shear failure has been observed in transverse tension tests using straight-sided coupons based on ASTM D3039. The notched coupon was designed to reduce the effects of edge initiated failure and produce the desired tensile failure. A limited set of tests were performed with partial pressurization of tubes to determine the transverse tensile strength in the absence of edge initiated failure. The transverse strength measured with the notched coupons was considerably higher than the straight-sided coupons, comparable to the tube results, and closer to the maximum possible strength based on maximum fiber strain. Further investigations of the effects of the observed biaxial stress state and stress concentrations in the notched geometry are needed.

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