G. Promis 1, A. Gabor 1 and P. Hamelin 1 1

Transcription

1 INVESTIGATION OF PULTRUDED FIBRE REINFORCED MINERAL POLYMER BASED COMPOSITE STRUCTURAL ELEMENTS AND CONFINED BY 3D WEAVING TECHNOLOGY FOR STRUCTURAL APPLICATIONS G. Promis 1, A. Gabor 1 and P. Hamelin 1 1 Civil Engineering and Environmental Laboratory (LGCIE) - Site Bohr, University Claude Bernard Lyon1, 82, Boulevard Niels Bohr, Campus de la DOUA, Villeurbanne Cedex patrice.hamelin@univ-lyon1.fr ABSTRACT In this paper the mechanical behaviour in bending of textile reinforced mineral matrix composite beams is analyzed. Two types of textile reinforcement made in E-glass fibers are considered: unidirectional rovings and chopped strand mat (CSM). The mineral matrix is an inorganic phosphate cement (IPC) and presents the advantage to be nonalkaline. The beams are hollow with square cross section, they are realized by bonding together composite plates having the dimensions 200x10x1 cm. The plates are manufactured by pultrusion process, using the roving and the mineral matrix. In the first part of the paper, we present the manufacturing process, and the mechanical characteristics of the composite itself in tension and shear are assessed. During the characterization tests, it has been emphasized that the shear behaviour is the weak point of the composite. Therefore, in order to enhance the mechanical behaviour of the beams, specimens have been strengthened using three types of reinforcement: internal reinforcement with struts, 3D external braiding and continuous wrapping by CSM reinforced mineral matrix. In the second part of the paper, the test in 4 point bending of four beams is presented. The analysis of the experimental results is governed by the search for a correlation between the phenomena observed at material level and those occurred during the tests of the structural elements. KEYWORDS textile reinforcement, mineral matrix composite, pultrusion, 3D braiding process, bending behaviour. INTRODUCTION Composite materials have a large utilisation in several industries, such as automotive, space or medicine. Newly developed composites used in civil engineering have to combine mechanical performances and architectural freedom, satisfying environmental and sustainable development criteria. Mineral matrix composites could be a solution for these requirements (Brockmann, T. and Brameshuber, W. (2005)). Mineral matrices are characterised by a fragile failure. Even if they have a good mechanical behaviour in compression, they have to be strengthened in order to ameliorate their weakness in tension or shear. The reinforcement can be of different types and origins: glass, carbon, vegetal, continuous fibres or textiles (Roye, A. and Gries, T. (2005)). These composites have the advantage of an automatic industrial process (Hegger, J.and Voss, S. (2005)), with a quite exact control of the transformation parameters (orientation of fibers, 3D positioning of the textiles, thickness, volume fraction ratio, ). The observed damaged mechanisms occur at different levels: impregnation (Hegger, J.and Voss, S. (2005)), influence of the geometry of the textile reinforcement (Peled, A. (2005)), cracking of the composite material (Cuypers et al. (2005)). The applications using fibre reinforced mineral matrix composites or textile reinforced concrete is increasing and focus also on loadbearing structural elements. Consequently detailed information is needed for the safe design of these loadbearing structures (Magalhaes, A. et al. (1996)). INTRINSIC CHARACTERISTICS OF THE COMPOSITE 419

2 The matrix of the composite has been developed at the Vrije Universiteit Brussel (VUB) and it is a new nonalkaline mineral polymer named inorganic phosphate cement (IPC). The matrix is composed by a meta calcium silicate (wollastonite), phosphoric acid and zinc oxide. The calcium silicate reacts with the metal ions of the phosphoric acid, forming a 3D reticular ceramic matrix. The IPC is a new material combining the flexibility of polyester resins with the properties of the ceramics. The obtained ceramic paste can be generally reinforced by glass fibres or textiles in order to obtain a composite with increased toughness, tensile strength and stiffness. This new material shows some advantages: low manufacturing cost based on reactive mineral powders, a noninflammable behaviour, an environmentally friendly composition and a chemically resistant behaviour. Manufacturing procedure For the realization of the beams, plates have been manufactured using pultrusion technique. The pultrusion process is similar to that used in the case of organic polymer matrix composites (e.g. epoxy, polyurethane) with some modifications of the impregnation bath and the shaping die. The scheme of the pultrusion process is presented in Figure 1. Figure 1: Outline of the pultrusion process The reinforcement consists of classical unidirectional E-glass roving having a linear weight of 735 tex which passes trough an impregnation bath. During the process, the bath is filled with the paste of IPC. It is important to continuously prepare and refresh the contents of the bath because of the relatively short pot-life (approximately 20 min.) of the mixture. The filaments are guided by three rectangular baffles having uniformly disposed ranges of holes of different diameters: =4 mm on the front baffle, =6 mm on the intermediate one and respectively =10 mm on the rear one (see Figure 2). After the impregnation of the filaments in the bath, they pass through calibration pipes having the same diameters as the holes of the rear baffle. The next step is the final shaping and curing in an oven: the composite passes first through a classical die at 40 C and through an open die at a temperature of 70 C. The pulling unit ensures the advancement of the process at a speed of 20 cm/min. The fibre volume fraction ratio obtained here is estimated to about 22%. The characteristics of the matrix and E-glass rovings are summarized in Table 1. Figure 2: Disposition of the holes on the impregnation bath Mechanical properties of the composite in tension, compression and shear In a first approach, for the evaluation of the mechanical properties, the characteristics in tension, compression and shear have been considered. For the tensile tests six specimens have been cut out from the pultruded plates along the longitudinal direction, having the dimensions of 330x47x9 mm. Testing conditions are similar to those described by the standard ISO applied for organic polymer composites. The test are made on a universal testing machine: the load is measured by a 200 kn cell and the longitudinal and transverse strain are recorded by strain gauges at a frequency of 10 Hz. The load is displacement controlled and has a rate of 1 mm/min. The observed non-linear behaviour, with an initial stiffness in the non-cracked zone degraded by the apparition of 420

