F Kaya/Ceramics Intemational 33(2007)279-28 Table 1 3. Results and discussion Some physical properties of the composite plates Sample no Average pore Bulk density Tensile In the experiments, two different composite plates with diameter(nm) (g/cm) (%) strength(MPa) different porosity level were tested, named sample 1 which 200 24 0 contains 30% porosity and sample 2 with a porosity level of 2.6 20% as shown in Table 1. A typical test sample prepared from the oxide/oxide composite panels is shown in Fig. 2. The AE results obtained from the sample I are shown in temperature to establish the damage initiation and progression Fig. 3 indicating that up to tensile stress value of 30 MPa,no in real time. First time to date, the effects of porosity and pore acoustic event has taken place, which may indicate that size of the composite on the ae parameters are also examined. specimen deforms elastically up to that point of loading. Fig 3 lso shows that the maximum stress level of the sample l is just 2. Experimental work over 100 MPa. During elastic deformation each fibre lay up within the composite moves in shear due to tensile loading as In the present work the 8HS satin woven mullite(Nextel the schematic illustration of the shear movement of the ceramic 720, 3M, USA)fibre reinforced alumina matrix composites fibre mats is shown in Fig 4 were tested as some of the physical properties of the samples a detailed examination of acoustic parameters such are shown in Table 1. Composite plates tested here contain energy and amplitude values given in Fig 3 has shown that different amount of porosity and a constant volume fraction of acoustic events could be categorised into two distinct sub O% reinforcement fibres [91 groups, as shown in Fig. 5. When the energy values of acoustic Tensile tests were performed on an Instron 5584 tension events are plotted against the amplitude values it could be machine using a cross-head speed of 0. 1 mm/s and tensile observed that acoustic events fit in a band, where amplitude elongation was measured with an extensometer with a gauge values ranges between 50 and 78 dB, while the energy values length of 10 mm. During the loading acoustic activities of the vary between 1 and 1(Joule x 10-8). However, there is a composite specimens were monitored simultaneously using distinct tail at higher energy values of 10 x 10(Joule x10). two nano-30 acoustic sensors with resonance frequency of which could be identified as fibre fractures as shown in Fig. 5 300 kHz. Location of the acoustic sensors on the tensile Results shown in Figs. 3 and 5 clearly explain that amplitude specimens is also given in Fig. 1 levels indicate how loud the acoustic hit occurres while the A 40 dB preamplifier was used individually for each channel energy values indicate how much energy is released due and the activities were captured and analysed using the Mistras fracture of the component. Both of these parameters could be software system loaded into a PC used to identify high stiffness materials such as reinforcement fibres. In ceramic matrix composites both ceramic matrix and the ceramic fibres posses high stiffness, therefore the amplitude values for the fracture of ceramic matrix and fibres could be expected to be comparable in their values. However, both strength(tensile strength of a mullite fibre is approximately 1200 MPa)and the thickness of a ceramic fibre mat (here Centimetre scale 8 mm Fig 1. Schematic diagram of the tensile specimen showing the dimensions of Fig. 2. Woven mullite fibre-reinforced alumina ceramic matrix composite the tensile specimen and location of the acoustic sensors. panel testedtemperature to establish the damage initiation and progression in real time. First time to date, the effects of porosity and pore size of the composite on the AE parameters are also examined. 2. Experimental work In the present work the 8HS satin woven mullite (NextelTM 720, 3M, USA) fibre reinforced alumina matrix composites were tested as some of the physical properties of the samples are shown in Table 1. Composite plates tested here contain different amount of porosity and a constant volume fraction of 40% reinforcement fibres [9]. Tensile tests were performed on an Instron 5584 tension machine using a cross-head speed of 0.1 mm/s and tensile elongation was measured with an extensometer with a gauge length of 10 mm. During the loading acoustic activities of the composite specimens were monitored simultaneously using two nano-30 acoustic sensors with resonance frequency of 300 kHz. Location of the acoustic sensors on the tensile specimens is also given in Fig. 1. A 40 dB preamplifier was used individually for each channel and the activities were captured and analysed using the Mistras software system loaded into a PC. 3. Results and discussion In the experiments, two different composite plates with different porosity level were tested, named sample 1 which contains 30% porosity and sample 2 with a porosity level of 20% as shown in Table 1. A typical test sample prepared from the oxide/oxide composite panels is shown in Fig. 2. The AE results obtained from the sample 1 are shown in Fig. 3 indicating that up to tensile stress value of 30 MPa, no acoustic event has taken place, which may indicate that specimen deforms elastically up to that point of loading. Fig. 3 also shows that the maximum stress level of the sample 1 is just over 100 MPa. During elastic deformation each fibre lay up within the composite moves in shear due to tensile loading as the schematic illustration of the shear movement of the ceramic fibre mats is shown in Fig. 4. A detailed examination of acoustic parameters such as energy and amplitude values given in Fig. 3 has shown that acoustic events could be categorised into two distinct sub￾groups, as shown in Fig. 5. When the energy values of acoustic events are plotted against the amplitude values it could be observed that acoustic events fit in a band, where amplitude values ranges between 50 and 78 dB, while the energy values vary between 13 and 16 (Joule 1018). However, there is a distinct tail at higher energy values of 10 103 (Joule 1018), which could be identified as fibre fractures as shown in Fig. 5. Results shown in Figs. 3 and 5 clearly explain that amplitude levels indicate how loud the acoustic hit occurres while the energy values indicate how much energy is released due to fracture of the component. Both of these parameters could be used to identify high stiffness materials such as reinforcement fibres. In ceramic matrix composites both ceramic matrix and the ceramic fibres posses high stiffness, therefore the amplitude values for the fracture of ceramic matrix and fibres could be expected to be comparable in their values. However, both strength (tensile strength of a mullite fibre is approximately 1200 MPa) and the thickness of a ceramic fibre mat (here 280 F. Kaya / Ceramics International 33 (2007) 279–284 Table 1 Some physical properties of the composite plates tested. Sample no Average pore diameter (nm) Bulk density (g/cm3 ) Porosity (%) Tensile strength (MPa) 1 200 2.4 30 102 2 90 2.6 20 155 Fig. 1. Schematic diagram of the tensile specimen showing the dimensions of the tensile specimen and location of the acoustic sensors. Fig. 2. Woven mullite fibre-reinforced alumina ceramic matrix composite panel tested.