Experimental and Numerical Evaluation of the Tensile Behavior of GFRP Laminates

Experimental and Numerical Evaluation of the Tensile Behavior of GFRP Laminates

P. Sampath Rao1

1. Professor ,

Department of Mechanical Engineering, Vijay Rural Engineering College, Nizamabad ,

JNT University Hyderabad ,T.S..INDIA

Abstract: 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. Fibre reinforced polymer composites are being used replacement material in various applications like marine boats aerospace, military and renewable energy generation. The reinforcement materials are highly hygroscopic nature and exposed to environment, water molecules travels along the reinforcement and affects the performance of the materials. In the present work an attempt has been made in establishing the investigation procedure to assess influence of moisture absorption coupled with temperature. And to estimate life cycle time of polymer composite components such as marine boats and submarine applications and under water applications. The results of the experimental investigation showed that the GFRP Composite laminate tested in different environmental condition under tensile loading. From the experimental results and numerical analysis, it was clear that the tensile behavior of the salt water conditioned specimens were significantly reduced. The results of the analytical predictions and numerical simulations are in good agreement with the experimental results.

Keywords: Glass fibre reinforced polymer, Environmental conditions, Resin transfer molding, and Tensile modulus.

I.INTRODUCTION

In recent years, Glass-fiber reinforced polymer (GFRP) have received considerable attention as structural materials due to their high strength-to-weight ratios in relation to conventional metals such as steel and aluminum. The advantages over conventional structural materials, composites are susceptible to heat and moisture when operating in harsh and changing environmental conditions. The main benefit of using the sandwich concept in structural composite components is its high bending stiffness and high strength to weight ratios [1]. In addition, composite materials are preferred over conventional materials because of its high corrosion resistance [2]. With its many advantages, composite structures have been widely used in the automotive, aerospace, marine and other industrial applications. This composite material also draws a lot of interest in the construction industry and is now beginning to be in use for civil engineering applications [3].

The researchers investigated on hygrothermal effects on composite laminates and they stated the hygrothermal effect causes a reduction of in-plane properties of laminates, where the ratio of the moisture absorption in composite laminates

is accelerated by the presence of cracks and voids in the laminate. For example, a study by Hertz et al. [4] in 1972 has shown that moisture absorption leads to changes in the thermo physical, mechanical, and chemical characteristics of the resin matrix by plasticization and hydrolysis. [5] Bao and Yee[6] investigated the moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites and found that short-term moisture diffusion interface has little effect on the fibermatrix interface. However, after prolonged moisture absorption, interfacial cracks develop in the composites. Vaddadi et al. [7] analyzed the evolutions of internal stresses within fiber- reinforced composites subjected to transient hygrothermal deformation. It was found that the stresses gradually increase from the exposed surface to the interior, as the moisture tends to expand the epoxy phase. The degradation of the reinforcements played an important role in strength reduction of fiber-reinforced composite as they are the major load-carrying constituents [8] and also the moisture conditions resulted in strength degradation. Karbhari and Zhang[10] investigated the durability of 2 and 4 layered specimens of E-Glass/Vinylester material and it was shown that the highest levels of tensile strength and modulus degradation for the deionized water immersed samples.

The objective of this work is analysis on the performance and durability of glass fiber reinforced polymer materials under tensile loading for different environmental conditions with varying temperature from room temperature to elevated temperature. In this investigation study the tensile behavior of glass fibre reinforced polymer composite material due to water absorption. Also life prediction analysis has been carried out by mathematical modeling.

1. EXPERIMENTAL SETUP

   2.1. Preparation of Test Samples

   The specimens for the present work are prepared using RTM Machine as shown in fig.1. The specifications for the laminate preparation are (i) injection pressures, 30-40 psi.

   (ii) Curing Temperature room temperature. The laminates obtained by RTM is the size 300mm x 300mm x 8mm.These laminates are sliced to standard ASTMD 638 tensile specimens of dimensions 250 mm x 30 mm x 8 mm as shown fig.2.

   Specifications of materials used in the composite laminate: Matrix: General purpose polyester resin (commercial Grade), Glass fibre: Saint Gobain makes E-Glass Chopped

   strand mat (stitched) 450g/s-m, the laminates are prepared with RTM process . The volume fraction of the

   Fig. 1 Resin transfer molding machine

   reinforcement loading about 40% which is found by Burn test the remaining is matrix about 60%

   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 were analyzed by employing exponential regression analysis life prediction mathematical 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 was being established as follows.

   Y (X )

   Y0 A1exp X X0 / t1…….. (2)

   Fig. 2 Specimens of GFRP laminate with dimensions of 250 mm×30 mm×8 mm

   1. Testing of the Laminates

      The laminates were tested under different environmental conditions at room temperature and constant elevated temperature in both water and salt water bath tubs. In total 120 specimens were soaked in water bath over period of 360 days at room temperature and 120 specimens were soaked in water bath over period of 60days at each constant temperature of 60ºC and 75ºC. Similarly way totally 120 specimens were soaked in salt water bath over period of 60days at each constant temperature 60ºC and 75ºC. Every 30 days specimens are taken from bath which are exposed to room temperature and for every10 days specimens are taken from bath which is maintained at 60ºC and 75ºC constant temperature and carried out tensile tests on universal testing machine. The load-stroke behavior obtained during testing was converted into engineering stress strain and calculated the elastic modulus. The tensile modulus was calculated using the below formulae

      Where stress was taken corresponding to strain percentage of 0.8, 1 and 1.2 from stress strain relation and averageit.

