Mechanical characterisation of basalt fibre reinforced plastic

Composites: Part B 42 (2011) 717–723

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Mechanical characterisation of basalt fibre reinforced plastic V. Lopresto a,⇑, C. Leone a, I. De Iorio b a b

Department of Materials and Production Engineering, University of Naples ‘‘Federico II’’, P.le Tecchio, 80, 80125 Naples, Italy Department of Aerospace Engineering, University of Naples ‘‘Federico II’’, P.le Tecchio, 80, 80125 Naples, Italy

a r t i c l e

i n f o

Article history: Received 20 May 2010 Received in revised form 20 January 2011 Accepted 27 January 2011 Available online 1 February 2011 Keywords: A. Polymer-matrix composites Basalt fibres A. Glass fibres B. Mechanical properties B. Impact behaviour

a b s t r a c t New perspectives have arisen on basalt fibre applications due to the potential low cost of this material together with its good mechanical performance, in particular at high temperature. The idea to fill these fibres into a polymer matrix is relatively recent and could offer very interesting perspectives that have not yet been sufficiently investigated. In this work, with the principal aim of evaluating the possibility to replace glass fibres in most of their applications, mechanical tests were carried out on comparable E-glass and basalt fibre reinforced plastic laminates. The latter were cut by square plates fabricated through vacuum bag technology. The results obtained on the two laminates were compared showing a high performance of the basalt material in terms of young modulus, compressive and bending strength, impact force and energy. These good properties suggest possible applications of basalt fibres in fields where glass composites are nowadays largely applied. The short-beam strength tests confirmed what above said by denoting an interfacial adhesion similar to that between E-glass and epoxy matrix. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction In the last years, the increasing interest in environmental issues has promoted the employment of natural fibres in polymer reinforcing [1–3]. Many types of natural fibres like sisal, kenaf, hem, flax, coconut and banana have been studied and applied. However, vegetal fibres are very sensitive to thermal and hygroscopic load and show limited mechanical properties due to the fibre extraction system, the difficulty in fibre arrangement, the fibre dimension and the interface strength. A possible solution that takes into account the environmental issues is represented by the use of mineral fibres like basalt. Since deep studies on this material are only recent, in the last 10 years a number of researchers have been investigating properties and behaviour of various composites made of continuous or short basalt fibres [4–15]. Obviously, a wide field concerning this material performance and its possible applications has not been investigated yet. In addition, some discrepancies among the results obtained by different authors have been observed. Thanks to the improvements in production technology, a new generation of basalt fibres is now available on market. Spun basalt fibres are obtained at high temperatures from selected melted basalt rocks [16]. The process technology is extremely similar to that used for glass fibres. The rock is first pre-treated and then melted to obtain continuous fibres. The melt flows into one or more bushings containing hundreds of small orifices. The basalt filaments are ⇑ Corresponding author. E-mail address: [email protected] (V. Lopresto). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.01.030

formed as the molten rock passes through these orifices. The filaments are then pulled over a roller. The advantages of this method are that neither precursor nor additives are necessary in the manufacturing process, with consequent economic gain and reduction of the environmental impact. Of course, on the basis of the specific kind of original rock employed, more than one category of basalt fibres with different chemical compositions can be obtained. As a consequence, not all the basalt fibres show the same mechanical and physical properties. In general, the positive features of this new generation of basalt fibres include sound insulation properties, excellent heat resistance (better than glass), good resistance to chemical attack and low water absorption [4]. For the latter reason they are suggested for applications requiring thermal insulation as well as for hot fluids transportation pipes. Another important characteristic is represented by the high mechanical performance comparable to that of glass fibre [5], that together with the lower cost could make this material suitable to potentially replace glass fibres in various industrial fields like aerospace, automotive, transportation and shipbuilding. Cziga´ny [17] asserted that the cheap basalt fibres can be efficiently applied in hybrid composite systems. They had already been adopted and studied as reinforcement in concrete matrix [7,18], where their high temperature properties such as the high fire resistance were evidenced, or in polymer matrices like epoxy [8], polypropylene [9–11] or phenol–formaldehyde resin [12,13]. The most important feature in composites, i.e. the fibre–matrix interface, has been studied in various basalt fibre-polymer matrix systems [6–8,14,15,19]. In [15], it was demonstrated that the

