Hydrothermal ageing effects on flexural properties of GFRP composite laminates

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 20, October 2013, pp Hydrothermal ageing effects on flexural properties of GFRP composite laminates P Sampath Rao* & M Manzoor Hussain Department of Mechanical Engineering, Jawaharlal Nehru Technological University, Hyderabad , India Received 10 January 2013; accepted 4 April 2013 The usage and replacement of conventional materials with polymer composite materials for engineering applications is always questioned by the end user, unless otherwise the research oriented reliable supporting certification is made available. The reinforcement materials are highly hygroscopic; the matrix material provides protection to the reinforcement. Since the composite component s edges are exposed to environment, water molecules travel along the reinforcement, that may cause damage to the interfacial bonding, further the performance of the composite laminate may get affected. This study of resistance of glass fiber reinforced polymer composite (GFRP) subjected to hydrothermal ageing and their flexural behavior under different temperature and exposure time. GFRP samples were prepared by using resin transfer molding (RTM) process and samples were exposed to different environmental conditions. Three point bending test is conducted and the test results indicate reduction in flexural modulus due to the degradation of GFRP under the environmental hydrothermal ageing conditions at 45 C strength decreases 23% to 70% and at 60 C and 75 C strength decreases 50% to 77% over exposure period of 60 days. The results show that strength degradation by moisture absorption and temperature effect over the lifecycle impacts of GFRP composite exposed to water and temperature for long period of time. The performances of test results have been predicted by mathematical modeling. Keywords: Glass fiber reinforced polymer composite (GFRP), Hydrothermal ageing, Flexural modulus, Retention ratio Glass fiber reinforced plastics materials are low cost, light-weight, and have good mechanical properties and thus have the potential for the use in structural applications such as chemical plants and pipelines which are subjected to aggressive environment. Therefore, data on the effects of moisture on retention of the mechanical properties of glass fiber reinforced composites during long-term environmental exposure are crucial for them to be utilized in outdoor applications. Glass fiber reinforced polymer composites (GFRP) show relatively low degradation in various corrosive environments in the unstressed state, however, they are very susceptible to stress corrosion, especially in dilute mineral acid environment 1,2. The environmental stress cracking characteristics of GFRP and (A-G) FRP were studied using CT (fracture mechanics) samples under constant tensile load and water environment. For GFRP the characteristics of crack length as a function of exposure time. Ductile aramid fibers seemed to protect the glass fiber reinforcement from stress cracking due to higher chemical resistance and complex failure mechanisms 3. Accelerated environmental ageing investigation of polyester/glass *Corresponding author ( sampathrao_polusani@yahoo.co.in) fiber reinforced composites (GFRPCs) was studied based on two kinds of alternating cycles, which provided humidity, temperature and ultraviolet radiation. The study dynamic mechanical analysis, for a range of temperatures and frequencies under tensile and three-point bending loadings, revealed that the aged materials gained in stiffness, whereas a small deterioration in strength was found 8. The main objective of this work is to investigate the effects on flexural strength of GFRP composite materials subjected to hydrothermal aging and its life prediction by mathematical modeling. In this work, the effects of environmental ageing on retention of flexural properties of glass fiber reinforced polymer composites (GFRP) are studied and qualitative correlation to between results from ageing and accelerated ageing is discussed. Finally, compared predicted values with experimental values. Experimental Procedure Production of laminates using resin transfer molding (RTM) The specimens for the present work are prepared using RTM machine. The resin transfer molding (RTM) machine, a closed mould process, consists of resin injection equipment, it has a hollow cylinder fitted with pressure gauge, valve and pressure pump and mould plates as shown Fig. 1. The materials used

