Long-term durability of bismaleimide composite in marine environments
Date of Issue2016-12-23
School of Mechanical and Aerospace Engineering
Fiber reinforced polymers (FRPs) have been increasingly used for marine and offshore drilling applications for over decades owing to their superior properties such as high strength and stiffness to weight ratio. The FRPs used for marine applications are generally exposed to a combination of long-term seawater exposure and a wide range of servicing temperatures. The life span of those applications is usually expected to be as long as several decades with little maintenance required. However, the uncertainty of the long-term durability due to lack of experimental data and prediction methods has always been a great concern for the design of composite structures. This study investigates the effects of seawater environment and elevated temperature on various mechanical properties of glass fiber reinforced bismaleimide (BMI) composites and estimates the long-term durability of the composite materials in such harsh environmental conditions. Firstly, the diffusion of seawater in the BMI composite at different temperatures was investigated. The water absorption by BMI composite was found to be a two-stage non-Fickian process. A fast first stage Fickian diffusion was followed by a slow second stage water absorption due to polymer relaxation. Increasing temperature largely accelerates the first stage diffusion but has no clear relationship with the maximum water absorption. A two-stage diffusion model and the Langmuir model were used in this study to describe the water absorption in BMI composite. The mathematic models showed good agreements with the experimental results. Secondly, the long-term seawater effects on the tensile, flexural and shear properties of the materials were investigated. Those material properties suffered 10-30% of loss after long-term exposure in seawater. Higher seawater temperature generally results in higher degradation of properties. The glass transition temperature (Tg) of the BMI composites with seawater absorption is considerably lowered due to the plasticization effect. However, the effect of water absorption below 50 °C was found to be reversible after a re-drying process, which implied no chemical changes taking place at this condition. Furthermore, an accelerated testing method for long-term durability prediction of FRPs was also reviewed and applied for estimating the viscoelastic, static, creep behaviors based on time-temperature superposition principle (TTSP). An automated shifting program and a formulation for constructing master curve were developed for applying the method on the dynamic tests results. The TTSP was found to be applicable to both dry and wet BMI specimens. For the dry specimen, the shift factors can be clearly expressed by two Arrhenius equations with different activation energies while they became more irregular for the wet specimen. It was obvious that the absorption of water or the chemical degradation affected the activation energies of the material. Based on the prediction, in a period of 50 years, the modulus of BMI material with seawater exposure is expected to degrade by as high as 18.4 % in 50 years compared with 10.9 % for dry condition at 200°C service temperature. Thirdly, the effect of seawater immersion at different temperatures on the delamination behavior of the composite was studied under mode I, mode II and mixed-mode I/II loadings. The fracture toughness was found to decrease monotonically with seawater exposure and increasing temperature except for pure mode II results, where it surged up nearly 20% over the dry specimen results. However, most specimens with seawater absorption exhibited increasing resistance curves with the delamination growth. This is mainly due to the plasticization of the matrix and higher ductility of the specimen after immersion. The Mode II fatigue delamination growth of the composite was also investigated by end-notched flexure tests. At a given normalized strain energy release rate, the delamination growth was found to be slower in wet specimens than in dry specimens. The experimental results were correlated with existing delamination criteria to determine the parameters for numerical models. In this study, the use of an experimental procedure based on J-integral was explored, and the result was compared with that of well-established ASTM methods. It showed that the J-integral method has wider selection of materials, provides better accuracy. An image-processing program was developed to measure the rotation angles of the specimen automatically, allowing the interlaminar fracture toughness to be monitored with lower requirements for image resolution, light and material surface compared to previous methods. Finally, fatigue tests were conducted to study stiffness degradation and fatigue life of the material under cyclic loading with the influence of seawater diffusion. Both on-axis and off-axis fatigue tests with various stress ratios were performed with tension-tension loading, and the results were presented in terms of statistical S-N curves. It was found that the fatigue behavior of wet specimens was slightly enhanced at high stress level. However, in general, the fatigue life has been significantly shortened with seawater exposure and elevated immersion temperature. The stiffness degradation curves show that the seawater-aged specimens had higher stiffness reduction in the first stage while slower degradation rate within the linear region (second stage). Overall, the wet specimens tend to have higher final stiffness at failure than the dry specimens. A stiffness degradation model incorporating the strain failure criterion was used to describe the damage accumulation in the BMI composite laminate statistically. The model is capable of predicting the statistical distribution of stiffness degradation curve. The life prediction agreed well with the result of statistical S-N curves and experimental data. The proposed model is attractive as it predicts not only the failure probability but also reflects damage accumulation in the material.