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1 mm

1 mm

1 mm

1 mm

(A)

(B)

1 mm

1 mm

(C)

(D)

Fig. 1. Picture of the used fabric: (A) basalt fabric, 200 g/m2, plain-weave, (B) E-glass fabric, 290 g/m2, plain-weave, (C) Basalt fibre composite, and (D) glass fibre composite.

interfacial region of basalt fibre reinforced polymer was more vulnerable than that of glass fibre reinforced composites after salt water immersion and moisture absorption. However, an excellent interfacial shear strength was found and in [6,19] it was asserted that basalt fibres form a better surface compared to glass fibres. On the other hand, it is known that glass fibres are susceptible to surface damage and suffer from a high sensitivity to alkaline conditions [20–23] whereas the chemically inert and stiffer carbon fibres present the well known disadvantage of high cost [24]. Basalt products have no toxic reaction with air or water, they are noncombustible and explosion-proof. When in contact with other chemicals they do not produce any chemical reaction that may damage health or the environment. At present no information about the risk related to very low fibre diameters is known. However, according to European law (97/69/Ce and 1907/2006) there should be no risk of toxicity for fibre diameters higher than 6 lm. In this work, with the aim of verifying the real opportunity to replace glass fibres and to enhance the already available experimental results by providing new data, E-glass and basalt fibre reinforced plastic laminates, with dimensions 300 mm  300 mm, were obtained by overlapping (0, 90) fabrics through vacuum bag

technology. Specimens cut from the square panels by a diamond saw were successively tested in order to characterize the basalt behaviour and compare the results between the two different reinforcements. Mechanical tests like tensile, bending, shear, compression and impact tests were carried out by means of a universal and a drop weight machine. By comparing the performance of the two composites, an overall better behaviour of basalt was evidenced, thus suggesting the possibility to use this material even in fields like the automotive, railway, shipbuilding and aerospace as well as in the chemical industry, where glass composites are already largely applied.

2. Materials and test procedures Basalt and glass fibre reinforced plastic laminates with 300 mm  300 mm in plane dimensions were obtained through vacuum bag technology. The different types of reinforcement employed were: basalt dry fabrics, 200 g/m2, plain-weave (warp 10F/ 10 mm, weft 10F/10 mm), tex 100, from ZLBM (De), and E-Glass dry fabric, 290 g/m2, plain-weave (warp 5F/10 mm, weft 5F/

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V. Lopresto et al. / Composites: Part B 42 (2011) 717–723 Table 1 Basic mechanical properties of the used basalt fibres.

a b c

Properties

Basalta

E-glassb

Filament diameter (lm) Density (kg/dm3) Tensile strength (MPa) Elastic modulus (GPa) Elongation at break (%) Max. service temp. (°C)

17 2.8 4800 90 3.15 650

7 2.54 3200 70 4.0 460c

ZLBM Muehlenbein KG datasheet. Della Betta Group datasheet. Average values from [25].

10 mm), tex 300 (Re 290/50 WEB from Della Betta Group, It). In Fig. 1, the frontal view of both the reinforcements adopted are reported (A, B) together width the lateral view of cross section micrograph of both composites (C, D). In Table 1, the basic mechanical properties of the adopted basalt fibres are reported and compared to those of the used E-glass fibres. In order to produce plates with the desired thickness, a sufficient number of plies were impregnated by an epoxy matrix (Becor I-SX10 + hardener SX10M) by hand roller, and overlapped on a release film placed on a glass tool. Then, they were covered with the peel ply, the bleeder and the plastic bag. After that, a curing stage at room temperature and at a vacuum level of -990 mbar was performed for 24 h. In Fig. 2, a schema of the lay up adopted for the panel fabrication is reported.

