F Kaya/Ceramics Intemational 33(2007)279-28 Overall, by analysing the Ae parameters shown in Figs. 3-8, properties. Based on the energy level of acoustic events, it is ombined with the microstructural observations presented in also found that during the tensile tests, delamination of the Figs. 9-11, it can be concluded that porosity level and pore size fibres from the matrix takes place with low energy level then affect the strength and acoustic events of the composite. As the multiple matrix cracks form with moderate energy level and omposite plates tested here have weak interfaces [5] between finally reinforcement fibres break with high energy level before fibres and ceramic matrix, acoustic activities with the lowest the composite's final failure. energy level can be classified as fibre debonding, activities with moderate energy level are considered heavily cracks formation References within the ceramic matrix and activities with the highest energy level characterise the fibre fracture. Fibre fracture takes place (1K.K Chawla, Ceramic Matrix Composites econd ed. Kluwer. Aca earlier in a composite sample with higher porosity level as the demic Press, Norvell (MA). Dordrecht, The Netherlands, 2003 large pores act as flaw sites. EA monitoring is shown to be an [2]CKaya. E.G.Butler,ASelcuk,ARBoccaccini,MH.Lewis,Mullite effective and rapid technique to evaluate the damage Nextel 720)fibre-reinforced mullite matrix composites exhibitin mechanisms in fibre-reinforced ceramic composites in real favourable therm roperties, J. Eur. Ceram Soc. 22(200 time. Using this technique, relationships between microstruc 2333-2342. ture and mechanical properties and their effects on damage [3] K.K. Chawla, Z.R. Zu, J-S. Ha, Processing, structure and properties of mullite fiber mullite matrix composites, J. Eur. Ceram. Soc. 16(1996) behaviour of composite materials can easily be correlated for 293-299 design purposes [4] C. Kaya, X Gu, I Dawery, E.G. Butler, Microstructural development of oven mullite fibre-reinforced mullite ceramic matrix composites by 4. Conclusion Sci. Technol. Adv Mater. 3(2002)35-44 5] C. Kaya. F. Kaya, H. Mor, Damage assessment of alumina fibre-rein orced mullite ceramic matrix composites subjected to cyclic fatigue at The damage mechanisms of mullite fibre(Nextel 720) ambient and elevated temperatures, J. Eur. Ceram Soc. 22(2002)447- reinforced alumina ceramic matrix composites with two 452. different porosity contents are investigated under tensile tests [6]C Kaya, E.G. Butler, A.R. Boccaccini, M. H. Lewis, Processing and using acoustic emission technique. Samples with high porosity Characterisation of mullite(NextelM720)fibre-reinforced mullite matrix nposites from hydrothermally processed mullite pI level(30%)and bigger average pore size(200 nm) show lower Krenkel, R, Naslain, H, Schneider, (Eds ) in: Proceedings of High tensile strength compared to the composite sample with lower temperature ceramic matrix composites, Wiley-VCH, Germany, 2001 porosity and pore size of 20%o and 90 nm, respectively. It is also 63964 found that porosity level influences the acoustic emission [7A.G. Beattie, Acoustic emission, principals and instrumentation, J response during the tensile loading, as both amplitude and the Acoust. Emission 2(1983)95-128 [8] F. Kaya. Ph. D. thesis, The University of Birmingham, UK, 2003. energy values of acoustic events of the sample with lower porosity level are observed to be higher than those with higher porosity contents. Acoustic emission results supported by the [10] G.N. Morscher, A.L. Gyekenyesil, The velocity and attenuation of SEM observations indicate that during the tensile tests of the acoustic emission waves in SiC/SiC composites loaded in tension, Comp Sci Technol.62(2002)117-1180. sample with higher level of porosity, large pores within the (11) G.N. Morscher, Stress-dependent matrix cracking in 2D woven SiC-fiber composite act as crack initiation sites and therefore many fibres reinforced melt-infiltrated SiC matrix composites, Comp. Sci. Technol. 