M.-L. Antti et al. /Journal of the European Ceramic Society 24(2004)565-578 is altered to some extent by anisotropy of elastic lay-ups (0/90 and +45)were produced from rectan gular plates of 300x 300 mm. Specimens with 0/90 fibre A monolithic ceramic might be expected to be close to orientation were obtained from two plates(A and B) an ideally notch sensitive material. However, here the processed in different batches; +45 specimens were measured strength will depend on the interaction of the taken from a third plate(C) stress field around the hole with the flaw population in The composites were plain weave with a unit cell of the material thus again introducing a notch size effect. approximately 1200 x 900 microns(Fig. 1). The warp Moreover, for such brittle solids the quantitative and weft fibre tows had average widths of 250 and 340 assessment of notch sensitivity is somewhat impractical microns, respectively. Furthermore, they exhibited up to since a unique value of the notch-free strength is difficult 0.5 deviation from the nominal orientation with respect to determine due to the stochastic nature of their to the side of the test bars in both the 0/90 and +450 fracture stress sam A number of semi-empirical models have been pro- posed to describe the notch sensitivity of long fibre, 3.2. Experimental procedure minate composites including the effective crack model proposed by Waddoups et al., I the point stress model2 Tensile tests were performed on centre circular not- and the average stress model. Primarily developed for ched straight-sided specimens with a length of 100 mm polymer matrix composites they have also proved for tests at ambient temperature and 200 mm for tests at to long fibre reinforced ceramics. Above all high-temperature. Their width was 12.5 mm. The cir- they can be used to predict the effect of hole size on the cular centre hole with its axis normal to the Lw surface basis of a limited number of experimental measurements (Fig. 2) was drilled with a carbide drill. The ratio in combination with fitting procedures. More recently, between hole diameter and specimen width(a/w) was more physically realistic models based on crack bridging one of three nominal values (0.1, 0.25 and 0.4). For mechanisms have been presented which also successfully practical reasons the actual values sometimes deviated predict the effects of notch geometry. However, appli- somewhat from the nominal cation of these models requires knowledge of the bridging forces, which usually implies additional experimental measurement. In the present work the results have been interpreted in terms of the Waddoups approach, not only because of its relative computational convenience but also because it was considered to be appropriate for the composite degraded by heat treatment which was assumed to have become embrittled making measure ment of an unnotched strength difficult 3. Experimental details 3.. Materia The material studied was a commercially available composite( Composite Optics, Inc, San Diego, USA) Fig. I. Fibre fabric structure of the composites. Loading direction + consisting of Nextel 720 fibres in a porous aluminosili cate matrix in the form of 3 mm thick plates. The plates LW surface consisted of 12 0/90 woven layers, with a density of 2.6 g/cm and a fibre volume of approximately 48% The fibre fabric is infiltrated with the matrix in a sol-gel process. After drying with a so-called vacuum-bag technique under low pressure and low temperature, the LT surfac composite is pressureless sintered. 3 Intermediate re- infiltration of pyrolysis steps are not necessary. The A⊙ matrix in these types of composites is characterised by a ansverse loadin porosity level around 30-40%o which renders the direction x Parallel loading matrix sufficiently weak to enable damage tolerance during loading. Test specimens of two different fibre Fig. 2. Definition of test bar surfaces.is altered to some extent by anisotropy of elastic properties.20 A monolithic ceramic might be expected to be close to an ideally notch sensitive material. However, here the measured strength will depend on the interaction of the stress field around the hole with the flaw population in the material thus again introducing a notch size effect. Moreover, for such brittle solids the quantitative assessment of notch sensitivity is somewhat impractical since a unique value of the notch-free strength is difficult to determine due to the stochastic nature of their fracture stress. A number of semi-empirical models have been pro￾posed to describe the notch sensitivity of long fibre, laminate composites including the effective crack model proposed by Waddoups et al.,21 the point stress model22 and the average stress model.22 Primarily developed for polymer matrix composites they have also proved applicable to long fibre reinforced ceramics. Above all they can be used to predict the effect of hole size on the basis of a limited number of experimental measurements in combination with fitting procedures. More recently, more physically realistic models based on crack bridging mechanisms have been presented which also successfully predict the effects of notch geometry.23 However, appli￾cation of these models requires knowledge of the bridging forces, which usually implies additional experimental measurement. In the present work the results have been interpreted in terms of the Waddoups approach, not only because of its relative computational convenience but also because it was considered to be appropriate for the composite degraded by heat treatment which was assumed to have become embrittled making measure￾ment of an unnotched strength difficult. 3. Experimental details 3.1. Material The material studied was a commercially available composite (Composite Optics, Inc, San Diego, USA) consisting of Nextel 720 fibres in a porous aluminosili￾cate matrix in the form of 3 mm thick plates. The plates consisted of 12 0
/90
woven layers, with a density of 2.6 g/cm3 and a fibre volume of approximately 48%. The fibre fabric is infiltrated with the matrix in a sol-gel process. After drying with a so-called vacuum-bag technique under low pressure and low temperature, the composite is pressureless sintered.13 Intermediate re￾infiltration of pyrolysis steps are not necessary. The matrix in these types of composites is characterised by a porosity level around 30–40%10 which renders the matrix sufficiently weak to enable damage tolerance during loading. Test specimens of two different fibre lay-ups (0/90
and 45
) were produced from rectan￾gular plates of 300300 mm. Specimens with 0/90 fibre orientation were obtained from two plates (A and B) processed in different batches; 45
specimens were taken from a third plate (C). The composites were plain weave with a unit cell of approximately 1200900 microns (Fig. 1). The warp and weft fibre tows had average widths of 250 and 340 microns, respectively. Furthermore, they exhibited up to 0.5
deviation from the nominal orientation with respect to the side of the test bars in both the 0/90 and 45
samples. 3.2. Experimental procedure Tensile tests were performed on centre circular not￾ched straight-sided specimens with a length of 100 mm for tests at ambient temperature and 200 mm for tests at high-temperature. Their width was 12.5 mm. The cir￾cular centre hole with its axis normal to the LW surface (Fig. 2) was drilled with a carbide drill. The ratio between hole diameter and specimen width (a/w) was one of three nominal values (0.1, 0.25 and 0.4). For practical reasons the actual values sometimes deviated somewhat from the nominal. Fig. 2. Definition of test bar surfaces. Fig. 1. Fibre fabric structure of the composites. Loading direction $. M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578 567