2 416 INDIAN J. ENG. MATER. SCI., OCTOBER 2013 for GFRP laminates are polyester resin (with density 1.35g/cm 3 manufactured by Ciba Geigy Ltd. and supplied by Northern Polymers, New Delhi, India) and glass fiber mats (woven fabric glass fiber with density 450g/cm 2 manufactured by Saint Gobian Ltd., India) with composition of 60% matrix (polyester resin) and 40% of glass fiber. The specifications for the laminate preparation are: (i) injection pressures Psi and (ii) curing temperature room temperature. The glass fiber mats are placed between the mould plates and clamped before sending the resin. The chemically combined resin that is resin mixed with 2% of accelerator (cobalt nathylene) and 2% of catalyst (methyl ethyl keypricperoxide) is poured inside the hollow cylinder through the valve present at top cap and immediately the valve is closed and the air is pumped into hollow cylinder up to maximum pressure of 40 Psi. The bottom valve of the cylinder is slowly released so that pressurized chemical resin enters in to the mould and it is spread equally in to all directions. To get a well-shaped laminate allow the mould to be idle for 4-5 h and laminate is solidified then unseal the mould separate the lower and upper mould parts. The laminate is slightly sticky to the mould surface and removed forcibly and laminate of mould shape is as shown Fig. 2. Since such laminate obtained is difficult to test, and to have three-point bending test for that laminates are sliced to standard ASTMD638 specimen of dimensions 250 mm 30 mm 8 mm as shown Fig. 3. Testing of the laminates For testing of laminates under environmental conditions, constant temperature water bath tub have been fabricated as shown in Fig. 4. The number of specimens are exposed in constant temperature water bath tub (as shown in Fig. 5) for period of 60 days. Every 10 days number of specimens are taken from bath, and carried out three-point bending test (Fig. 6). Load versus deflection curves are drawn from the test results and calculated the flexural modulus using formulae (which has been obtained from bending equation) E f = L 3 m / 4bd 3 (1) Where L is support span (specimen gauge length) (mm), b is width of test specimen (mm), d is depth or thickness of test specimen (mm), m is gradient (i.e., slope) of the initial straight-line portion of the load deflection curve, (P/D), (N/mm). Fig. 1 RTM Machine Fig. 2 GFRP composite laminate piece Fig. 3 Test pieces of GFRP laminate

3 RAO & HUSSAIN: FLEXURAL PROPERTIES OF GFRP COMPOSITE LAMINATES 417 Results and Discussion Fig. 4 Constant temperature water bath tub Fig. 5 Inside portion of the constant temperature water bath tub with specimens Three-point bending test The number of specimens of dimensions 250 mm 30 mm 8 mm are exposed to water bath at constant temperatures 45 C, 60 C and 75 C and the same are tested with three-point bending test. This is repeated for every 10 days, the results are noted and the same are displayed on load versus deflection curve in Figs 7, 8 and 9, respectively Flexural modulus of elasticity was calculated from graphs as shown in Figs 7, 8 and 9 for specimens exposed in water at constant temperatures 45 C, 60 C and 75 C respectively with defferent exposure times by using Eq. (1) and values are shown in Table 1, for example specimen exposed at 45 C for 10 days model calculation of modulus. The variation of flexural modulus with exposure time is shown in Fig. 10. A graph retention ratio versus exposure time is shown in Fig. 11. Performance prediction analysis An indirect indication of service life is obtained simply by comparison of the performance of materials under given test conditions, the one which shows the smaller change being deemed to perform better. To make a direct estimate of service life of materials, it is necessary to apply some form of extrapolation technique to their experimental data. The life estimation of GFRP composites in these environmental conditions are analyzed by employing linear regression analysis life prediction models. The life predication equation was derived on the basis of experimental data in terms of the degradation coefficient (decay constant), soaking time, minimum strength and exponential coefficient for different environmental conditions. Exponential linear regression provides powerful technique for fitting the best relationship between dependent and independent variables based on this technique life estimation of composite materials is being established as: Y (x) =Y 0 + A 1 exp- (x-x 0 )/t 1 (2) Fig. 6 Bending test on UTM Here Y (x) is dependent parameter, x is exposure time in terms of days, Y 0 is minimum strength property after long exposure of time, t 1 is the degradation coefficient or decay constant and A 1 is exponential coefficient which is determined by using experimental data.

