Ceramic stress concentration factors

Using Equation 2 and 602 MPa for xmaxi fh calculated value of xpredicted at room temperature was 448 MPa while the measured values from the torsion tests were in the range of 75.6 - 246.4 MPa. The measui >d values change to 128.8 410.6 MPa when the stress concentration factor (Kt = 1.7) associated with the change in diameter at the Joint (12.7 mm ceramic to 19.5 mm metal) Is accounted for. The measured values are 30 - 90 % of values predicted by FEA. The relatively large scatter In the strength values of the joints should be attributed to the handling of ceramic parts and to the difference in the ideal joints modelled and the real joints tested. 

The FEA values of ozz and ooo at 650"C were obtained from the joint model which was cooled down to room temperature and heated back up to esO C. As shown In Table 3, predicted Increased to 659.4 MPa due to the compressive principal stresses. This value exceeded the strength values of any components In the joint at 650 C and failure did not occur in the ceramic, The values of xmeasured (4,2-17.5 MPa) represents the strength of the weakest component, i.e., the interface region, at 650°C. In this case, the stress concentration factor was not applicable. 

Composites with 0-3 connectivity consist of a three-dimensional polymer matrix loaded with discrete ceramic particles. These are the simplest composites to make, and although they do not have the desirable stress concentration factors found in other connectivity patterns, certain distinct advantages make them extremely interesting. 

A flaw such as a simple spherical pore concentrates the stress on the bonds in the vicinity of the pore by a factor of two over the appHed stress (6) however, most ceramics contain imperfections that enhance the stress to a much greater degree, leading to severe strength reductions. A typical ceramic such as alumina is as much as one hundred times weaker than the theoretical strength. 

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