D. Hiilsenberg et al/ Composites: Part B 39(2008)362-373 developing during cooling is acting on the fibers. These are well distributed micro stresses which. at first. will not exceed the strength of the composite at no point. As the portion of BN-coated fibers in the composites amounts to 16-17 vol% only, the E-modulus of the matrix glass will p mechanical behavior will only be linear(Hooke 's law.If, 9 101 be predominant. Up to a bending stress of 180 MPa, the however, the bending stress exceeds this value, then the the fibers to slide despite the radial compressive stress. At005 what stress sliding sets in will depend on the thickness of fiber coating Fig. 16. Stress-strain- curves of two samples of a Nextel 440-fiber/25 nm The fiber pull-out is illustrated in Fig. 15 and does not BN-layer/8650 glass matrix composite with 15 vol% fiber content. seem to differ from the damage-tolerant behavior of other composites. The fracture aspect of the matrix, however, shows another interesting effect. The very small lumps of Due to the fact that the financial support of the research glass look like broken, hardened single-sheet safety glass. made from double-coated Nextel 440 fibers and an 8650 Therefore, the following preliminary conclusion concern- glass matrix will be have to be investigated further in the ing the failure mechanism of these special composites can future. They allow better mechanical properties to be be drawn: The fibers as well as the micro hardened glass matrix will break at the same time when stress is high xpected and strain very strong 4.5. Optical properties of the composites 4.4. Mechanical properties of Nextel 440-fibers/8650 matrix glass composites As described above, composites with Because of identical values of x and n for the fibers and C-coated fibers are black le matrix, these composites should be both damage-toler--BN-coated Nextel 440 fibers and 756 glass matrix are ant and transparent in the case of a bn coating of the white, i.e., because of the non-adapted optical refractive fibers. Fig. 16 shows the stress-strain curves for two test index they are not transparent, and samples of one and the same composite. The two curves- BN-coated Nextel 440 fibers and N-SK 4 glass matrix prove the variability of the mechanical properties(extre- are translucent white although their refractive indices mely deviating curves were chosen). The fibers seem to be are identical; the internal micro stresses(A), however clamped slightly, which increases strength in comparison cause light scattering with Fig. 10. The fiber pull-out occurs only in a very restricted stress range. From this, it follows that a sligl The composites with an 8650 glass matrix should now be difference in a, 1.e. F >M, will support damage tolerance transparent as both a and n for the fibers and the matrix better than completely identical values of a of both the fiber glass are identical. Indeed, they really are transparent, and the matrix but transparency is not as good as expected. As already pointed out above, the investigations were only carried out with fibers with a single bn-coating. The results of the wavelength-dependent transmission obtained for com- posites with an N-sK 4 glass matrix as well as an 8650 glass matrix are shown in Fig. 17. In the case of the N-sK glass matrix, transmission will increase in the visible wavelength range by up to 20%if the fiber is coated with TiO2, or only by up to 15% in the case of a BN/TiO2 double-layer coating. If 8650 glass is used as matrix, the transmission of the composite will be considerably higher already in the case of a fiber with a sin gle BN coating, increasing up to 45%. The composite wil be colorless transparent. However, colorless actually stand in contradiction to the behavior of the curve in Fig. 17 This fact can only be explained on the basis of the effects exerted by the Mie or the rayleigh scattering Mie scatter Fig. 15. Fiber pull-out and matrix fracture in a Nextel 440/BN, TiO,/ ing will occur if heterogeneities of the order of magnitude of the wavelength used are found in the glass, i.e., rangingdeveloping during cooling is acting on the fibers. These are well distributed micro stresses which, at first, will not exceed the strength of the composite at no point. As the portion of BN-coated fibers in the composites amounts to 16–17 vol% only, the E-modulus of the matrix glass will be predominant. Up to a bending stress of 180 MPa, the mechanical behavior will only be linear (Hooke’s law). If, however, the bending stress exceeds this value, then the BN interface between the fibers and the matrix will enable the fibers to slide despite the radial compressive stress. At what stress sliding sets in will depend on the thickness of fiber coating. The fiber pull-out is illustrated in Fig. 15 and does not seem to differ from the damage-tolerant behavior of other composites. The fracture aspect of the matrix, however, shows another interesting effect. The very small lumps of glass look like broken, hardened single-sheet safety glass. Therefore, the following preliminary conclusion concern￾ing the failure mechanism of these special composites can be drawn: The fibers as well as the micro hardened glass matrix will break at the same time when stress is high and strain very strong. 4.4. Mechanical properties of Nextel 440-fibers/8650 matrix glass composites Because of identical values of a and n for the fibers and the matrix, these composites should be both damage-toler￾ant and transparent in the case of a BN coating of the fibers. Fig. 16 shows the stress–strain curves for two test samples of one and the same composite. The two curves prove the variability of the mechanical properties (extre￾mely deviating curves were chosen). The fibers seem to be clamped slightly, which increases strength in comparison with Fig. 10. The fiber pull-out occurs only in a very restricted stress range. From this, it follows that a slight difference in a, i.e. aF > aM, will support damage tolerance better than completely identical values of a of both the fiber and the matrix. Due to the fact that the financial support of the research work carried out so far has come to a stop, composites made from double-coated Nextel 440 fibers and an 8650 glass matrix will be have to be investigated further in the future. They allow better mechanical properties to be expected. 4.5. Optical properties of the composites As described above, composites with – C-coated fibers are black, – BN-coated Nextel 440 fibers and 756 glass matrix are white, i.e., because of the non-adapted optical refractive index they are not transparent, and – BN-coated Nextel 440 fibers and N-SK 4 glass matrix are translucent white although their refractive indices are identical; the internal micro stresses (Da), however, cause light scattering. The composites with an 8650 glass matrix should now be transparent as both a and n for the fibers and the matrix glass are identical. Indeed, they really are transparent, but transparency is not as good as expected. As already pointed out above, the investigations were only carried out with fibers with a single BN-coating. The results of the wavelength-dependent transmission obtained for com￾posites with an N-SK 4 glass matrix as well as an 8650 glass matrix are shown in Fig. 17. In the case of the N-SK glass matrix, transmission will increase in the visible wavelength range by up to 20% if the fiber is coated with TiO2, or only by up to 15% in the case of a BN/TiO2 double-layer coating. If 8650 glass is used as matrix, the transmission of the composite will be considerably higher already in the case of a fiber with a sin￾gle BN coating, increasing up to 45%. The composite will be colorless transparent. However, colorless actually stands in contradiction to the behavior of the curve in Fig. 17. This fact can only be explained on the basis of the effects exerted by the Mie or the Rayleigh scattering. Mie scatter￾ing will occur if heterogeneities of the order of magnitude of the wavelength used are found in the glass, i.e., ranging Fig. 15. Fiber pull-out and matrix fracture in a Nextel 440/BN, TiO2/ N-SK 4 glass composite. Fig. 16. Stress–strain-curves of two samples of a Nextel 440-fiber/25 nm BN-layer/8650 glass matrix composite with 15 vol% fiber content. D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373 371