K.L. Choy et al./ Materials Science and Engineering 4278 (2000)187-194 this type of end tab was not a suitable representative of Specimen dimensions as recommended by ASTM, BSI and Rae the composite. Subsequently, aluminium end tabs were d(mm) b(mm) /(mm) investigated. Using sample D, a specimen of 25 mm by 100 mm was cut(as recommended by ASTM). Tapered ASTM aluminium end tabs were produced(about 4 mm thick), d+1 with a tab length of approximately 30 mm, leaving a RAE Conclusion sample gauge length of 40 mm. To prevent slippage d+10 form the testing machine, 'Heavy Duty Araldite' was Minimum S for interlaminar shear determination testing rig. used to bond the aluminium tabs. The results proved positive, failure of the specimen occurred in the gauge 2. 4. Image analysis section, without any end tab slippage. Alur minium e tabs were implemented for the final testing The fabricated composites were sectioned, ground ASTM [8], and Royal Aerospace Establishment [91 and polished for image analysis. Both the volume frac- tandards were studied with regards to determining tion of fibres and porosity of the as-fabricated com suitable tensile test dimensions. A large specimen width posites were determined using image analysis coupled was chosen to avoid edge effects and cut edges were with optical microscopy. The errors inherent in this ground down to avoid frictional effects. Testing was conducted on an instron model 4206 at a strain rate of include representativeness of the sample, quality of 2x 10-4 analysis method are well established [5, 6] and sources sample preparation, operator bias and instrument er rors. The main errors found to be incurred during these 2. 6. Interlaminar shear properties particular tests were setting of detector thresholds for feature discrimination and the inhomogeneous mi One important deficiency which affects the applica tion of most fibrous ceramic composites is interlaminar rostructure of the composite materials. At the lowest shear strength(ILS). Laminated and 2-d prefom-rein magnification, such features were readily distinguish ble but one had to compromise between the available forced CMCs are more susceptible to failure in the time for such measurements with respect to the desired matrix-rich interlaminar regions. Such interlaminar fail- ure, or delamination may lead to loss of stiffness and accuracy. possible structural failure. An understanding of failure The outcome of the analysis yielded quantitative mechanisms leading to ILS is therefore crucial information on the volume fraction of fibres V. The The short beam shear test method is found to be data were averaged and presented with their 95% confi- simple and relatively inexpensive for determining shear dence limits. Furthermore, the porosity was also esti- properties. The shear stress distribution along the thick mated but was limited to being determined in the 0 ness of the specimen is a parabolic function, which plies where fibres are observed end on(performed at reaches a maximum at the neutral axis and zero at the higher magnification). This analysis was not performed upper/lower surfaces. However, due to non-constant on the plies where the fibres were transversely aligned bending moments along the shear plane, the maximum due to inaccuracies introduced by fibre tearing mecha shear stress does not occur on the neutral axis [10. A nisms during the sample preparation. Thus, the values more appropriate testing procedure would make use of for porosity are restricted to the bulk of the 0 plies and the thin-walled tube torsion test, thought to exhibit hence, do not represent the matrix rich regions existing pure shear [11]. However, the short beam shear test was between the laminae chosen due to its overriding simplicity. Short beam shear testing in three-point bending re- quires a span between the outer loading points which is 2.5. Tensile testing small enough to induce the shear failure and prevent any tensile failure in the test specimen. The maximum Problems with implementing tensile testing for CMCs shear stress is given as t=3/4(P/bd) and the maxi- are well documented [l], and attempts have been made mum tensile/compressive stresses occurring at the sur o perfect techniques, including elaborate specimen faces for rectangular bar is given by, preparation involving cast integrated epoxy end tabs c=3/2(PS/b]2). Thus, the ratio of tensile(or com 7]. It has been recommended that procedures used for pressive) stress to shear stress yields a parameter, lymer matrix composites be followed [], as well as namely 2S/d. As a rough working rule for laminated established standards [8, 9]. Glass reinforced plastic and CMCs, tensile or compressive failure will occur at aluminium tabs were both investigated as potential end S/d> 20 and shear at S/d<5 [12]. Suitable specimen tabs dimensions were chosen in accordance to ASTM [13], reinforced plastic end tabs was observed. Therefore, Establishment [15] as shown in Table val Aerospace Failure of the specimens in the region of the glass British Standards Institution [14] and RK.