   2. 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

2. RESULTS AND DISCUSSIONS

   The laminates were tested under different environmental conditions at room temperature and constant elevated temperature in both water and salt water bath tubs with tensile test and noted the results. From the results stress- strain graphs are drawn and calculated tensile modulus of the specimens within the elastic limits choosing three points in a straight line portion of stress-strain relation. For example specimen soaked at 600C in water for 10days tensile modulus is calculated as Tensile Modulus (10 Days), E1= (8/0.7+10/0.9+10.5/1)/3*100 N/mm2 =1.12 Gpa

   The laminates are soaked to water bath at room temperature, and are tested with tensile test. This is repeated for every 30 days, the final experimental and numerical results are noted and the same displayed in Fig.3.The laminates are exposed at constant temperatures 600C and 750C in water bath, tested with tensile test. At each temperature this is repeated for every 10 days, the final experimental and numerical results are noted and the same displayed in Fig.4 and 5 and also shown in table.1. The laminates are exposed or soaked at constant temperatures 600C and 750C in saltwater bath, tested with tensile test. At each temperature this is repeated for every 10 days, the final experimental and numerical results are noted and the same displayed in Fig.6 and 7 and also shown in table.1.

   The experimental results revealed mechanical properties of the GFRP (E-Glass/Polyester) samples subjected to aging at room temperature and elevated temperature in water as shown in fig.3and 4 from the experimental results (shown in figure4), initially the tensile strength appeared to be increased as soaking (exposure) time is increasing this is because of cross linking reaction in polyester resin is still in progress up to 2 to 3 weeks of laminate preparation. The degradation due to water molecular sleep age is dominated by cross linkage speed. After that rapid reduction in mechanical properties is observed and gradual decrease is observed over the exposure time. The regression analysis is performed for each of the time steps and this yields a set of

   exponential linear relationships between the tensile modulus and exposure time based on the analysis derive the mathematical equation.

   0.6

   0.4

   0.2

   0

   Tensile Modulus in GPa

   The results shows rapid reduction for the specimen soaked to salt water as compared to specimens exposed to water because of more moisture interference in fibre matrix in the saltwater than water. The samples subjected to aging at the constant temperature water bath (60ºC and 750C) showed a hyperbolic decrement in the tensile modulus. On the whole it is observed that tensile modulus decreased to some extent with the presence of moisture, temperature and reduction in tensile module is more salt water conditioned specimen then normal water. There is significant reduction in modulus because of loosing bonding strength of the polyester resin at temperature. It is clear that the tensile modulus rapidly decreases due to hydrothermal aging because moisture generally affects any property which is dominated by the matrix and/or interface. However the tensile strength being a fibre dominated property the strength reduction occurs only if the fibres themselves are affected by hydrothermal environmental conditions.

   1.2

   1

   0.8

   Experimental value

   Specimen at RT in water

   0

   500 1000

   Exposure time in days

   Fig.3 Tensile test- Numerical (predicted) and experimental tensile modulus of GFRP Specimens exposed to room temperature

   Fig.4 Tensile test- Numerical (predicted) and experimental tensile modulus of GFRP Specimens exposed 600C in water

   Fig.5 Tensile test- Numerical (predicted) and experimental tensile modulus of GFRP Specimens exposed 750C in water

   Fig.6 Tensile test- Numerical (predicted) and experimental tensile modulus of GFRP Specimens exposed 600C in saltwater

   The regression analysis is performed for each of the time steps and this yields a set of exponential linear relationships between the tensile modulus and exposure time at different conditions as shown table1 and figures 3,4,5,6 and 7. The obtained predicted values are compared to experimental data and they are similar and good agreement between them. Based on this analysis it has to be noted that as temperature increases the predicted or numerical values are increases that indicate rate of degradation increase. The life estimation of composite materials has been possible by prediction models. It is expected to maintain minimum strength after 120 days over life cycle of period at 600C, 750C in water and saltwater under this environmental impact. According to prediction analysis at elevated temperatures predicted values and experimental values are comparable.

