The hoop stress–strain curves on the outer surface of the sample are constructed using the measured hoop strain and the calculated hoop stress (from internal pressure). The results are shown in Fig. 46.5. The strain in the top figure is measured with strain gage, the strain in the bottom figure is measured with DIC method. The hysteresis of the loading-unloading cycles are clearly seen for each loading and unloading cycle in the top stress–strain curves based on the strain gage data. The hysteresis is not as clear in the stress–strain curve based on the DIC hoop strains (lower figure), the last three hysteresis loops can be vaguely seen. The reason is the lower strain resolution and the low density of data points due to sustainable frame rate limit. The bandwidth of the data bus (from CCD camera to computer) and the write speed of the hard drive limits the usable frame rate to 2 frame per second. A “knee” (bend) is clearly indicated in the stress–strain curve from strain gage data. The knee corresponds to an internal pressure of 69 MPa (10,000 psi). For ceramic composite, the “knee” is typically observed in the uniaxial tension test where the knee indicates the PLS (Proportional Limit Strain); for the case of the SiCf-SiCmsample it is believed to be the stress level when cracking of the SiC matrix occurs [10]. At the knee point, the hoop stress at the outside diameter is around 160 MPa, the hoop strain is 850 m-strains based on DIC data and 550 m-strains based on the strain gage measurement.; this difference in the strain value is also found in Fig. 46.4. 0 50 100 150 200 250 0 500 1000 1500 2000 Hoop Stress (Mpa) Gage Hoop Strain (microstrains) 1Kpsi 2Kpsi 4Kpsi 6Kpsi 8Kpsi 10Kpsi 12Kpsi 14K psi 0 50 100 150 200 250 0 500 1000 1500 2000 Hoop Stress ( Mpa) DIC Hoop strains (microstrains) 10Kpsi 12Kpsi 14Kpsi “Knee” – 10K psi 155MPa a b Fig. 46.5 Hoop stress and strain on the outside surface of the sample. Strain gage (top) and DIC Method (bottom) 392 L.H. Alva et al.
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