D. Hiilsenberg et al Composites: Part B 39(2008)362-373 led-out nber surface remained fber channel Fig 13. Ti-presence, measured by EDX, in the debonded areas of the composite Nextel 440/BN, TiO2/756 glass desized fibers. The different thicknesses of the layers do not ing from the differences in thermal expansion. Both curves seem to exert a particularly strong influence indicate a completely brittle-elastic behavior. The fiber 2 The composites are white-translucent, not transparent. being coated only with TiO2, the mechanical properties his results despite adapted refractive indices- from of the composite will not improve either. This curve will the thermally caused internal micro stresses cover the two curves described above and is therefore not The stress-strain curves of different Nextel 440 fibers/N- drawn. As the TiO2 reacts with the N-sK 4 glass, no differ sk 4 glass matrix composites can be seen in Fig 14 ence to the behavior of the desized fiber can be stated It becomes immediately evident that none of the samples ter a bn coating has been carried out, the com- breaks damage-tolerant. Furthermore, all samples present posite properties will change greatly. Here, however, the nearly the same curve ramp in the coordinate origin, i.e., effect exerted by a single BN layer(curve 2)is not as strong nearly the same E-modulus. It is determined by the as the effect exerted by a BN/TiO2 double layer(curves 3, 5 mechanical properties of the matrix glass rather than by and 6). The reasons for this are explained in the preceding the relatively low fiber volume portion. This is the bad chapter. The definitive fracture of all composites, however, point about the results obtained is brittle. If the bending stress exceeds a value of about The very good points, however, are the high tensile 180 MPa and increases further, the sample will stretch strength of the composites, the high fracture strain, and more than below this value. Depending on the type of coat the large area below the stress-strain curve ing, the fracture strain is 8-17 times higher than in the case The pure, non-reinforced hot-pressed matrix glass of non-reinforced glass, and can even reach the unexpect- (curve 1)presents a tensile strength whose value is the same edly high value of 3.5%. The explanation of this result as specified by the manufacturer for the compact glass has not completely been convincing so far. It could be, so material (cf. Table 2). The composite with desized fibers to say, a competition taking place between internal micro (curve 4) presents even worse values, which is due to chem- stresses caused by Ao or clampings, and the fiber pull-out ical reactions taking place between the fibers and the caused by the turbostratic BN layers. Over the whole per matrix during hot pressing, and to internal stresses result- iod when loading is exerted, the radial compressive stress ed fiber FVc 8% 2.40nm BN. FVC 16% 5-35nm BN-30nm TiO. FVc 4001 3-40nm BN-110nm TiO2, FVC 16% 6-100nm BN-40nm TiO2, FVC 3 50 Fig. 14. Stress-strain-curves of Nextel 440-fibers/N-SK 4-glass matrix composites. The fiber-coating and the fiber-volume-content varydesized fibers. The different thicknesses of the layers do not seem to exert a particularly strong influence. The composites are white-translucent, not transparent. This results – despite adapted refractive indices – from the thermally caused internal micro stresses. The stress–strain curves of different Nextel 440 fibers/N￾SK 4 glass matrix composites can be seen in Fig. 14. It becomes immediately evident that none of the samples breaks damage-tolerant. Furthermore, all samples present nearly the same curve ramp in the coordinate origin, i.e., nearly the same E-modulus. It is determined by the mechanical properties of the matrix glass rather than by the relatively low fiber volume portion. This is the bad point about the results obtained. The very good points, however, are the high tensile strength of the composites, the high fracture strain, and the large area below the stress–strain curve. The pure, non-reinforced hot-pressed matrix glass (curve 1) presents a tensile strength whose value is the same as specified by the manufacturer for the compact glass material (cf. Table 2). The composite with desized fibers (curve 4) presents even worse values, which is due to chem￾ical reactions taking place between the fibers and the matrix during hot pressing, and to internal stresses result￾ing from the differences in thermal expansion. Both curves indicate a completely brittle-elastic behavior. The fiber being coated only with TiO2, the mechanical properties of the composite will not improve either. This curve will cover the two curves described above and is therefore not drawn. As the TiO2 reacts with the N-SK 4 glass, no differ￾ence to the behavior of the desized fiber can be stated. Only after a BN coating has been carried out, the com￾posite properties will change greatly. Here, however, the effect exerted by a single BN layer (curve 2) is not as strong as the effect exerted by a BN/TiO2 double layer (curves 3, 5 and 6). The reasons for this are explained in the preceding chapter. The definitive fracture of all composites, however, is brittle. If the bending stress exceeds a value of about 180 MPa and increases further, the sample will stretch more than below this value. Depending on the type of coat￾ing, the fracture strain is 8–17 times higher than in the case of non-reinforced glass, and can even reach the unexpect￾edly high value of 3.5%. The explanation of this result has not completely been convincing so far. It could be, so to say, a competition taking place between internal micro stresses caused by Da or clampings, and the fiber pull-out caused by the turbostratic BN layers. Over the whole per￾iod when loading is exerted, the radial compressive stress Fig. 13. Ti4+-presence, measured by EDX, in the debonded areas of the composite Nextel 440/BN, TiO2/756 glass. Fig. 14. Stress–strain-curves of Nextel 440-fibers/N-SK 4-glass matrix composites. The fiber-coating and the fiber-volume-content vary. 370 D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373