Accelerated thermal ageing of epoxy resin and 3-D carbon fiber/epoxy braided composites
Introduction
In the design of the new generation of aircraft structures, three-dimensional (3-D) braided composite is currently widely used due to its excellent through-thickness properties, high damage tolerance and fatigue resistance [1]. The aerospace applications require a long service life of materials, in particularly in harsh environments (temperature, oxidizing environment, etc.). Hence, the effects of thermal ageing on the mechanical properties of the 3-D braided composites are considered vitally important for aircraft design.
Currently, the studies have mainly concentrated on characterizing degradation and damage initiation. Tsotsis [2], [3], [4], [5] found that the weight loss during thermo-gravimetric test could not be used as criteria for material acceptance. The properties’ changes, such as mechanical properties [6], [7], [8], [9], [10], [11], [12], physical properties [13], thermal conductivity [14] and the development of microcracks [15], [16], [17] caused by thermal treatment have been extensively studied and documented. Compared with isothermal ageing, thermal cycling can accelerate the damage processes and especially the matrix crack propagation from the surfaces to the core of the laminate [18].
It was found that there are two step changes in the mechanical properties under thermal ageing [19]. In the first consolidation stage, owing to the post-curing reaction, there are improvements in the mechanical properties. In the second degradation stage, the mechanical properties decreased significantly. For pure epoxy, the thermo-oxidative ageing can be characterized by three phases [20]. The first phase is dominated by the polymer viscoelastic behavior and by stress relaxation at high temperature. The second phase is characterized by thermo-oxidative matrix shrinkage and change of the instantaneous matrix elastic modulus with time. The third phase is attributed to the development of microcracks.
In the thermal degradation of composite materials, the oxygen pressure is an accelerating factor [21], [22], [23]. The thermal ageing process was controlled by the oxygen diffusion and the thickness of oxidized layer [24], [25], [26]. Instrumented ultra-micro indentation was used to characterize the mechanical property of an oxidized epoxy polymer. And the experimental measures agreed well with the predicted values of an oxidation model [27], [28].
The degradation mechanism also strongly depends on the specimen geometry and anisotropy [29], [30]. Nam and Seferis [31] found that the property degradation was significantly dependent on the fiber orientation pertaining to the composite anisotropy. The work of Mlyniec et al. [32], similarly, concluded that the alignment of the reinforcing fibers would affect long term damping performance of the carbon/epoxy composites. Stability of the modal damping of unidirectional carbon/epoxy laminates is affected mainly by the properties of the fiber–matrix interface, while the quasi-isotropic laminates depends mainly on long-term properties of the matrix. Surfaces with different microstructural characteristics, on the other hand, could be expected to exhibit different oxidation behavior [11], [33].
The degradation is often investigated under accelerated conditions at elevated temperatures. As described above, the mechanical properties of composite are more dependent on the matrix and thus more sensitive to thermal ageing. In this paper, both pure epoxy resin and three-dimensional carbon/epoxy braided composites were exposed to an isothermal high-temperature environment. Their micro-morphologies and compressive behaviors after thermal ageing were presented. From the experimental results, we also investigated the influence of the temperature on the thermal ageing mechanisms.
Section snippets
Materials and specimens
The materials used in this study were T700S-12K carbon fiber supplied by Toray Inc. (Japan) and JA-02 epoxy resin supplied by Changshu Jiafa Chemical Inc. (China). The 3-D braided preform (shown in Fig. 1) with square cross section of 11 × 11 braiding yarn arrays, was manufactured with a four-step 1 × 1 braiding technique. Epoxy resin was injected into the braided preform with vacuum assisted resin transfer molding (VARTM) technique. Curing process followed a stepwise program of 90 °C for 2 h, 110 °C
Color change
Figs. 4 [34] and 5 show the color change of specimens during the thermal ageing. The epoxy cube turned black when exposure time reached 16 days at 180 °C, while there is no significant change in the specimens aged at 90 °C and 110 °C (Fig. 4a). Fig. 4b shows that the oxidized layer, which is indicated by the darker region, is distinctly visible near the surfaces exposed to air. The inner core of epoxy cubes might be protected from oxidation and thus appeared a lighter color. Compared with the pure
Conclusions
The thermal ageing of pure resin and 3-D carbon fiber/epoxy braided composites were investigated experimentally. Five ageing temperature points were selected according to the glass transition temperatures of pure resin (about 110 °C) and composite (about 130 °C). The specimens have been exposed to air for 1, 2, 4, 8, and 16 days at 90 °C, 110 °C, 120 °C, 130 °C and 180 °C, respectively. The micro-observation technologies and ATR-FTIR spectroscopy were conducted to illustrate the microstructure changes
Acknowledgements
The authors acknowledge the financial supports from the Chang Jiang Scholars Program and National Science Foundation of China (Grant Number 11272087 and 11572085). The financial supports from Foundation for the Fok Ying Tong Education Foundation (Grant No. 141070), Shu-Guang project (Grant No. 14SG31) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation, the Fundamental Research Funds for the Central Universities of China and DHU Distinguished Young
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