Different mechanical tests were carried out, according to the ASTM specifications, on both composites: tensile, compression, flexural, shear and low velocity impact test. In order to obtain the required specimen geometries for the above mentioned tests, panels with various thicknesses were obtained by overlapping different numbers of layers. Furthermore, since the difference in aerial weight between the two adopted reinforcements resulted in diverse layer thicknesses, a different number of plies was considered for the two kinds of fibres to obtain the same results in terms of panel thickness. In Tables 2–4 the mechanical tests, the standard test methods, the stacking sequences and the sample geometrical specifications for both materials are reported for each different test. After panel production, the final fibre volume fractions were measured through matrix burning tests according to the ASTM D3171 specifications. Fibre volume fractions of 51% and 46% were found respectively for basalt and glass laminates realised for Table 4 Sample characteristics for the low velocity impact test. Test

St. test method

Reinforcement

Stacking sequence

Thickness (mm)

Plane dimension (mm)

Low velocity impact test

EN 6038

Basalt Glass

[(0/90)]20 [(0/90)]10

3 3

90  90 90  90

7

6 5

8 4 3 2 1 Fig. 2. Schema of the lay up adopted for panel fabrication: (1) glass tool, (2) sealant tape, (3) release film, (4) reinforcement, (5) peel ply, (6) bleeder, (7) vacuum bag, and (8) valve to the vacuum pump.

Table 2 Sample characteristics for the tensile and compressive tests.

a

Test

Standard test method

Reinforcement

Stacking sequence

Thickness (mm)

Width (mm)

Total length (mm)

G.l.a (mm)

Tensile

ASTM D 3039/D 3039M-00

Compression

ASTM D 695–02a

Basalt Glass Basalt Glass

[(0/90)]16 [(0/90)]8 [(0/90)]25 [(0/90)]10

2.5 2.5 4.0 4.0

25 25 12.7 12.7

280 280 79 79

25a 25a – –

An averaging axial extensometer Instron mod. 2620–604 with a gauge length of 25 mm was used.

Table 3 Sample characteristics for the flexural and shear tests. Test

Standard test method

Reinforcement

Stacking sequence

Thickness (mm)

Width (mm)

Span (mm)

Flexural

ASTM D 790-03

Shear

ASTM D 2344/D 2344M–00

Basalt Glass Basalt Glass

[(0/90)]16 [(0/90)]10 [(0/90)]64 [(0/90)]36

2.6 2.6 10 10

13 13 20 20

40 40 60 60

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tensile, bending and shear tests, whereas 47% and 40% fractions were found for panels built for compressive and impact characterisation. The MTS Alliance RT/50 Universal Machine was used for all the mechanical tests except for the impact ones that were carried out on a Ceast Fractovis MK4 falling weight machine. The latter device allows to vary the impact energy by changing the impactor mass and the falling height. In the present campaign, impact tests were carried out up to complete penetration of the coupons, by setting

suitable parameters in order to have an impact energy of about 100 J. The employed instrumented impactor was cylindrical in shape with a 19.8 mm diameter hemispherical nose and struck the specimens, simply supported on a ring with a 50 mm internal diameter, in the centre. The force–displacement curve was recorded after each impact test by the DAS4000 acquisition program. Penetration energies and maximum forces were measured and compared. On the whole, at least five tests per each condition were performed.

30

Young modulus (GPa)

Tensile

Compression

20

10

0 Basalt

Glass

Material Fig. 3. Comparison of tensile, flexural and compressive Young modulus between basalt and E-glass fibre composites.

800 Tensile

Ultimate strength (MPa)

3. Results and discussion

Flexural

Flexural 600

Compression

400

200

0 Basalt

Glass

Material Fig. 4. Comparison of tensile, flexural and compressive ultimate strength between basalt and E-glass fibre composites.

