Hygroscopic aspects of epoxy/carbon fiber composite laminates in aircraft environments

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Abstract

In this study, various hygroscopic effects of such parameters as hygrothermal temperature, matrix volume ratio (Vm), void volume ratio (Vv), specimen thickness, lay-up sequence and internal stress were investigated for epoxy/carbon fiber composite laminates. The specimen thickness and lay-up sequence had little effect on the through-the-thickness water absorption behavior of composite laminates, but the other parameters affected the moisture absorption rate and equilibrium water uptake in different ways and intensities. The glass transition temperature of composite laminates was strongly affected and linearly decreased by the quantity of equilibrium water uptake. A characteristic length of moisture migration through the unidirectional laminates was proposed as a function of fiber angle to the exposed laminate surface. In this approach, the fibers imbedded in the matrix were assumed to act as a barrier to the penetrating water molecules, and the developed model was well compared with the experimental results.

Introduction

The absorbed water molecules in polymer composite materials are known to have significant effects on their physical and chemical properties of matrix as well as on their final performance of composite structures especially in their long-term utilization. The absorbed water usually depresses the glass transition temperature Tg by plasticizing the polymer network and also affects mechanical performance and long-term durability of high-performance applications [1], [2], [3], [4], [5], [6]. Longitudinal properties dominated by fiber properties in the unidirectional composites may not drop so noticeably but such properties as compression strength and intralaminar/interlaminar shear strength are significantly affected by the absorbed moisture especially at high temperatures. The primary and secondary composite structures used in state of the art aircraft usually experience the repeated absorption/desorption of water in a wide range of humidity and temperature. This type of non-mechanical fatigue is considered to be closely associated with the long-term durability of composite materials especially when the water absorption is accompanied with high environmental variations of temperature and pressure as well as mechanical load variations during the service usage of aircraft. Accordingly, for the applications of high performance composite materials in aircraft environments, the composite materials have been strongly required to satisfy the hot–wet mechanical properties designated in the material specifications, where the specimens are tested after being exposed to water at high temperatures for specified time [7].

A change in temperature and moisture content usually induces hygrothermal forces and moment resultants as well as dimensional changes in the composite body. In addition, the thermal stresses produced during the cooling process of composite laminate after cure at elevated temperature could be so combined with those hygrothermal stresses induced moisture absorption [8], [9]. The resulting hygrothermal and mechanical stresses combined with each other may be sufficiently large enough to influence the failure of the laminate and thus should not be neglected in modern design analysis and lifetime estimation. Furthermore, the recursive changes of internal stresses due to water absorption–desorption processes may induce fatigue damage in the inter- and intralaminar region of composite laminates influencing long-term durability and performance of composite [10]. In addition, in the field of composite applications utilizing electric and electromagnetic characteristics of carbon fiber reinforced composite, the water absorption is one of key issues that should be addressed. The composite laminates exposed to water for a time usually swell and thus the interfacial characteristics and local stress concentration of matrix and fibers tend to change and then consequently influence the electric conductivity. Accordingly, the electric resistivity closely related with conductivity is known to be proportional to the square of the moisture weight gain in the longitudinal direction of epoxy/carbon fiber composite laminate [11]. Especially in such applications of aircraft radome fabricated with glass fiber reinforced composites or antenna with carbon fiber reinforced composites, the absorbed water can strongly influence the aircraft safety since the dielectric properties and the electromagnetic characteristics of these functional and safety-related composite structures can be easily deteriorated by the moisture absorbed in the structure [12].

The objective of this study is to investigate various aspects and concerns encountered by water absorption in the case of 177°C (350°F) curable unidirectional carbon fiber/epoxy composite systems that are widely and increasingly used as secondary and primary structures for aircraft applications. Focusing on the composite applications in aircraft environment, a significant depression of glass transition temperature due to the abnormal water uptake behavior was identified in hygrothermal temperature conditions. Various hygroscopic aspects of composite laminates were investigated by examining the effects of matrix volume ratio Vm, void volume ratio Vv, specimen thickness, lay-up sequence, applied bending load, etc. In addition, the characteristic diffusion length of water molecules was derived to predict the anisotropic diffusion rates for composite specimens with different fiber volume ratios and fiber angles.

Section snippets

Fick's second law of diffusion

For a relatively large and thin composite plate, the water concentration in the thickness z-direction may be governed by the one-dimensional diffusion equation of Fick's second law [13], viz.:c(z,t)t=Dz2c(z,t)2z(0≤z≤L,t>0)c=ci(0<z<L,t≤0)c=c(z=0,L,t>0)where t is time; L, thickness of specimen; z, distance measured from the bottom surface; subscripts ‘i’ and ‘∞’ represent ‘initial’ and ‘full saturated’ states, respectively; and Dz, moisture diffusion coefficient in the z-direction. The

Experimental

The prepreg system used in this study is T7G145/F584-4 (Hexcel Co., USA) carbon/epoxy unidirectional tape, specified by McDonnell Douglas as DMS2224 (Class 1, Type T, Grade 2). Several fundamental test requirements and test data for unidirectional prepreg DMS2224 are summarized in Table 1. In hygrothermal cycling studies, AS4/3501-6 (Hercules Co., USA) prepreg system has also been used. The prepregs are cured up to 177°C in the autoclave or hot-press by using a standard curing and consolidation

Results and discussion

Fig. 8(a) shows the normalized percent weight gain of the composite specimens as a function of time divided by the specimen thickness (t/L). It is observed that the moisture absorption behavior is exhibiting a linear relation in the initial stages of absorption, M/M<0.5, for isothermal water absorption temperatures of 35, 70, 85 and 95°C, respectively. The diffusion coefficients can be determined by Eq. (7) from the initial slopes of the linear relations, and the results are shown in Fig. 8(b)

Conclusions

Various aspects of hygrothermal effects on the carbon fiber/epoxy composites for aircraft applications have been investigated. Measuring moisture absorption and physical changes of composite laminates, it is observed that the glass transition temperature decreases linearly with the increment of moisture absorption. It is also found that the moisture absorption behavior is affected by the hygrothermal temperature history. The diffusion coefficient increases with the increment of matrix volume

Acknowledgements

This work was supported by GRANT No. KOSEF 96-0502-0601-2 from the Korea Science and Engineering Foundation.

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