-L. Choy et al. / Materials Science and Engineering A278 (2000) 187–194 189 Table 3 Specimen dimensions as recommended by ASTM, BSI and RAE Source d (mm) b (mm) l (mm) S (mm) ASTM 6 – 4 – d d BSI 3 10 20–25 5d+1 RAE 2 5d 5d+10 5d Fixed 7 10 5d+10 da Conclusion a Minimum S for interlaminar shear determination testing rig. this type of end tab was not a suitable representative of the composite. Subsequently, aluminium end tabs were investigated. Using sample D, a specimen of 25 mm by 100 mm was cut (as recommended by ASTM). Tapered aluminium end tabs were produced (about 4 mm thick), with a tab length of approximately 30 mm, leaving a sample gauge length of 40 mm. To prevent slippage form the testing machine, ‘Heavy Duty Araldite’ was used to bond the aluminium tabs. The results proved positive, failure of the specimen occurred in the gauge section, without any end tab slippage. Aluminium end tabs were implemented for the final testing. ASTM [8], and Royal Aerospace Establishment [9] standards were studied with regards to determining suitable tensile test dimensions. A large specimen width was chosen to avoid edge effects and cut edges were ground down to avoid frictional effects. Testing was conducted on an Instron model 4206 at a strain rate of 2×10−4 s−1 . 2.6. Interlaminar shear properties One important deficiency which affects the applica￾tion of most fibrous ceramic composites is interlaminar shear strength (ILS). Laminated and 2-d prefom-rein￾forced CMCs are more susceptible to failure in the matrix-rich interlaminar regions. Such interlaminar fail￾ure, or delamination may lead to loss of stiffness and possible structural failure. An understanding of failure mechanisms leading to ILS is therefore crucial. The short beam shear test method is found to be very simple and relatively inexpensive for determining shear properties. The shear stress distribution along the thick￾ness of the specimen is a parabolic function, which reaches a maximum at the neutral axis and zero at the upper/lower surfaces. However, due to non-constant bending moments along the shear plane, the maximum shear stress does not occur on the neutral axis [10]. A more appropriate testing procedure would make use of the thin-walled tube torsion test, thought to exhibit pure shear [11]. However, the short beam shear test was chosen due to its overriding simplicity. Short beam shear testing in three-point bending re￾quires a span between the outer loading points which is small enough to induce the shear failure and prevent any tensile failure in the test specimen. The maximum shear stress is given as tc=3/4{P/bd} and the maxi￾mum tensile/compressive stresses occurring at the sur￾faces for a rectangular bar is given by, sc=3/2{PS/bd2 }. Thus, the ratio of tensile (or com￾pressive) stress to shear stress yields a parameter, namely 2S/d. As a rough working rule for laminated CMCs, tensile or compressive failure will occur at S/d\20 and shear at S/dB5 [12]. Suitable specimen dimensions were chosen in accordance to ASTM [13], British Standards Institution [14] and Royal Aerospace Establishment [15] as shown in Table 3. 2.4. Image analysis The fabricated composites were sectioned, ground and polished for image analysis. Both the volume frac￾tion of fibres and porosity of the as-fabricated com￾posites were determined using image analysis coupled with optical microscopy. The errors inherent in this analysis method are well established [5,6] and sources include representativeness of the sample, quality of sample preparation, operator bias and instrument er￾rors. The main errors found to be incurred during these particular tests were setting of detector thresholds for feature discrimination and the inhomogeneous mi￾crostructure of the composite materials. At the lowest magnification, such features were readily distinguish￾able but one had to compromise between the available time for such measurements with respect to the desired accuracy. The outcome of the analysis yielded quantitative information on the volume fraction of fibres Vf . The data were averaged and presented with their 95% confi- dence limits. Furthermore, the porosity was also esti￾mated but was limited to being determined in the 0° plies where fibres are observed end on (performed at higher magnification). This analysis was not performed on the plies where the fibres were transversely aligned due to inaccuracies introduced by fibre tearing mecha￾nisms during the sample preparation. Thus, the values for porosity are restricted to the bulk of the 0° plies and hence, do not represent the matrix rich regions existing between the laminae. 2.5. Tensile testing Problems with implementing tensile testing for CMCs are well documented [1], and attempts have been made to perfect techniques, including elaborate specimen preparation involving cast integrated epoxy end tabs [7]. It has been recommended that procedures used for polymer matrix composites be followed [1], as well as established standards [8,9]. Glass reinforced plastic and aluminium tabs were both investigated as potential end tabs. Failure of the specimens in the region of the glass reinforced plastic end tabs was observed. Therefore