F Kaya/Ceramics Intemational 33(2007)279. Tensile 2 140 120910x 1x10 60100x10 0,03 1x103 00 0000,050.100,150 train amplitude o stress v strain Fig. 7. Tensile test and AE results of sample 2 showing that maximum tensile strength of this sample (155 MPa)is 50% higher than that of sample 1 acoustic response of sample I shown in Fig 3, both energy and amplitude values. As a result there is a direct relationship the amplitude values of this particular specimen are observed to between the tensile strength and the acoustic emission response be higher (see Figs. 3 and 6). Minimum and maximum of the materials as high strength compact material emits high amplitude values recorded from sample I are lower than those level of acoustic activities. recorded during the tensile testing of sample 2, because the Compared to sample 1(see Fig. 6), high-energy value porosity values of these specimens are different as given in acoustic events which are identified as fibre fractures, occur Table 1. Pores in sample I are approximately 200 nm in earlier in sample 2, as shown in Fig 8. An acoustic event with diameters, whereas the size of the pores that observed in sample absolute energy level higher than 10 Joule x10 is recorded 2 is about 90 nm. The porosity levels of sample 1 and sample 2 in 20th second of the monotonic loading, which may indicate are calculated to be 30 and 20%o, respectively. Correspondingly, that the damage tolerant behaviour observed in low interfacial bulk densities of two samples are measured to be 2.4 and 2.6g/ strength composites is in action in sample 2. This may indicate cm, respectively. that stress transfer from the relatively weak matrix ligaments to It is also observed from the Fig. 7 that the tensile strength of the fibre bundles is occurring more effectively when the the sample 2 is approximately 50% higher than that of the porosity content is low. From the initial fibre fracture, the events sample 1(maximum tensile strength of sample l is measured to of the fibre fractures are also observed to be regular, i.e. couple be 102 MPa, whereas the tensile strength of the sample 2 is of fibre bundles fracture every 10 s, as shown in Fig 8 measured to be 155 MPa). Therefore, it could be decisively said Detail microstructural observations were also carried out that high porosity content decreases the tensile strength of find out the effects of the porosity level and pore size on the composites. Porosity content of the ceramic samples is also fracture behaviour of the composite plates. Fig. 9a shows the found to be infuencing the acoustic emission response during cross-sectional SEM micrograph of the sample with 20% the tensile loading, as both amplitude and the energy values of porosity with an average pore size of 90 nm whilst the SEM acoustic events recorded during the tensile loading of sample 2 image of the fracture surface of the composite plate having 30% are observed to be higher than those recorded during the tensile porosity with an average pore size of 300 nm is shown in loading of the sample 1(see Figs. 3 and 7). This result also Fig 9b explains that each pore is actually a discontinuity in the abs energy material therefore under tensile load, higher stresses than that of.- the rest of the cross-section are concentrated around the pores, which may then increase the possibility of crack initiation Therefore, as the loading increases matrix ligaments located g between the pores fracture as this process continues pores 2 10000 merge in to a bigger discontinuity, decreasing the cross-section 100 increases leading to a rapid fracture. Acoustic emission results are affected by the loss of the tensile strength, because as Time, second explained above if the energy generated by the fracture is high, Fig 8. Absolute energy versus time of occurrence plot for sample 2 indicating relevant acoustic event possesses high-level energy and that damage tolerant behaviour occurs in this particular sampleacoustic response of sample 1 shown in Fig. 3, both energy and the amplitude values of this particular specimen are observed to be higher (see Figs. 3 and 6). Minimum and maximum amplitude values recorded from sample 1 are lower than those recorded during the tensile testing of sample 2, because the porosity values of these specimens are different as given in Table 1. Pores in sample 1 are approximately 200 nm in diameters, whereas the size of the pores that observed in sample 2 is about 90 nm. The porosity levels of sample 1 and sample 2 are calculated to be 30 and 20%, respectively. Correspondingly, bulk densities of two samples are measured to be 2.4 and 2.6 g/ cm3 , respectively. It is also observed from the Fig. 7 that the tensile strength of the sample 2 is approximately 50% higher than that of the sample 1 (maximum tensile strength of sample 1 is measured to be 102 MPa, whereas the tensile strength of the sample 2 is measured to be 155 MPa). Therefore, it could be decisively said that high porosity content decreases the tensile strength of composites. Porosity content of the ceramic samples is also found to be influencing the acoustic emission response during the tensile loading, as both amplitude and the energy values of acoustic events recorded during the tensile loading of sample 2 are observed to be higher than those recorded during the tensile loading of the sample 1 (see Figs. 3 and 7). This result also explains that each pore is actually a discontinuity in the material therefore under tensile load, higher stresses than that of the rest of the cross-section are concentrated around the pores, which may then increase the possibility of crack initiation. Therefore, as the loading increases matrix ligaments located between the pores fracture as this process continues pores merge in to a bigger discontinuity, decreasing the cross-section area, rapidly. As a result the net stress on the entire specimen increases leading to a rapid fracture. Acoustic emission results are affected by the loss of the tensile strength, because as explained above if the energy generated by the fracture is high, relevant acoustic event possesses high-level energy and amplitude values. As a result there is a direct relationship between the tensile strength and the acoustic emission response of the materials as high strength compact material emits high level of acoustic activities. Compared to sample 1 (see Fig. 6), high-energy value acoustic events which are identified as fibre fractures, occur earlier in sample 2, as shown in Fig. 8. An acoustic event with absolute energy level higher than 103+ Joule 108 is recorded in 20th second of the monotonic loading, which may indicate that the damage tolerant behaviour observed in low interfacial strength composites is in action in sample 2. This may indicate that stress transfer from the relatively weak matrix ligaments to the fibre bundles is occurring more effectively when the porosity content is low. From the initial fibre fracture, the events of the fibre fractures are also observed to be regular, i.e. couple of fibre bundles fracture every 10 s, as shown in Fig. 8. Detail microstructural observations were also carried out to find out the effects of the porosity level and pore size on the fracture behaviour of the composite plates. Fig. 9a shows the cross-sectional SEM micrograph of the sample with 20% porosity with an average pore size of 90 nm whilst the SEM image of the fracture surface of the composite plate having 30% porosity with an average pore size of 300 nm is shown in Fig. 9b. 282 F. Kaya / Ceramics International 33 (2007) 279–284 Fig. 7. Tensile test and AE results of sample 2 showing that maximum tensile strength of this sample (155 MPa) is 50% higher than that of sample 1. Fig. 8. Absolute energy versus time of occurrence plot for sample 2 indicating that damage tolerant behaviour occurs in this particular sample.