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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

As shown in Figure 4.33,

As shown in Figure 4.33, the uniaxial compression strength of the preforms ranged between 1.1 MPa (TOPC10) and 245 MPa for the foamed preform AODY30. The compression strength of the AGPC15 with the glassy frit binder was lower than that of the AOPC20. The FATO hybrid reference preform had a strength of 7.0 MPa. Uniaxial compression strength (MPa) /MPa 300 200 18 16 14 12 10 8 6 4 2 0 AOPC20 AGPC15 TOPC10 AODY30 FATO Figure 4.33 Uniaxial compression strength σc of the preforms. The isostatic compression test was used in order to reproduce the conditions in the squeeze cast infiltration experiments. As the compression strength σc of the AODY30 preform was above the maximum pressure of the porosimeter, the isostatic testing method was omitted for this preform. The relative compressions ciso of the preforms made with pore forming additives as a function of the applied mercury pressure are presented in Figure 4.34. As an incremental pressurisation programme was used, the curves show a step-like form. The results may be categorized into two groups. The first group consisted of the AOPC20 and TOPC20 preforms which exhibited marginal compression up to 1 MPa and subsequently a steep increase in the degree of compression. Up to the maximum applied pressure, the curves described a close to 127

logarithmic compression behaviour, leading to relative volumetric compressions of 0.32 and 0.33 for AOPC20 and TOPC20, respectively. After pressure release to an ambient pressure of 0.1 MPa, a spring back to a constant compression of 0.27 occurred. Relative Rel. volumetric compression c () iso 0.5 0.4 0.3 0.2 0.1 AOPC20 AGPC15 MOPC20 TOPC20 0.0 0.1 1 10 100 Isostatic pressure P Piso (MPa) /MPa iso Figure 4.34 Isostatic compression and pressure relief behaviour of the AOPC20, AGPC15, MOPC20 and TOPC20 preforms. The second group consisted of the MOPC20 and AGPC15 preforms with no significant compression up to 10 MPa. After that pressure, a steep increase in the degree of compression occurred. Compression reached similar levels to AOPC20 and TOPC20 at an applied pressure of 100 MPa. After pressure release, a spring back to 0.26 and 0.29 was measured for AGPC15 and MOPC20 respectively. Similar to the compression behaviour of AOPC20 and TOPC20, the TOPC10 preform and the FATO hybrid preform started to compress after reaching an isostatic pressure of about 1 MPa, as shown in Figure 4.35. FATO showed the greatest compression of all the preforms which was 0.45 at the maximum load of 212 MPa. After pressure relief, a spring back to 0.40 took place. 128

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