64 fracture as soon as the tests start resulting in lower mechanical (2004)1311-1319Overall, by analysing the AE parameters shown in Figs. 3–8, combined with the microstructural observations presented in Figs. 9–11, it can be concluded that porosity level and pore size affect the strength and acoustic events of the composite. As the composite plates tested here have weak interfaces [5] between fibres and ceramic matrix, acoustic activities with the lowest energy level can be classified as fibre debonding, activities with moderate energy level are considered heavily cracks formation within the ceramic matrix and activities with the highest energy level characterise the fibre fracture. Fibre fracture takes place earlier in a composite sample with higher porosity level as the large pores act as flaw sites. EA monitoring is shown to be an effective and rapid technique to evaluate the damage mechanisms in fibre-reinforced ceramic composites in real time. Using this technique, relationships between microstruc￾ture and mechanical properties and their effects on damage behaviour of composite materials can easily be correlated for design purposes. 4. Conclusion The damage mechanisms of mullite fibre (Nextel 720TM) reinforced alumina ceramic matrix composites with two different porosity contents are investigated under tensile tests using acoustic emission technique. Samples with high porosity level (30%) and bigger average pore size (200 nm) show lower tensile strength compared to the composite sample with lower porosity and pore size of 20% and 90 nm, respectively. It is also found that porosity level influences the acoustic emission response during the tensile loading, as both amplitude and the energy values of acoustic events of the sample with lower porosity level are observed to be higher than those with higher porosity contents. Acoustic emission results supported by the SEM observations indicate that during the tensile tests of the sample with higher level of porosity, large pores within the composite act as crack initiation sites and therefore many fibres fracture as soon as the tests start resulting in lower mechanical properties. Based on the energy level of acoustic events, it is also found that during the tensile tests, delamination of the fibres from the matrix takes place with low energy level then multiple matrix cracks form with moderate energy level and finally reinforcement fibres break with high energy level before the composite’s final failure. References [1] K.K. Chawla, Ceramic Matrix Composites, second ed, Kluwer, Aca￾demic Press, Norvell (MA), Dordrecht, The Netherlands, 2003. [2] C. Kaya, E.G. Butler, A. Selcuk, A.R. Boccaccini, M.H. Lewis, Mullite (Nextel 720TM) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties, J. Eur. Ceram. Soc. 22 (2002) 2333–2342. [3] K.K. Chawla, Z.R. Zu, J-S. Ha, Processing, structure and properties of mullite fiber mullite matrix composites, J. Eur. Ceram. Soc. 16 (1996) 293–299. [4] C. Kaya, X. Gu, I. Dawery, E.G. Butler, Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing, Sci. Technol. Adv. Mater. 3 (2002) 35–44. [5] C. Kaya, F. Kaya, H. Mori, Damage assessment of alumina fibre-rein￾forced mullite ceramic matrix composites subjected to cyclic fatigue at ambient and elevated temperatures, J. Eur. Ceram. Soc. 22 (2002) 447– 452. [6] C. Kaya, E. G. Butler, A. R. Boccaccini, M. H. Lewis, Processing and Characterisation of mullite (NextelTM 720) fibre-reinforced mullite matrix composites from hydrothermally processed mullite precursor, in: W., Krenkel, R., Naslain, H., Schneider, (Eds.), in: Proceedings of High temperature ceramic matrix composites, Wiley-VCH, Germany, 2001, pp. 639–644. [7] A.G. Beattie, Acoustic emission, principals and instrumentation, J. Acoust. Emission 2 (1983) 95–128. [8] F. Kaya, Ph.D. thesis, The University of Birmingham, UK, 2003. [9] G.N. Morscher, Modal acoustic emission of damage accumulation in a woven SiC/SiC composite, Comp. Sci. Technol. 59 (1999) 687–697. [10] G.N. Morscher, A.L. Gyekenyesi1, The velocity and attenuation of acoustic emission waves in SiC/SiC composites loaded in tension, Comp. Sci. Technol. 62 (2002) 1171–1180. [11] G.N. Morscher, Stress-dependent matrix cracking in 2D woven SiC-fiber reinforced melt-infiltrated SiC matrix composites, Comp. Sci. Technol. 64 (2004) 1311–1319. 284 F. Kaya / Ceramics International 33 (2007) 279–284