3 cracks. This is explained by the big difference between the failure strain of the matrix and that of the textile reinforcement. Table 1: Physical properties of the composite's components IPC matrix E-glass roving Density(kg/m 3 ) Linear weight (tex) Elastic modulus (MPa) Shear modulus (MPa) Poisson s coefficient Tensile strength (MPa) Shear strength (MPa) MPa 160 MPa 140 MPa 120 MPa Area I Area II Stress [MPa] 100 MPa 80 MPa 60 MPa 40 MPa 20 MPa 0 MPa 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% Strain [%] Figure 3: Stress strain curves obtained by tensile tests Concerning the behaviour in compression, several authors consider (Berthelot, J. M. (1999)) that in general the behaviour of a composite is the same in compression and in tension in the early stage of the behaviour (same Young modulus). For the IPC, this is confirmed by Cuypers with a failure strength of the composite equal to approximately 60 MPa, that corresponds to the failure strength of the pure IPC. Therefore, we consider a tensile/compression constitutive equation for the composite as shown in the Figure Stress [MPa] 38 E = MPa E = E = MPa 0.15 Strain [%] Figure 4: Constitutive equation of the composite in tension/compression In order to estimate the shear resistance of the composite, 3 point bending test has been performed on short beam specimens. This test method involves loading a plate under three-point bending with the dimensions such that an interlaminar shear failure is induced. This method measures the "apparent" interlaminar shear strength of composite materials. In the present case, 4 specimens of 10 mm thickness have been tested having a thickness/span ratio of 5 (Figure 5). The shear strength is calculated with the formula: 421

4 3F τ =, (1) 4S where F and S stand for the applied load and the cross section area, respectively. For the shear strength of the composite an average value of 6.8 MPa has been obtained. CHARACTERISTICS OF BEAMS Figure 5: Outline of the interlaminar shear stress A first beam has been realized assembling together the plates obtained by the pultrusion process in a manner to have a hollow square cross section. The four plates forming the beam have been bonded together using a bicomponent epoxy resin, hardening at room temperature. This first beam is referenced B mm 97 mm 10 mm 87 mm 107 mm 87 mm 107 mm Figure 6: Cross section and experimental setup of the beam B1 Knowing that the shear characteristics of the composite material are not satisfactory, three other beams, having the same geometrical configuration as the initial beam B1, have been realized with different types of reinforcements. - internal strengthening: consists in the bonding at the interior of the beam vertical struts, cutted-out from the pultruted plates, having a spacing of 200mm (Figure 7). The struts are placed in a way that the fibre reinforcement is oriented vertically in order to have a maximum efficiency. The reference of this beam is BS2. - confinement strengthening: an external 3D braiding is realized with carbon strands (1000dtex, 4.1 mm width) spaced at 20 mm and inclined at 45. The average thickness of the braiding is about 1 mm. The braiding process is a classical one used in the industry. In the present case 36 carriers have been used for supporting the roving rollers. The beam is fixed on a support and travelled across the braiding device (Figure 8). This beam is referenced BC3. - external strengthening: the beam is wounded around with a continuous layer of IPC composite reinforced with a 300 g/m² E-glass chopped strand mat (CSM). A hand lay-up technique similar to that used for classic FRP composites is employed for the realization of the winding directly on the beam. The additional thickness of this reinforcement is about 4.1 mm. This latter beam is referenced BW4. The mechanical properties of the CSM reinforced composite are presented in Table 2. Its behaviour is bilinear as for the pultruded composite but with other characteristic values. 422

