Accelerated aging effects on carbon fiber/epoxy composites
Introduction
Carbon fiber reinforced polymer (CFRP) composites have emerged as the materials of choice in many aeronautic structures due to their combination of properties, which include high strength and modulus-to-weight ratio and corrosion resistance. With the ever-increasing use of these materials, the influence of environmental service conditions on their durability has become an ongoing concern to the aeronautic industry and airline companies. Such structures must be designed to withstand the operational conditions imposed throughout the service life of the structural component. Thus, the influence of environmental factors such as moisture, temperature and UV radiation on the durability, as well as the reliability and safety of CFRP aeronautic structures must be better understood and considered in the design.
Traditionally, polymer aging has been classified into two main categories: physical and chemical aging. Physical aging [1] may be characterized by changes in molecular conformation of the material without changing the structural integrity of the molecules. This type of aging process is therefore reversible. It is the manifestation of a slow evolution of the polymer towards thermodynamic equilibrium with time dependent changes in volume, enthalpy, entropy and mechanical properties. Physical aging may occur during long time exposures of the material to elevated temperatures below the Tg. The changes in molecular conformation result in increases in Young's modulus, yield stress, density, viscosity, and reductions in creep and stress relaxation. Previous studies [2] have shown that physical aging may cause material embrittlement as well. In the case of chemical aging, there is an irreversible degradation of the molecular structure caused by mechanisms such as chain scission, changes in crosslink density, oxidation and depolymerization. In most applications, the two types of aging (physical and chemical) occur simultaneously.
High temperatures are known to accelerate the degradation process of polymers [3]. Also, thermal cycling is known to produce cracks in laminates and crack growth may be accelerated in oxidative atmospheres [4]. Elevated air and oxygen pressures accelerate the rate of thermo-oxidative degradation in polymeric composite materials as compared to samples tested at atmospheric pressure [5].
Light exposure may increase the concentration of free radicals in the polymer, which is a sign of degradation [6]. Photodegradation initiated by solar radiation results in absorption of UV radiation by chromophores and in activation of excited states in macromolecules. This degradation process may affect mechanical properties of polymeric films due to chain scission and crosslinking of macromolecules [7].
The consequences of water absorption on polymer properties have been investigated in many studies and some of the effects reported include plastification, a reduction in glass transition temperature, an increase in creep and stress relaxation, and a decrease in mechanical strength and elastic modulus [8], [9], [10], [11], [12], [13]. The absorbed moisture may also react with unreacted groups of the epoxy resin structure and therefore influence the aging process [14]. For composites without protective painting, UV-radiation may degrade the polymeric surface of the composite, thereby exposing the fibers while further increasing moisture absorption [15], [16]. This combination of moisture absorption and UV degradation may produce significant effects on mechanical properties, particularly on elastic modulus and interlaminar shear strength [15], [16].
In a previous study of the mechanisms of hydrothermal degradation in hot-wet conditions of graphite-epoxy laminates, interlaminar shear strength tests indicated that the combination of high relative humidity with high temperature may produce a significant reduction in strength [15]. In another study, hydrothermal aging of carbon-epoxy caused a decrease in modulus and strength, an increase in strain to failure and a reduction in wear resistance [17]. Hydrothermal cycling has also been reported as causing delamination [18].
Aging is normally associated with the deterioration of CFRP properties, although slight improvements in mechanical properties as a result of aging have also been reported. In one of these studies reported in the literature, interlaminar Mode I fracture toughness of a carbon-epoxy composite showed a variable trend towards slight decreases or slight increases caused by hydrothermal aging [19]. Another study [20] suggests that tensile and compressive properties of carbon-epoxy composites may improve during aging at the initial consolidation stage. In this case, the improvement in mechanical properties was attributed to post-curing reactions. A degradation stage followed when the composite properties are significantly decreased; this is attributed to a deterioration of the matrix and weakening of the fiber-matrix interface.
Although many studies regarding aging of carbon-epoxy composites are available in the literature, the effects of aging and the extent of those effects on properties of these materials are not fully understood. In this study, carbon-epoxy composites were exposed to alternating cycles of UV-A radiation and water condensation in an accelerated aging chamber, and the effects of aging on the material were evaluated by Fourier-Transform Infrared (FTIR) Spectroscopy, mechanical tests (interlaminar shear strength and compressive strength), Scanning Electron Microscopy (SEM) and also in terms of mass variation.
Section snippets
Experimental
The material tested in this investigation was HexPly® AS4/8552, a carbon/epoxy unidirectional prepreg tape from Hexcel Corporation, with a fiber volume fraction of 57% and a fiber weight ratio of 65%. The laminates were vacuum bagged and cured in an autoclave using a two-step cure cycle based on manufacturer specifications. The temperature was first increased to 110 °C and held for 1 h at a pressure of 700 kPa. Then, the temperature was increased to 180 °C and held for 2 h at the same pressure,
Mass variation
Mass loss caused by accelerated aging is shown in Table 1.
All samples showed mass loss caused by accelerated aging. Possible reasons for the difference in mass loss of ILSS and compression samples are the presence of tabs on compression samples and the difference in exposed area of material in relation to sample volume.
Scanning Electron Microscopy (SEM)
SEM analysis allowed for observing the consolidation of the laminate layers and carbon fiber/polymer matrix distribution along the cross-sectional area. Fig. 1 (a) shows the
Conclusions
The aim of this study was to investigate the effects of aging on carbon fiber-epoxy samples exposed to alternating cycles of water condensation at 50 °C and UVA radiation at 80 °C for a period of 3 months. All samples showed mass loss caused by the aging procedure. SEM investigations show matrix loss and degradation of the material at the fiber/matrix interface, with change in surface texture and exposure of carbon fibers of the aged material. FTIR Spectroscopy suggests chemical changes in the
Acknowledgements
The authors would like to acknowledge the Brazilian funding agencies CNPq and CAPES for financial support.
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