Basalt and glass fibre reinforced plastic specimens were subjected to tensile, bending and compressive standard tests. The obtained results will be hereafter discussed and the analysed experimental data will be reported and compared in the following figures. The reported data consist of the mean values of five or more tests together with the standard deviations represented by vertical bars. As it can be noted from Fig. 3, in all the carried out tests good results were obtained with the basalt composite in terms of young modulus: this material, in fact, showed 35–42% higher values compared to the tested glass counterpart. This result is consistent with [6,22,23,26], where a commercial basalt fibre presented an at least 16% higher modulus than E-glass fibre, but it is in disagreement with what asserted in [27] where the detailed properties of basalt fibre in epoxy matrix compared to glass showed a basalt Young’s modulus comparable to that of the corresponding glass–epoxy composite. On the basis of the data presented in Fig. 3, the mechanical behaviour of the two materials in terms of stiffness seems not to be influenced by the particular test since no substantial variations were found among the modulus obtained through the three different mechanical tests. The discussion concerning the ultimate strength is slightly different since, although only in the tensile test case, glass specimens showed a better behaviour (Fig. 4). On the contrary, in literature no significant difference in tensile, bending and compressive strengths was found between basalt epoxy and the corresponding glass epoxy reinforced by BGF 443 fabric studied in [27]. Also in [6,22,23,26] an equivalent tensile strength was found for the basalt composite compared to E-glass system. Moreover, it is possible to observe that basalt fibre reinforced plastics showed similar tensile and bending strength, meaning that the major bending failure happened in tensile way. The same did not happen for glass fibre specimens that showed different and very low flexural and compressive ultimate strength compared to the basalt ones. These results indicate different bending failure mechanisms. By observing the failure surface after the bending tests, represented in Figs.

Fig. 5. Bending failures: up (A) and bottom (B) side of the tested specimens.

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30 Flexural Compression 20

To overcome the difficulty in obtaining two laminates having different reinforcing and different material density with the same fibre volume fraction, the same data discussed above were rearranged in Figs. 6 and 7. In these figures, starting from the measured fibre volume fraction percentages, a fixed value of 50% of an equivalent composite was considered. Even in this way, although the glass laminate presented higher modulus values compared to what

10

400

0 Basalt (50%)

Glass (50%)

Material Fig. 6. Comparison of tensile, flexural and compressive Young modulus between basalt and E-glass fibre composites having the same fibre volume fraction of 50%.

800

Specific strength (MPa*dm3/kg)

Young modulus (GPa)

Tensile

Ultimate strength (MPa)

Tensile

Tensile Flexural 300

200

100

0 Basalt

Flexural 600

Compression

Glass

Material

Compression

Fig. 9. Tensile, flexural and compressive specific strength of basalt and E-glass composites.

400

50

200 40

Basalt (50%)

Glass (50%)

Material Fig. 7. Comparison of tensile, flexural and compressive ultimate strength between basalt and E-glass fibre composites having the same fibre volume fraction of 50%.

5A and B, different failure mechanisms were recognised: in Fig. 5A, the upper side of the specimen showed damage only in the case of glass composite, whereas the opposite happened on the bottom side (Fig. 5B). As already asserted, this behaviour denotes a compressive failure mode for glass fibre and a tensile failure mode for the basalt fibre when subjected to bending load.

Fsbs (MPa)

0

20

10

0 Basalt

Glass

Material Fig. 10. Short-beam strength (Fsbs) for basalt and E-glass fibre reinforced matrix.

16

25 Tensile Flexural

12

Compression

8

4

Specific Fsbs (MPa*dm3/kg)

Specific modulus (GPa*dm3/kg)

30

20

15

10

5

0

0 Basalt

Glass

Material Fig. 8. Tensile, flexural and compressive specific modulus of basalt and E-glass composites.

Basalt

Glass

Material Fig. 11. Comparison between the specific short-beam strength (Fsbs/density) for basalt and E-glass fibre reinforced matrix.

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shown in Fig. 3, all the results confirmed the same trend: a 45% increased Young’s modulus, independently of the specific experimental test, and a better ultimate flexural and compressive strength for basalt fibre was found. A better behaviour consisting of a 24% higher tensile strength for the glass fibre was confirmed too. Because of the difference in density between the fibres, with the aim of giving a more helpful comparison, the specific properties, that is the ratio between the analysed mechanical properties and the material density, were reported for each single panel (Figs. 8

150

15

120

12

90

9

60

6

30

3

0

Maximum force (kN)

Absorbed energy (J)

Absorbed energy Maximum force

0 Basalt

Glass

Material

Basalt

Fig. 12. Absorbed impact energy and maximum impact force of basalt and E-glass composites.

