18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS INFLUENCE OF HYDROTHERMAL ENVIRONMENT ON MECHANICAL BEHAVIOR OF FIBER REINFORCED POLYMERS: CHARACTERIZATION AND MODELING S. Cao, B. Wang*, G. Q. Jiao Dept. of Engineering Mechanics, Northwestern Polytechnical University, Xi’an, P.R.China * Corresponding author (b.wang@nwpu.edu.cn) Keywords : Carbon fiber reinforced polymer; Hydrothermal effect; Diffusion; Simulation; Compressive properties Type A specimens were prepared for open-hole 1 Introduction compression tests and there is a hole of 6mm Due to their desirable specific strength and specific diameter in its center. The stacking sequence stiffness properties, carbon fiber reinforced polymer specimens is [+45/0/-45/90] 4S . matrix cUUomposites (CFRP) has been used in Prior to exposing to hydrothermal environment, different industrial sectors. However, polymer resin type A specimens were preconditioned by can absorb water from the surrounding environment drying at 80 o C for 164 hours. The immersion followed by diffusion of water into all of composite. tests were conducted at 70 o C with temperature- The presence of water would result in the controlled water bath for 344 hours. After plasticization and hydrolysis of polymer matrix, and immersion, open-hole compression tests were weakening the fiber-matrix interface [1-2]. carried out on the wet specimens immediately. Additionally, the difference of the amount of water- sorption between fiber and matrix would leads to Compression tests were also conducted on the different volumetric expansions, which causes specimens that had not been hydrothermally localized stress and strain field in the composite [3]. exposed. It has been reported that hydrothermal environment Type B specimens were used for the witness of influences the mechanical behavior of CFRPs relative weight gain, and immersion test was greatly, especially the properties dominated by the conducted at the same condition. During the testing, matrix or the interface [2, 4]. type B specimens were periodically weighed with an The intent of this research was to better understand electronic balance (precision 0.01mg). The amount the aging of CFRPs immersed in water and the of absorbed water in specimens was calculated as degradation of mechanical properties, and to predict equation (1). their long term behavior. Water-sorption is W W M i b 100% (1) experimentally determined by gravimetric methods. i W Fickian diffusion model and finite element analysis b Where: M i is relative weight gain, % ; W i is current were employed to describe the water-sorption. In specimen mass, g ; W b is oven-dry specimen mass, g . addition, the comparison of compressive strength between dry specimens and water-immersed Finite element analysis was carried out using the specimens is performed in this article. A finite finite element software ABAQUS. The mass element model is also employed to simulate the diffusion analysis and static stress analysis were mechanical behavior after immersion. employed to model transient moisture diffusion and compression test respectively. 2 Materials and methods 3 Result The material used in this study was T700/9916 carbon fiber-reinforced epoxy laminate. The 3.1 Water-sorption tests result fiber volume fraction was around 65%. The Both experimental and theoretical water-sorption dimensions of specimens are given in Table 1. curves versus the square root of time are plotted in
the Fig. 1. The solid line is the theoretical Fickian Considering each laminate of the specimen as diffusion curve obtained by calculating the equation orthotropic material, the values of diffusivity and (2) proposed by Shen and Springer [5]. The solubility were calculated by the combination of experimental data shows a similar trend to the solid experimental results and the method of interpolation curve, which suggests the water-sorption behaviour to provide the appropriate solution for transient of this material followed Fickian diffusion. The moisture diffusion. After computing and simulating theoretical result exhibits a lower rate than the repeatedly, the material parameters were estimated as D 11 =0.0147 mm 2 /h , D 22 = D 33 =0.0038 mm 2 /h , s =1.02, experimental data. This is because the thickness of where D 11 is the diffusivity in the direction parallel plates in the equation (2) is small enough to neglect to the fibers, and D 22 and D 33 are the diffusivities in edge effects and only the faces perpendicular to z direction are concerned. However, the thickness of the directions normal to the fibers[5]. specimens in this study can’t neglect and water can Because water diffuses into the specimen from all diffuse into the specimens from all the faces of the faces, a solid element model within the piles to specimens, which leads to greater experimental model the sequence of laminate was employed. The results. maximum moisture content was specified as the D t boundary condition on the faces contacting the water n 2 2 z exp (2 1) ( ) m m h 2 environment. The entire specimen had zero moisture 8 (2) G i 1 content at the beginning of mass diffusion procedure. m m 2 n 2 (2 1) n m i 0 Fig. 2 shows that the simulated result provides a Where: m b is the initial weight of the moisture in the good agreement with the experimental data. The material, g ; m m is the weight of moisture in the water diffusion model with the same diffusivity and material when the material is fully saturated, g ; D Z is solubility is also conducted on the open-hole the diffusivity of the material in the direction normal compressive specimens. And the water content to the surface, mm 2 /h ; h is the thickness of distribution at different local regions at t=344h is specimens, mm ; t is time, h . illustrated in Fig. 3. Non-uniform water distribution can be observed and the specimen was not saturated. 3.2 Open-hole compression tests result The surface of specimen and the edge of hole have The results of compressive tests are given in Fig. 4. higher water concentration. After immersed in hot water for 344 hours, 3.4 Open-hole compression test modelling considerable reduction in the strength has been measured. With a relative weight gain of 0.718%, The simulation of open-hole compression test was the average strength decreased about 6 % and the carried out after mass diffusion analysis. dispersion degree of compressive strength It has been reported that the properties of the aggravated. Same failure mode which was shear composites are not dependent on the duration of crippling damage can be observed in both dry exposure but mainly on the water content [2]. The specimens and immersed specimens. influence of hydrothermal environment on mechanical properties can be considered as the 3.3 Water diffusion modelling decrease of rigidity due to water uptake. It is In the mass diffusion analysis of ABAQUS, the difficulty to introduce the water distribution directly governing equations are an extension of Fick’s into the static stress analysis model. In addition, equations [6]: temperature and water concentration have the same s degree of freedom in ABAQUS. Thereby, the water J D s z ( ) (3) distribution was introduced into the mechanical z analysis model as the temperature initial condition Where D is the diffusivity, s is the solubility of the and a sequentially coupled thermal-mechanical diffusing material in the base material, ϕ is the analysis was carried out. In this way, hydrothermal “normalized concentration”, ϕ =c/s , where c is the effects can be simplified as the influence of mass concentration of the diffusing material, J is the temperature in the analysis. flux of concentration of the diffusing phase. The user subroutine UMAT was employed to predict the compressive strength and the failure patterns of
Recommend
More recommend
