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Engineering Materials 2E VOLUME2 Episode 9

Tham khảo tài liệu 'engineering materials 2e volume2 episode 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The statistics of brittle fracture and case study 191 Fig. 18.6. Á flat-faced pressure window. The pressure difference generates tensile stresses in the low-pressure face. Table 18.1 Properties of soda glass Modulus E GPa 74 Compressive strength ơc MPa 1000 Modulus of rupture sr MPa 50 Weibull modulus m 10 Time exponent n 10 Fracture toughness KC MPa m1 2 0.7 Thermal shock resistance AT K 84 Poisson s ratio v for ceramics is close to 0.3 so that R2 ơrnax Ap Rr. 18.12 The material properties of window glass are summarised in Table 18.1. To use these data to calculate a safe design load we must assign an acceptable failure probability to the window and decide on its design life. Failure could cause injury so the window is a critical component we choose a failure probability of 10-6. The vacuum system is designed for intermittent use and is seldom under vacuum for more than 1 hour so the design life under load is 1000 hours. The modulus of rupture ơr 50 MPa measures the mean strength of the glass in a short-time bending test. We shall assume that the test sample used to measure ơr had dimensions similar to that of the window otherwise a correction for volume is necessary and that the test time was 10 minutes. Then the Weibull equation eqn. 18.9 for a failure probability of 10-6 requires a strength-reduction factor of 0.25. And the static fatigue equation eqn. 18.10 for a design life of 1000 hours t t test 104 requires a reduction factor of 0.4. For this critical component a design stress Ơ 50 MPa X 0.25 X 0.4 5.0 MPa meets the requirements. We apply a further safety factor of S 1.5 to allow for uncertainties in loading unforeseen variability and so on. 192 Engineering Materials 2 Pressure p Fig. 18.7. Á hemispherical pressure window. The shape means that the glass is everywhere in compression. We may now specify the dimensions of the window. Inverting eqn. 18.12 gives 1 2 t i s p 0.17. 18.13 R I G A window designed to these specifications should withstand a pressure .