5 Figure 7: Overview of the beam BS2, strengthened with struts (geometry and failure after test) EXPERIMENTAL STUDY Figure 8: Overview of the braiding process used for the beam BC3 Table 2: Mechanical characteristics of the CSM composite CSM composite Non cracked Cracked Reinforcement volume fraction (%) 16 Young s modulus Limit stress (MPa) Limit strain (%) All the four beams have been tested in four point bending with a usual instrumentation in order to realize the analysis of the mechanical behaviour. The beams having a length of 2000 mm, the distance between the bearings is established at 1890 mm; loads are applied at the third of the span (630 mm), as shown in Figure 6. The instrumentation of the beams is made in order to ensure the acquisition of the following parameters: applied load, mid span displacement, strain fields in the different zones of the beams. The test is force controlled. Analysis of the global behaviour Figure 9 illustrates the behaviour of the beams in terms of applied load and mid-span deflection: it seems to be a linear relation between the two parameters until the maximum load supported by the beams, which in the case of beams B1 and BC3 corresponds to the failure load. Concerning the evolution of stiffnesses, for the beams B1, BS2 and BC3 their are quasi identical (1160 N/mm) with low discrepancy (less than 5\%). This is fully understandable since little material was added, with low effect on the geometrical and mechanical configuration of the beam. On the other hand, for the last beam an increase of 12% of the stiffness is noticed since in this case the total thickness of the external layers is about 8.2 mm. The failure of the beam B1 is due exclusively to shear, with a quasi instantaneous development of a horizontal crack at the mid-height of the beam, parallel to the orientation of the reinforcing roving strands. The slipping between the upper and lower part of the beam is about 4.7 mm, measured at the level of the bearings. The maximum deflection is equal to 14.5 mm (0.73% of the span). The beam BS2 shows a reduced strength compared to that of the reference beam B1, its failure is due also to shear, with the same longitudinal crack at the mid-height of the beam. In fact, the struts have a damaging effect instead of reinforcing: the struts reduce the buckling length of the fibres in the compression zone, inducing supplementary longitudinal stresses. The carbon fibre braided beam BC3 shows an increase in strength of about 30%, the maximum deflection is 18.2 mm (1% of the span). In this case, the shear failure have been accompanied by the local debonding of the braiding. For the fourth beam BW4, wrapped with external layers, the increase in ultimate load is very important, approximately 423

6 135\%. The beam still failed in shear, as the crack started at the zone where the load has been applied and developed at 45 can testify (Figure 9) Shear force (dan) Mid-span displacement (mm) Figure 9: Global response of beams in terms of shear force mid-span displacement curves CONCLUSIONS In order to realize structural elements it is important to develop adequate manufacturing process for the composite material. For this, the pultrusion technique has been adapted to the use of IPC composite material. The pultruded plates show some deficiency in terms of shear capacity, therefore an improvement of its performance is necessary. The external confinement by carbon fiber braiding increase the shear capacity in the range of 25-30\%, but the weak adhesion between the beam and the braiding (only mechanical confinement) limits its effect. The external wrapping with CSM reinforced IPC composite is the most effective, with an increase of the shear load in the range of 135%. The maximum flexural loadbearing capacity is not reached, thus an improvement of the shear behaviour is still recommendable. ACKNOWLEDGMENTS This work was supported by the Contex-T project of the Sixth Framework Programme (Priority 3) of the European Union, to which the authors are very grateful. REFERENCES Berthelot, J. M. (1999). Matériaux composites : Comportement mécanique et analyse des structures, 3ème édition, Edition TEC & DOC, Paris, ISBN Brockmann, T. and Brameshuber, W. (2005). Matrix development for the production technology of textile reinforced concrete (TRC) structural elements, Composites in Construction (CCC 2005), Third International Conference, France, Lyon, Cuypers, H., Van Itterbeeck, P., De Bolster, E. and Wastiels, J. (2005), Durability of cementitious composites Composites in Construction (CCC 2005), Third International Conference, France, Lyon, Hegger, J.and Voss, S. (2005). Textile reinforced concrete Bearing behavior, design, applications, Composite in Construction,Composites in Construction (CCC 2005), Third International Conference, France, Lyon, p Magalhaes, A., Marques, A., Oliveira, F., Soukatchoff, P. and Castro, P. (1996). Mechanical behaviour of cementitious matrix composites, Cement & Concrete Composites, Vol. 18, Peled, A. (2005). Textile cement based composites, effects of fabric geometry, fabric type and processing, Composite in Construction, Third International Conference, France, Lyon, p , 9-22 Peled, A. (2005). Textile cement based composites, effects of fabric geometry, fabric type and processing, Composite in Construction, Third International Conference, France, Lyon, Roye, A. and Gries, T. (2005). Tensile behavior of rovings, textiles and concrete elements, Composites in Construction (CCC 2005), Third International Conference, France, Lyon,