According to the principle of synchronous demodulation,the modulation information in the output signal of vibro-acoustic modulation test was investigated and the nonlinear interaction of sound and vibration with the crack interface in a metallic rod was analyzed.A swept signal excitation was used to obtain a proper high frequency parameter in the test of aluminum rod samples with different sizes of cracks and the modulation information in the output signal was analyzed.It was found that the ultrasound wave in the structure can be separated into two parts.One is the wave which passes through the crack interface modulated by the low frequency vibration,and other wave has no any changes.The nonlinear modulation model was modified and a quantitative method for crack detection was proposed.It is demonstrated by comparing with the ultrasonic C-scanning that this method can be used to estimate the fatigue crack size in the metallic rod.
At the turn of the century, sharp changes in Sino-Indian relations have drawn wide attention of the international community. Relations between the two countries, improved and developed since they restored exchanges of ambassadors in 1976, suffered heavy setbacks because of India' nuclear tests in May 1998. But both sides have proceeded in all cases from the overall public interest and made joint efforts, hence quickly overcoming their temporary difficulties. Ever since June 1999, the two nations have restored and developed ties on the basis of the Five Principles. The past five years have witnessed obvious improvement and rapid development of relations between the two sides. This can be seen from the following respects.
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