An Investigation on Strength Degradation of Gfrp Laminates Under Environmental Impact

The usage and replacement of conventional materials with polymer composite materials for engineering applications is always questioned by the end user, unless otherwise supported by the authentic published research. The reinforcement materials are highly hygroscopic; the matrix material provides protection to the reinforcement. Since the edges of composite components and surface are exposed to environment, water molecules travel along the reinforcement, which can damage the interfacial bonding, further the performance of the composite laminate may get affected. In this scenario the investigation related to this aspect, requires quantitative assessment which is carried out to estimate the damage associated by accelerated simulation of the real time situation provides information to ext rapolate the effects of the moisture associated damage to the composite laminate. In this present work an attempt has been made in establishing the investigation procedure to estimate the influence of moisture absorption on strength degradation coupled with temperature. To estimate the life cycle time of polymer composite components such as marine boats and components related to submarine applicat ions. From the test results, it is established reduced that the tensile strength and flexural modulus were reduced significantly, of the specimens subjected to water soaking and varying temperature.


Introduction
Glass fibre reinforced plastic materials are of lo w cost, light in weight, have good mechanical properties thus having potential to use them for structural applications such as equipment for chemical plants and pipelines which are subjected to aggressive environment. Therefore, the informat ion comprising the effects of moisture absorption at higher temperatures on the mechanical properties of g lass reinforced co mposites is very essential. Glass fibre reinforced po ly mer co mposites (GFRP) show relatively low degradation in various corrosive environments in the unstressed state, however, they are very susceptible to stress corrosion, especially in d ilute mineral acid environment [1,2]. Krystyna Imielinska discussed in detail about the environmental stress cracking characteristics of GFRP and (A-G) FRP which were studied using CT (fracture mechanics) samples under.
Constant tensile load and water environ ment. For GFRP the characteristics of crack length as a function exposure time (upto3 months).Ductile aramid fibres seemed to protect the glass fibre rein forcement fro m stress cracking due to higher chemical resistance and complex failure mechanis ms [3].Accelerated environ mental ageing study of polyester/glass fibre reinforced co mposites (GFRPCs) were studied based on two kinds of alternating cycles, which provided humid ity, temperature and u ltraviolet radiat ion 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 s ma ll deteriorat ion in strength was found [6] Tensile and fle xu ral strength of bamboo fibre reinforced polypropylene co mposite and bamboo-glass fibre reinforced polypropylene hybrid co mposite were reduced [8]. On similar way experimental work has been done with polyester-glass fibre reinforced co mposites. The environmental stress cracking failure due to temperature and mo isture has been studied for g lass fibre reinfo rced composites [9]. Moisture does not only affect the adhesive bond of the bonded system in service, but also during the application of FRP on concrete surface. Tests on CFRP bonded to concrete with init ially da mp surface using a modified cantilever beam indicated reduction in bond strength when compared to specimens with initially dry concrete surface [11]. Since the failu re under effect of mo isture generally occurs by either concrete delaminating or concrete-epoxy interface separation. The effects of variable mo isture conditions on the fracture toughness of concrete/FRP bonded system are studied by means of the peel and shear fracture toughness determined fro m the conditioned test specimens. Moisture conditions can result in strength degradation [13].The ma in object ive of this work is to investigate the effects of hydro aging and hydrothermal aging environmental (constant temperature water bath) conditions on the performance and durability of glass fibre reinforced poly mer.

Preparati on of Test Samples
The specimens for the present work are prepared using RTM Machine. The specifications for the laminate preparation are (i) in jection 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 d imensions 250 mm x 30 mm x 8 mm (as shown fig.1). These Specimens are immersed in the water bath at room temperature for a period of 180days and other set of specimens immersed inconstant temperature water bath which is maintained at 60 0 Cfor a period of 60days (as shown fig.2&3).