   Fig.7 Tensile test- Numerical (predicted) and experimental tensile modulus of GFRP Specimens exposed 750C in saltwater

   Table1 Tensile modulus of GFRP Composite laminates soaked to water and salt water at 600C and 750C

   Exposure time (days)	Specimen exposed in water and salt water-Tensile modulus (GPa)
   	Specimen in water at 600C		Specimen in water at 750C		Specimen in salt water at 600C		Specimen in salt water at 750C
   	Predicted	Experimental	Predicted	Experimental	Predicted	Experimental	Predicted	Experimental
   0	0.970	0.970	0.970	0.970	0.970	0.970	0.97	0.970
   10	1.118	1.12	1.110	1.116	0.974	0.979	0.905	0.925
   20	0.829	0.825	0.813	0.809	0.843	0.825	0.771	0.725
   30	0.707	0.75	0.690	0.687	0.730	0.701	0.664	0.676
   40	0.654	0.680	0.639	0.657	0.634	0.642	0.578	0.620
   50	0.632	0.623	0.620	0.615	0.552	0.528	0.509	0.488
   60	0.629	0.615	0.611	0.605	0.482	0.492	0.454	0.452
   70	0.618	–	0.608	–	0.421	–	0.412	–
   80	0.617	–	0.607	–	0.370	–	0.312	–
   90	0.616	–	0.606	–	0.326	–	0.302	–
   100	0.615	–	0.605	–	0.288	–	0.252	–
   110	0.612	–	0.604	–	0.256	–	0.222	–
   120	0.609	–	0.604	–	0.228	–	0.212	–
   130	0.607	–	0.604	–	0.205	–	0.192	–
   140	0.606	–	0.604	–	0.185	–	0.182	–
   150	0.605	–	0.604	–	0.164	–	0.162	–
   160	0.605		0.604		0.164		0.152
   170	0.605		0.604		0.164		0.152

3. CONCLUSIONS

The investigation showed a remarkable reduction in mechanical strength (tensile modulus) of GFRP composite laminates which are subjected to different environmental conditions over soaking time. The tensile strength values of the specimens are decreases over exposure period of 60 days in water and salt water at constant temperature. As per the results tensile modulus initially slightly increases and gradual decrease over long period and expected to maintain

considerable minimum strength over service of the life cycle. The following points drawn from Experimental and theoretical results (numerical results obtained from mathematical modeling).

i). The presence of moisture or water particles in the matrix, fibre-matrix interface and also attack on the glass fibres are all the reasons for reduction of properties due to hydrothermal impact.

ii).The tensile modulus reduction is more in hydrothermal aging because of temperature is a key factor for accelerated aging in the processes of water diffusion and chemical degradation.

1. The investigation noticed that aging at elevated temperatures, strength degradation is more in salt water exposure when compared to normal water.

2. It is worth noticing that aging at elevated temperatures will also cause color change in samples.

3. Prediction analysis stated that the material posses minimum strength over the life cycle.

4. The predictive mathematical models have showed good agreement between experimental data and theoretical values. The numerical results are similar when compared to experimental results. Therefore life of the laminate can be estimated reasonably.

REFERENCES

1. Belouettar, S, Abbadi, A, Azari, Z, Belouettar, R, Freres, P. Experimental investigation of static and fatigue behaviour of composite honeycomb materials using four point bending tests. Composite Structures 2008; 87(3), pp 265-273.

2. Russo, A, Zuccarello, B. Experimental and numerical evaluation of the mechanical behaviour of GFRP sandwich panels. Composite Structures 2007; 81, pp 575-586.

3. Keller, T. Material tailored use of FRP composites in bridge and building construction, Swiss Federal Institute of Technology Lausanne, Switzerland, 2006.

4. Hertz J, Christian JL, Varlas M, et al. Advanced composites applications for spacecraft and missiles, Phase I, Fila report, AFML-TR-186, Vol. II, General Dynamics – Convair Division, San Diego California, 1972.

5. Bao L-R and Yee AF. Effect of temperature on moisture absorption in a bismaleimide resin and its fibre composites.

   Polymer 2002; 43: 39873997

6. Bao L-R and Yee AF. Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fibre Composites Part I: Uni-weave composites. Compos Sci. Technol 2000; 62: 20992110.

7. Vaddadi P, Nakamura T and Singh RP. Transient hygrothermal stresses in fibre reinforced composites. A heterogeneous characterization approach. Composites: Part A 2003; 34: 719730.

8. Han M and Nairn JA. Hygrothermal Ageing of Polyimide Matrix Composites, Comp.: Part A, 2003, 34:979-986.

9. Imielinska K. Environmental stress cracking in e-glass And aramid/glass epoxy Composites, Kompozyty (Comp), 2006, 6:19-23.

10. Karbhari VM and Zhang S. E-glass/Vinylester Composites in Aqueous EnvironmentsI: Experimental Results, Appl. Comp. Mater., 2003, 10:19-48.

11. Mula S, Bera T, Ray PK and Ray BC. Effects of Hydrothermal Aging on Mechanical Behavior of Sub-zero Weathered GFRP Composites, Jour. of Reinf. Plast. and Comp. 2006,6, p. 673-680.

12. Wan B, Petrou MF and Harries K. The effects of the presence of water on the durability of bond between CFRP and concrete, Jour. of Reinf. Plast. Comp., 2006, 25(8):875-890.

13. P.Sampath Rao, and Dr. M. Manzoor Husain, Hydrothermal ageing effects on Flexural Properties of gfrp composite laminates, Indian Journal of Engineering and Material Science Volume-20 October 2013, pp 415-424,Publisher: CSIR- NISCAIR.

14. Mouzakis ED, Zoga H and Galiotis C. Accelerated environmental ageing study of polyester/glass fiber reinforced composites (GFRPCs), Jour. Comp.: Part B, 2008, 39, p. 467- 475