Effect of specimen thicknesses on water absorption and flexural strength of CFRP laminates subjected to water or alkaline solution immersion

https://doi.org/10.1016/j.conbuildmat.2019.03.009Get rights and content

Highlights

  • Long-term aging test of CFRP laminates subjected to different solutions are made.

  • Effects of specimen thickness on aging test of CFRP laminates are discussed.

  • New analytical models of accelerated factors are proposed.

  • The proposed models are applied to predict long-term properties of CFRP laminates.

Abstract

In this paper, an experimental research was undertaken to investigate the effect of specimen thicknesses on water absorptions and flexural strengths of wet lay-up CFRP laminates subjected to distilled water or alkaline solution immersion up to 180 days. Test results showed that the water uptake and flexural strength retention of CFRP laminates were significantly affected by the adopted specimen thickness. For the same aging time, the water uptake of CFRP laminates decreased in the early stage of immersion and increased in the later stage of immersion with the increase of specimen thickness. Meanwhile, the flexural strength retention generally increased as specimen thickness increased. In addition, a new thickness-based accelerated method for hygrothermal aging test of CFRP laminates was proposed. The accelerated factors of the water uptake and flexural strength retention of CFRP laminates were theoretically deduced. The proposed analytical model of accelerated factors was verified with current test data, and then applied to predict long-term properties of CFRP laminates. Compared with the traditional temperature-based accelerated method, the new thickness-based accelerated method is much easier to apply to predict long-term properties of CFRP laminates with good accuracy.

Introduction

Carbon Fiber Reinforced Polymer (CFRP) is increasingly used in civil infrastructures as reinforcing rods, externally bonding or wrapping for strengthening, and even all-composite structural systems [1], [2], [3], [4], [5], [6], [7], [8]. CFRP is generally exposed to aggressive environment during the long service life of these infrastructures [9], [10], [11], [12]. Therefore, the long-term environmental effects (e.g., moisture, temperature, pH, etc.) on the mechanical properties of CFRP (e.g., tensile strength, stiffness, interfacial properties, etc.) have been greatly concerned in recent decades [13], [14], [15], [16], [17].

The durability of CFRPs is influenced by several factors, such as the fiber volume fraction, the manufacturing process, and environmental exposure conditions [18]. Among these factors, the water absorption is one of the main reasons for the degradation of mechanical properties of CFRP composites after long-term duration in service [19]. Therefore, a large number of studies have been undertaken to evaluate the water uptake of CFRP composites and its influence on the mechanical properties of CFRP by aging tests. Karbhari and Xian [20] found that the moisture uptake response of CFRP could be characterized by a two-stage model, which indicated an initial diffusion-based response followed by a relaxation-based one. They also suggested that the long-term properties of pultruded CFRP composites were mainly affected by moisture uptakes through plasticization, swelling, and hydrolysis of the resin matrix. Kafodya et al. [21] investigated water uptakes of pultruded unidirectional CFRP plates immersed in water and seawater with a duration of 20 weeks, and found that the type of solution had little effect on the degradation of CFRPs. Almeida et al. [22] studied the effect of hygrothermal conditioning on mechanical properties of carbon fiber/epoxy filament-wound composite laminates. With the increase of aging time, the trend of water uptake was well fitted with the Fickian model, and the shear strength and modulus of specimens reduced to about 30% and 38%, respectively. Based on aging tests, polyurethane-based pultruded carbon fiber plates (CFRPUs) were conditioned in water, seawater and alkali solution to evaluate their water uptakes and the degradation of mechanical properties [23], [24], [25]. It was found that CFRPUs showed a better moisture resistance than epoxy-based CFRPs. After 1-year duration with all the solutions, tensile properties of CFRPUs kept basically constant while epoxy-based CFRP plates showed noticeable decrease in the tensile strength.

Besides the aforementioned pultruded products, the wet lay-up CFRP laminates are also commonly used in civil engineering projects. Due to the imperfection in hand application, the durability of wet lay-up CFRP laminates is different from that of pultruded products. Zhang et al. [26] evaluated the effect of moisture uptake on bending strengths of wet lay-up CFRP laminates immersed in water for 7 and 14 days. Experimental results revealed that the moisture uptake resulted in a significant degradation of bending strength of CFRPs. To further investigate the degradation mechanism of wet lay-up CFRP laminates due to water immersion, Takeda et al. [27] adopted fiber Bragg grating sensors to monitor water absorptions. Based on the monitoring technique, the swelling strain and coefficient of moisture expansion of unidirectional CFRP laminates were estimated. Saadatmanesh et al. [28] investigated the residual strength of unidirectional CFRP laminates subjected to alkaline and acidic solutions. Test results showed that the tensile mechanical properties of the wet lay-up CFRP laminates were scarcely affected by the humid environments after a duration of 27 months. Through aging tests, Sciolti et al. [29] found that the tensile strength of wet lay-up CFRP laminates decreased by 30% after immersed in distilled water for 30 weeks.

Based on the aforementioned studies, the trend of water absorption and the degradation of mechanical properties of CFRP composites can be investigated by immersing specimens in different solutions. However, it is still insufficient to use the current findings to quantificationally evaluate the long-term properties of CFRP composites subjected to hygrothermal environments for two reasons.