Specifications of materi als used in the composite lami nate:
Matrix: General purpose polyester resin (commercial Grade) Glass fibre: Saint Gobain ma kes E-Glass Chopped strand mat (stitched) 450g/s-m, the laminates are prepared with RTM process . The volume fract ion of the reinforcement loading about 40% which is found by Burn test the remaining is matrix About 60%

Experi mentation
The present work is focused to understand influence of the water soaking time which leads to reduce GFRP composite laminate's tensile strength. The experiments were carried out on number of specimens moulded by RTM and exposed to water bath which is maintained at room temperature and constant temperature at 60 0 C. Every 30 days specimens are taken fro m bath which are exposed to roo m temperature and fo r every10 days specimens are taken fro m bath which is maintained at 60 0 C temperature, and carried out tensile tests, 3-point bending tests to determine the young's and flexural modulus which are exposed to various periods. rious Figure 5. Tensile testing on UTM

Results-Tensile and Flexural Modulus
The laminates are exposed to water bath at roo m temperature, and are tested with tensile and 3-point bending test. This is repeated for every 30 days , the results are noted and the same d isplayed in g raphs 1&3 .The laminates are exposed at constant temperature 60 0 C, tested with tensile and 3-po int bending test. This is repeated for every 10 days, and plotted graphs2&4 with the obtained results. Fro m stress-strain graphs tensile young's modulus of the specimens are calculated with in the elastic limits choosing three points in a straight line portion and shown in graphs5and6. The specimen exposed for 60 0 C fo r 10days tensile modulus is calcu lated as Young's Modulus (10 Days), E 1 = (8/0.7+10/0.9+10.5/1)/3*100 N/mm 2 =1.101Gpa Fle xu ral modulus of elasticity is calculated with graphs 3 and 4 for specimens exposed at room temperature and constant temperature 60 0 C with defferent exposure times by using following equation and the results are shown in graphs5and 6. L = Support span (laminate length), (mm) M=slope of the deflection curve in initial straight portion (N/ mm) b = Width of test beam, (mm) d = Depth of tested beam, (mm) The specimen exposed at room temperature for 180days flexu ral modulus is calculated as E f1 =L 3 *M/4bd 3 E f1 =220 3 *0.0333*10 3 /4*30*8 3 =5.7764Gpa Figure 6. Three point bending testing on UTM Graph 1. Tensile t est-specimens exposed to room temperature (st ress V/S Strain at various exposed time 30 to180days) Graph 2. T ensile t est-Specimen exposed to constant temperature 600C (stress V/s Strain at various exposed time 10 to 60 days) Graph 3. Three point bending t est-Specimen exposed at room temperature (Load v/s Deflection exposed time 30 to 180days) Graph 4. Three point bending t est-Specimen exposed at temperature 60degrees (Load v/s deflection exposed time 10 to 60days)

Discussions
The experimental results reveal that the GFRP (E-Glass/Polyester) samples subjected to waters absorption at room temperature (results as shown in Graph5) the tensile modules has been rapidly reduced till 90 days and there a moderate reduction in it, whereas the flexural modulus has sharp reduction in the first 60 days and then a gradual reduction is observed . The samples subjected to aging at the constant temperature water bath (60ºC) showed a hyperbolic decrement in the tensile strength and the flexural modules (as shown in Graph6). On the whole it was observed that tensile modulus decreased to some e xtent with the presence of mo isture and temperature.
There is significant reduction in modulus because of loosing bonding strength of the polyester resin and fibre inter phase at room temperature. It is clear that the flexural modulus rapidly decreases with hydro aging and hydrothermal aging, because moisture generally affects any property which is do minated by the matrix and/or interface. However the flexu ral strength being a fibre dominated property the strength reduction occurs only if the fibres themselves are affected by hydrothermal environ mental conditions..
The GFRP specimen showed a 66.7% reduction in the young's modules due to the increase in the water bath temperature, i.e., there is a considerable effect of the bath temperature on the mechanical properties of the GFRP composites.

Conclusions
Fro m the investigation it is observed that there is a remarkable reduction in mechanical strength (young's and flexu ral modulus) is observed in GFRP co mposite laminates which are subjected to different environ mental conditions a exposure time . The flexural strength values of the specimens are decreased with exposure period of 60days in water at constant temperature. As per the results initially rapid reduction in mechanical properties is observed and gradual decrease is observed during next phase.
The follo wing important conclusions were drawn fro m test results. a). The presence of mo isture or water part icles in the matrix, fibre-matrix interface and also attack on the glass fibres are all the reason for reduction of properties is due to interfacial bond damage. b).The tensile and fle xu ral modulus reduction is more in Hygrothermal aging when compare to hydro aging because the Temperature accelerated the aging processes c).It is worth noticing that aging at higher temperatures caused colour change in samples. e).The change in tensile properties of laminates during aging are due to of Hygrothermal degradation of g lass fibre, matrix interface.