and 9). Even in this case, all the already discussed trends were confirmed, thus they do not require further discussion. Following the same practice, also the shear strength results obtained from the short beam tests were processed in the same way. For all samples, the observed failure mode was the interlaminar shear one, according to what reported in the ASTM D2344. In opposition to what happened for the other properties, although the basalt short-beam strength value (Fsbs) appeared higher than the glass one (Fig. 10), no significant differences were found comparing the specific Fsbs, that is the Fsbs/density ratios, (Fig. 11). This result is consistent with what found by Liu et al. in [27] as it indicates that a good interfacial adhesion can be obtained by embedding basalt fibres in epoxy matrix. On the contrary, in [15] a more vulnerable interface compared to glass-matrix adhesion was found. To better point out this crucial issue, the broken specimens should be observed through an electronic microscope (SEM) after the shear tests. However, by visually inspecting the specimens after the tests, it was possible to confirm in both cases a correct failure mechanism in shear mode as suggested by the standard. Beyond the impact characterisation, essential for applications in the aeronautical and mechanical fields where crash aspects represent a key point, no substantial differences were found between the two compared systems. This outcome is shown in Fig. 12, where a slightly higher force value was found for E-glass whereas an 11% higher energy absorption capability was noted for basalt. Of course, the latter difference could be explained by the diversity in fibre volume fractions, since in [28] the authors demonstrated that this is a fundamental parameter in governing the impact energy aspects. However, by looking at Fig. 13, a larger delaminated area was observed on the E-glass specimens, thus denoting a higher bent to damage. To confirm this, a huge experimental impact cam-

Impact side

Glass

Back side

Fig. 13. Impact damage.

V. Lopresto et al. / Composites: Part B 42 (2011) 717–723 Table 5 Measured composite materials properties. Test

Properties

Tensile

Us (MPa) E (GPa) Us (MPa) E (GPa) Us (MPa) E (GPa) Fsbs (MPa) Abs. Energy (J) Fmax (kN)

Flexural Compression Shear Impact

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References

Glass

Basalt

Value

St. dev.

Value

St. dev.

582 16 224 15 174 14 36 81 97

12.22 0.51 12.52 1.43 14.78 2.29 1.76 5.60 0.67

506 25 505 23 280 22 41 92 96

20.71 1.10 22.77 1.11 18.22 1.37 0.20 2.35 2.68

paign, varying the impact energy and supporting the experimental tests with microscopic and ultrasonic evaluations, would be necessary. Finally, in Table 5 the measured properties for all the performed tests are reported in order to offer a helpful comparison between the two studied materials.

4. Conclusions By comparing the results of the mechanical tests carried out on equivalent basalt and E-glass fibre reinforced plastic laminates, it was possible to figure out the opportunity to replace glass with basalt as filler in the epoxy matrix in applications where glass composites are already largely applied. In fact, basalt composite showed a 35–42% higher Young’s modulus as well as a better compressive strength and flexural behaviour, whereas a higher tensile strength was found for glass material. The mechanical behaviour of both the analyzed systems in terms of stiffness seems not to be influenced by the specific test so that the small variations in impact response are simply due to the difference in fibre volume fraction. Moreover, different bending failure mechanisms were observed: compressive for glass fibre and tensile for basalt under bending load. The short-beam strength was quite similar for both the materials under investigation as the Fsbs/density ratio was considered. Therefore, a quite good interfacial adhesion, not worse than the one between E-glass and epoxy matrix, was confirmed. Of course, we do not know if the better basalt behaviour is due to the different areal weight but at this stage the problem was overcame normalising the properties respect to the density confirming the results. Even if additional data were provided within the present work, the need of further investigations about a relatively young material such as the basalt fibre composite is evident. The interfacial adhesion examination through electronic microscope (SEM) as well as the analysis of material properties at high temperature could be very useful in order to offer new perspectives in the fields of engineering.

Acknowledgements CIRTIBS Research Centre of University of Naples Federico II is acknowledged for providing the facilities and the financial support to develop the present research work.

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