First, the test temperatures, immersion durations and test methods were deliberately designed to keep consistent in most studies. However, the adopted specimen thicknesses were generally different from each other [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. This indicates that experimental results based on specimens with different thicknesses are non-comparable and thus cannot be directly used to establish a unified prediction model. It is mainly because that the time needed to reach the fully permeation of CFRP composite is greatly related to the thickness of the specimen when the diffusion rate of immersion solution maintains constant. A similar concern had already been made to the durability of glass fiber reinforced polymer (GFRP) bars with different diameters [30]. It was found that the degradation of mechanical properties of the GFRP bars was significantly influenced by specimens’ diameters. However, to the authors' knowledge, such effect has not been considered in the existing durability studies on FRP laminates with different thicknesses, which hampers the development of comparative analysis [31], [32], [33].

Second, the immersion time of the above mentioned aging tests are greatly short compared to the real/designed service life of CFRP composites, thus it is unreasonable to evaluate their long-term durability using such short-term experimental results [34]. To solve this problem, a temperature-based accelerated method (TAM) was proposed to better evaluate the long-term durability of FRP composites [35], [36], [37]. Using this method, FRP composites are usually tested at a high temperature to accelerate the degradation process subjected to aggressive environments, and then the test results based on a relative short aging time can be transformed to evaluate the long-term properties of FRP materials. It was found that, however, the structure of the resin could change from a glassy state to a viscoelastic state when the glass transition temperature Tg is reached. Therefore, the use of too high temperature close to Tg could not only accelerate the degradation rate but also modify the thermomechanical-induced degradation mechanisms themselves, leading to an underestimation of the durability of FRP materials [38]. For this reason, TAM is generally recommended to be used under a restricted temperature, which limits its application in practice because of the unstable and low accelerated efficiency. Therefore, it is necessary to develop a more effective accelerated testing method to better evaluate the durability of FRP composites. It should be noted that the aging time of FRP laminates needed to achieve a given degradation level is dependent on the thickness of specimens. If the effect of specimen thickness on properties of FRP laminates exposed to aggressive environments is clearly explored, it is possible to establish a new accelerated testing method based on the specimen thickness.

The main targets of this paper are to (1) evaluate the effect of specimen thickness on the water absorption and flexural strength of CFRP laminates immersed in distilled water and alkaline solution, and (2) propose a new thickness-based accelerated method to better predict the water absorption and flexural strength of CFRP laminates. An experimental program was conducted to investigate the water absorption and flexural strength of wet lay-up CFRP laminates with different thicknesses. The effect of specimen thickness on the water absorption and flexural strength retention of CFRP laminates was evaluated based on test results, which suggested that a thickness-based accelerated method could be developed to accelerate the hygrothermal aging test of CFRP laminates. The accelerated factors of water absorption and flexure strength retention of CFRP laminates were respectively deduced and verified with test results of the current study.

Section snippets

Materials and specimen preparation

The unidirectional carbon fiber sheet (UT70-30 produced by Toray Co., Ltd.) was used in this study. The weight and nominal thickness of the carbon fiber sheet are 300 g/m2 and 0.167 mm, respectively. According to the manufacturer, the tensile strength, tensile modulus and elongation at break of the carbon fiber sheet are 4077 MPa, 245 GPa, and 1.51%, respectively. The resin (JGN-T produced by Kaihua Co., Ltd.) adopted in current study was a two-component epoxy with main and curing agents mixed

Water absorption

Table 3 shows the tested mean water uptake and CV (coefficient of variation) for each group of specimens. Water uptake curves of CFRP laminates with four thicknesses immersed in distilled water or alkaline solution are shown in Fig. 2. It can be seen that the trend of water uptake curves is composed of ascending and descending phases for both distilled water and alkaline solution. In the ascending branch, water uptake increased almost linearly with the increase of square root of aging time

Analytical model of accelerated factors

The above test results clearly show that the specimen thickness has great influences on both water absorption and flexural strength of CFRP laminates. It can be seen from Fig. 2, Fig. 4 that different aging times are needed to achieve the same level of water uptake or flexural strength retention for CFRP laminates with different thicknesses. Hence, it is possible to accelerate the aging effect on CFRP laminates by changing the specimen thickness. In this section, such effect is quantitatively

Model validation

The analytical results suggest that a thicker specimen needs more aging time to reach a given water uptake and/or flexural strength retention. The water uptake or flexural strength retention after aging is positively correlated to the ratio of aging time and square of thickness, i.e.,kth2=cwhere t is the aging time, h is the thickness of CFRP specimens, k is the proportionality factor, and c is the water uptake or flexural strength retention of CFRP laminates after aging. For the same immersion

Conclusions

In this study, the effect of specimen thickness on the water absorption and flexural strength of wet lay-up CFRP laminates immersed in distilled water or alkaline solution was investigated. An analytical study was developed to deduce the accelerated factor of the thickness-based accelerated method for hygrothermal aging of the CFRP laminates. Based on the results, the following conclusions can be drawn.

1. In this study, the trend of water uptake of the wet lay-up CFRP laminates conditioned in

Conflict of interest

The authors declared that there are no conflicts of interest.

Acknowledgements

The financial supports from the National Key R&D Program of China with Grant No. 2017YFC0703000, the National Natural Science Foundation of China with Grant Nos. 51878120 and 51778102, the Fundamental Research Funds for the Central Universities with Grant No. DUT18LK35, the Natural Science Foundation of Liaoning Province of China with Grant No. 20180550763, are greatly acknowledged.

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