Comparatively analysis of the effects of water / seawater on the composites made of E-glass woven fabrics and chopped fibres.

Cerbu, Camelia ; Ciofoaia, Vasile ; Teodorescu-Draghicescu, Horatiu 等

1. INTRODUCTION

Nowadays, it is known the great interest for the researches concerning the effects of the aggressive environment (water, seawater, thermal cycles, chemical solution etc.) on the mechanical characteristics of the composite materials (Aktas et al., 2008; Cerbu, 2007).

On the other hand, the manufacturing technology could also affect the mechanical behaviour of the composite subjected to the aggressive environments. A previous paper (Cerbu et al., 2008) showed the effects of the technology on the mechanical behaviour of some laminated composite reinforced with E-glass woven fabrics.

Herein, the results of the tensile test were used to mechanically characterize the effects of the moisture absorption on the composite material studied.

2. MATERIALS AND METHODS

The first of all, a laminated composite plate is manufactured. It may note that 8 layers of the laminated composites are made of woven fabrics EWR300 / polyester Copoly 7233 while 10 layers are reinforced with chopped E-glass fibres. The different layers are laid alternately. A higher pressure was used in the moulding step of the technology. Some characteristics of the EWR300 woven fabrics made of continue E-glass fibres, are known: weight [gamma] = 315 [+ or -] 5% g/[mm.sup.2]; thickness g = 0.3 [+ or -] 0.05mm ; tensile strength [sigma] = 1750 N/ a narrow strip of 50x0.3[mm.sup.2] in case of the warp; tensile strength [sigma] = 2500 N / a narrow strip of 50x0.3 [mm.sup.2] in case of the weft.

[FIGURE 1 OMITTED]

Then, the specimens were cut from the plate for the tensile test. The dimensions of the specimens were established according to the actual European standards (***SR EN ISO 527, 2000).

Before the tensile test, some specimens were immersed in water at room temperature during 5255 hours while the other specimens were kept in natural seawater (Black Sea) for 5810 hours. The water and seawater absorption was periodically recorded by considering of recommendations of the actual European Standards for plastics (***SR EN ISO 62, 2008). With this purpose in view, some specimens were firstly dried during three days at 40[degrees]C before immersion.

After immersion, the both dried and wet specimens were subjected to tensile test. The results obtained in the tensile test for the wet specimens were compared with the ones obtained in case of the dried specimens (blank test).

The speed of loading during the tensile test was 1 mm/min.

Before each tensile test of a specimen, the dimensions of the cross-section were accurately measured and then, they were considered as input data in the software program of the machine. The testing equipment allowed us to record pairs of values (force F and elongation [DELTA]1, stress [sigma] and strain [epsilon]) in form of files having 100-150 recordings. The testing machine also gave us the results of a statistical calculus for the set of the specimens tested.

3. RESULTS AND DISCUSSIONS

Figure 1 shows the absorption data recorded in case of the composite specimens immersed in water / seawater.

[FIGURE 2 OMITTED]

The parabolic trend line of the absorption data is drawn with continuous line in case of the water absorption while it is represented with dash line in case of the seawater absorption. It may be easily observed that the absorption curve recorded in case of the specimens immersed in water, is located below the one recorded in case of the specimens kept in seawater. There is a quite small difference between the two absorption curves recorded. It follows that the diffusivity of the water and seawater inside the composite material, has approximately the same value.

The quantity of the absorbed water represented 0,451 % in weight after 5255 hours of immersion while the seawater absorbed was 0,376 % after 5810 hours of immersion.

It may observe that the absorption curves have an asymptotic behaviour after [approximately equal to] 3600 hours. Therefore, it may be assumed that the composite specimens approached the saturation point.

Experimental results recorded during tensile tests in case of all specimens tested, may be graphically drawn by using [sigma] - [epsilon] coordinates (fig. 2).

Analysing the [sigma]-[epsilon] curves it may easily observe that the specimens immersed in water for 5255 hours are more rigid than the both dried specimens and specimens immersed in seawater, respectively.

