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ULTRASONIC PULSE TRANSMIT-RECEIVER METHOD FOR DETECTING AND MONITORING OF FATIGUE DAMAGE

机译:超声波脉冲发射-接收方法的疲劳损伤检测与监测

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摘要

A dedicated ultrasound transmission method is used for in-situ detection and monitoring of fatigue cracks. For this purpose, piezoelectric transducers are embedded at the ends of hourglass-shaped specimens of HSLA steel. One transducer emits constant-amplitude time-limited pulses which travel as longitudinal and surface waves. The waves are received by the other transducer and their times-of-flight and amplitudes are readily monitored by a digitizing oscilloscope. During constant amplitude fatigue cycling at constant R, a decrease in surface wave amplitude indicates crack initiation while crack propagation is sensed by the change in the amplitude of both waves. During initial fatigue cycles, the transmitted wave amplitudes may decrease due to cyclic plastic strain and the accumulation of dislocations. In subsequent cycles, the lattice defects reach a saturation level giving rise to a steady state level of the transmitted wave amplitudes. Wave amplitudes monitored during saturation stage serve as a reference for subsequent measurements of amplitude decrease that indicates the growth of a fatigue crack. Changing the applied load in the early stage of fatigue cycling has a little effect on the saturation level. Such an effect diminishes with fatigue cycling and does not interfere with the ability of detecting crack initiation. The method is capable of monitoring the entire history of fatigue damage evolution from initial strain hardening, through strain saturation, crack nucleation, crack growth to failure.
机译:专用的超声波传输方法用于原位检测和监测疲劳裂纹。为此,将压电换能器嵌入在HSLA钢的沙漏形试样的末端。一个换能器发出恒定振幅的限时脉冲,该脉冲作为纵向波和表面波传播。这些波由另一个换能器接收,并且它们的飞行时间和幅度可以通过数字示波器轻松监控。在以恒定R进行的恒定振幅疲劳循环过程中,表面波振幅的减小表示裂纹开始,而通过两个波幅的变化感测到裂纹扩展。在初始疲劳周期中,由于周期性塑性应变和位错的积累,透射波的振幅可能会降低。在随后的循环中,晶格缺陷达到饱和水平,从而使透射波幅度达到稳态水平。在饱和阶段监测到的波振幅可作为后续测量振幅下降的参考,这些振幅下降指示疲劳裂纹的增长。在疲劳循环的早期阶段改变施加的载荷对饱和度几乎没有影响。这种作用随着疲劳循环而减小,并且不会干扰检测裂纹萌生的能力。该方法能够监视从初始应变硬化到应变饱和,裂纹成核,裂纹扩展到破坏的整个疲劳损伤演化历史。

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