4 418 INDIAN J. ENG. MATER. SCI., OCTOBER 2013 Fig.7 Specimen exposed in water at 45 C constant temperature for different exposed times (Load versus deflection curve for exposure time 10 days to 60 days) The GFRP composite materials exposed in water at constant temperatures 45 C, 60 C and 75 C. The mathematical equations for flexural modulus was established by experimental data given in Eqs (3)-(5) and graphically represented in Fig.12 (a-c) At 45 C Y (x i ) = exp- (x i -10)/ (3) At 60 C Y (x i ) = exp- (x i )/ (4) At 75 C Y (x i ) = exp-(x i )/ (5)

5 RAO & HUSSAIN: FLEXURAL PROPERTIES OF GFRP COMPOSITE LAMINATES 419 Fig. 8 Specimen exposed in water at 60 constant temperature for different exposed times (load versus deflection curve for exposure time 10 days to 60 days) The values of flexural property are calculated by using above equations. The predicted values of flexural property are compared with experimental values and shown in Table 2. The present work focused on the investigation on effect of moisture and temperature on GFRP composites for that number of specimens are fabricated and exposed to accelerated hydrothermal

6 420 INDIAN J. ENG. MATER. SCI., OCTOBER 2013 Fig. 9 Specimen exposed in water at 75 C constant temperature for different exposed times (load versus deflection curve for exposure time 10 days to 60 days) environmental conditions by immerse in constant temperature water bath tub at temperatures of 45 C, 60 C and 75 C for period of 60 days. Every 10 days specimens are taken from bath and carried out the three-point bending test and determined the flexural modulus. Figs 7-9 show the three-point bending test results at constant temperatures 45 C, 60 C and 75 C over exposure periods of 10, 20, 30, 40, 50 and 60 days. The flexural modulus is calculated with help of these figures (graphs) and given in Table 1. The test results show that initially rapid reduction in flexural modulus over exposure time of 30 days thereafter significant reduction in modulus because of loosing bonding strength of the polyester resin. From the test results as shown in Fig.10 (a, b), it is clear that flexural modulus of the conditioned specimens was significantly reduced due to the degradation of GFRP under the environmental hydrothermal aging conditions at 45 C strength decreases 23% to 70% and at 60 C and 75 C strength decreases 50% to 77% over exposure period of 60 days. The results showed that strength degradation by moisture absorption and temperature effect over

7 RAO & HUSSAIN: FLEXURAL PROPERTIES OF GFRP COMPOSITE LAMINATES 421 Fig.10 Variation of flexural modulus with exposure time Fig.11 Retention ratio versus exposure time Fig. 12 The graphical mathematical modeling of GFRP composite laminates at constant temperatures (a) 45 C, (b) 60 C and (c) 75 C

8 422 INDIAN J. ENG. MATER. SCI., OCTOBER 2013 Fig. 13 Figure shows failure behavior of the composite materials after bending test S. No Exposure time in days the lifecycle impacts of GFRP composite exposed to water and temperature for long period of time. The results show that the effect of temperature as well as exposure time, at particular temperature the flexural modulus decreases with increasing exposure time and at same time at particular exposure period the flexural modulus decreases with increase in temperature. Typical failure modes observed in test specimens are shown in Fig. 13. It has been shown that moisture plasticizes the polyester matrix and reduces the glass transition temperature. Also presence of moisture and temperature at the fiber-matrix interface reduces the strength of the composite material. Thus, moisture Table 1 Flexural modulus of elasticity of GFRP laminates at different temperatures with exposure time Flexural modulus of elasticity at constant temperature at 45 C in Gpa Un exposed flexural modulus of elasticity = Gpa Retention Retention ratio ratio Flexural modulus of elasticity at constant temperature at 60 C in Gpa Flexural modulus of elasticity at constant temperature at 75 C in Gpa Retention ratio Exposure time Table2 Predicted values of flexural modulus in comparison with experimental values of GFRP composite laminates at different temperature conditions Predicted value, GPa Flexural modulus for specimens exposed in water at constant temperatures Exposed at 45 C Exposed at 60 C Exposed at 75 C Experimental value, GPa Error percentage Predicted value, GPa Experimental value, GPa Error percentage Predicted value, GPa Experimental value, GPa Error percentage