The mean values of some mechanical characteristics (Young's modulus E, tensile stress [[sigma].sub.max] at maximum load) are graphically plotted (fig. 3, 4) while other experimental results may be found in the table 1.

It may be noted that Young's modulus was computed on the linear portion of the a--s curve. Generally speaking, it may also remark that the Young's modulus E has the highest value (81134 MPa) in case of the composite specimens tested after 5255 hours of water immersion. But the maximum tensile stress [[sigma].sub.max] decreases with 13.13 % due to the water absorption.

A little degradation 3.046 % of the Young's modulus E (from 18910 MPa down to 18334 MPa) was recorded after 5810 hours of immersion in seawater (fig. 3). On the other hand, the maximum tensile stress [[sigma].sub.max] increases a little due to the seawater absorption (fig. 4).

4. CONCLUSIONS

It could observe that the polymer composite reinforced with both E-glass woven fabrics and E-glass chopped fibres became more rigid due to the water absorption while the maximum tensile stress [[sigma].sub.max] decreased. Since the quantity of the water absorbed inside the specimens is approximately the same, the nature of the environment could be the cause of the increasing of the stiffness after immersion in water. The reasons could be the modifications of polyester resins induced by seawater absorption like other researchers (Visco et al., 2008) published.

Results concerning to the distinct effects of the both water and seawater, on the composite material tested, could be used to design composite structures that work in wet environment such as boat hull, vessel, sanitary-engineering installations etc. By using the method of finite elements, these results will be used to analyse the changing of strains and displacement in such as members after material degradation.

5. ACKNOWLEDGEMENT

The research described within the present paper was possible owing to a national scientific project exploratory research, Project ID_191/2007, supported by Ministry of Education and Research of Romania.

6. REFERENCES

Aktas, A. & Uzun, I. (2008). Sea water effect on pinned-joint glass fibre composite materials, Composite Structures J., 85 (2008) pp 59-63, ISSN 0263-8223

Cerbu, C. (2007). Aspects concerning the degradation of the elastic and strength characteristics of the glass / polymer composite material due to the moisture absorption, Plastic Materials J., 44, nr. 2, (june 2007), pp 97-102, ISSN 0025-5289

Cerbu, C.; Ciofoaia V. & Curtu I. (2008). The effects of the manufacturing on the mechanical characteristics of the E-glass /epoxy composites, Proceedings of The 12th International Research / Expert Conference "Trends in the development of machinery and associated technology", pp 229-232, ISBN 978-9958-617-41-6, Istanbul, Turkey, august, 2008, Faculty of Mec. Eng. in Zenica

Visco, A.M.; Calabrese, L. & Cianciafara P. (2008). Modification of polyester resin based composites induced by seawater absorption, Composites J.: Part A, 39 (2008) pp 805-814, ISSN 1359-835X

***(2000) SR EN ISO 527. Plastics. Determination of tensile properties, European Committee for Standardization, Brussels, 2000

***(2008) SR EN ISO 62, Plastics. Determination of water absorption, European Committee for Standardization, Brussels, 2008

Tab. 1. Mean values of some mechanical characteristics
obtained by tensile test

 Mechanical Dried Wet specimens
 characteristics specimen
 After immersion After immersion
 in water in seawater

Load at maximum load 17427 15487 20224
(N)

Strain at maximum load 0.0126 0,0041762 0,011631

Work from preload to 6057 4265 6585
maximum load (N-mm)

Extension at Maximum 0,63001 0,50115 0,58154
Load (mm)

Fig. 3. Experimental results concerning Young's modulus E

Young's modulus E [MPa]

Dried specimens 18910
After immersion in water 81134
After immersion in seawater 18334

Note: Table made from bar graph.

Fig. 4. Experimental results concerning the tensile stress
[[sigma].sub.max]

Maximum tensile stress
[MPa]

Dried specimens 134,0
After immersion in water 116,4
After immersion in seawater 142,55

Note: Table made from bar graph.