9 RAO & HUSSAIN: FLEXURAL PROPERTIES OF GFRP COMPOSITE LAMINATES 423 generally affects any property which is dominated by the matrix and/or interface. However, the flexural strength being a fiber dominated property the strength reduction occurs only if the fibers themselves are affected by hydrothermal environmental conditions. It has been shown that water can cause degradation at fiber level in glass fibers. Degradation is initiated by water extracting ions from the fiber, thereby altering its structure. These ions combine with water form bases which itch and pit the fiber surface, and leads to premature failure of the fibers. The retention ratio is calculated for flexural modulus compare with modulus of exposed specimens at different exposure times at different temperatures to modulus of unexposed specimen as shown in Table 1. This retention ratio steadily decreases over exposure period of 40 days and on further exposure the retention ratio decreases slowly as shown in Fig.11(a, b). The regression analysis is performed for each of the time steps and this yields a set of exponential linear relationships between the flexural modulus and exposure time at different constant temperatures. The relationships so obtained are shown in Eqs (3)-(5) can be used to determine the flexural modulus of the composite specimen at different time steps for different temperatures. For predictions of response due to immersion in water at 45 C, 60 C and 75 C the values of flexural modulus at each time step are obtained by substituting the exposure time in days in the Eqs (3)-(5), the values are listed in Table 2. The values of predicted flexural modulus thus obtained are compared to experimentally obtained data. The percentage error between the experimental and predicted values is also given in Table 2. The percentage error is calculated according to the equation, Percentage error = [(predicted value experimental value) 100/ experimental value] The prediction values of flexural modulus, for the specimens immersed in water at 45 C and 60 C, are slightly lower than the experimental values but at 75 C prediction values higher than experimental values. It has to be noted that as temperature increases the predicted values are increased that indicates rate of degradation increases. The life estimation of composite materials has possible prediction models. Conclusions The investigation showed a remarkable reduction in mechanical strength (flexural modulus) of GFRP composites which are exposed to different constant temperatures over different exposure times. The flexural strength values of the specimens are decreased over exposure period of 60 days in water at constant temperature. As per the results initially rapid reduction and gradual decrease over long period and expected to maintain considerable minimum strength over service as shown in Fig. 10(a,b). The moisture present in matrix of composite material at constant temperature causes matrix swelling, inter-phase debonding, physical damage of matrix inter-phase and hydrolysis of composite materials are the main reasons for the reduction in flexural strength. The mathematical analysis able shows life service estimation of the materials as per this investigation the rate of degradation increases with the increase in exposure time and temperature. The predicted values agree well with the experimental values for the specimens exposed 45 C, but not as well for those exposed to 75 C. The following conclusions may be drawn for this study: (i) From the test results retention ratio was calculated and it is initially reduced rapidly then gradually decreased over long period of exposure and expected to maintain considerable minimum strength over service. (ii) The presence of moisture or water particles in the matrix, fiber-matrix interface and also attack on the glass fibers are all the reasons for the reduction of properties due to environmental impact. For example parts made of GFRP materials and exposed to environment, the water particles travel along reinforcement and damage the interfacial bonding between glass fiber and matrix then parts get affected. (iii) The reduction in flexural modulus of GFRP laminate under accelerated hydrothermal aging in the processes of water diffusion and chemical degradation, temperature is a key factor. (iv) The results show temperature effect on GFRP composite materials that strength decreases 23% to 70% at 45 C and 50% to 77% at 60 C and 75 C over exposure period of 60 days considered. (v) The change flexural properties of GFRP during environmental aging is a consequence of hydrothermal degradation of glass fiber deboning at the fiber/matrix interface, dissolution and degradation of polymer matrix.

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